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Strengths and limitations of zircon Lu-Hf and O isotopes in modelling crustal growth
Strengths and limitations of zircon Lu-Hf and O isotopes in modelling crustal growth Justin L. Payne, David J. McInerney, Karin M. Barovich, Christopher L. Kirkland, Norman J. Pearson, Martin Hand PII: DOI: Reference:
S0024-4937(15)00467-3 doi: 10.1016/j.lithos.2015.12.015 LITHOS 3786
To appear in:
LITHOS
Received date: Accepted date:
7 August 2015 17 December 2015
Please cite this article as: Payne, Justin L., McInerney, David J., Barovich, Karin M., Kirkland, Christopher L., Pearson, Norman J., Hand, Martin, Strengths and limitations of zircon Lu-Hf and O isotopes in modelling crustal growth, LITHOS (2016), doi: 10.1016/j.lithos.2015.12.015
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ACCEPTED MANUSCRIPT Strengths and limitations of zircon Lu-Hf and O isotopes in modelling crustal growth
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Justin L. Payne a*, David J. McInerneyb, Karin M. Barovichc, Christopher L. Kirklandd, Norman J.
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a
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Pearsone and Martin Handc
School of Natural and Built Environments, Mawson Lakes Campus, University of South Australia, SA
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5095 Australia b
Centre for Tectonics, Resources and Exploration (TRaX), University of Adelaide, SA 5005 Australia
d
Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCFS) and Centre
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c
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School of Civil, Environmental and Mining Engineering, University of Adelaide, SA 5005 Australia
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for Exploration Targeting (CET), Department of Applied Geology, Curtin University, WA 6845
e
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Australia
Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC (CCFS),
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Macquarie University, NSW 2109 Australia
*Corresponding Author: Email: [email protected]
Abstract: The robust nature of the mineral zircon, combined with our analytical ability to readily acquire insitu uranium-lead (U-Pb), lutetium-hafnium (Lu-Hf) and oxygen (O) isotopic data, has resulted in a rapid rise in the use of zircon isotopic datasets for studying both the generation of continental crust and its growth through Earth history. In such studies there has been a strong focus on developing
ACCEPTED MANUSCRIPT methods to determine the timing and/or proportion of juvenile magmatic addition to the continental crust. One widespread approach to determine the timing of crustal growth has been the
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construction or fitting of ‘reworking arrays’ to regional Hf isotopic datasets. Simple stochastic
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models are presented which highlight that in many cases apparent reworking arrays are much more
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likely to represent a process of on-going dilution and refertilisation of ancient crust, consistent with “Hot Zone” models of granitoid generation and the need to refertilise lower crustal reservoirs to maintain magmatism. A new compilation of magmatic rock zircon Lu-Hf and O isotope data is used
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to demonstrate that the use of mantle-like O isotope data as a screening tool for “meaningful” Hf model ages is also unlikely to be reliable, with independently constrained data indicating that as few
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as 14 % of Hf model ages provide a meaningful indicator of the timing of crustal growth. The limitations of Hf model ages are discussed with regard to existing approaches for continental growth
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and we demonstrate that popular inverse modelling approaches suffer from a bias created by both
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the use of model ages and numerical artefacts. In an effort to address some of the limitations within
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existing models, we develop stochastic models based on joint calibration of multiple datasets which
Keywords:
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allow for more unique solutions.
Zircon, Lu-Hf isotopes, O isotopes, Continental growth, Crustal evolution
Highlights:
Juvenile addition to “hot zones” yields Lu-Hf arrays mimicking crustal reworking
New zircon Hf-O isotope compilation indicates < 14 % of Hf model ages are reliable
Inverse modelling of global continental growth is limited by numerical artefacts
Joint Hf-O modelling can provide improved constraints on crustal generation
ACCEPTED MANUSCRIPT Contents 1. Introduction 2. Lu-Hf and O isotope systematics
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3. A Persistent Problem: the non-unique isotopic composition of granitoids 3.1 Mixed versus non-mixed – Are Hf model ages meaningful?
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3.1.1 An Oxygen isotope filter for Hf model ages? 3.1.2 Reworking Arrays to show crust generation
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3.2 Reworking Arrays interpreted as crust generation
3.2.1 Lu/Hf compositions of reworked sources
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3.2.2 The isotopic composition of reworked “New Crust” 3.2.3 Fertility of the crust for reworking
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3.2.4 How do linear trends form in regional Hf isotope datasets?
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4. Use of Hf and O isotope data in zircon for assessing global continental growth 4.1 Inverse modelling of global continental growth
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4.1.1 The U-Pb and Hf-only approach 4.1.2 The U-Pb and Hf-O isotope approach
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4.2 Temporal Bias of Hf model age datasets 4.3 Can the zircon record resolve global continental growth? 4.3.1 Stochastic models using O and Hf isotope data
4.4 Limitations of the zircon record 5. Conclusions Acknowledgements References
ACCEPTED MANUSCRIPT 1. Introduction The mechanisms by which the continents grow and are recycled remains a fundamental question in
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the Earth sciences (Belousova et al., 2010; Cawood et al., 2013; Dhuime et al., 2011; Hawkesworth et
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al., 2010; Kamber, 2015). The areal extent, relief and composition of continental crust exerts a
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strong control on global albedo, nutrient supply to the oceans, establishment and maintenance of a habitable planet with ecological diversity, and local and global climate and weather patterns. Hence, the composition, volume and freeboard of the continental crust throughout Earth history has
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significant consequences for atmosphere and biosphere evolution (Campbell and Squire, 2010;
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Kasting, 2013; McKenzie et al., 2014; Peters and Gaines, 2012; Rey and Coltice, 2008). The variety of existing models for the volume of the continents throughout Earth history (Fig. 1) highlights the
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complexity of continental growth and the lack of consensus on the topic. Much of the uncertainty
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surrounding the growth of the continents undoubtedly results from the paucity of geological material available from the first 1.0 – 2.0 billion years of Earth history (Cawood et al., 2013). Our
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inability to directly study the complete geological record from early Earth requires the use of a variety of geochemical, mineralogical and geophysical proxies for the evolution of the continents.
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Numerous lines of evidence have been used to indicate the changing nature of global tectonics throughout Earth history (e.g. igneous whole rock geochemistry (Campbell, 2003; Keller and Schoene, 2012; Smithies et al., 2007), inclusions in diamond (Shirey and Richardson, 2011), osmium isotopes (Pearson et al., 2007), atmospheric oxidation (Campbell and Allen, 2008), changing P-T conditions of metamorphism (Brown, 2006; Stern, 2005)) and associated with this, changes in the rates of growth and destruction of the continental crust. One important way to chart the development of continental crust through time is the use of isotopic systems within the mineral zircon as quantitative proxies for crustal growth and reworking (Belousova et al., 2010; Campbell and Allen, 2008; Condie et al., 2011; Dhuime et al., 2012; Hawkesworth and Kemp, 2006; Iizuka et al., 2010; Kemp et al., 2006; Næraa et al., 2012; Rino et al., 2004; Spencer et al., 2015; Van Kranendonk and Kirkland, 2013; Van Kranendonk et al., 2015; Voice et al., 2011).
ACCEPTED MANUSCRIPT Zircon is a generally robust and strongly refractory mineral that can survive multiple recycling events. It is this robustness and its ability to form in a range of melt compositions, allied with the fact
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that it incorporates uranium (U) and thorium (Th), but excludes lead (Pb), that provides an
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indispensable tool to chart crustal growth through time (e.g. Scherer et al., 2007). The incorporation
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of hafnium (Hf) as a structural element within zircon allows the Lu-Hf isotope system to be used to provide information on the protolith age, addition of juvenile material from the mantle to the crust and/or crustal reworking. In this regard, zircon Lu-Hf has largely replaced whole-rock Sm-Nd as the
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most common isotopic system used in the study of continental growth due in part to the capability for rapid data collection and the ability to link the crustal growth information to the U-Pb
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crystallisation age of the mineral. Analysis of oxygen isotopes in igneous zircon crystals of known age can also be used to trace the evolution of crustal recycling and crust-mantle interactions. Zircon
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crystals diffuse oxygen slowly, even under high-temperature conditions, hence their measured δ18O
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values approximate the crystallization values, provided that no late alteration has occurred (Page et
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al., 2007; Peck et al., 2001). Incorporation of crustal materials that have undergone low temperature weathering or alteration processes increases the 18O values of a melt and also the zircons crystallised from the melt (Valley et al., 2005). Therefore O isotopes have frequently been regarded
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as an effective way to screen zircons, and particularly zircon with juvenile Hf isotope signatures, for the effects of crustal contamination (Dhuime et al., 2012; Hawkesworth and Kemp, 2006).
The ability to rapidly obtain U-Pb age data and Lu-Hf along with O isotope data from single zircon grains by in-situ analytical techniques (SIMS and LA-(MC)ICP-MS) has enabled the acquisition of large datasets with the number of data now in the many thousands for each data type (Belousova et al., 2010; Dhuime et al., 2012; Iizuka et al., 2010; Payne et al., 2015; Roberts and Spencer, 2015). A number of recent studies (Arndt, 2013; Nebel et al., 2014; Roberts and Spencer, 2015) have discussed some of the limitations of various approaches to the use of Lu-Hf and O isotope data in
ACCEPTED MANUSCRIPT zircons. This contribution builds upon the discussion of a number of these studies by presenting new datasets and detailed analysis of existing models to better illustrate some of the necessary
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considerations when using zircon isotopic datasets for interpreting crustal growth. We conclude
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with examples of modelling approaches that hold promise for testing geodynamic models and
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outline a number of issues that remain to be resolved.
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2. Lu-Hf and O isotope systematics
The radioactive decay of 176Lu to 176Hf is useful for studies of continental crust generation due to the
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differing behaviour of Lu and Hf during partial melting within the mantle. Compared to Hf, Lu is less incompatible and preferentially remains in the mantle resulting in a comparatively high Lu/Hf value 176
Lu/177Hf = 0.0384 – 0.0390: Dhuime et al., 2011; Griffin et
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for the depleted mantle (modern day
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Lu/177Hf values typically cited in the range ca. 0.008 to 0.025 (Hawkesworth and Kemp,
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crustal
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al., 2000). The incompatible behaviour of Hf results in its enrichment in the continental crust with
2006; Kemp et al., 2006; Rudnick and Gao, 2003; Vervoort and Blichert-Toft, 1999; Vervoort and Patchett, 1996; Wang et al., 2011). The decay of
Lu to
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Hf then produces relatively high
Hf/177Hf ratios in the mantle (Lu-rich) and low 176Hf/177Hf ratios in the crust (Lu-poor).
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To assist with interpretation of Lu-Hf isotopic data the epsilon notation (Box 1) is commonly used. The epsilon () parameter is defined as parts per 10,000 deviation from a Chondritic Uniform Reservoir (CHUR). Rocks with positive epsilon values (176Hf/177Hf higher than CHUR) at their time of formation are commonly considered to be sourced from dominantly juvenile mantle and/or recently formed crustal material. Those with negative epsilon values (176Hf/177Hf lower than CHUR) are described as evolved and represent re-working of old crustal material or a low Lu/Hf reservoir. Depleted mantle model ages are also commonly determined by back calculating the timing of extraction of a nominal protolith source material from the model depleted mantle composition (Box
ACCEPTED MANUSCRIPT 1). As the Lu/Hf ratio of zircon is not representative of the ratio in the crust from which the host melt was derived a two-stage Depleted Mantle (Crustal) model age TDMc is used. The method proposed
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for zircon by Griffin et al. (2002), and widely adopted since, is to assume an average bulk crustal
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composition of 176Lu/177Hf = 0.015 for the calculation. Some studies determine the value to be used
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for the crustal composition by assessment of trends within regional datasets (e.g. Pietranik et al., 2008) or assign different 176Lu/177Hf compositions based upon the measured O isotope composition of the zircon (e.g. Li et al., 2012). An obvious limitation with two stage model ages is uncertainty
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regarding the appropriate value for the 176Lu/177Hf crustal composition (Roberts and Spencer, 2015). We discuss this aspect specifically in Section 3.2.1. When calculating a TDMc the possible variance in Lu/177Hf of the protolith results in a range of a few hundred million years, depending on how much
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the initial zircon composition differs from the Depleted Mantle (Box 1). A second limitation of TDMc
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ages is the composition used for the Depleted Mantle itself. As discussed by Dhuime et al. (2011)
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and Arndt (2013), it is unlikely that crust generated from the mantle has a composition equal to the
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model Depleted Mantle, which is frequently modelled as a simplistic linear evolution from CHUR to the value of modern day mid-ocean ridge basalts as a proxy of mantle compositions. Dhuime et al. (2011) proposed a New Crust (NC) model composition anchored at the present day by the
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composition of new crust generated in modern island arcs. This equates to an isotopically enriched source with calculated model ages (TNCc) up to a few hundred million years younger than typical TDMc model age calculations. Arndt (2013) uses the range of Hf values (0 to +12) in mafic volcanics in modern arcs to argue that using the New Crust estimate of Dhuime et al. is still likely to underestimate the amount of juvenile material incorporated into new crust and provide erroneous model ages. Although uncertainty exists surrounding the crust and mantle source compositions used for model ages, the primary limitation of TDMc or TNCc ages is the likelihood that a given igneous rock results from the mixing of melts derived from multiple sources or rock packages and hence the model ages also represent a mixing between different age components (e.g. Arndt and Goldstein, 1987; Roberts and Spencer, 2015). These limitations will be discussed further in Section 4.
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O isotope data are reported using delta notation with normalisation to Vienna Standard Mean
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Ocean Water (VSMOW, Box 2). O isotope data in zircon can provide a robust indicator of the isotopic
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composition of the magma from which the zircon crystallised. Some fractionation occurs between
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the parent magma and zircon with 18O in zircon typically on the order of 1-1.5 ‰ higher than the magma. This effect is compositionally dependent with Lackey et al. (2008) defining the general
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relationship 18O(WR-Zircon) ≈ 0.0612(wt% SiO2) – 2.50 ‰. Zircon that crystallises from mantle derived magmas has a limited range of compositions centred around 5.3 ± 0.3 (1 SD, Valley et al.,
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1998). Valley et al. describes a tighter range of zircon compositions (5.32 ± 0.17, 1 SD) for zircon from the Kimberley region and hence it may be the case that in some regions the widely used value
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of 5.3 ± 0.3 (1 SD) represents an over-estimate of the range of mantle-derived magma compositions.
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We use 5.3 ± 0.3 (1 SD) in this review to maintain consistency with recent studies on continental growth (e.g. Dhuime et al., 2012). Fractional crystallisation of mafic magmas can result in
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fractionation of up to 1.5‰ (Muehlenbachs and Byerly, 1982; Taylor and Sheppard, 1986). However, the largest fractionation of O isotopes occurs during low temperature processes and as a result O
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isotope data in magmatic rocks are highly sensitive to incorporation of supracrustal material and fluid alteration. Box 2 shows the common 18O range of various rock types and emphasises the higher 18O values found in sedimentary rocks. Of the rock types listed in Box 2, crustally-derived melts are most likely to be sourced from siliciclastic sediments (sandstones, shales and clays), altered basalts and common igneous rocks (gabbros and granitoids). Of these it is probable that only mafic rocks and their altered equivalents have not significantly changed 18O composition throughout Earth history. Zircon-bearing igneous rocks clearly show a compositional evolution with a wider range of 18O compositions since the end of the Archean (Fig. 2a, Payne et al., 2015; Roberts and Spencer, 2015; Spencer et al., 2014; Valley et al., 2005; Van Kranendonk and Kirkland, 2013; Van Kranendonk et al., 2015). Limited data for siliciclastic sedimentary rocks also indicate a broadened
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An additional consideration in the use of zircon oxygen isotopes is the potential for incorporation of
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H2O into metamict zircon, related to equalizing zones of local charge imbalance (Pidgeon et al., 2013;
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Van Kranendonk et al., 2015). Elevated 16O1H/16O measurements are able to indicate disturbance of the O isotope system in zircon grains that otherwise appear pristine (e.g. oscillatory zoning, concordant U-Pb systematics). These recent studies raise concerns regarding the primary nature of O
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isotope data in ancient zircons, as self-induced crystal damage is a function of both crystal chemistry
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and time.
The combined use of the Lu-Hf and O isotope systems in zircon allows for some level of
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interpretation on whether the host magmas incorporated juvenile or evolved material and if they
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incorporated supracrustal material (Hawkesworth and Kemp, 2006; Kemp et al., 2009; Kirkland et al., 2013b; Wang et al., 2011). The systems are most useful when end-member compositions (i.e. direct
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mantle products or evolved supracrustal melts) are indicated, but theutility decreases for any compositions between these end-members due to the potential for a wide range of non-unique
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solutions for the data. In the following sections we highlight scenarios that may lead to the overinterpretation of information obtained from Hf and O isotope data in zircon and discuss approaches for resolving some of the current limitations in applying Hf and O isotope data to crustal growth.
3. A persistent problem: Are model ages meaningful?
A key consideration is the meaning of Hf model ages and whether it is generally possible to use them to determine crustal growth events directly from zircon Hf(-O) isotope data. The advent of zircon Hf isotope data has driven a resurgence in the direct application of isotopic model ages as accurate
ACCEPTED MANUSCRIPT indicators of the age of protolith separation from the mantle. As outlined in Section 2, Hf model ages are highly model dependent due to the range of
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Lu/177Hf compositions of crustal reservoirs and
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the isotopic composition of mantle reservoirs. The significance and implications of the mixing of
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melt sources has long been recognised for Nd isotope data and model ages (Arndt and Goldstein,
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1987), but the same issue appears to be under-represented in discussions on zircon Hf model ages. Although there is increasing discussion on the limitations of model ages (Arndt, 2013; Bell et al., 2011; Roberts and Spencer, 2015; Zeh et al., 2011), they are still routinely applied and two methods
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are commonly used to assign meaning to Hf model ages on a regional or global scale. The first uses O isotope data as a filter (e.g. Dhuime et al., 2012; Hawkesworth et al., 2010; Wang et al., 2011) while
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the second uses patterns, termed ‘reworking arrays’, within the Hf isotope data to infer ages of crustal generation (e.g. Kemp et al., 2006; Kemp et al., 2015; Kemp et al., 2010; Kirkland et al.,
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2013b; Petersson et al., 2015; Zeh et al., 2014). In this section we highlight some of the limitations
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that still remain with both of these approaches.
3.1 An Oxygen isotope filter for Hf model ages?
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In an attempt to identify juvenile material from zircon isotopic spectra it has been proposed that zircon with a 18O signature that is mantle-like (i.e. 5.3 ± 0.3 ‰, 1SD) either indicates derivation of the parent magma directly from the mantle or from re-melting of a previously mantle-derived rock (Dhuime et al., 2012; Hawkesworth et al., 2010). The inference is that the Hf isotopic signature of the zircon faithfully records the addition of juvenile material to the continental crust, with calculated model ages providing a meaningful age of crustal growth. This premise has recently been questioned by Roberts and Spencer (2015), Iizuka et al. (2010) and Nebel et al. (2011) among others. To demonstrate, in a semi-quantitative manner, the shortcomings of this method for determining crustal growth we have compiled zircon Hf and O isotope data from igneous rocks for which the Hf and O isotope analysis were conducted on the same individual grains. The compilation includes data
ACCEPTED MANUSCRIPT from 167 igneous rocks with a total of 2586 zircons (excluding 200 inherited zircon grains) ranging in age from 3451 Ma to 4.4 Ma. The advantage of this dataset over detrital zircon datasets is that it
Each zircon is classified as either “mantle-like” (within
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in the Supplementary Information.
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allows a direct link to the original host magma petrogenesis. All data and data sources are provided
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uncertainty of 5.3 ± 0.6) or “non-mantle-like” based upon its O isotope composition, and is then further classified as either derived from mantle-derived magmas, purely reworking of continental crust or a mixture of mantle and crustal melts. The difference in age between the age of the zircon
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and its New Crust model age (TNCc) is also calculated and referred to as the Age.
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Fig. 3 provides a schematic representation of the information obtained from this compilation of igneous rock data. The compiled data allows us to determine which zircons/rocks yield Hf model
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ages that are likely to reflect an actual crustal generation event. In the compilation, 29.5 % of
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analysed zircons have a mantle-like O-isotope composition with 23.1 % having a mantle-like Oisotope composition and an evolved Hf model age more than 200 Myr older than their crystallisation
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age (Age>200 Myr). Using the interpretation of Hawkesworth et al. (2010) or Dhuime et al. (2012) for example, this would imply that 23.1% of the igneous rocks were formed solely from reworking of
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mafic crust (or the fractionally-crystallised intermediate to felsic products of mafic magmas) in order for the zircons to yield meaningful model ages. However, using the published petrogenetic models of rocks in the dataset, only 7.7 % of zircons have a mantle-like O composition and are derived purely from reworking of pre-existing crust with no additional new mantle input. The data suggest, including mantle-like O isotope composition zircons with juvenile Hf (Age<200 Myr) compositions (6.4 %) and mantle-like zircons derived from purely crustal reworking, that only 14.1 % of Hf model ages can be considered as meaningful indicators of the time of crust generation. However, this should be considered a maximum as the possibility still exists for both mixing of melts derived from mantle-like crust of different ages and/or non-depleted mantle compositions of the original mantlederived melts. This analysis based on actual petrogenetic information indicates that there is little geological basis to consider O isotope data in zircon as a unique robust filter for the validity of Hf
ACCEPTED MANUSCRIPT model ages as a measure of the time of crust generation. A limitation with the compilation used to reach this conclusion is the relatively small number of magmatic rocks included (n=167, zircon
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n=2585). However, based upon studies of granitoid systems such as Kemp et al. (2009), it seems the
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most likely bias exhibited by the compilation is towards rocks formed purely by crustal melting. In
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Kemp et al., all the modelled Lachlan Orogen granitoid suites contained greater than 20% mantle contribution, even in the ‘S-type’ granite suites that are most commonly considered examples of crustal reworking. This suggests, if anything, the results obtained from our compilation remain an
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over estimate of the proportion of zircons that provide reliable model ages that reflect new crust
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generation.
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3.2 ‘Reworking Arrays’ interpreted as crust generation
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‘Reworking arrays’ are constructed from Hf isotope data when a regression can be fitted to a subset
crust value of
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of regional data along a line that is interpreted to represent crustal evolution (often assigned a bulk Lu/177Hf = 0.015). The intersection of the regressed line with a Depleted Mantle
growth line is then interpreted to be an age of major crustal growth in the region with the slope of
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the line reflecting the 176Lu/177Hf ratio of the reworked crust. Perhaps the most notable discussion of ‘reworking arrays’ in the literature involves the Lu-Hf isotopic data from the Jack Hills zircon and the implications for existence and/or evolution of Hadean continental crust (Amelin et al., 1999; Bell et al., 2011; Kemp et al., 2010; Nebel et al., 2014). Examples from younger geological terrains are also found in the literature (e.g. Kemp et al., 2006; Næraa et al., 2012; Petersson et al., 2015; Pietranik et al., 2008). As the formation and reworking of the Hadean crust may well be significantly different from the remainder of Earth history, the following discussion is focused on the generation of magmatic crust in the post-Hadean. For a ‘reworking array’ to provide a reliable age constraint on an original crust forming event, all reworking events must occur without the addition of any material from crust of a differing age, or juvenile material from the mantle (but see caveat of Section 3.2.4
ACCEPTED MANUSCRIPT and Kirkland et al. (2013)). Additionally, for the isotopic composition of rocks formed in a crustal growth event to record their approximate age they must be derived from the mantle and emplaced
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with little or no mixing of crustal melts of a significantly different age. Bulk or upper crustal
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reservoirs are inherently mixtures of multiple crustal types/compositions/ages and therefore cannot
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produce an accurate single crustal growth age. The most likely source material capable of producing a valid source array will be mafic under-plated crust, preserved oceanic plateaux and island arcs or possibly the mafic lower portions of a continental arc. Two important factors to consider are the
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likely compositions of the possible source reservoirs and the fertility of the source regions.
3.2.1 Lu/Hf compositions of reworked sources
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One of two approaches can be used for assigning a reworked source composition to a ‘reworking
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array’. The first is to assign a particular crustal 176Lu/177Hf composition and assess whether the data
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fit a regression line of that composition. The second approach is to determine a regression line and assess whether the 176Lu/177Hf composition indicated by that line represents an appropriate crustal composition (e.g. Ge et al., 2014; Kemp et al., 2006; Kirkland et al., 2013b; Zeh et al., 2014). Given
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that we are considering the possibility for ‘reworking arrays’ to provide a meaningful age of crustal generation there is likely to be a limited range of 176Lu/177Hf values that should be considered when attempting to constrain a crust generation age. Island arcs, while lithologically simpler than continental arcs, still yield compositions ranging from mafic (or ultramafic) to felsic. However, lithologies in the lower crust of arcs that are likely to be sourced during crustal reworking are predominantly mafic or ultramafic (Clarke et al., 2000; Daczko and Halpin, 2009; Ducea et al., 2015; Jagoutz and Schmidt, 2012; Saleeby et al., 2003). As an example, lithologies from the lower crust of the Kohistan Island Arc, Pakistan, yield average 176Lu/177Hf values of 0.048 ± 0.023 or 0.069 ± 0.029 (excluding ultramafics and dependant upon the volume estimations used (Jagoutz and Schmidt, 2012)). Further estimates for the composition of potential crustal source rocks are taken from the
ACCEPTED MANUSCRIPT upper crustal components of modern island arcs. To maintain consistency with the New Crust of Dhuime et al. (2011) we have used the GEOROC databases for the Aegean, Aleutian, Bismarck
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Honshu, Izu Bonin, Kamchatka, Kermadec, Lesser Antilles, Luzon, Mariana, Scotia, Sunda and Tonga
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Lu/177Hf values for all the compiled arcs range from 0.017 to 0.039, hence all are
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1. The median
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arcs. The 176Lu/177Hf distributions for mafic lithologies within these arcs are given in Fig. 4a and Table
above the bulk continental crust value of 0.015 (Table 1). The data encompass the average compositions of the bulk, upper, mid and lower continental crust (Rudnick and Gao, 2003). However,
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as crustal melt generation invariably averages crustal compositions by incremental and progressive extraction of a few percent partial melting from a large volume of rock, the continual extraction of
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material from a single intrusion with a bulk or upper crustal composition is improbable. This indicates that reworking of arc crust with no additional input should fall along an array with Lu/177Hf values of at least 0.017 but in all likelihood, particularly given the lower crustal
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compositions of the Kohistan Arc , more likely to be significantly higher and on the order of 0.025.
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The compositional range of non-arc mafic under-plated crust is perhaps best represented by the composition of continental Large Igneous Provinces (LIPS) and/or oceanic plateaus. Although it is
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quite likely that ultramafic material is a significant component of a mafic underplate, this material is unlikely to melt to produce significant volumes of zircon-bearing granitoids without refertilisation and thus modification to its isotopic systems. Therefore we use the mafic components of LIPs and plateaus as a proxy for the composition of non-arc under-plated mafic crust, keeping in mind that fractional crystallisation and crustal assimilation processes may mean the basalts provide an underestimate of the
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Lu/177Hf ratios of the under-plated crust. Fig. 4b provides a summary of
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Lu/177Hf data from ten continental flood basalt provinces (green lines) and three oceanic plateaus
(black lines). The median values for the majority of these provinces fall above the bulk continental crust value but they are generally closer to a 176Lu/177Hf ratio of 0.015 and a number fall on or below this ratio. These data provide the option for non-arc mafic under-plated crust to be a potential reservoir source for nominal bulk crustal compositions in ‘reworking arrays’ based upon the
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Lu/177Hf ratios. However, in order for the ‘reworking arrays’ to provide a meaningful age of crust
formation the reworked source material must still be isotopically juvenile when it formed (see 4.2.2).
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The final composition that may be considered as source material capable of producing a reliable
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‘reworking array’ is the oceanic crust of a passive margin. While the oceanic crust does in general
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have an isotopic composition that is closer to the model Depleted Mantle composition, it is still heterogeneous, as demonstrated by the wide array of documented E-MORB, T-MORB and HIMU compositions (Hofmann, 2003; Kamber, 2011). Most importantly, the Lu/Hf composition of oceanic
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crust is incompatible with the concept that it may provide reliable crustal growth ages with the 176
Lu/177Hf value of 616 ocean floor
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commonly used crustal reworking array values. The median
basalts analysed by Jenner and O’Neill (2012) is 0.028, considerably higher than many nominated
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crustal reworking values.
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3.2.2 The isotopic composition of reworked “New Crust” Based upon the above summary it is unlikely that a valid crustal ‘reworking array’ will evolve using a
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bulk crustal 176Lu/177Hf value of 0.015 or, depending on the tectonic setting, potentially not even an ‘island arc’ value of 0.018. Crust produced from a single growth event capable of providing a meaningful depleted mantle model age seems likely to yield a
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Lu/177Hf value >0.02, a value
consistent with some published arrays (Amelin et al., 1999; Næraa et al., 2012). In the case that a ‘reworking array’ is produced then it is still necessary to consider the original isotopic composition of the reworked source. The commonly applied source model for new crust is Depleted Mantle with a modern day Hf value of +17. Although most geoscientists recognise that the composition of the mantle is highly heterogeneous it is not uncommon to read statements such as “the 1.48–1.36 Ga Atype granites cannot represent products of juvenile mantle melts; if they had been derived from melting of a mantle source, they would have εHf values close to the depleted-mantle evolution curve
ACCEPTED MANUSCRIPT (e.g. values of about +10 to +12 at ∼1.4 Ga)” (Goodge and Vervoort, 2006). In that particular example the interpretation of predominantly crustal origin for the granites is demonstrated to be
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correct using a combination of zircon and whole rock geochemistry. It is arguably the exception
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rather than the rule that the mantle source for crustal generation has a model DM composition and
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when considering crustal growth (i.e. the addition of mantle material to the crust) this should be accounted for. Fig. 5 displays the Hf(T) range for the provinces given in Fig. 4a and 4b. The Hf(T) data provided are recalculated from Nd(T) data which are used due to the greater number of data points
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available for the provinces. These data highlight how little of the mafic material yields an isotopic composition that would provide a depleted mantle model age that fits precisely with its timing of
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formation. Of particular note is the observation that each of the flood basalt provinces that have Lu/177Hf values akin to bulk continental crust (0.015) have Hf(t)(recalculated) values significantly
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below the DM and in most cases the provinces are similar to CHUR (Table 1). As this dataset has not
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been filtered for crustal contamination the continental flood basalt provinces may be biased towards 176
Lu/177Hf
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isotopically evolved crustal compositions. However, this would also account for low
values and hence it does not increase the likelihood of the flood basalt provinces as suitable reworked crust sources. Crustal contamination should be less significant for the oceanic arcs and not
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relevant for oceanic plateau or mid-ocean ridge basalts. The compilation of Hoffman (2003) displays a continuum of Nd values for the Atlantic, Pacific and Indian MORBs from +14 to +3 with some Indian MORB samples as low as -6. Given even the ‘simplest’ products of DM melting show a modern day range of 11 epsilon units it is hard to argue that the calculation of DM model ages (from reworking arrays or directly) is justifiable. It seems logical to treat both modern and ancient depleted mantle reservoirs as a range of compositions rather than a single linear model. This diversity and the subsequent impact on model ages is increasingly being recognised (e.g. Arndt, 2013; Roberts and Spencer, 2015) and in some instances the logical application of a range of mantle compositions is used and plotted on Hf diagrams (e.g. Avigad et al., 2015; Kemp et al., 2006).
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3.2.3 Fertility of the crust for reworking
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The varying fertility of crustal rocks is well known. The primary concern with regard to isotopic crustal reworking arrays is the capacity of a given lower crustal reservoir to continuously generate
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new intra-crustal melts without any addition of mantle-derived melts or fluids that would create a mixed isotopic signature.
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Melt generation in the crust occurs most readily when fluxed directly by H2O or via dehydration melt reactions involving the breakdown of muscovite, biotite or hornblende. Wet melting is likely to be
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self-limiting as once melt forms and is able to leave the rock it will also mobilise other fluids, thereby effectively dehydrating the system and stalling further melt production. As discussed by Brown
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(2010), in “the absence of a significant influx of aqueous fluid, metapelites, metagreywackes, meta-
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andesites, and some amphibolites may yield 10–50 vol.% H2O-undersaturated melt at attainable
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crustal temperatures (Clemens, 2006; Clemens and Vielzeuf, 1987; Thompson, 1990).” This melt is interpreted to be progressively extracted as it forms in most instances. The case for on-going melt extraction is provided by the incremental assembly of granitoid plutons and suites (e.g. Annen et al.,
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2015; Miller et al., 2011) and also by the preservation of granulite facies rocks. If significant melt is retained within the system then granulite facies rocks will be retrogressed to amphibolite facies assemblages during the post-peak P-T-t cycle. White and Powell (2002) estimates that greater than 50 – 70% of the generated melt must be extracted in order to preserve anhydrous granulite facies mineral assemblages. The extraction of melt and resultant decrease in bulk fertility subsequently requires higher crustal temperatures (or refertilisation) in order to generate a suitable amount of melt to form new granitoid rocks. That crustal fertility changes is evidenced by the changing composition of granitoid rocks within an orogen with the last magmatic event often represented by ferroan, alkali-, HFSE, Cl-, and F-rich granitoids and/or charnockites that have been variably interpreted to represent incorporation of melt derived from previously dehydrated and melt-
ACCEPTED MANUSCRIPT removed crust (Eby, 1992; Martin, 2006; Martin et al., 2012; Whalen et al., 1987). The need for crustal refertilisation also highlights the conceptual utility of models for granitoid formation such as
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the Hot Zone models of Annen et al. (2006) which inherently refertilise the lower crust through
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introduction of mafic magmas and volatiles (Smith, 2014; Solano et al., 2012).
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3.2.4 How do linear trends form in regional Hf isotope datasets?
The foregoing discussion places significant constraints on the likelihood of a crustal reworking array
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providing a reliable estimate of the age of crustal growth. Geological, geochemical and isotopic evidence indicates that the majority of granitoids contain some mantle component which implies
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the injection of new mafic melts into the site of granite generation. The additional requirement for
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refertilisation to enable continued reworking also implies that any crustal material formed is
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volumetrically diluted by later crustal additions during reworking. This makes a reworking array that represents isotopically static reworking of crust of a single age exceedingly unlikely. However, the fact remains that a number of studies have presented evidence for Hf isotope arrays interpreted to
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be consistent with continued reworking of a single crustal growth event (e.g. Kirkland et al., 2013b; Petersson et al., 2015). This suggests it is worthwhile considering how these linear arrays could be formed in regional datasets. To assist with this we use the Musgrave Province, Central Australia, as a case study. The Musgrave Province records ca. 500 million years of magmatism and has been cited as a terrain that preserves evidence for a linear crustal reworking array (Kirkland et al., 2013b).
An important consideration in the potential production of meaningful reworking arrays is outlined for the Musgrave Provinceby Kirkland et al. (2013b). Kirkland et al. indicate that among several juvenile input events within the Musgrave Province a ca. 1900 Ma crustal growth event was significant. The primary supporting evidence for this interpretation is an apparent crustal reworking
ACCEPTED MANUSCRIPT array supported by zircon Lu-Hf-O isotope data from magmatic rocks that formed from ca. 1600 Ma to ca. 1100 Ma. The magmatic suites include interpreted arc-like rocks at ca. 1600 Ma ranging
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through to the HFSE-rich, ferroan, calc-alkalic to alkali-calcic granites and charnockites of the
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Pitjantjatjara Supersuite at ca. 1200 Ma. There is a general lack of direct geological evidence for
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magmatism at ca. 1900 Ma in the Musgrave Province which Kirkland et al. (2013b) suggested may reflect underplating processes with limited upper crustal remnants. However, other evidence of cryptic juvenile crust production at ca. 1900 Ma has been proposed from neighbouring terrains. The
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Rudall Province (Kirkland et al., 2013a), Capricorn Orogen, buried Madura Province (Kirkland et al., 2015) and north Gawler Craton (Reid et al., 2014) have all been interpreted to have some degree of
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juvenile input at 1900 – 1950 Ma, including the presence of a ca. 1920 Ma granite in the Gawler Craton (Reid et al., 2014). A key discussion in Kirkland et al. is the concept that the addition of
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juvenile mantle-derived melt into high field strength element rich crust will have little affect on the
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isotopic signature of the resultant magma. The calculations of Kirkland et al. suggested for bulk
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assimilation, up to 50% MORB-like melt was required to move the Hf(t) value by more than 1 epsilon unit and in the case of partial melting even more MORB-like material would be required. This is cited as evidence that the HFSE-rich Pitjantjatjara Supersuite granites reliably reflect the original
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composition of the crust they were derived from and cite a reworking array indicating a 1900 Ma crustal growth event. It is worth noting that Smithies et al. (2011) indicate the Pitjantjatjara granites may contain between 30 – 50% mantle-derived melt, allowing for, but not dictating, some slight movement of the isotopic composition of the granites. Although the potential for a c. 1.9 Ga reworking array exists we outline a number of additional considerations below that are of general relevance to other regions. These considerations do not diminish the potential importance of the isotopic inertia of HFSE-rich crustal melts and this is a consideration we revisit in later sections.
To demonstrate some important features of reworking arrays we discuss a simple model of ca. 1900 Ma crust production and reworking and illustrate why more complex processes are likely to be
ACCEPTED MANUSCRIPT necessary in the case of the Musgrave Province (and elsewhere). One important requirement for static reworking of a single source is that lower crustal compositions must remain effectively fixed.
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Prior to the formation of the Pitjantjatjara Supersuite, the Musgrave Province had undergone ~400
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Myr of magmatism including interpreted arc magmatism at ca. 1600 Ma at which time it is clear that
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new mantle additions to the crust occurred (Kirkland et al., 2013b; Wade et al., 2006). Hence, any single lower crustal reservoir (i.e. a 1900 Ma crust) would have been progressively melted and intruded by mantle-derived magmas (e.g. MASH or Hot Zone, Annen et al., 2006; Hildreth and
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Moorbath, 1988; Solano et al., 2012). Each intrusion of mantle-derived magmas will progressively dilute the pre-existing lower crust with the added material and thus it would be highly unlikely that
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crust generated at 1900 Ma could retain either its original lithological form or its original isotopic composition through 400 Myr of magmatic activity. As the Pitjantjatjara Supersuite is the only suite
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potentially indicative of melting of a granulitic lower crust component (e.g. Ferroan, alkali, HFSE-rich
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magmas) it supports continual refertilisation of the lower crust in the preceding 400 Myrs – helping
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to support the isotopic modification of the lower crust through incorporation of mantle-derived fluids and magmas. The second additional consideration is the composition of pre-1200 Ma suites. The ca. 1600-1200 Ma suites have Hf concentrations that suggest any crustally-derived melts had
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lower HFSE concentrations and hence were more sensitive to isotopic modification by incorporation of juvenile material (e.g. ca. 1600 Ma suite - Hf = 4 – 9 ppm (Wade et al. 2006), ca. 1300 Ma Wankanki Suite – Avg. Hf = 5.7 ppm and ca. 1400 Ma Papulankutja Suite – Avg. Hf = 9.6 ppm (Kirkland et al., 2013)). Hence, although the Pitjantjatjara Supersuite may not have its isotopic composition easily modified, the pre-1200 Ma suites are less likely to record a crustal composition that would define the apparent 1.9 Ga crustal reworking array.
This analysis once again leaves us with the question of how linear, apparent reworking arrays form. To provide some insight into this question we use some simple models that attempt to approximate magmatism within a terrain, again using the Musgrave Province as an example. The models use
ACCEPTED MANUSCRIPT stochastic generation of ‘zircon’ Hf isotope data through a number of different scenarios related to the manner in which the lower crustal composition evolves during magmatic events. The mantle
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composition used is a normally distributed New Crust (Dhuime et al., 2011) with an upper 2SD on
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model DM and a conservative Hf concentration of 2ppm. The starting old crust in Fig. 6a and 6b is
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normally distributed with a starting DM composition centred upon 3 Ga and a 2SD range of DM ages of 2.65 – 3.35 Ga. Magmatic events are modelled at 1600, 1400, 1300 and 1200 Ma and the mantle proportions for each event are 0.9 ± 0.05, 0.65 ± 0.05, 0.65 ± 0.05 and 0.8 ± 0.05 (1SD), respectively.
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The latter 3 events are based on the I- and A-type mantle proportions calculated for the Lachlan Fold Belt granites by Kemp et al. (2009). The distribution centred upon 0.9 ± 0.05 for the 1600 Ma event
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is used as an estimate of the proportion of mantle-derived melts in arc magmatism and is influenced by the estimates of 0.8-0.9 mantle melt proportion for the ca 1600 Ma suite in the Musgrave
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Province (Wade et al., 2006). Crustal melt Hf concentrations are 5.3 ppm (Upper Crust of Rudnick
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and Gao, 2003) with the exception of the 1200 Ma event which uses a concentration of 15 ppm,
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adopted from the composition of the HFSE-rich Pitjantjatjara Supersuite (Kirkland et al., 2013b). The isotopic composition of the lower crust in the models evolves as described below.
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Fig. 6a demonstrates the model data generated using the above criteria and a lower crustal composition that is purely ca. 3 Ga old crust (OC) at 1600 Ma, is 50:50 old crust and the generated 1600 Ma crust (NC16) at 1400 Ma, is 40:40:20 OC, NC16 and the generated 1400 Ma crust (NC14) at 1300 Ma and 32:32:16:20 OC, NC16, NC14 and the generated 1300 Ma crust at 1200 Ma. This represents a gradual dilution of pre-existing old crust with new material as it is generated, 50:50 for the arc-event and 80:20 for subsequent events. We have chosen to use the recently generated crust as the dilutant, as opposed to a depleted mantle composition, as this gives a conservative estimate of the dilution of the older crustal component. Similarly the dilutant proportions used are ones that are likely to be conservative in light of models such as the Hot Zone of Annen et al. (2006). The results of Fig. 6a yield a linear trend in the data that could be considered consistent with a reworking
ACCEPTED MANUSCRIPT array. A best-fit line for the data yields a DM crustal growth age of ca. 2.0 Ga and a 176Lu/177Hf slope of 0.0153. This is despite the fact that there is no crust generated at 2.0 Ga. While this is only a
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model, the significance of the model lies in the fact that we have allowed for the gradual change in
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composition of the lower crust. When the lower crustal composition evolves solely along the
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composition of the initial 3 Ga reservoir the results of Fig. 6b are obtained. The data rapidly drop below CHUR reflecting the increased crustal melt proportion (0.35) at 1400 and 1300 Ma and the increased Hf concentration (15 ppm) in the crustal melt at 1200 Ma. Thus with lower crustal dilution
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present, the on-going magmatism can moderate the composition of the lower crust and, although the crust proportions, concentrations and compositions change, the resultant magmatic data tend to
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cluster around CHUR.
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Fig. 6c and 6d consider the case in which a homogeneous lower crust does form at ca. 1900 Ma with
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a 176Lu/177Hf ratio of 0.018. We have used the same mixing models for the magmatic events, with the
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1600 Ma and 1200 Ma events being similar to the petrogenesis published for the age-equivalent suites in the Musgrave Province. In both Fig. 6c and 6d it is immediately apparent that the ‘granites’ generated are more juvenile than those seen in the Musgrave Province (Fig. 6d). Furthermore the
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reworking array in Fig. 6c has a
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Lu/177Hf ratio that is significantly higher than the original crust.
While the model of Fig. 6a does not prove that the Musgrave Province rocks formed by reworking and progressive dilution of a ca. 3 Ga lower crust, Fig. 6c indicates that the rocks are unlikely to have formed by simple reworking of ca. 1.9 Ga new crust, particularly given the petrogenetic constraints provided by whole rock studies (Smithies et al., 2011; Wade et al., 2006). Fig. 6e and 6f show the model results obtained from incorporation of ca. 3.0 Ga crust into the 1900 Ma model of Fig. 6c. In both cases the Archean crust component is static in the sense that it is not diluted over time and effectively stays fertile throughout the magmatic events (without refertilisation that causes isotopic modification). This therefore represents an extreme case of contamination as it is likely the older crust would not remain this predominant and its influence would decrease over time, leading to
ACCEPTED MANUSCRIPT more juvenile signatures for the ca. 1200 Ma magmatism. Fig. 6e demonstrates that an apparent crustal array can be generated using a 1900 Ma crust contaminated by older material. However, the
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model age provided in such a scenario may be insensitive to the actual age of the 1900 Ma crust and
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is largely a function of the age and composition of the 1600 Ma magmatism. Fig. 6g displays the
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same model as Fig. 6e but uses a 2200 Ma crust instead of the 1900 Ma crust. This model returns an apparent reworking array with an age of 1.86 Ga, suggesting the younger juvenile event may have
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limited control on the age provided by the reworking array in some instances.
The models presented in Fig. 6 provide a mechanism by which continued magmatism is able to
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produce linear arrays that could be incorrectly interpreted as purely crustal reworking arrays. It allows for continued refertilisation of the lower crust, is compatible with constraints on mantle input
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into granitoid suites (Kemp et al., 2009) and is consistent with lower crust dilution effects of
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proposed MASH/Hot Zone models (Annen et al., 2006; Solano et al., 2012). We suggest that
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meaningful crustal reworking arrays are probably an exception in the geological record and the likelihood of them producing a meaningful crustal growth age is small. Instead, the assumption should be for progressively diluted crust with consideration given to a potential older initial crustal
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reservoir, as would normally be the case for mixing to produce a single zircon or rock model age. In this regard, as noted by Bell et al. (2011), the most crustally evolved individual zircon Hf isotope values in a terrain may effectively represent pure crustal reworking of an initial juvenile cratonic protolith with little impact from magma mixing with a juvenile component (e.g. Ge et al., 2014). Zircon crystals with less extreme values are then considered to represent magma mixing and it remains difficult to directly determine subsequent rates or timing of crustal growth after initial cratonic protolith formation.
ACCEPTED MANUSCRIPT 4. Use of Hf and O isotope data in zircon for assessing global continental growth A variety of recent studies (e.g. Belousova et al., 2010; Dhuime et al., 2012) use Hf and O isotope
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data to undertake inverse modelling of the isotopic datasets in an attempt to determine the timing
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of juvenile material addition into the continents (i.e. continental growth). An important limitation
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and/or consideration for all such modelling approaches is the issue of mixed Hf model ages (Section 3 and Roberts and Spencer, 2015) and the potential non-unique nature of mixing between multiple reservoirs. In this section we review a number of recent studies and their methods for dealing with
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mixed model ages and investigate further possibilities for modelling continental growth.
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4.1 Inverse modelling of global continental growth
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Two recent studies that have addressed the growth of the continents using global detrital zircon UPb and Hf (±O) isotopic datasets are Belousova et al.(2010), who used a Hf-only approach, and
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Dhuime et al. (2012) who adopted a combined Hf and O isotope approach. These studies are discussed as their underlying principles are commonly applied in other global or terrain-scale
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modelling.
Belousova et al. use U-Pb and Lu-Hf isotope data from a global dataset of modern and ancient detrital zircon (n = 16,445) to calculate the proportion of juvenile material added to the crust through time, and from this produce an integrated curve that was interpreted to represent continental growth through time. The dataset was divided into 100 Myr intervals based upon the UPb age or Depleted Mantle (Crustal) model age (TDMc). Zircons with an U-Pb age for a given time interval but an older TDMc were inferred to represent reworking of evolved crustal material while zircon with a model age within the same time interval as the crystallisation age were considered to represent contribution of juvenile crust during that time period. The number of juvenile zircons was then divided by the total number of juvenile and evolved zircons to provide the proportion of
ACCEPTED MANUSCRIPT juvenile material added during each time interval. The crustal growth curve is derived by integration of the juvenile material addition curve. Belousova et al. acknowledge the likelihood of mixed
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model/crust-formation ages but assume that it is largely negated as mixing will be averaged out over
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the entire duration of Earth history.
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Dhuime et al. (2012) follow the approach of Hawkesworth et al. (2010) in adopting the use of O isotope data in zircon in an attempt to resolve the issue of mixed model ages. O isotopes are used to
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identify mantle-like zircon compositions (18O = 5.3 ± 0.6) that either represent new crustal material derived from the mantle or mantle-derived material that has later been re-melted but thought to
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still preserve a valid mantle model age. For 100 Myr time slices these “non-mixed” model ages are then divided by the total number of model ages for that time slice. Dhuime et al. nominate this as
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the proportion of new crust formation and produce two mathematical relationships (for pre- and
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post-3.2 Ga) which they consider to be representative of new crust formation through time. These relationships are then applied to the model ages of all data in a larger U-Pb and Hf dataset to
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produce the number of Calculated New Crust Ages through time that is used to calculate a juvenile crustal growth curve.
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4.1.1 The U-Pb and Hf-only approach The use of the U-Pb age data in conjunction with the Hf model age data (e.g. Belousova et al.) creates the potential for the calculated Juvenile Proportion curve to be highly dependent upon the U-Pb age distribution, which itself has been suggested as biased due to time varying zircon preservation potential (Cawood et al., 2013; Hawkesworth et al., 2009). Comparison of the Juvenile Proportion curve with the U-Pb age distribution highlights an inverse correlation of peaks and troughs in the two datasets (Fig. 7a and 7b). An inverse correlation may be expected to be a numerical artefact of the method, as the number of U-Pb ages is in the denominator of the Juvenile Proportion calculation. To investigate this apparent numerical dependence of the Juvenile Proportion curve on the U-Pb age distribution we have used the U-Pb age distribution of the
ACCEPTED MANUSCRIPT available Terranechron dataset (n = 12,375) of Belousova et al. in a simple forward model. Synthetic Hf isotope compositions were generated for each U-Pb age data point and used to calculate
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synthetic juvenile crust production curves following the method of Belousova et al.. The synthetic Hf
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isotope data were allowed to range between the reservoir compositions of the Depleted Mantle and 176
Lu/177Hf = 0.0118
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an extreme crustal composition of mafic crust generated at 4.55 Ga by using
(Continental Flood Basalt, Low Ti Siberian Traps (Farmer, 2003), as a proxy for early mafic crust (see Supp. Information A for alternative compositions). In this model the Hf isotope composition was
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assigned a random proportion (pseudo-random number generation algorithm, Microsoft Excel™ 2007) of depleted mantle composition with the remaining isotope composition assigned a random
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value between the depleted mantle and 4.55 Ga mafic crust (Fig. 7a – Random Hf + Random Juvenile). This model represents a random mixing of pre-existing crust and mantle-derived magmas
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and is considered to be a simple approximation of granitoid formation. The model replicates the
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shape of the Belousova et al. juvenile proportion curve (Fig. 7a) and closely mimics the proportion of
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juvenile material for much of Earth history. However, the synthetic data does not reproduce the distribution of Hf data within individual 100 Myr time slices (see Supp. Information A, Fig. S1). The similarity in calculated juvenile proportion but overall difference in Hf highlights that the calculated
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juvenile proportion curve is highly dependent upon the U-Pb age distribution and divorced from the Hf isotope data. Given that the proportion of juvenile material incorporated into each synthetic data point is known it is also possible to further assess the validity of the juvenile proportion returned via the Belousova et al. equation. The mean of the juvenile proportion in each 100 Myr time slice in the synthetic data is approximately 0.5 (Fig. 7c). This is expected for randomly generated data which exist within the range 0 to 1. This constant proportion of juvenile input contradicts the curve calculated using the Belousova et al. approach and further suggests this approach may not produce a model that is capable of accurately identifying juvenile input or new crustal growth. 4.1.2 The U-Pb and Hf-O isotope approach
ACCEPTED MANUSCRIPT The combined use of Hf and O isotopes in studies such as Dhuime et al. contains an underlying assumption that zircon with a mantle-like O isotopic composition yields a Hf model age that
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represents crust formation. By arguing for a non-mixed model age for zircon with mantle-like O
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isotope composition but an old Depleted Mantle model age, Dhuime et al. effectively assert that
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every zircon of this type is produced solely by intra-crustal melting of mafic or fractionallycrystallised igneous rocks. In the dataset of Dhuime et al., 45.1% of all analysed zircons have a mantle-like O composition and 40.4% of all analysed zircons have a mantle-like O composition but a
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Hf model age more than 200 Myr older than their crystallisation age. In order to retain a non-mixed model age this implies that ~ 40% of the magmatic rocks were generated by reworking of igneous
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crust without incorporation of any juvenile mantle-derived melts. This is in stark contrast to the findings of studies such as Kemp et al. (2009; 2007) that demonstrate in the Lachlan Fold Belt the A-
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type granites with mantle-like O isotope compositions contain on the order of 90% juvenile mantle-
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derived melts. Furthermore, Kemp et al. (2009) highlight that almost all granitic rocks contain
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mantle-derived melts with even the ‘S-type’ granite suites containing greater than 20% juvenile material. These examples from such a well-studied region, considered in context of the discussion in Section 3.1, question the assumptions that mantle-like O isotope data can be used uncritically as a
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tool to identify pristine Hf model ages.
The inability to use O isotope data as a simple filter
combined with the demonstrated numerical artefacts introduced by Hf-only inverse modelling indicates that simple inverse modelling approaches are unable to produce reliable temporal information on continental growth.
4.2 Temporal Bias of Hf model age datasets In the previous sections we have discussed the limitations created by mixed Hf model ages, it is also worth considering the specific bias created in the recent geological record through the use of Hf model ages. Roberts and Spencer (2015) highlight that the use of Hf model ages is likely to bias
ACCEPTED MANUSCRIPT crustal growth models towards more growth early in Earth history. A further complexity created by the use of Hf model ages in inverse modelling approaches becomes evident in the Phanerozoic. The
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bias created by use of model ages in the Belousova et al. and Dhuime et al. studies results in a
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perceived decrease and increase of juvenile crust addition in the Phanerozoic, respectively.
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Hf model ages will in almost all cases be significantly older than the formation age of the host magmatic rock. This is logical as the majority of zircon-bearing rocks are likely to contain a component of enriched mantle-derived melts, subducted crust or continental crust that will yield an
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isotopically evolved composition pulling the data away from the depleted mantle composition to
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yield an older model age. Bias is created in the recent geological record when model ages are used as a temporal reference point. In the study of Belousova et al. juvenile crust addition is detected by
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either juvenile Hf isotopic values in a given time-slice or by zircon crystals from subsequent
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magmatic events that yield a model age in the given time-slice of interest. This creates a bias as it is not possible to have zircons from future magmatic events yield a model age from previous recent
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time slices. Given the average difference in age between the crystallisation age of a zircon and its TDMc is typically on the order of 500 Myr (Fig. 8) this suggests analysis of the last 500 Ma (as a
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minimum) of the zircon record using the method of Belousova et al. will be biased against the recording of juvenile crustal addition by the exclusion of magmas and zircons that have not yet formed.
The study of Dhuime et al. shows a significant increase in the proportion of juvenile material in global magmatism in the last 1000 Myr with the most recent two time slices modelled yielding 100 % juvenile magma proportions. While we have demonstrated the use of O isotope data to identify nonmixed model ages is unlikely to be accurate, it is certainly true that zircons produced only from mantle-derived melts or re-melting of a mafic lower crust are highly likely to yield ‘mantle-like’ O isotope values. Additionally the only magmas that can produce zircon with Hf model ages in the last few hundred million years will almost certainly be derived directly from the mantle or re-melting of a
ACCEPTED MANUSCRIPT recently added mafic lower crust. Hence, by default, zircons with recent model ages should have ‘mantle-like’ O isotope values and, following the methods of Dhuime et al., will produce an apparent
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increase in the proportion of juvenile material in global magmatism. The effect will become less
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apparent moving back in the zircon record and the upward inflection of juvenile proportions (Fig. 1B
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in Dhuime et al.) from ca. 1500 Ma onwards may largely represent the decreasing dilution of juvenile model ages by reworked crust model ages. This inference is supported by the relatively consistent ‘mantle-like’zircon proportions in the last 1500 Myrs when calculated using the U-Pb age of the
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zircons as opposed to the Hf model age (Payne et al., 2015).
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The temporal bias introduced by Hf model ages is a further obstacle to their use and, combined with the issue of mixed model ages, lead us to conclude that the direct application of Hf model ages as
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crustal separation ages in continental growth studies should be abandoned. In the following sections
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we discuss alternative methods to obtain information on crustal growth from Hf and O isotope data
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in zircon.
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4.3 Can the zircon record resolve global continental growth? Despite the limitations of current inverse modelling approaches, it remains that zircon arguably provides the most complete record of crustal processes throughout Earth history and holds the greatest promise for resolving the growth of the continents. The primary application of Hf and O isotope data from zircon is likely to be for delineating the rate of juvenile addition to continental magmatism and identifying the nature and contributions of pre-existing crustal reservoirs. The direct inversion of Hf model age data effectively requires the zircon data to be binary in the sense that each zircon/event represents either crustal reworking or juvenile growth. Given that the majority of zircon bearing granitoids are a mixture of mantle- and crustally-derived magmas the approach of continental growth modelling should also attempt to resolve this mixing on an event-by-event or
ACCEPTED MANUSCRIPT time slice basis. The significance of such an approach is that it gives full recognition to the possibility that the majority of crustal growth may occur during time periods with significant reworking of old,
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isotopically evolved crustal material with little apparent zircon isotope evidence for juvenile input.
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Such a scenario is particularly well demonstrated by the Pitjantjatjara Supersuite in the Musgrave
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Province. This event is likely to have had more than a 50% contribution from the mantle and hence may have generated in excess of 150,000 km3 of new crust (total suite volume of 75,000km2 x 4km thick, Smithies et al., 2011) while producing granitoids with Hf isotopic compositions centred around
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CHUR (Kirkland et al., 2013b). Inverse modelling is unable to fully resolve progressive crustal growth of this type and hence is likely to bias new crust generation toward the early Earth (Roberts and
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Spencer, 2015). A number of recent studies have developed a range of analysis approaches that do
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growth throughout Earth history.
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not rely on direct inverse modelling and each seeks to constrain variation in crustal recycling or
Iizuka et al. (2010) used mixing calculations with time dependent incorporation of pre-existing
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reservoirs to calculate a Reworking Index. This index characterizes the amount of categorised input that is required to produce mean Hf values in modern detrital zircon (river) datasets (Fig. 9). The
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Iizuka et al. study is significant as it recognises the combined importance of granitoid crust, mafic crust and juvenile input into magmatism and generates models with a variety of input parameters to test the validity of the resultant Reworking Index trends. The strength of the Iizuka et al. approach is potentially limited by the use of mean Hf values only and, as noted by Iizuka et al., uncertainty regarding the applicability of the power law model used for the availability of pre-existing crust for reworking. Van Kranendonk and Kirkland (2013) use O isotopes in zircon along with magmatic rock trace element compositions and scan statistics to highlight periods of increased crustal recycling and sediment subduction at ca. 1.2 Ga. Their approach does not quantify rates but rather focuses on the changes in rates of reworking, as identified through geochemical proxies, to identify evolutionary shifts in global tectonics. Similarly, Van Kranendonk et al. (2015) uses change-point analysis of mean zircon O isotope values to highlight changes in O isotope data at ca. 3.2 and 2.6 Ga (Fig. 9). The 3.2
ACCEPTED MANUSCRIPT Ga change is interpreted to represent the onset of steep subduction. The approach of Van Kranendonk and others (2013, 2015) is not able to determine rates of continental growth, rather just
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episodes where this may be enhanced. However, the multi-variate approach of Van Kranendonk and
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Kirkland (2013) is a valuable example as it demonstrates correlations can exist between large
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datasets of O isotope data in zircon crystals and the chemistry of magmatic rocks. Spencer et al. (2014) note the broadening of O isotope distributions in time frames associated with supercontinent formation and/or collisional tectonics. The mean zircon O isotope compositions are modelled using
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sediment and mantle O isotope compositions through time to generate a crustal reworking index (Fig. 9), as proxied by the proportion of sediment required to be incorporated into mantle melts to
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generate the mean zircon composition. The modelling approach of Iizuka et al. in many respects underlies the approach of Payne et al. (2015) that uses three-component stochastic modelling and
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time varying compositions of magmatic and siliciclastic crust to determine best-fit models for the
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zircon O isotope distributions in 100 Myr time slices. The use of distributions as opposed to mean
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values allows for a greater degree of model differentiation than would otherwise be possible. The best-fit models argue against systematic changes in crustal generation over time and instead O isotope data suggest the most significant variation in juvenile input into magmatism occurs on
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shorter (100-200 Myr) timescales. This approach is limited by the relatively simple model for the incorporation of pre-existing igneous and siliciclastic sedimentary crust (zircon or whole rock data from the preceding 100 Myrs (also tested at 300 Myrs)). A second potential weakness lies in the whole-rock siliciclastic O isotope compilation that may be biased towards seafloor sediments in the last 500 Myrs, or alternatively biased away from seafloor sediments prior to this time period depending upon the importance of subducted sediments in controlling the O isotope composition of magmas. The prescriptive nature of mantle-crust proportion distribution models used (e.g. uniform or truncated normal distributions) is also likely to be a limitation of the modelling approach.
ACCEPTED MANUSCRIPT 4.3.1 Stochastic models using O and Hf isotope data With the exception of Van Kranendonk and Kirkland (2013), each of these studies is limited by their
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reliance upon a single data type. In the case of Payne et al. (2015) this results in a large number of
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statistically viable models for each 100 Myr time slice. For the purposes of this review a number of
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models have been constructed that build upon the work of Payne et al. (2015), to demonstrate analytical approaches that we feel hold potential for future research and highlight some of the limitations and issues surrounding continental growth modelling. We adopt stochastic modelling
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approaches as they provide the ability to generate distributions for isotopic data that can be directly
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tested against observed datasets. This allows for greater refinement of model characteristics than is possible using approaches that use singular measures such as mean values of datasets. As a result,
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obtained best-fit models are likely to be more unique and the approach is better suited to
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undertaking sensitivity testing of input data and model parameters. Stochastic models using Hf and O isotope data were calculated using the zircon Hf isotope datasets
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of Roberts and Spencer (2015) and the zircon O isotope dataset of Payne et al. (2015) updated with additional data included in Supplementary Data Table 1. The complete O isotope data input file is
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provided in Supplementary Data Table 2. Determining the appropriate Hf reservoir composition is significantly more complex than for O isotope reservoirs due to the time varying nature of Hf isotope reservoirs (e.g. Iizuka et al., 2010). For the purposes of this discussion, a very simple approach is adopted. The Hf isotope composition of the siliciclastic sedimentary crust has been estimated using a new compilation of Nd isotope data in siliciclastic sedimentary rocks (drawing from Condie (2014) and Champion (2013)) and the ‘All Sediment’ relationship of Vervoort et al. (1999) (Hf = 1.67Nd + 2.82, Fig. 10, Data and sources are given in Supplementary Data Table 3). The sedimentary data are then divided into 500 Myr timeslices as per Payne et al. (2015). The Hf isotope composition of the pre-existing igneous crust is derived from zircon Hf data from the preceding 100 Myr with resampling to provide distributions of the appropriate size (e.g. 10,000 or 1000 data points). The
ACCEPTED MANUSCRIPT calculated Hf values are then shifted by -1.2 units to represent an expected change in value in 100 Myrs from crust with a bulk 176Lu/177Hf value of 0.015.
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The simplest model constructed uses the approach of Payne et al. (2015) but extends that work by
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testing the obtained O isotope model results against the global Hf isotope datasets. Only 6 models
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out of 2772 models tested provided statistically viable results for the Hf isotope data. This is interpreted to be a direct result of the simple mantle proportion distributions used for the O isotope
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models as the Hf isotope data in each time slice have much greater topography and variation than the O isotope data (Supp. Information B). This limitation can be overcome through the use of
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variable mantle proportion distributions that are calibrated to fit the data. Mantle proportion is divided into 10 bins between 0 and 1, with each bin assigned a uniform probability density. Bin
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probability densities were calibrated such that the Kolmogorov–Smirnov (K-S) statistic between
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simulated and observed zircon distributions was minimized, using the “constrOptim” function in R, based on the Nelder-Mead method (Nelder and Mead, 1965). The graphical output of the variable
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mantle proportion models are provided in Supplementary Information B. Using O isotope data a large number of viable results (KS-test returns a p-value > 0.05, see Payne et al. (2015)) can be
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obtained with widely varying Igneous and Sedimentary crust ratios (I:S) for each time slice. Due to the calibration of the mantle proportion distributions this likely represents an over-fitting of the data with quite disparate mantle proportion distributions obtained depending upon the I:S ratio. Similarly when only Hf isotope data are used, the majority of time slices can be adequately modelled with a range of I:S ratios, but to a much lesser extent than for the O isotope data due to the greater complexity of the Hf isotope distributions. Of note is the inability to fit the Hf isotope data for many of the Hadean and early Archean time slices. The Hf data for these time intervals are largely negative (e.g.Bell et al., 2011; Nebel et al., 2014; and Supp. Information B) and hence the depleted mantle and sedimentary crust compositions used are too juvenile to be able to provide a significant contribution. The inability to return highly biased mantle proportions (e.g. near 0) hints at a nonoptimal calibration routine for extreme distributions. When using a joint calibration routine with
ACCEPTED MANUSCRIPT both Hf and O isotope data only four time slices (900-1000 Ma, 1300-1400 Ma, 1400-1500 Ma, 16001700 Ma) return statistically viable results for both datasets. A number of others return results with
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a reasonable visual fit but fail the KS-test (e.g. 600 – 700 Ma).
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The poor results of the joint calibration emphasize two features that are likely to be limiting factors.
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The first is the need for linked Hf and O isotope datasets. In this study we have used global datasets with the assumption that the large datasets will effectively average out the varying tectonic settings and mechanisms of igneous crust generation such that the Hf and O isotope datasets will be
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unbiased relative to each other for the purposes of the modelling. Given the O isotope dataset is
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much smaller it is quite possible that this is not the case and future modelling efforts will benefit from the sole use of Hf and O isotope data collected from the same zircon grains, requiring more
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data collection. The second consideration is the inherent complexity of global isotopic reservoirs.
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That isotopic reservoirs used in these models are potentially inadequate is demonstrated by the use of a comparatively juvenile Hf sedimentary rock distribution for the Hadean to early Archean time
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slices that display evolved zircon compositions. Many of the limitations associated with reservoir compositions may be related to variability of reservoir compositions both temporally (e.g. Hadean to
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Archean sediments) and spatially (e.g. reworking of an Archean craton versus a Proterozoic island arc terrane by a Neoproterozoic event). Global averaging and the use of distributions for isotopic reservoirs may partially resolve this issue but an improved approach would be to construct models at a terrain or continent scale in the first instance in order to better calculate relevant reservoir compositions. A continent or terrain-based approach would allow for factors such as variable fertility and composition of the lower crust to be accounted for and considerations such as ‘isotopic inertia’ in a given terrain/event to be modelled. That these issues become apparent when using joint calibration methods highlights that they are able to provide comparatively unique solutions (or not as the case may be) for continental growth (e.g. Fig. 11) and better enable identification of uncertainties and limitations in the modelling approach.
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4.4 Limitations of the zircon record
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The robustness and longevity of zircon within the geological record provides an exceptional opportunity to study the growth of the continental crust. However, numerous authors have
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questioned how representative the zircon record is of continental growth or, more precisely, the generation of continental crust forming magmas from the mantle reservoir. Ongoing discussion
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focuses on whether the episodic nature of the zircon U-Pb age histograms (and magmatic rocks in general) correlates to episodic increases in magmatic activity within the Earth (Arndt and Davaille,
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2013; Arndt, 2013; Condie, 1998, 2004; O'Neill et al., 2007) or is largely a function of collisional tectonic settings preferentially reworking and preserving rocks and zircon on the continents
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(Cawood et al., 2013; Condie et al., 2011; Hawkesworth et al., 2009; Hawkesworth et al., 2010; Kemp
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et al., 2006; Spencer et al., 2015; Voice et al., 2011). If the use of the zircon isotope record is focused on determining only the relative proportions of juvenile input into continental magmatism, as
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opposed to volumetric growth directly, then the varying abundance of zircons as a function of preservation potential becomes less important. However, preferential preservation during
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supercontinent formation, for example, may still bias the zircon record towards magmas dominated by crustal reworking (Voice et al., 2011) and effectively disguise the level of juvenile input at that time.
The potential also exists for differential zircon fertility of magmatic events (Dickinson, 2008; Moecher and Samson, 2006) to bias the zircon record away from juvenile crustal growth that has a large component of mafic magmatism (e.g. oceanic plateaux and to a lesser extent oceanic island arcs)(Cawood et al., 2013). In these scenarios a period of crustal growth would be best recorded by subsequent magmatism that shows an increased level of relatively juvenile material in the reworked crust component. Overcoming these issues will require integration of other data types to complement the information that can be derived from the zircon record. Ideally, data such as trace
ACCEPTED MANUSCRIPT element and Nd isotope compositions of the sedimentary record and igneous rock trace elements can be used for joint calibration of models with zircon data, or at the very least testing of models
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produced solely from zircon datasets. In order for such approaches to be fully realised there needs
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to be development of quantitative or semi-quantitative models for considerations such as the
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isotopic signature for recycling of the upper crust into sedimentary basin, zircon fertility of magmatic suites (e.g. Dickinson, 2008), probablisitic relationships for the correlation of zircon and whole rock characteristics and improved constraints on suitable reservoir compositions (Iizuka et al., 2010;
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Payne et al., 2015). Given the regional variation that occurs at any given timeframe within Earth history, particularly of crustal reservoir compositions, it is likely that global crustal generation
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models will need to be compiled from models constructed at the terrane or craton-scale in the first
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instance.
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To determine the volume of the continental crust over Earth history, models for the rates of new crustal generation over time will need to be augmented by models for the rates of destruction of
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continental crust over time, such as determined for the modern day Earth (Scholl and von Huene, 2007; Stern and Scholl, 2010). As the recycling of crustal materials into the mantle via subduction
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also influences the composition of the new crust that is generated, the interdependence of models for crustal growth and destruction may play a pivotal role in accurately determining the history of continental growth on Earth.
5. Conclusions The global zircon record is undoubtedly the most comprehensive dataset available for studying changing tectonic regimes and continental growth and recycling throughout Earth history. The compilation and analysis of Lu-Hf and O isotope data collected from zircon in igneous rocks suggests the majority of zircon-bearing granitoids and subsequently the zircon isotopic signatures represent
ACCEPTED MANUSCRIPT mixtures of melt derived from multiple mantle and crustal sources. Coupled with the non-depleted mantle composition of many mantle reservoirs there is little evidence to suggest that Hf model ages
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are reliable, direct indicators of the timing of juvenile crust addition to the continents. Simple
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stochastic models incorporating the premise of refertilisation of the lower crust through ‘Hot
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Zone’/MASH process demonstrates that apparent crustal reworking arrays in Hf isotope data can be easily produced through progressive dilution of ancient crust during later magmatic events. Combined, these considerations indicate a new generation of modelling approaches are required in
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order to determine global continental growth from the zircon isotopic record. We suggest models using joint calibration of Lu-Hf and O zircon isotope data coupled with other trace element and
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isotopic information hold promise for extracting continental growth from the Earth’s fragmentary
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geological record.
ACCEPTED MANUSCRIPT Acknowledgements: Anthony Reid, Rian Dutch, David Kelsey and Tom Raimondo are thanked for on-going discussions
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surrounding the use of zircon for studying crustal generation and evolution. Priya is thanked for
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assistance with an early compilation of Hf-O isotope data in granitoids. Chris Spencer is thanked for
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comments and Jade Starr Lackey and an anonymous reviewer are thanked for constructive reviews that improved the paper. This is contribution 339 from TRAX, 681 from the ARC Centre of Excellence for Core to Crust Fluid Systems (http://www.ccfs.mq.edu.au) and 1043 in the GEMOC Key Centre
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(http://www.gemoc.mq.edu.au).
ACCEPTED MANUSCRIPT FIGURE CAPTIONS Box 1 – Lu-Hf Isotope systematics
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Box 2 – O isotope systematic
Figure 1 – Example crustal growth models after Cawood et al. (2013) showing the variation present
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in models of global continental growth over time. See discussion in Section 4 on limitations of the Hf
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isotope data of Belousova et al. (2010) and Dhuime et al. (2012).
Figure 2 – Compilation of O isotope data in zircon from the compilations of Valley et al. (2015) and
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Payne et al. (2015) with additional data as provided in Supplementary Appendix 1. Black line represents the maximum value envelope of Valley et al. b) O isotope compilation of siliciclastic
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sedimentary rocks through time after Payne et al. (2015). Both datasets show an expanding range of
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O isotope compositions towards the modern-day.
Figure 3 – Schematic column plot showing a new compilation of zircon Hf-O isotope data from magmatic rocks including the published petrogenetic interpretation for rock formation (i.e. melt sources). The compilation indicates that 23.1% of zircons have a mantle-like O-isotope composition and an evolved Hf model age. However, the petrogenetic information indicates that only 7.7% of the zircons have a mantle-like O isotope composition and are derived solely from crustal reworking. This suggests a maximum of 13.5% (including 5.8 % of juvenile mantle-derived zircons) of Hf model ages in zircon can be considered meaningful. This is a maximum as it does not take into account heterogeneity of the lower crust and non-DM starting compositions.
ACCEPTED MANUSCRIPT Figure 4 – 176Lu/177Hf distributions and median values for a range of ‘mafic’ rocks that may create new crust for later reworking to produce a ‘meaningful’ model age or reworking array. a) Island arcs
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using the subset used by Dhuime et al. for the New Crust model (Range Used - 45-55 wt% SiO2). b)
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Continental large igneous provinces and oceanic plateaux (Caribbean, Kerguelen and Ontong-Java:
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(Range Used - 40-55 wt% SiO2). Data are sourced from pre-compiled datasets on the GEOROC database (http://georoc.mpch-mainz.gwdg.de/georoc/). These data indicate that island arc crust is unlikely to generate crustal reworking arrays with 176Lu/177Hf slopes of 0.015 or 0.018 in most cases.
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LIPs may be amenable to the formation of reworking arrays but see Fig. 5 for limitations.
Figure 5 – Nd isotope data (with n given) for the datasets used in Figure 4. Data has been
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recalculated to Hf(t) using the IAV array of Vervoort et al. (1999) for a) and the Mantle Array of
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Chauvel et al. (2008) for b). Nd isotope data are used to ensure enough data are available for all
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locations. Green dots in b) represent DM model composition at the age of each event. Few of the locations have a distribution overlapping the model DM isotopic composition (as noted by Dhuime et al. (2011), Arndt (2013)). Of note is the general correlation between crustally evolved eHf values and
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bulk crust like 176Lu/177Hf compositions of the LIPs (Fig. 4b).
Figure 6 – Stochastic models approximating the generation of granitoid magmas over a 400 Myr period, loosely based upon the geological history of the Musgrave Province. Four magmatic events are modelled: an arc at 1600 Ma, ‘I-type’ magmatism at 1400 and 1300 Ma and ferroan, alkali, HFSE (anorogenic) magmatism at 1200 Ma. Parameters of the models are given in the text. The model (a) using ca. 3 Ga crust with progressive dilution of the old lower crustal reservoir by each magmatic event. Differing crustal compositions and juvenile inputs produce a pseudo-‘reworking array’ with an apparent crustal 176Lu/177Hf ratio of 0.0153. Juvenile crust produced at 1900 Ma with equivalent
ACCEPTED MANUSCRIPT subsequent magmatism produces an array with a flat 176Lu/177Hf ratio of 0.0232, significantly higher
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than the assigned true crustal composition of 0.018 for the juvenile crust.
Figure 7 – a) Juvenile proportion curve of Terranechron® dataset in Belousova et al. (2010) (red line)
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with juvenile proportion curves for synthetic Hf datasets produced using random Hf isotopic compositions and the Terranechron® U-Pb age distribution. Random Hf + Random Juvenile (blue line)
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uses a random component of depleted mantle Hf isotopic composition with the remaining component a random composition between the depleted mantle and 4.55 Ga mafic crust. Blue
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shaded field represents the range of results obtained from 100 models generated using the Random Hf + Random Juvenile method. b) Terranechron® U-Pb age distribution is provided to highlight the
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inverse correlation between peaks and minimums in the juvenile proportion curves and U-Pb age
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distribution. These models highlight the dependence of the Belousova et al. method on the U-Pb age distribution with little dependence upon Hf isotope data. c) Actual juvenile proportion of Random Hf
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approach.
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+ Random juvenile model compared to the juvenile proportion calculated using the Belousova et al.
Figure 8 – The mean (solid line) and range (field) of the difference in age between the U-Pb ages and two-stage DM ages for zircon in the dataset of Belousova et al. (2010). Demonstrates the general age difference is on the order of 500 Myr and increases slightly towards the modern day. This age difference results in bias being introduced when using zircon model ages as geochronological constraints on O isotope data.
Figure 9 – Estimates of crustal reworking calculated for magmatism during Earth history: Solid Black line – Reworking index of Iizuka et al. (2010) representing the proportion of crust incorporated into
ACCEPTED MANUSCRIPT magmatism; Solid orange line – Reworking Index of Spencer et al. (2014) representing the incorporation of sedimentary material into magmatism; and, grey shaded column plot – Proportion
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of crustal materials incorporated into magmatism from Payne et al. (Payne et al., 2015). Vertical
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orange dashed line represents the timing of an interpreted peak in crustal reworking from Van
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Kranendonk and Kirkland (2013). Vertical blue dashed lines represent inflection points in the level of
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crustal recycling from Van Kranendonk et al. (2015). See text for details on methods used.
Figure 10 – Siliciclastic sediment Hf isotope compositions throughout Earth history as determined by
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recalculation of whole rock Nd isotope data. Data compilation is provided in Supplementary Data
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Table 3.
Figure 11 - Example model results for the 1300 – 1400 Ma time-slice. Column 1 is taken from the
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best-fit model result of Payne et al. (2015). Columns 2 to 4 demonstrate the additional constraints that can be obtained through using both Hf and O isotope data to constrain the models for mantle
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proportion distributions through time. It is worth noting that a number of the best fit models, particularly for Hf-only calibrations return bimodal mantle proportion distributions. For the time slice pictured the joint calibration model can be seen to combine the results of the single system calibrations. The final sedimentary proportion obtained also represents a compromised value between the single system calibrations.
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Lu/
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Hf and Hf compositions of potential crustal reservoirs for reworking - Median and Percentile Values Island Arcs
5% Media n 95%
Hf
5% Media n 95%
Mariana
Scotia
Sunda
Tonga
0.019
0.018
0.008
0.019
0.009
0.010
0.020
0.034
0.027
0.019
0.029
0.021
0.022
0.039
0.049 13.1
0.038 8.8
0.026 5.1
0.046 9.2
0.044 8.7
0.032 3.6
0.054 8.8
15.6
14.8
13.0
10.0
14.5
13.3
7.8
14.2
16.8
16.4
15.0
12.5
16.9
16.6
12.2
16.6
Honshu
Izu Bonin
Kamchatka
Kermadec
0.007
0.010
0.017
0.008
0.021
0.008
0.018
0.020
0.033
0.019
0.037
0.016
0.031 3.4
0.031 10.8
0.056 13.0
0.038 -0.4
0.057 13.8
0.033 13.8
8.7
14.8
14.5
8.8
16.0
10.1
18.3
15.8
13.2
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Table 1 -
Flood Basalt Provinces
Oceanic Plateaus Carribean Kerguele Onton Colombia n g Java n
Deccan
Etendeka Parana
Franklin
KarooFerrar
Mackenzie
North Atlantic
Siberian Traps
Tarim Basin
YellowstoneSnake River
0.012
0.006
0.008
0.017
0.006
0.012
0.007
0.003
0.005
0.012
0.010
0.008
0.020
0.019
0.014
0.015
0.018
0.018
0.018
0.016
0.017
0.010
0.018
0.030
0.014
0.028
0.044
0.026
0.025
0.022
0.025
0.038
0.039
0.025
0.012
0.081
0.039
0.094
-5.0 -19.6 -11.6 -22.1 -9.2 5% Media -0.4 1.9 -4.2 9.2* -2.1 -0.6 n 8.1 9.7 9.7 13.7 7.6 95% *n=4, hence percentiles are not calculated and province is not plotted in Figure 5. ^Values for Ontong-Java Plateau exclude 5 highly negative epsilon values (seen in Figure 5)
-21.8
-14.1
-8.9
-11.6
0.055 12.8
-3.9
7.4^
5.5
1.2
-2.6
-4.0
9.1
-0.3
10.0
12.2
6.4
6.7
10.1
17.1
9.4
11.5
177
Hf
H 5% Media n 95%
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Lu/ f
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ACCEPTED MANUSCRIPT Payne et al. - Strengths and limitations of zircon Lu-Hf and O isotopes in modelling crustal growth
Highlights: Juvenile addition to “hot zones” yields Lu-Hf arrays mimicking crustal reworking
New zircon Hf-O isotope compilation indicates < 14 % of Hf model ages are reliable
Inverse modelling of global continental growth is limited by numerical artefacts
Joint Hf-O modelling can provide improved constraints on crustal generation
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S0024-4937(15)00467-3 doi: 10.1016/j.lithos.2015.12.015 LITHOS 3786
To appear in:
LITHOS
Received date: Accepted date:
7 August 2015 17 December 2015
Please cite this article as: Payne, Justin L., McInerney, David J., Barovich, Karin M., Kirkland, Christopher L., Pearson, Norman J., Hand, Martin, Strengths and limitations of zircon Lu-Hf and O isotopes in modelling crustal growth, LITHOS (2016), doi: 10.1016/j.lithos.2015.12.015
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Strengths and limitations of zircon Lu-Hf and O isotopes in modelling crustal growth
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Justin L. Payne a*, David J. McInerneyb, Karin M. Barovichc, Christopher L. Kirklandd, Norman J.
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a
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Pearsone and Martin Handc
School of Natural and Built Environments, Mawson Lakes Campus, University of South Australia, SA
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5095 Australia b
Centre for Tectonics, Resources and Exploration (TRaX), University of Adelaide, SA 5005 Australia
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Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCFS) and Centre
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c
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School of Civil, Environmental and Mining Engineering, University of Adelaide, SA 5005 Australia
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for Exploration Targeting (CET), Department of Applied Geology, Curtin University, WA 6845
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Australia
Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC (CCFS),
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Macquarie University, NSW 2109 Australia
*Corresponding Author: Email: [email protected]
Abstract: The robust nature of the mineral zircon, combined with our analytical ability to readily acquire insitu uranium-lead (U-Pb), lutetium-hafnium (Lu-Hf) and oxygen (O) isotopic data, has resulted in a rapid rise in the use of zircon isotopic datasets for studying both the generation of continental crust and its growth through Earth history. In such studies there has been a strong focus on developing
ACCEPTED MANUSCRIPT methods to determine the timing and/or proportion of juvenile magmatic addition to the continental crust. One widespread approach to determine the timing of crustal growth has been the
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construction or fitting of ‘reworking arrays’ to regional Hf isotopic datasets. Simple stochastic
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models are presented which highlight that in many cases apparent reworking arrays are much more
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likely to represent a process of on-going dilution and refertilisation of ancient crust, consistent with “Hot Zone” models of granitoid generation and the need to refertilise lower crustal reservoirs to maintain magmatism. A new compilation of magmatic rock zircon Lu-Hf and O isotope data is used
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to demonstrate that the use of mantle-like O isotope data as a screening tool for “meaningful” Hf model ages is also unlikely to be reliable, with independently constrained data indicating that as few
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as 14 % of Hf model ages provide a meaningful indicator of the timing of crustal growth. The limitations of Hf model ages are discussed with regard to existing approaches for continental growth
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and we demonstrate that popular inverse modelling approaches suffer from a bias created by both
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the use of model ages and numerical artefacts. In an effort to address some of the limitations within
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existing models, we develop stochastic models based on joint calibration of multiple datasets which
Keywords:
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allow for more unique solutions.
Zircon, Lu-Hf isotopes, O isotopes, Continental growth, Crustal evolution
Highlights:
Juvenile addition to “hot zones” yields Lu-Hf arrays mimicking crustal reworking
New zircon Hf-O isotope compilation indicates < 14 % of Hf model ages are reliable
Inverse modelling of global continental growth is limited by numerical artefacts
Joint Hf-O modelling can provide improved constraints on crustal generation
ACCEPTED MANUSCRIPT Contents 1. Introduction 2. Lu-Hf and O isotope systematics
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3. A Persistent Problem: the non-unique isotopic composition of granitoids 3.1 Mixed versus non-mixed – Are Hf model ages meaningful?
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3.1.1 An Oxygen isotope filter for Hf model ages? 3.1.2 Reworking Arrays to show crust generation
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3.2 Reworking Arrays interpreted as crust generation
3.2.1 Lu/Hf compositions of reworked sources
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3.2.2 The isotopic composition of reworked “New Crust” 3.2.3 Fertility of the crust for reworking
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3.2.4 How do linear trends form in regional Hf isotope datasets?
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4. Use of Hf and O isotope data in zircon for assessing global continental growth 4.1 Inverse modelling of global continental growth
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4.1.1 The U-Pb and Hf-only approach 4.1.2 The U-Pb and Hf-O isotope approach
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4.2 Temporal Bias of Hf model age datasets 4.3 Can the zircon record resolve global continental growth? 4.3.1 Stochastic models using O and Hf isotope data
4.4 Limitations of the zircon record 5. Conclusions Acknowledgements References
ACCEPTED MANUSCRIPT 1. Introduction The mechanisms by which the continents grow and are recycled remains a fundamental question in
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the Earth sciences (Belousova et al., 2010; Cawood et al., 2013; Dhuime et al., 2011; Hawkesworth et
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al., 2010; Kamber, 2015). The areal extent, relief and composition of continental crust exerts a
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strong control on global albedo, nutrient supply to the oceans, establishment and maintenance of a habitable planet with ecological diversity, and local and global climate and weather patterns. Hence, the composition, volume and freeboard of the continental crust throughout Earth history has
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significant consequences for atmosphere and biosphere evolution (Campbell and Squire, 2010;
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Kasting, 2013; McKenzie et al., 2014; Peters and Gaines, 2012; Rey and Coltice, 2008). The variety of existing models for the volume of the continents throughout Earth history (Fig. 1) highlights the
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complexity of continental growth and the lack of consensus on the topic. Much of the uncertainty
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surrounding the growth of the continents undoubtedly results from the paucity of geological material available from the first 1.0 – 2.0 billion years of Earth history (Cawood et al., 2013). Our
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inability to directly study the complete geological record from early Earth requires the use of a variety of geochemical, mineralogical and geophysical proxies for the evolution of the continents.
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Numerous lines of evidence have been used to indicate the changing nature of global tectonics throughout Earth history (e.g. igneous whole rock geochemistry (Campbell, 2003; Keller and Schoene, 2012; Smithies et al., 2007), inclusions in diamond (Shirey and Richardson, 2011), osmium isotopes (Pearson et al., 2007), atmospheric oxidation (Campbell and Allen, 2008), changing P-T conditions of metamorphism (Brown, 2006; Stern, 2005)) and associated with this, changes in the rates of growth and destruction of the continental crust. One important way to chart the development of continental crust through time is the use of isotopic systems within the mineral zircon as quantitative proxies for crustal growth and reworking (Belousova et al., 2010; Campbell and Allen, 2008; Condie et al., 2011; Dhuime et al., 2012; Hawkesworth and Kemp, 2006; Iizuka et al., 2010; Kemp et al., 2006; Næraa et al., 2012; Rino et al., 2004; Spencer et al., 2015; Van Kranendonk and Kirkland, 2013; Van Kranendonk et al., 2015; Voice et al., 2011).
ACCEPTED MANUSCRIPT Zircon is a generally robust and strongly refractory mineral that can survive multiple recycling events. It is this robustness and its ability to form in a range of melt compositions, allied with the fact
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that it incorporates uranium (U) and thorium (Th), but excludes lead (Pb), that provides an
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indispensable tool to chart crustal growth through time (e.g. Scherer et al., 2007). The incorporation
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of hafnium (Hf) as a structural element within zircon allows the Lu-Hf isotope system to be used to provide information on the protolith age, addition of juvenile material from the mantle to the crust and/or crustal reworking. In this regard, zircon Lu-Hf has largely replaced whole-rock Sm-Nd as the
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most common isotopic system used in the study of continental growth due in part to the capability for rapid data collection and the ability to link the crustal growth information to the U-Pb
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crystallisation age of the mineral. Analysis of oxygen isotopes in igneous zircon crystals of known age can also be used to trace the evolution of crustal recycling and crust-mantle interactions. Zircon
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crystals diffuse oxygen slowly, even under high-temperature conditions, hence their measured δ18O
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values approximate the crystallization values, provided that no late alteration has occurred (Page et
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al., 2007; Peck et al., 2001). Incorporation of crustal materials that have undergone low temperature weathering or alteration processes increases the 18O values of a melt and also the zircons crystallised from the melt (Valley et al., 2005). Therefore O isotopes have frequently been regarded
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as an effective way to screen zircons, and particularly zircon with juvenile Hf isotope signatures, for the effects of crustal contamination (Dhuime et al., 2012; Hawkesworth and Kemp, 2006).
The ability to rapidly obtain U-Pb age data and Lu-Hf along with O isotope data from single zircon grains by in-situ analytical techniques (SIMS and LA-(MC)ICP-MS) has enabled the acquisition of large datasets with the number of data now in the many thousands for each data type (Belousova et al., 2010; Dhuime et al., 2012; Iizuka et al., 2010; Payne et al., 2015; Roberts and Spencer, 2015). A number of recent studies (Arndt, 2013; Nebel et al., 2014; Roberts and Spencer, 2015) have discussed some of the limitations of various approaches to the use of Lu-Hf and O isotope data in
ACCEPTED MANUSCRIPT zircons. This contribution builds upon the discussion of a number of these studies by presenting new datasets and detailed analysis of existing models to better illustrate some of the necessary
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considerations when using zircon isotopic datasets for interpreting crustal growth. We conclude
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with examples of modelling approaches that hold promise for testing geodynamic models and
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outline a number of issues that remain to be resolved.
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2. Lu-Hf and O isotope systematics
The radioactive decay of 176Lu to 176Hf is useful for studies of continental crust generation due to the
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differing behaviour of Lu and Hf during partial melting within the mantle. Compared to Hf, Lu is less incompatible and preferentially remains in the mantle resulting in a comparatively high Lu/Hf value 176
Lu/177Hf = 0.0384 – 0.0390: Dhuime et al., 2011; Griffin et
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for the depleted mantle (modern day
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Lu/177Hf values typically cited in the range ca. 0.008 to 0.025 (Hawkesworth and Kemp,
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crustal
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al., 2000). The incompatible behaviour of Hf results in its enrichment in the continental crust with
2006; Kemp et al., 2006; Rudnick and Gao, 2003; Vervoort and Blichert-Toft, 1999; Vervoort and Patchett, 1996; Wang et al., 2011). The decay of
Lu to
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Hf then produces relatively high
Hf/177Hf ratios in the mantle (Lu-rich) and low 176Hf/177Hf ratios in the crust (Lu-poor).
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To assist with interpretation of Lu-Hf isotopic data the epsilon notation (Box 1) is commonly used. The epsilon () parameter is defined as parts per 10,000 deviation from a Chondritic Uniform Reservoir (CHUR). Rocks with positive epsilon values (176Hf/177Hf higher than CHUR) at their time of formation are commonly considered to be sourced from dominantly juvenile mantle and/or recently formed crustal material. Those with negative epsilon values (176Hf/177Hf lower than CHUR) are described as evolved and represent re-working of old crustal material or a low Lu/Hf reservoir. Depleted mantle model ages are also commonly determined by back calculating the timing of extraction of a nominal protolith source material from the model depleted mantle composition (Box
ACCEPTED MANUSCRIPT 1). As the Lu/Hf ratio of zircon is not representative of the ratio in the crust from which the host melt was derived a two-stage Depleted Mantle (Crustal) model age TDMc is used. The method proposed
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for zircon by Griffin et al. (2002), and widely adopted since, is to assume an average bulk crustal
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composition of 176Lu/177Hf = 0.015 for the calculation. Some studies determine the value to be used
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for the crustal composition by assessment of trends within regional datasets (e.g. Pietranik et al., 2008) or assign different 176Lu/177Hf compositions based upon the measured O isotope composition of the zircon (e.g. Li et al., 2012). An obvious limitation with two stage model ages is uncertainty
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regarding the appropriate value for the 176Lu/177Hf crustal composition (Roberts and Spencer, 2015). We discuss this aspect specifically in Section 3.2.1. When calculating a TDMc the possible variance in Lu/177Hf of the protolith results in a range of a few hundred million years, depending on how much
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the initial zircon composition differs from the Depleted Mantle (Box 1). A second limitation of TDMc
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ages is the composition used for the Depleted Mantle itself. As discussed by Dhuime et al. (2011)
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and Arndt (2013), it is unlikely that crust generated from the mantle has a composition equal to the
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model Depleted Mantle, which is frequently modelled as a simplistic linear evolution from CHUR to the value of modern day mid-ocean ridge basalts as a proxy of mantle compositions. Dhuime et al. (2011) proposed a New Crust (NC) model composition anchored at the present day by the
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composition of new crust generated in modern island arcs. This equates to an isotopically enriched source with calculated model ages (TNCc) up to a few hundred million years younger than typical TDMc model age calculations. Arndt (2013) uses the range of Hf values (0 to +12) in mafic volcanics in modern arcs to argue that using the New Crust estimate of Dhuime et al. is still likely to underestimate the amount of juvenile material incorporated into new crust and provide erroneous model ages. Although uncertainty exists surrounding the crust and mantle source compositions used for model ages, the primary limitation of TDMc or TNCc ages is the likelihood that a given igneous rock results from the mixing of melts derived from multiple sources or rock packages and hence the model ages also represent a mixing between different age components (e.g. Arndt and Goldstein, 1987; Roberts and Spencer, 2015). These limitations will be discussed further in Section 4.
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O isotope data are reported using delta notation with normalisation to Vienna Standard Mean
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Ocean Water (VSMOW, Box 2). O isotope data in zircon can provide a robust indicator of the isotopic
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composition of the magma from which the zircon crystallised. Some fractionation occurs between
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the parent magma and zircon with 18O in zircon typically on the order of 1-1.5 ‰ higher than the magma. This effect is compositionally dependent with Lackey et al. (2008) defining the general
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relationship 18O(WR-Zircon) ≈ 0.0612(wt% SiO2) – 2.50 ‰. Zircon that crystallises from mantle derived magmas has a limited range of compositions centred around 5.3 ± 0.3 (1 SD, Valley et al.,
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1998). Valley et al. describes a tighter range of zircon compositions (5.32 ± 0.17, 1 SD) for zircon from the Kimberley region and hence it may be the case that in some regions the widely used value
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of 5.3 ± 0.3 (1 SD) represents an over-estimate of the range of mantle-derived magma compositions.
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We use 5.3 ± 0.3 (1 SD) in this review to maintain consistency with recent studies on continental growth (e.g. Dhuime et al., 2012). Fractional crystallisation of mafic magmas can result in
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fractionation of up to 1.5‰ (Muehlenbachs and Byerly, 1982; Taylor and Sheppard, 1986). However, the largest fractionation of O isotopes occurs during low temperature processes and as a result O
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isotope data in magmatic rocks are highly sensitive to incorporation of supracrustal material and fluid alteration. Box 2 shows the common 18O range of various rock types and emphasises the higher 18O values found in sedimentary rocks. Of the rock types listed in Box 2, crustally-derived melts are most likely to be sourced from siliciclastic sediments (sandstones, shales and clays), altered basalts and common igneous rocks (gabbros and granitoids). Of these it is probable that only mafic rocks and their altered equivalents have not significantly changed 18O composition throughout Earth history. Zircon-bearing igneous rocks clearly show a compositional evolution with a wider range of 18O compositions since the end of the Archean (Fig. 2a, Payne et al., 2015; Roberts and Spencer, 2015; Spencer et al., 2014; Valley et al., 2005; Van Kranendonk and Kirkland, 2013; Van Kranendonk et al., 2015). Limited data for siliciclastic sedimentary rocks also indicate a broadened
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An additional consideration in the use of zircon oxygen isotopes is the potential for incorporation of
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H2O into metamict zircon, related to equalizing zones of local charge imbalance (Pidgeon et al., 2013;
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Van Kranendonk et al., 2015). Elevated 16O1H/16O measurements are able to indicate disturbance of the O isotope system in zircon grains that otherwise appear pristine (e.g. oscillatory zoning, concordant U-Pb systematics). These recent studies raise concerns regarding the primary nature of O
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isotope data in ancient zircons, as self-induced crystal damage is a function of both crystal chemistry
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and time.
The combined use of the Lu-Hf and O isotope systems in zircon allows for some level of
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interpretation on whether the host magmas incorporated juvenile or evolved material and if they
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incorporated supracrustal material (Hawkesworth and Kemp, 2006; Kemp et al., 2009; Kirkland et al., 2013b; Wang et al., 2011). The systems are most useful when end-member compositions (i.e. direct
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mantle products or evolved supracrustal melts) are indicated, but theutility decreases for any compositions between these end-members due to the potential for a wide range of non-unique
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solutions for the data. In the following sections we highlight scenarios that may lead to the overinterpretation of information obtained from Hf and O isotope data in zircon and discuss approaches for resolving some of the current limitations in applying Hf and O isotope data to crustal growth.
3. A persistent problem: Are model ages meaningful?
A key consideration is the meaning of Hf model ages and whether it is generally possible to use them to determine crustal growth events directly from zircon Hf(-O) isotope data. The advent of zircon Hf isotope data has driven a resurgence in the direct application of isotopic model ages as accurate
ACCEPTED MANUSCRIPT indicators of the age of protolith separation from the mantle. As outlined in Section 2, Hf model ages are highly model dependent due to the range of
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Lu/177Hf compositions of crustal reservoirs and
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the isotopic composition of mantle reservoirs. The significance and implications of the mixing of
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melt sources has long been recognised for Nd isotope data and model ages (Arndt and Goldstein,
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1987), but the same issue appears to be under-represented in discussions on zircon Hf model ages. Although there is increasing discussion on the limitations of model ages (Arndt, 2013; Bell et al., 2011; Roberts and Spencer, 2015; Zeh et al., 2011), they are still routinely applied and two methods
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are commonly used to assign meaning to Hf model ages on a regional or global scale. The first uses O isotope data as a filter (e.g. Dhuime et al., 2012; Hawkesworth et al., 2010; Wang et al., 2011) while
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the second uses patterns, termed ‘reworking arrays’, within the Hf isotope data to infer ages of crustal generation (e.g. Kemp et al., 2006; Kemp et al., 2015; Kemp et al., 2010; Kirkland et al.,
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2013b; Petersson et al., 2015; Zeh et al., 2014). In this section we highlight some of the limitations
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that still remain with both of these approaches.
3.1 An Oxygen isotope filter for Hf model ages?
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In an attempt to identify juvenile material from zircon isotopic spectra it has been proposed that zircon with a 18O signature that is mantle-like (i.e. 5.3 ± 0.3 ‰, 1SD) either indicates derivation of the parent magma directly from the mantle or from re-melting of a previously mantle-derived rock (Dhuime et al., 2012; Hawkesworth et al., 2010). The inference is that the Hf isotopic signature of the zircon faithfully records the addition of juvenile material to the continental crust, with calculated model ages providing a meaningful age of crustal growth. This premise has recently been questioned by Roberts and Spencer (2015), Iizuka et al. (2010) and Nebel et al. (2011) among others. To demonstrate, in a semi-quantitative manner, the shortcomings of this method for determining crustal growth we have compiled zircon Hf and O isotope data from igneous rocks for which the Hf and O isotope analysis were conducted on the same individual grains. The compilation includes data
ACCEPTED MANUSCRIPT from 167 igneous rocks with a total of 2586 zircons (excluding 200 inherited zircon grains) ranging in age from 3451 Ma to 4.4 Ma. The advantage of this dataset over detrital zircon datasets is that it
Each zircon is classified as either “mantle-like” (within
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in the Supplementary Information.
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allows a direct link to the original host magma petrogenesis. All data and data sources are provided
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uncertainty of 5.3 ± 0.6) or “non-mantle-like” based upon its O isotope composition, and is then further classified as either derived from mantle-derived magmas, purely reworking of continental crust or a mixture of mantle and crustal melts. The difference in age between the age of the zircon
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and its New Crust model age (TNCc) is also calculated and referred to as the Age.
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Fig. 3 provides a schematic representation of the information obtained from this compilation of igneous rock data. The compiled data allows us to determine which zircons/rocks yield Hf model
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ages that are likely to reflect an actual crustal generation event. In the compilation, 29.5 % of
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analysed zircons have a mantle-like O-isotope composition with 23.1 % having a mantle-like Oisotope composition and an evolved Hf model age more than 200 Myr older than their crystallisation
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age (Age>200 Myr). Using the interpretation of Hawkesworth et al. (2010) or Dhuime et al. (2012) for example, this would imply that 23.1% of the igneous rocks were formed solely from reworking of
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mafic crust (or the fractionally-crystallised intermediate to felsic products of mafic magmas) in order for the zircons to yield meaningful model ages. However, using the published petrogenetic models of rocks in the dataset, only 7.7 % of zircons have a mantle-like O composition and are derived purely from reworking of pre-existing crust with no additional new mantle input. The data suggest, including mantle-like O isotope composition zircons with juvenile Hf (Age<200 Myr) compositions (6.4 %) and mantle-like zircons derived from purely crustal reworking, that only 14.1 % of Hf model ages can be considered as meaningful indicators of the time of crust generation. However, this should be considered a maximum as the possibility still exists for both mixing of melts derived from mantle-like crust of different ages and/or non-depleted mantle compositions of the original mantlederived melts. This analysis based on actual petrogenetic information indicates that there is little geological basis to consider O isotope data in zircon as a unique robust filter for the validity of Hf
ACCEPTED MANUSCRIPT model ages as a measure of the time of crust generation. A limitation with the compilation used to reach this conclusion is the relatively small number of magmatic rocks included (n=167, zircon
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n=2585). However, based upon studies of granitoid systems such as Kemp et al. (2009), it seems the
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most likely bias exhibited by the compilation is towards rocks formed purely by crustal melting. In
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Kemp et al., all the modelled Lachlan Orogen granitoid suites contained greater than 20% mantle contribution, even in the ‘S-type’ granite suites that are most commonly considered examples of crustal reworking. This suggests, if anything, the results obtained from our compilation remain an
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over estimate of the proportion of zircons that provide reliable model ages that reflect new crust
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generation.
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3.2 ‘Reworking Arrays’ interpreted as crust generation
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‘Reworking arrays’ are constructed from Hf isotope data when a regression can be fitted to a subset
crust value of
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of regional data along a line that is interpreted to represent crustal evolution (often assigned a bulk Lu/177Hf = 0.015). The intersection of the regressed line with a Depleted Mantle
growth line is then interpreted to be an age of major crustal growth in the region with the slope of
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the line reflecting the 176Lu/177Hf ratio of the reworked crust. Perhaps the most notable discussion of ‘reworking arrays’ in the literature involves the Lu-Hf isotopic data from the Jack Hills zircon and the implications for existence and/or evolution of Hadean continental crust (Amelin et al., 1999; Bell et al., 2011; Kemp et al., 2010; Nebel et al., 2014). Examples from younger geological terrains are also found in the literature (e.g. Kemp et al., 2006; Næraa et al., 2012; Petersson et al., 2015; Pietranik et al., 2008). As the formation and reworking of the Hadean crust may well be significantly different from the remainder of Earth history, the following discussion is focused on the generation of magmatic crust in the post-Hadean. For a ‘reworking array’ to provide a reliable age constraint on an original crust forming event, all reworking events must occur without the addition of any material from crust of a differing age, or juvenile material from the mantle (but see caveat of Section 3.2.4
ACCEPTED MANUSCRIPT and Kirkland et al. (2013)). Additionally, for the isotopic composition of rocks formed in a crustal growth event to record their approximate age they must be derived from the mantle and emplaced
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with little or no mixing of crustal melts of a significantly different age. Bulk or upper crustal
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reservoirs are inherently mixtures of multiple crustal types/compositions/ages and therefore cannot
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produce an accurate single crustal growth age. The most likely source material capable of producing a valid source array will be mafic under-plated crust, preserved oceanic plateaux and island arcs or possibly the mafic lower portions of a continental arc. Two important factors to consider are the
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likely compositions of the possible source reservoirs and the fertility of the source regions.
3.2.1 Lu/Hf compositions of reworked sources
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One of two approaches can be used for assigning a reworked source composition to a ‘reworking
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array’. The first is to assign a particular crustal 176Lu/177Hf composition and assess whether the data
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fit a regression line of that composition. The second approach is to determine a regression line and assess whether the 176Lu/177Hf composition indicated by that line represents an appropriate crustal composition (e.g. Ge et al., 2014; Kemp et al., 2006; Kirkland et al., 2013b; Zeh et al., 2014). Given
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that we are considering the possibility for ‘reworking arrays’ to provide a meaningful age of crustal generation there is likely to be a limited range of 176Lu/177Hf values that should be considered when attempting to constrain a crust generation age. Island arcs, while lithologically simpler than continental arcs, still yield compositions ranging from mafic (or ultramafic) to felsic. However, lithologies in the lower crust of arcs that are likely to be sourced during crustal reworking are predominantly mafic or ultramafic (Clarke et al., 2000; Daczko and Halpin, 2009; Ducea et al., 2015; Jagoutz and Schmidt, 2012; Saleeby et al., 2003). As an example, lithologies from the lower crust of the Kohistan Island Arc, Pakistan, yield average 176Lu/177Hf values of 0.048 ± 0.023 or 0.069 ± 0.029 (excluding ultramafics and dependant upon the volume estimations used (Jagoutz and Schmidt, 2012)). Further estimates for the composition of potential crustal source rocks are taken from the
ACCEPTED MANUSCRIPT upper crustal components of modern island arcs. To maintain consistency with the New Crust of Dhuime et al. (2011) we have used the GEOROC databases for the Aegean, Aleutian, Bismarck
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Honshu, Izu Bonin, Kamchatka, Kermadec, Lesser Antilles, Luzon, Mariana, Scotia, Sunda and Tonga
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Lu/177Hf values for all the compiled arcs range from 0.017 to 0.039, hence all are
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1. The median
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arcs. The 176Lu/177Hf distributions for mafic lithologies within these arcs are given in Fig. 4a and Table
above the bulk continental crust value of 0.015 (Table 1). The data encompass the average compositions of the bulk, upper, mid and lower continental crust (Rudnick and Gao, 2003). However,
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as crustal melt generation invariably averages crustal compositions by incremental and progressive extraction of a few percent partial melting from a large volume of rock, the continual extraction of
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material from a single intrusion with a bulk or upper crustal composition is improbable. This indicates that reworking of arc crust with no additional input should fall along an array with Lu/177Hf values of at least 0.017 but in all likelihood, particularly given the lower crustal
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compositions of the Kohistan Arc , more likely to be significantly higher and on the order of 0.025.
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The compositional range of non-arc mafic under-plated crust is perhaps best represented by the composition of continental Large Igneous Provinces (LIPS) and/or oceanic plateaus. Although it is
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quite likely that ultramafic material is a significant component of a mafic underplate, this material is unlikely to melt to produce significant volumes of zircon-bearing granitoids without refertilisation and thus modification to its isotopic systems. Therefore we use the mafic components of LIPs and plateaus as a proxy for the composition of non-arc under-plated mafic crust, keeping in mind that fractional crystallisation and crustal assimilation processes may mean the basalts provide an underestimate of the
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Lu/177Hf ratios of the under-plated crust. Fig. 4b provides a summary of
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Lu/177Hf data from ten continental flood basalt provinces (green lines) and three oceanic plateaus
(black lines). The median values for the majority of these provinces fall above the bulk continental crust value but they are generally closer to a 176Lu/177Hf ratio of 0.015 and a number fall on or below this ratio. These data provide the option for non-arc mafic under-plated crust to be a potential reservoir source for nominal bulk crustal compositions in ‘reworking arrays’ based upon the
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Lu/177Hf ratios. However, in order for the ‘reworking arrays’ to provide a meaningful age of crust
formation the reworked source material must still be isotopically juvenile when it formed (see 4.2.2).
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The final composition that may be considered as source material capable of producing a reliable
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‘reworking array’ is the oceanic crust of a passive margin. While the oceanic crust does in general
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have an isotopic composition that is closer to the model Depleted Mantle composition, it is still heterogeneous, as demonstrated by the wide array of documented E-MORB, T-MORB and HIMU compositions (Hofmann, 2003; Kamber, 2011). Most importantly, the Lu/Hf composition of oceanic
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crust is incompatible with the concept that it may provide reliable crustal growth ages with the 176
Lu/177Hf value of 616 ocean floor
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commonly used crustal reworking array values. The median
basalts analysed by Jenner and O’Neill (2012) is 0.028, considerably higher than many nominated
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crustal reworking values.
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3.2.2 The isotopic composition of reworked “New Crust” Based upon the above summary it is unlikely that a valid crustal ‘reworking array’ will evolve using a
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bulk crustal 176Lu/177Hf value of 0.015 or, depending on the tectonic setting, potentially not even an ‘island arc’ value of 0.018. Crust produced from a single growth event capable of providing a meaningful depleted mantle model age seems likely to yield a
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Lu/177Hf value >0.02, a value
consistent with some published arrays (Amelin et al., 1999; Næraa et al., 2012). In the case that a ‘reworking array’ is produced then it is still necessary to consider the original isotopic composition of the reworked source. The commonly applied source model for new crust is Depleted Mantle with a modern day Hf value of +17. Although most geoscientists recognise that the composition of the mantle is highly heterogeneous it is not uncommon to read statements such as “the 1.48–1.36 Ga Atype granites cannot represent products of juvenile mantle melts; if they had been derived from melting of a mantle source, they would have εHf values close to the depleted-mantle evolution curve
ACCEPTED MANUSCRIPT (e.g. values of about +10 to +12 at ∼1.4 Ga)” (Goodge and Vervoort, 2006). In that particular example the interpretation of predominantly crustal origin for the granites is demonstrated to be
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correct using a combination of zircon and whole rock geochemistry. It is arguably the exception
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rather than the rule that the mantle source for crustal generation has a model DM composition and
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when considering crustal growth (i.e. the addition of mantle material to the crust) this should be accounted for. Fig. 5 displays the Hf(T) range for the provinces given in Fig. 4a and 4b. The Hf(T) data provided are recalculated from Nd(T) data which are used due to the greater number of data points
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available for the provinces. These data highlight how little of the mafic material yields an isotopic composition that would provide a depleted mantle model age that fits precisely with its timing of
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formation. Of particular note is the observation that each of the flood basalt provinces that have Lu/177Hf values akin to bulk continental crust (0.015) have Hf(t)(recalculated) values significantly
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below the DM and in most cases the provinces are similar to CHUR (Table 1). As this dataset has not
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been filtered for crustal contamination the continental flood basalt provinces may be biased towards 176
Lu/177Hf
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isotopically evolved crustal compositions. However, this would also account for low
values and hence it does not increase the likelihood of the flood basalt provinces as suitable reworked crust sources. Crustal contamination should be less significant for the oceanic arcs and not
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relevant for oceanic plateau or mid-ocean ridge basalts. The compilation of Hoffman (2003) displays a continuum of Nd values for the Atlantic, Pacific and Indian MORBs from +14 to +3 with some Indian MORB samples as low as -6. Given even the ‘simplest’ products of DM melting show a modern day range of 11 epsilon units it is hard to argue that the calculation of DM model ages (from reworking arrays or directly) is justifiable. It seems logical to treat both modern and ancient depleted mantle reservoirs as a range of compositions rather than a single linear model. This diversity and the subsequent impact on model ages is increasingly being recognised (e.g. Arndt, 2013; Roberts and Spencer, 2015) and in some instances the logical application of a range of mantle compositions is used and plotted on Hf diagrams (e.g. Avigad et al., 2015; Kemp et al., 2006).
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3.2.3 Fertility of the crust for reworking
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The varying fertility of crustal rocks is well known. The primary concern with regard to isotopic crustal reworking arrays is the capacity of a given lower crustal reservoir to continuously generate
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new intra-crustal melts without any addition of mantle-derived melts or fluids that would create a mixed isotopic signature.
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Melt generation in the crust occurs most readily when fluxed directly by H2O or via dehydration melt reactions involving the breakdown of muscovite, biotite or hornblende. Wet melting is likely to be
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self-limiting as once melt forms and is able to leave the rock it will also mobilise other fluids, thereby effectively dehydrating the system and stalling further melt production. As discussed by Brown
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(2010), in “the absence of a significant influx of aqueous fluid, metapelites, metagreywackes, meta-
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andesites, and some amphibolites may yield 10–50 vol.% H2O-undersaturated melt at attainable
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crustal temperatures (Clemens, 2006; Clemens and Vielzeuf, 1987; Thompson, 1990).” This melt is interpreted to be progressively extracted as it forms in most instances. The case for on-going melt extraction is provided by the incremental assembly of granitoid plutons and suites (e.g. Annen et al.,
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2015; Miller et al., 2011) and also by the preservation of granulite facies rocks. If significant melt is retained within the system then granulite facies rocks will be retrogressed to amphibolite facies assemblages during the post-peak P-T-t cycle. White and Powell (2002) estimates that greater than 50 – 70% of the generated melt must be extracted in order to preserve anhydrous granulite facies mineral assemblages. The extraction of melt and resultant decrease in bulk fertility subsequently requires higher crustal temperatures (or refertilisation) in order to generate a suitable amount of melt to form new granitoid rocks. That crustal fertility changes is evidenced by the changing composition of granitoid rocks within an orogen with the last magmatic event often represented by ferroan, alkali-, HFSE, Cl-, and F-rich granitoids and/or charnockites that have been variably interpreted to represent incorporation of melt derived from previously dehydrated and melt-
ACCEPTED MANUSCRIPT removed crust (Eby, 1992; Martin, 2006; Martin et al., 2012; Whalen et al., 1987). The need for crustal refertilisation also highlights the conceptual utility of models for granitoid formation such as
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the Hot Zone models of Annen et al. (2006) which inherently refertilise the lower crust through
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introduction of mafic magmas and volatiles (Smith, 2014; Solano et al., 2012).
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3.2.4 How do linear trends form in regional Hf isotope datasets?
The foregoing discussion places significant constraints on the likelihood of a crustal reworking array
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providing a reliable estimate of the age of crustal growth. Geological, geochemical and isotopic evidence indicates that the majority of granitoids contain some mantle component which implies
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the injection of new mafic melts into the site of granite generation. The additional requirement for
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refertilisation to enable continued reworking also implies that any crustal material formed is
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volumetrically diluted by later crustal additions during reworking. This makes a reworking array that represents isotopically static reworking of crust of a single age exceedingly unlikely. However, the fact remains that a number of studies have presented evidence for Hf isotope arrays interpreted to
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be consistent with continued reworking of a single crustal growth event (e.g. Kirkland et al., 2013b; Petersson et al., 2015). This suggests it is worthwhile considering how these linear arrays could be formed in regional datasets. To assist with this we use the Musgrave Province, Central Australia, as a case study. The Musgrave Province records ca. 500 million years of magmatism and has been cited as a terrain that preserves evidence for a linear crustal reworking array (Kirkland et al., 2013b).
An important consideration in the potential production of meaningful reworking arrays is outlined for the Musgrave Provinceby Kirkland et al. (2013b). Kirkland et al. indicate that among several juvenile input events within the Musgrave Province a ca. 1900 Ma crustal growth event was significant. The primary supporting evidence for this interpretation is an apparent crustal reworking
ACCEPTED MANUSCRIPT array supported by zircon Lu-Hf-O isotope data from magmatic rocks that formed from ca. 1600 Ma to ca. 1100 Ma. The magmatic suites include interpreted arc-like rocks at ca. 1600 Ma ranging
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through to the HFSE-rich, ferroan, calc-alkalic to alkali-calcic granites and charnockites of the
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Pitjantjatjara Supersuite at ca. 1200 Ma. There is a general lack of direct geological evidence for
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magmatism at ca. 1900 Ma in the Musgrave Province which Kirkland et al. (2013b) suggested may reflect underplating processes with limited upper crustal remnants. However, other evidence of cryptic juvenile crust production at ca. 1900 Ma has been proposed from neighbouring terrains. The
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Rudall Province (Kirkland et al., 2013a), Capricorn Orogen, buried Madura Province (Kirkland et al., 2015) and north Gawler Craton (Reid et al., 2014) have all been interpreted to have some degree of
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juvenile input at 1900 – 1950 Ma, including the presence of a ca. 1920 Ma granite in the Gawler Craton (Reid et al., 2014). A key discussion in Kirkland et al. is the concept that the addition of
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juvenile mantle-derived melt into high field strength element rich crust will have little affect on the
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isotopic signature of the resultant magma. The calculations of Kirkland et al. suggested for bulk
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assimilation, up to 50% MORB-like melt was required to move the Hf(t) value by more than 1 epsilon unit and in the case of partial melting even more MORB-like material would be required. This is cited as evidence that the HFSE-rich Pitjantjatjara Supersuite granites reliably reflect the original
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composition of the crust they were derived from and cite a reworking array indicating a 1900 Ma crustal growth event. It is worth noting that Smithies et al. (2011) indicate the Pitjantjatjara granites may contain between 30 – 50% mantle-derived melt, allowing for, but not dictating, some slight movement of the isotopic composition of the granites. Although the potential for a c. 1.9 Ga reworking array exists we outline a number of additional considerations below that are of general relevance to other regions. These considerations do not diminish the potential importance of the isotopic inertia of HFSE-rich crustal melts and this is a consideration we revisit in later sections.
To demonstrate some important features of reworking arrays we discuss a simple model of ca. 1900 Ma crust production and reworking and illustrate why more complex processes are likely to be
ACCEPTED MANUSCRIPT necessary in the case of the Musgrave Province (and elsewhere). One important requirement for static reworking of a single source is that lower crustal compositions must remain effectively fixed.
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Prior to the formation of the Pitjantjatjara Supersuite, the Musgrave Province had undergone ~400
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Myr of magmatism including interpreted arc magmatism at ca. 1600 Ma at which time it is clear that
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new mantle additions to the crust occurred (Kirkland et al., 2013b; Wade et al., 2006). Hence, any single lower crustal reservoir (i.e. a 1900 Ma crust) would have been progressively melted and intruded by mantle-derived magmas (e.g. MASH or Hot Zone, Annen et al., 2006; Hildreth and
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Moorbath, 1988; Solano et al., 2012). Each intrusion of mantle-derived magmas will progressively dilute the pre-existing lower crust with the added material and thus it would be highly unlikely that
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crust generated at 1900 Ma could retain either its original lithological form or its original isotopic composition through 400 Myr of magmatic activity. As the Pitjantjatjara Supersuite is the only suite
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potentially indicative of melting of a granulitic lower crust component (e.g. Ferroan, alkali, HFSE-rich
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magmas) it supports continual refertilisation of the lower crust in the preceding 400 Myrs – helping
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to support the isotopic modification of the lower crust through incorporation of mantle-derived fluids and magmas. The second additional consideration is the composition of pre-1200 Ma suites. The ca. 1600-1200 Ma suites have Hf concentrations that suggest any crustally-derived melts had
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lower HFSE concentrations and hence were more sensitive to isotopic modification by incorporation of juvenile material (e.g. ca. 1600 Ma suite - Hf = 4 – 9 ppm (Wade et al. 2006), ca. 1300 Ma Wankanki Suite – Avg. Hf = 5.7 ppm and ca. 1400 Ma Papulankutja Suite – Avg. Hf = 9.6 ppm (Kirkland et al., 2013)). Hence, although the Pitjantjatjara Supersuite may not have its isotopic composition easily modified, the pre-1200 Ma suites are less likely to record a crustal composition that would define the apparent 1.9 Ga crustal reworking array.
This analysis once again leaves us with the question of how linear, apparent reworking arrays form. To provide some insight into this question we use some simple models that attempt to approximate magmatism within a terrain, again using the Musgrave Province as an example. The models use
ACCEPTED MANUSCRIPT stochastic generation of ‘zircon’ Hf isotope data through a number of different scenarios related to the manner in which the lower crustal composition evolves during magmatic events. The mantle
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composition used is a normally distributed New Crust (Dhuime et al., 2011) with an upper 2SD on
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model DM and a conservative Hf concentration of 2ppm. The starting old crust in Fig. 6a and 6b is
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normally distributed with a starting DM composition centred upon 3 Ga and a 2SD range of DM ages of 2.65 – 3.35 Ga. Magmatic events are modelled at 1600, 1400, 1300 and 1200 Ma and the mantle proportions for each event are 0.9 ± 0.05, 0.65 ± 0.05, 0.65 ± 0.05 and 0.8 ± 0.05 (1SD), respectively.
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The latter 3 events are based on the I- and A-type mantle proportions calculated for the Lachlan Fold Belt granites by Kemp et al. (2009). The distribution centred upon 0.9 ± 0.05 for the 1600 Ma event
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is used as an estimate of the proportion of mantle-derived melts in arc magmatism and is influenced by the estimates of 0.8-0.9 mantle melt proportion for the ca 1600 Ma suite in the Musgrave
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Province (Wade et al., 2006). Crustal melt Hf concentrations are 5.3 ppm (Upper Crust of Rudnick
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and Gao, 2003) with the exception of the 1200 Ma event which uses a concentration of 15 ppm,
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adopted from the composition of the HFSE-rich Pitjantjatjara Supersuite (Kirkland et al., 2013b). The isotopic composition of the lower crust in the models evolves as described below.
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Fig. 6a demonstrates the model data generated using the above criteria and a lower crustal composition that is purely ca. 3 Ga old crust (OC) at 1600 Ma, is 50:50 old crust and the generated 1600 Ma crust (NC16) at 1400 Ma, is 40:40:20 OC, NC16 and the generated 1400 Ma crust (NC14) at 1300 Ma and 32:32:16:20 OC, NC16, NC14 and the generated 1300 Ma crust at 1200 Ma. This represents a gradual dilution of pre-existing old crust with new material as it is generated, 50:50 for the arc-event and 80:20 for subsequent events. We have chosen to use the recently generated crust as the dilutant, as opposed to a depleted mantle composition, as this gives a conservative estimate of the dilution of the older crustal component. Similarly the dilutant proportions used are ones that are likely to be conservative in light of models such as the Hot Zone of Annen et al. (2006). The results of Fig. 6a yield a linear trend in the data that could be considered consistent with a reworking
ACCEPTED MANUSCRIPT array. A best-fit line for the data yields a DM crustal growth age of ca. 2.0 Ga and a 176Lu/177Hf slope of 0.0153. This is despite the fact that there is no crust generated at 2.0 Ga. While this is only a
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model, the significance of the model lies in the fact that we have allowed for the gradual change in
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composition of the lower crust. When the lower crustal composition evolves solely along the
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composition of the initial 3 Ga reservoir the results of Fig. 6b are obtained. The data rapidly drop below CHUR reflecting the increased crustal melt proportion (0.35) at 1400 and 1300 Ma and the increased Hf concentration (15 ppm) in the crustal melt at 1200 Ma. Thus with lower crustal dilution
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present, the on-going magmatism can moderate the composition of the lower crust and, although the crust proportions, concentrations and compositions change, the resultant magmatic data tend to
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cluster around CHUR.
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Fig. 6c and 6d consider the case in which a homogeneous lower crust does form at ca. 1900 Ma with
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a 176Lu/177Hf ratio of 0.018. We have used the same mixing models for the magmatic events, with the
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1600 Ma and 1200 Ma events being similar to the petrogenesis published for the age-equivalent suites in the Musgrave Province. In both Fig. 6c and 6d it is immediately apparent that the ‘granites’ generated are more juvenile than those seen in the Musgrave Province (Fig. 6d). Furthermore the
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reworking array in Fig. 6c has a
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Lu/177Hf ratio that is significantly higher than the original crust.
While the model of Fig. 6a does not prove that the Musgrave Province rocks formed by reworking and progressive dilution of a ca. 3 Ga lower crust, Fig. 6c indicates that the rocks are unlikely to have formed by simple reworking of ca. 1.9 Ga new crust, particularly given the petrogenetic constraints provided by whole rock studies (Smithies et al., 2011; Wade et al., 2006). Fig. 6e and 6f show the model results obtained from incorporation of ca. 3.0 Ga crust into the 1900 Ma model of Fig. 6c. In both cases the Archean crust component is static in the sense that it is not diluted over time and effectively stays fertile throughout the magmatic events (without refertilisation that causes isotopic modification). This therefore represents an extreme case of contamination as it is likely the older crust would not remain this predominant and its influence would decrease over time, leading to
ACCEPTED MANUSCRIPT more juvenile signatures for the ca. 1200 Ma magmatism. Fig. 6e demonstrates that an apparent crustal array can be generated using a 1900 Ma crust contaminated by older material. However, the
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model age provided in such a scenario may be insensitive to the actual age of the 1900 Ma crust and
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is largely a function of the age and composition of the 1600 Ma magmatism. Fig. 6g displays the
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same model as Fig. 6e but uses a 2200 Ma crust instead of the 1900 Ma crust. This model returns an apparent reworking array with an age of 1.86 Ga, suggesting the younger juvenile event may have
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limited control on the age provided by the reworking array in some instances.
The models presented in Fig. 6 provide a mechanism by which continued magmatism is able to
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produce linear arrays that could be incorrectly interpreted as purely crustal reworking arrays. It allows for continued refertilisation of the lower crust, is compatible with constraints on mantle input
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into granitoid suites (Kemp et al., 2009) and is consistent with lower crust dilution effects of
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proposed MASH/Hot Zone models (Annen et al., 2006; Solano et al., 2012). We suggest that
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meaningful crustal reworking arrays are probably an exception in the geological record and the likelihood of them producing a meaningful crustal growth age is small. Instead, the assumption should be for progressively diluted crust with consideration given to a potential older initial crustal
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reservoir, as would normally be the case for mixing to produce a single zircon or rock model age. In this regard, as noted by Bell et al. (2011), the most crustally evolved individual zircon Hf isotope values in a terrain may effectively represent pure crustal reworking of an initial juvenile cratonic protolith with little impact from magma mixing with a juvenile component (e.g. Ge et al., 2014). Zircon crystals with less extreme values are then considered to represent magma mixing and it remains difficult to directly determine subsequent rates or timing of crustal growth after initial cratonic protolith formation.
ACCEPTED MANUSCRIPT 4. Use of Hf and O isotope data in zircon for assessing global continental growth A variety of recent studies (e.g. Belousova et al., 2010; Dhuime et al., 2012) use Hf and O isotope
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data to undertake inverse modelling of the isotopic datasets in an attempt to determine the timing
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of juvenile material addition into the continents (i.e. continental growth). An important limitation
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and/or consideration for all such modelling approaches is the issue of mixed Hf model ages (Section 3 and Roberts and Spencer, 2015) and the potential non-unique nature of mixing between multiple reservoirs. In this section we review a number of recent studies and their methods for dealing with
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mixed model ages and investigate further possibilities for modelling continental growth.
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4.1 Inverse modelling of global continental growth
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Two recent studies that have addressed the growth of the continents using global detrital zircon UPb and Hf (±O) isotopic datasets are Belousova et al.(2010), who used a Hf-only approach, and
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Dhuime et al. (2012) who adopted a combined Hf and O isotope approach. These studies are discussed as their underlying principles are commonly applied in other global or terrain-scale
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modelling.
Belousova et al. use U-Pb and Lu-Hf isotope data from a global dataset of modern and ancient detrital zircon (n = 16,445) to calculate the proportion of juvenile material added to the crust through time, and from this produce an integrated curve that was interpreted to represent continental growth through time. The dataset was divided into 100 Myr intervals based upon the UPb age or Depleted Mantle (Crustal) model age (TDMc). Zircons with an U-Pb age for a given time interval but an older TDMc were inferred to represent reworking of evolved crustal material while zircon with a model age within the same time interval as the crystallisation age were considered to represent contribution of juvenile crust during that time period. The number of juvenile zircons was then divided by the total number of juvenile and evolved zircons to provide the proportion of
ACCEPTED MANUSCRIPT juvenile material added during each time interval. The crustal growth curve is derived by integration of the juvenile material addition curve. Belousova et al. acknowledge the likelihood of mixed
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model/crust-formation ages but assume that it is largely negated as mixing will be averaged out over
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the entire duration of Earth history.
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Dhuime et al. (2012) follow the approach of Hawkesworth et al. (2010) in adopting the use of O isotope data in zircon in an attempt to resolve the issue of mixed model ages. O isotopes are used to
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identify mantle-like zircon compositions (18O = 5.3 ± 0.6) that either represent new crustal material derived from the mantle or mantle-derived material that has later been re-melted but thought to
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still preserve a valid mantle model age. For 100 Myr time slices these “non-mixed” model ages are then divided by the total number of model ages for that time slice. Dhuime et al. nominate this as
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the proportion of new crust formation and produce two mathematical relationships (for pre- and
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post-3.2 Ga) which they consider to be representative of new crust formation through time. These relationships are then applied to the model ages of all data in a larger U-Pb and Hf dataset to
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produce the number of Calculated New Crust Ages through time that is used to calculate a juvenile crustal growth curve.
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4.1.1 The U-Pb and Hf-only approach The use of the U-Pb age data in conjunction with the Hf model age data (e.g. Belousova et al.) creates the potential for the calculated Juvenile Proportion curve to be highly dependent upon the U-Pb age distribution, which itself has been suggested as biased due to time varying zircon preservation potential (Cawood et al., 2013; Hawkesworth et al., 2009). Comparison of the Juvenile Proportion curve with the U-Pb age distribution highlights an inverse correlation of peaks and troughs in the two datasets (Fig. 7a and 7b). An inverse correlation may be expected to be a numerical artefact of the method, as the number of U-Pb ages is in the denominator of the Juvenile Proportion calculation. To investigate this apparent numerical dependence of the Juvenile Proportion curve on the U-Pb age distribution we have used the U-Pb age distribution of the
ACCEPTED MANUSCRIPT available Terranechron dataset (n = 12,375) of Belousova et al. in a simple forward model. Synthetic Hf isotope compositions were generated for each U-Pb age data point and used to calculate
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synthetic juvenile crust production curves following the method of Belousova et al.. The synthetic Hf
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isotope data were allowed to range between the reservoir compositions of the Depleted Mantle and 176
Lu/177Hf = 0.0118
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an extreme crustal composition of mafic crust generated at 4.55 Ga by using
(Continental Flood Basalt, Low Ti Siberian Traps (Farmer, 2003), as a proxy for early mafic crust (see Supp. Information A for alternative compositions). In this model the Hf isotope composition was
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assigned a random proportion (pseudo-random number generation algorithm, Microsoft Excel™ 2007) of depleted mantle composition with the remaining isotope composition assigned a random
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value between the depleted mantle and 4.55 Ga mafic crust (Fig. 7a – Random Hf + Random Juvenile). This model represents a random mixing of pre-existing crust and mantle-derived magmas
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and is considered to be a simple approximation of granitoid formation. The model replicates the
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shape of the Belousova et al. juvenile proportion curve (Fig. 7a) and closely mimics the proportion of
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juvenile material for much of Earth history. However, the synthetic data does not reproduce the distribution of Hf data within individual 100 Myr time slices (see Supp. Information A, Fig. S1). The similarity in calculated juvenile proportion but overall difference in Hf highlights that the calculated
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juvenile proportion curve is highly dependent upon the U-Pb age distribution and divorced from the Hf isotope data. Given that the proportion of juvenile material incorporated into each synthetic data point is known it is also possible to further assess the validity of the juvenile proportion returned via the Belousova et al. equation. The mean of the juvenile proportion in each 100 Myr time slice in the synthetic data is approximately 0.5 (Fig. 7c). This is expected for randomly generated data which exist within the range 0 to 1. This constant proportion of juvenile input contradicts the curve calculated using the Belousova et al. approach and further suggests this approach may not produce a model that is capable of accurately identifying juvenile input or new crustal growth. 4.1.2 The U-Pb and Hf-O isotope approach
ACCEPTED MANUSCRIPT The combined use of Hf and O isotopes in studies such as Dhuime et al. contains an underlying assumption that zircon with a mantle-like O isotopic composition yields a Hf model age that
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represents crust formation. By arguing for a non-mixed model age for zircon with mantle-like O
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isotope composition but an old Depleted Mantle model age, Dhuime et al. effectively assert that
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every zircon of this type is produced solely by intra-crustal melting of mafic or fractionallycrystallised igneous rocks. In the dataset of Dhuime et al., 45.1% of all analysed zircons have a mantle-like O composition and 40.4% of all analysed zircons have a mantle-like O composition but a
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Hf model age more than 200 Myr older than their crystallisation age. In order to retain a non-mixed model age this implies that ~ 40% of the magmatic rocks were generated by reworking of igneous
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crust without incorporation of any juvenile mantle-derived melts. This is in stark contrast to the findings of studies such as Kemp et al. (2009; 2007) that demonstrate in the Lachlan Fold Belt the A-
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type granites with mantle-like O isotope compositions contain on the order of 90% juvenile mantle-
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derived melts. Furthermore, Kemp et al. (2009) highlight that almost all granitic rocks contain
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mantle-derived melts with even the ‘S-type’ granite suites containing greater than 20% juvenile material. These examples from such a well-studied region, considered in context of the discussion in Section 3.1, question the assumptions that mantle-like O isotope data can be used uncritically as a
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tool to identify pristine Hf model ages.
The inability to use O isotope data as a simple filter
combined with the demonstrated numerical artefacts introduced by Hf-only inverse modelling indicates that simple inverse modelling approaches are unable to produce reliable temporal information on continental growth.
4.2 Temporal Bias of Hf model age datasets In the previous sections we have discussed the limitations created by mixed Hf model ages, it is also worth considering the specific bias created in the recent geological record through the use of Hf model ages. Roberts and Spencer (2015) highlight that the use of Hf model ages is likely to bias
ACCEPTED MANUSCRIPT crustal growth models towards more growth early in Earth history. A further complexity created by the use of Hf model ages in inverse modelling approaches becomes evident in the Phanerozoic. The
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bias created by use of model ages in the Belousova et al. and Dhuime et al. studies results in a
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perceived decrease and increase of juvenile crust addition in the Phanerozoic, respectively.
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Hf model ages will in almost all cases be significantly older than the formation age of the host magmatic rock. This is logical as the majority of zircon-bearing rocks are likely to contain a component of enriched mantle-derived melts, subducted crust or continental crust that will yield an
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isotopically evolved composition pulling the data away from the depleted mantle composition to
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yield an older model age. Bias is created in the recent geological record when model ages are used as a temporal reference point. In the study of Belousova et al. juvenile crust addition is detected by
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either juvenile Hf isotopic values in a given time-slice or by zircon crystals from subsequent
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magmatic events that yield a model age in the given time-slice of interest. This creates a bias as it is not possible to have zircons from future magmatic events yield a model age from previous recent
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time slices. Given the average difference in age between the crystallisation age of a zircon and its TDMc is typically on the order of 500 Myr (Fig. 8) this suggests analysis of the last 500 Ma (as a
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minimum) of the zircon record using the method of Belousova et al. will be biased against the recording of juvenile crustal addition by the exclusion of magmas and zircons that have not yet formed.
The study of Dhuime et al. shows a significant increase in the proportion of juvenile material in global magmatism in the last 1000 Myr with the most recent two time slices modelled yielding 100 % juvenile magma proportions. While we have demonstrated the use of O isotope data to identify nonmixed model ages is unlikely to be accurate, it is certainly true that zircons produced only from mantle-derived melts or re-melting of a mafic lower crust are highly likely to yield ‘mantle-like’ O isotope values. Additionally the only magmas that can produce zircon with Hf model ages in the last few hundred million years will almost certainly be derived directly from the mantle or re-melting of a
ACCEPTED MANUSCRIPT recently added mafic lower crust. Hence, by default, zircons with recent model ages should have ‘mantle-like’ O isotope values and, following the methods of Dhuime et al., will produce an apparent
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increase in the proportion of juvenile material in global magmatism. The effect will become less
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apparent moving back in the zircon record and the upward inflection of juvenile proportions (Fig. 1B
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in Dhuime et al.) from ca. 1500 Ma onwards may largely represent the decreasing dilution of juvenile model ages by reworked crust model ages. This inference is supported by the relatively consistent ‘mantle-like’zircon proportions in the last 1500 Myrs when calculated using the U-Pb age of the
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zircons as opposed to the Hf model age (Payne et al., 2015).
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The temporal bias introduced by Hf model ages is a further obstacle to their use and, combined with the issue of mixed model ages, lead us to conclude that the direct application of Hf model ages as
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crustal separation ages in continental growth studies should be abandoned. In the following sections
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we discuss alternative methods to obtain information on crustal growth from Hf and O isotope data
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in zircon.
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4.3 Can the zircon record resolve global continental growth? Despite the limitations of current inverse modelling approaches, it remains that zircon arguably provides the most complete record of crustal processes throughout Earth history and holds the greatest promise for resolving the growth of the continents. The primary application of Hf and O isotope data from zircon is likely to be for delineating the rate of juvenile addition to continental magmatism and identifying the nature and contributions of pre-existing crustal reservoirs. The direct inversion of Hf model age data effectively requires the zircon data to be binary in the sense that each zircon/event represents either crustal reworking or juvenile growth. Given that the majority of zircon bearing granitoids are a mixture of mantle- and crustally-derived magmas the approach of continental growth modelling should also attempt to resolve this mixing on an event-by-event or
ACCEPTED MANUSCRIPT time slice basis. The significance of such an approach is that it gives full recognition to the possibility that the majority of crustal growth may occur during time periods with significant reworking of old,
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isotopically evolved crustal material with little apparent zircon isotope evidence for juvenile input.
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Such a scenario is particularly well demonstrated by the Pitjantjatjara Supersuite in the Musgrave
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Province. This event is likely to have had more than a 50% contribution from the mantle and hence may have generated in excess of 150,000 km3 of new crust (total suite volume of 75,000km2 x 4km thick, Smithies et al., 2011) while producing granitoids with Hf isotopic compositions centred around
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CHUR (Kirkland et al., 2013b). Inverse modelling is unable to fully resolve progressive crustal growth of this type and hence is likely to bias new crust generation toward the early Earth (Roberts and
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Spencer, 2015). A number of recent studies have developed a range of analysis approaches that do
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growth throughout Earth history.
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not rely on direct inverse modelling and each seeks to constrain variation in crustal recycling or
Iizuka et al. (2010) used mixing calculations with time dependent incorporation of pre-existing
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reservoirs to calculate a Reworking Index. This index characterizes the amount of categorised input that is required to produce mean Hf values in modern detrital zircon (river) datasets (Fig. 9). The
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Iizuka et al. study is significant as it recognises the combined importance of granitoid crust, mafic crust and juvenile input into magmatism and generates models with a variety of input parameters to test the validity of the resultant Reworking Index trends. The strength of the Iizuka et al. approach is potentially limited by the use of mean Hf values only and, as noted by Iizuka et al., uncertainty regarding the applicability of the power law model used for the availability of pre-existing crust for reworking. Van Kranendonk and Kirkland (2013) use O isotopes in zircon along with magmatic rock trace element compositions and scan statistics to highlight periods of increased crustal recycling and sediment subduction at ca. 1.2 Ga. Their approach does not quantify rates but rather focuses on the changes in rates of reworking, as identified through geochemical proxies, to identify evolutionary shifts in global tectonics. Similarly, Van Kranendonk et al. (2015) uses change-point analysis of mean zircon O isotope values to highlight changes in O isotope data at ca. 3.2 and 2.6 Ga (Fig. 9). The 3.2
ACCEPTED MANUSCRIPT Ga change is interpreted to represent the onset of steep subduction. The approach of Van Kranendonk and others (2013, 2015) is not able to determine rates of continental growth, rather just
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episodes where this may be enhanced. However, the multi-variate approach of Van Kranendonk and
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Kirkland (2013) is a valuable example as it demonstrates correlations can exist between large
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datasets of O isotope data in zircon crystals and the chemistry of magmatic rocks. Spencer et al. (2014) note the broadening of O isotope distributions in time frames associated with supercontinent formation and/or collisional tectonics. The mean zircon O isotope compositions are modelled using
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sediment and mantle O isotope compositions through time to generate a crustal reworking index (Fig. 9), as proxied by the proportion of sediment required to be incorporated into mantle melts to
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generate the mean zircon composition. The modelling approach of Iizuka et al. in many respects underlies the approach of Payne et al. (2015) that uses three-component stochastic modelling and
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time varying compositions of magmatic and siliciclastic crust to determine best-fit models for the
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zircon O isotope distributions in 100 Myr time slices. The use of distributions as opposed to mean
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values allows for a greater degree of model differentiation than would otherwise be possible. The best-fit models argue against systematic changes in crustal generation over time and instead O isotope data suggest the most significant variation in juvenile input into magmatism occurs on
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shorter (100-200 Myr) timescales. This approach is limited by the relatively simple model for the incorporation of pre-existing igneous and siliciclastic sedimentary crust (zircon or whole rock data from the preceding 100 Myrs (also tested at 300 Myrs)). A second potential weakness lies in the whole-rock siliciclastic O isotope compilation that may be biased towards seafloor sediments in the last 500 Myrs, or alternatively biased away from seafloor sediments prior to this time period depending upon the importance of subducted sediments in controlling the O isotope composition of magmas. The prescriptive nature of mantle-crust proportion distribution models used (e.g. uniform or truncated normal distributions) is also likely to be a limitation of the modelling approach.
ACCEPTED MANUSCRIPT 4.3.1 Stochastic models using O and Hf isotope data With the exception of Van Kranendonk and Kirkland (2013), each of these studies is limited by their
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reliance upon a single data type. In the case of Payne et al. (2015) this results in a large number of
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statistically viable models for each 100 Myr time slice. For the purposes of this review a number of
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models have been constructed that build upon the work of Payne et al. (2015), to demonstrate analytical approaches that we feel hold potential for future research and highlight some of the limitations and issues surrounding continental growth modelling. We adopt stochastic modelling
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approaches as they provide the ability to generate distributions for isotopic data that can be directly
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tested against observed datasets. This allows for greater refinement of model characteristics than is possible using approaches that use singular measures such as mean values of datasets. As a result,
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obtained best-fit models are likely to be more unique and the approach is better suited to
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undertaking sensitivity testing of input data and model parameters. Stochastic models using Hf and O isotope data were calculated using the zircon Hf isotope datasets
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of Roberts and Spencer (2015) and the zircon O isotope dataset of Payne et al. (2015) updated with additional data included in Supplementary Data Table 1. The complete O isotope data input file is
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provided in Supplementary Data Table 2. Determining the appropriate Hf reservoir composition is significantly more complex than for O isotope reservoirs due to the time varying nature of Hf isotope reservoirs (e.g. Iizuka et al., 2010). For the purposes of this discussion, a very simple approach is adopted. The Hf isotope composition of the siliciclastic sedimentary crust has been estimated using a new compilation of Nd isotope data in siliciclastic sedimentary rocks (drawing from Condie (2014) and Champion (2013)) and the ‘All Sediment’ relationship of Vervoort et al. (1999) (Hf = 1.67Nd + 2.82, Fig. 10, Data and sources are given in Supplementary Data Table 3). The sedimentary data are then divided into 500 Myr timeslices as per Payne et al. (2015). The Hf isotope composition of the pre-existing igneous crust is derived from zircon Hf data from the preceding 100 Myr with resampling to provide distributions of the appropriate size (e.g. 10,000 or 1000 data points). The
ACCEPTED MANUSCRIPT calculated Hf values are then shifted by -1.2 units to represent an expected change in value in 100 Myrs from crust with a bulk 176Lu/177Hf value of 0.015.
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The simplest model constructed uses the approach of Payne et al. (2015) but extends that work by
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testing the obtained O isotope model results against the global Hf isotope datasets. Only 6 models
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out of 2772 models tested provided statistically viable results for the Hf isotope data. This is interpreted to be a direct result of the simple mantle proportion distributions used for the O isotope
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models as the Hf isotope data in each time slice have much greater topography and variation than the O isotope data (Supp. Information B). This limitation can be overcome through the use of
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variable mantle proportion distributions that are calibrated to fit the data. Mantle proportion is divided into 10 bins between 0 and 1, with each bin assigned a uniform probability density. Bin
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probability densities were calibrated such that the Kolmogorov–Smirnov (K-S) statistic between
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simulated and observed zircon distributions was minimized, using the “constrOptim” function in R, based on the Nelder-Mead method (Nelder and Mead, 1965). The graphical output of the variable
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mantle proportion models are provided in Supplementary Information B. Using O isotope data a large number of viable results (KS-test returns a p-value > 0.05, see Payne et al. (2015)) can be
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obtained with widely varying Igneous and Sedimentary crust ratios (I:S) for each time slice. Due to the calibration of the mantle proportion distributions this likely represents an over-fitting of the data with quite disparate mantle proportion distributions obtained depending upon the I:S ratio. Similarly when only Hf isotope data are used, the majority of time slices can be adequately modelled with a range of I:S ratios, but to a much lesser extent than for the O isotope data due to the greater complexity of the Hf isotope distributions. Of note is the inability to fit the Hf isotope data for many of the Hadean and early Archean time slices. The Hf data for these time intervals are largely negative (e.g.Bell et al., 2011; Nebel et al., 2014; and Supp. Information B) and hence the depleted mantle and sedimentary crust compositions used are too juvenile to be able to provide a significant contribution. The inability to return highly biased mantle proportions (e.g. near 0) hints at a nonoptimal calibration routine for extreme distributions. When using a joint calibration routine with
ACCEPTED MANUSCRIPT both Hf and O isotope data only four time slices (900-1000 Ma, 1300-1400 Ma, 1400-1500 Ma, 16001700 Ma) return statistically viable results for both datasets. A number of others return results with
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a reasonable visual fit but fail the KS-test (e.g. 600 – 700 Ma).
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The poor results of the joint calibration emphasize two features that are likely to be limiting factors.
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The first is the need for linked Hf and O isotope datasets. In this study we have used global datasets with the assumption that the large datasets will effectively average out the varying tectonic settings and mechanisms of igneous crust generation such that the Hf and O isotope datasets will be
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unbiased relative to each other for the purposes of the modelling. Given the O isotope dataset is
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much smaller it is quite possible that this is not the case and future modelling efforts will benefit from the sole use of Hf and O isotope data collected from the same zircon grains, requiring more
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data collection. The second consideration is the inherent complexity of global isotopic reservoirs.
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That isotopic reservoirs used in these models are potentially inadequate is demonstrated by the use of a comparatively juvenile Hf sedimentary rock distribution for the Hadean to early Archean time
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slices that display evolved zircon compositions. Many of the limitations associated with reservoir compositions may be related to variability of reservoir compositions both temporally (e.g. Hadean to
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Archean sediments) and spatially (e.g. reworking of an Archean craton versus a Proterozoic island arc terrane by a Neoproterozoic event). Global averaging and the use of distributions for isotopic reservoirs may partially resolve this issue but an improved approach would be to construct models at a terrain or continent scale in the first instance in order to better calculate relevant reservoir compositions. A continent or terrain-based approach would allow for factors such as variable fertility and composition of the lower crust to be accounted for and considerations such as ‘isotopic inertia’ in a given terrain/event to be modelled. That these issues become apparent when using joint calibration methods highlights that they are able to provide comparatively unique solutions (or not as the case may be) for continental growth (e.g. Fig. 11) and better enable identification of uncertainties and limitations in the modelling approach.
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4.4 Limitations of the zircon record
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The robustness and longevity of zircon within the geological record provides an exceptional opportunity to study the growth of the continental crust. However, numerous authors have
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questioned how representative the zircon record is of continental growth or, more precisely, the generation of continental crust forming magmas from the mantle reservoir. Ongoing discussion
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focuses on whether the episodic nature of the zircon U-Pb age histograms (and magmatic rocks in general) correlates to episodic increases in magmatic activity within the Earth (Arndt and Davaille,
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2013; Arndt, 2013; Condie, 1998, 2004; O'Neill et al., 2007) or is largely a function of collisional tectonic settings preferentially reworking and preserving rocks and zircon on the continents
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(Cawood et al., 2013; Condie et al., 2011; Hawkesworth et al., 2009; Hawkesworth et al., 2010; Kemp
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et al., 2006; Spencer et al., 2015; Voice et al., 2011). If the use of the zircon isotope record is focused on determining only the relative proportions of juvenile input into continental magmatism, as
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opposed to volumetric growth directly, then the varying abundance of zircons as a function of preservation potential becomes less important. However, preferential preservation during
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supercontinent formation, for example, may still bias the zircon record towards magmas dominated by crustal reworking (Voice et al., 2011) and effectively disguise the level of juvenile input at that time.
The potential also exists for differential zircon fertility of magmatic events (Dickinson, 2008; Moecher and Samson, 2006) to bias the zircon record away from juvenile crustal growth that has a large component of mafic magmatism (e.g. oceanic plateaux and to a lesser extent oceanic island arcs)(Cawood et al., 2013). In these scenarios a period of crustal growth would be best recorded by subsequent magmatism that shows an increased level of relatively juvenile material in the reworked crust component. Overcoming these issues will require integration of other data types to complement the information that can be derived from the zircon record. Ideally, data such as trace
ACCEPTED MANUSCRIPT element and Nd isotope compositions of the sedimentary record and igneous rock trace elements can be used for joint calibration of models with zircon data, or at the very least testing of models
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produced solely from zircon datasets. In order for such approaches to be fully realised there needs
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to be development of quantitative or semi-quantitative models for considerations such as the
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isotopic signature for recycling of the upper crust into sedimentary basin, zircon fertility of magmatic suites (e.g. Dickinson, 2008), probablisitic relationships for the correlation of zircon and whole rock characteristics and improved constraints on suitable reservoir compositions (Iizuka et al., 2010;
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Payne et al., 2015). Given the regional variation that occurs at any given timeframe within Earth history, particularly of crustal reservoir compositions, it is likely that global crustal generation
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models will need to be compiled from models constructed at the terrane or craton-scale in the first
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instance.
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To determine the volume of the continental crust over Earth history, models for the rates of new crustal generation over time will need to be augmented by models for the rates of destruction of
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continental crust over time, such as determined for the modern day Earth (Scholl and von Huene, 2007; Stern and Scholl, 2010). As the recycling of crustal materials into the mantle via subduction
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also influences the composition of the new crust that is generated, the interdependence of models for crustal growth and destruction may play a pivotal role in accurately determining the history of continental growth on Earth.
5. Conclusions The global zircon record is undoubtedly the most comprehensive dataset available for studying changing tectonic regimes and continental growth and recycling throughout Earth history. The compilation and analysis of Lu-Hf and O isotope data collected from zircon in igneous rocks suggests the majority of zircon-bearing granitoids and subsequently the zircon isotopic signatures represent
ACCEPTED MANUSCRIPT mixtures of melt derived from multiple mantle and crustal sources. Coupled with the non-depleted mantle composition of many mantle reservoirs there is little evidence to suggest that Hf model ages
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are reliable, direct indicators of the timing of juvenile crust addition to the continents. Simple
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stochastic models incorporating the premise of refertilisation of the lower crust through ‘Hot
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Zone’/MASH process demonstrates that apparent crustal reworking arrays in Hf isotope data can be easily produced through progressive dilution of ancient crust during later magmatic events. Combined, these considerations indicate a new generation of modelling approaches are required in
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order to determine global continental growth from the zircon isotopic record. We suggest models using joint calibration of Lu-Hf and O zircon isotope data coupled with other trace element and
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isotopic information hold promise for extracting continental growth from the Earth’s fragmentary
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geological record.
ACCEPTED MANUSCRIPT Acknowledgements: Anthony Reid, Rian Dutch, David Kelsey and Tom Raimondo are thanked for on-going discussions
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surrounding the use of zircon for studying crustal generation and evolution. Priya is thanked for
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assistance with an early compilation of Hf-O isotope data in granitoids. Chris Spencer is thanked for
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comments and Jade Starr Lackey and an anonymous reviewer are thanked for constructive reviews that improved the paper. This is contribution 339 from TRAX, 681 from the ARC Centre of Excellence for Core to Crust Fluid Systems (http://www.ccfs.mq.edu.au) and 1043 in the GEMOC Key Centre
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(http://www.gemoc.mq.edu.au).
ACCEPTED MANUSCRIPT FIGURE CAPTIONS Box 1 – Lu-Hf Isotope systematics
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Box 2 – O isotope systematic
Figure 1 – Example crustal growth models after Cawood et al. (2013) showing the variation present
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in models of global continental growth over time. See discussion in Section 4 on limitations of the Hf
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isotope data of Belousova et al. (2010) and Dhuime et al. (2012).
Figure 2 – Compilation of O isotope data in zircon from the compilations of Valley et al. (2015) and
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Payne et al. (2015) with additional data as provided in Supplementary Appendix 1. Black line represents the maximum value envelope of Valley et al. b) O isotope compilation of siliciclastic
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sedimentary rocks through time after Payne et al. (2015). Both datasets show an expanding range of
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O isotope compositions towards the modern-day.
Figure 3 – Schematic column plot showing a new compilation of zircon Hf-O isotope data from magmatic rocks including the published petrogenetic interpretation for rock formation (i.e. melt sources). The compilation indicates that 23.1% of zircons have a mantle-like O-isotope composition and an evolved Hf model age. However, the petrogenetic information indicates that only 7.7% of the zircons have a mantle-like O isotope composition and are derived solely from crustal reworking. This suggests a maximum of 13.5% (including 5.8 % of juvenile mantle-derived zircons) of Hf model ages in zircon can be considered meaningful. This is a maximum as it does not take into account heterogeneity of the lower crust and non-DM starting compositions.
ACCEPTED MANUSCRIPT Figure 4 – 176Lu/177Hf distributions and median values for a range of ‘mafic’ rocks that may create new crust for later reworking to produce a ‘meaningful’ model age or reworking array. a) Island arcs
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using the subset used by Dhuime et al. for the New Crust model (Range Used - 45-55 wt% SiO2). b)
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Continental large igneous provinces and oceanic plateaux (Caribbean, Kerguelen and Ontong-Java:
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(Range Used - 40-55 wt% SiO2). Data are sourced from pre-compiled datasets on the GEOROC database (http://georoc.mpch-mainz.gwdg.de/georoc/). These data indicate that island arc crust is unlikely to generate crustal reworking arrays with 176Lu/177Hf slopes of 0.015 or 0.018 in most cases.
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LIPs may be amenable to the formation of reworking arrays but see Fig. 5 for limitations.
Figure 5 – Nd isotope data (with n given) for the datasets used in Figure 4. Data has been
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recalculated to Hf(t) using the IAV array of Vervoort et al. (1999) for a) and the Mantle Array of
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Chauvel et al. (2008) for b). Nd isotope data are used to ensure enough data are available for all
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locations. Green dots in b) represent DM model composition at the age of each event. Few of the locations have a distribution overlapping the model DM isotopic composition (as noted by Dhuime et al. (2011), Arndt (2013)). Of note is the general correlation between crustally evolved eHf values and
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bulk crust like 176Lu/177Hf compositions of the LIPs (Fig. 4b).
Figure 6 – Stochastic models approximating the generation of granitoid magmas over a 400 Myr period, loosely based upon the geological history of the Musgrave Province. Four magmatic events are modelled: an arc at 1600 Ma, ‘I-type’ magmatism at 1400 and 1300 Ma and ferroan, alkali, HFSE (anorogenic) magmatism at 1200 Ma. Parameters of the models are given in the text. The model (a) using ca. 3 Ga crust with progressive dilution of the old lower crustal reservoir by each magmatic event. Differing crustal compositions and juvenile inputs produce a pseudo-‘reworking array’ with an apparent crustal 176Lu/177Hf ratio of 0.0153. Juvenile crust produced at 1900 Ma with equivalent
ACCEPTED MANUSCRIPT subsequent magmatism produces an array with a flat 176Lu/177Hf ratio of 0.0232, significantly higher
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than the assigned true crustal composition of 0.018 for the juvenile crust.
Figure 7 – a) Juvenile proportion curve of Terranechron® dataset in Belousova et al. (2010) (red line)
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with juvenile proportion curves for synthetic Hf datasets produced using random Hf isotopic compositions and the Terranechron® U-Pb age distribution. Random Hf + Random Juvenile (blue line)
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uses a random component of depleted mantle Hf isotopic composition with the remaining component a random composition between the depleted mantle and 4.55 Ga mafic crust. Blue
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shaded field represents the range of results obtained from 100 models generated using the Random Hf + Random Juvenile method. b) Terranechron® U-Pb age distribution is provided to highlight the
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inverse correlation between peaks and minimums in the juvenile proportion curves and U-Pb age
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distribution. These models highlight the dependence of the Belousova et al. method on the U-Pb age distribution with little dependence upon Hf isotope data. c) Actual juvenile proportion of Random Hf
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approach.
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+ Random juvenile model compared to the juvenile proportion calculated using the Belousova et al.
Figure 8 – The mean (solid line) and range (field) of the difference in age between the U-Pb ages and two-stage DM ages for zircon in the dataset of Belousova et al. (2010). Demonstrates the general age difference is on the order of 500 Myr and increases slightly towards the modern day. This age difference results in bias being introduced when using zircon model ages as geochronological constraints on O isotope data.
Figure 9 – Estimates of crustal reworking calculated for magmatism during Earth history: Solid Black line – Reworking index of Iizuka et al. (2010) representing the proportion of crust incorporated into
ACCEPTED MANUSCRIPT magmatism; Solid orange line – Reworking Index of Spencer et al. (2014) representing the incorporation of sedimentary material into magmatism; and, grey shaded column plot – Proportion
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of crustal materials incorporated into magmatism from Payne et al. (Payne et al., 2015). Vertical
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orange dashed line represents the timing of an interpreted peak in crustal reworking from Van
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Kranendonk and Kirkland (2013). Vertical blue dashed lines represent inflection points in the level of
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crustal recycling from Van Kranendonk et al. (2015). See text for details on methods used.
Figure 10 – Siliciclastic sediment Hf isotope compositions throughout Earth history as determined by
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recalculation of whole rock Nd isotope data. Data compilation is provided in Supplementary Data
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Table 3.
Figure 11 - Example model results for the 1300 – 1400 Ma time-slice. Column 1 is taken from the
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best-fit model result of Payne et al. (2015). Columns 2 to 4 demonstrate the additional constraints that can be obtained through using both Hf and O isotope data to constrain the models for mantle
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proportion distributions through time. It is worth noting that a number of the best fit models, particularly for Hf-only calibrations return bimodal mantle proportion distributions. For the time slice pictured the joint calibration model can be seen to combine the results of the single system calibrations. The final sedimentary proportion obtained also represents a compromised value between the single system calibrations.
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Hf and Hf compositions of potential crustal reservoirs for reworking - Median and Percentile Values Island Arcs
5% Media n 95%
Hf
5% Media n 95%
Mariana
Scotia
Sunda
Tonga
0.019
0.018
0.008
0.019
0.009
0.010
0.020
0.034
0.027
0.019
0.029
0.021
0.022
0.039
0.049 13.1
0.038 8.8
0.026 5.1
0.046 9.2
0.044 8.7
0.032 3.6
0.054 8.8
15.6
14.8
13.0
10.0
14.5
13.3
7.8
14.2
16.8
16.4
15.0
12.5
16.9
16.6
12.2
16.6
Honshu
Izu Bonin
Kamchatka
Kermadec
0.007
0.010
0.017
0.008
0.021
0.008
0.018
0.020
0.033
0.019
0.037
0.016
0.031 3.4
0.031 10.8
0.056 13.0
0.038 -0.4
0.057 13.8
0.033 13.8
8.7
14.8
14.5
8.8
16.0
10.1
18.3
15.8
13.2
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Table 1 -
Flood Basalt Provinces
Oceanic Plateaus Carribean Kerguele Onton Colombia n g Java n
Deccan
Etendeka Parana
Franklin
KarooFerrar
Mackenzie
North Atlantic
Siberian Traps
Tarim Basin
YellowstoneSnake River
0.012
0.006
0.008
0.017
0.006
0.012
0.007
0.003
0.005
0.012
0.010
0.008
0.020
0.019
0.014
0.015
0.018
0.018
0.018
0.016
0.017
0.010
0.018
0.030
0.014
0.028
0.044
0.026
0.025
0.022
0.025
0.038
0.039
0.025
0.012
0.081
0.039
0.094
-5.0 -19.6 -11.6 -22.1 -9.2 5% Media -0.4 1.9 -4.2 9.2* -2.1 -0.6 n 8.1 9.7 9.7 13.7 7.6 95% *n=4, hence percentiles are not calculated and province is not plotted in Figure 5. ^Values for Ontong-Java Plateau exclude 5 highly negative epsilon values (seen in Figure 5)
-21.8
-14.1
-8.9
-11.6
0.055 12.8
-3.9
7.4^
5.5
1.2
-2.6
-4.0
9.1
-0.3
10.0
12.2
6.4
6.7
10.1
17.1
9.4
11.5
177
Hf
H 5% Media n 95%
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ACCEPTED MANUSCRIPT Payne et al. - Strengths and limitations of zircon Lu-Hf and O isotopes in modelling crustal growth
Highlights: Juvenile addition to “hot zones” yields Lu-Hf arrays mimicking crustal reworking
New zircon Hf-O isotope compilation indicates < 14 % of Hf model ages are reliable
Inverse modelling of global continental growth is limited by numerical artefacts
Joint Hf-O modelling can provide improved constraints on crustal generation
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