Effects of thermal annealing and chemical abrasion on ca. 3.5 Ga metamict zircon and evidence for natural reverse discordance: Insights for UPb LA-ICP-MS dating

Accepted Manuscript Effects of thermal annealing and chemical abrasion on ca. 3.5Ga metamict zircon and evidence for natural reverse discordance: Insights for U-Pb LA-ICP-MS dating

Daniel Wiemer, Charlotte M. Allen, David T. Murphy, Irina Kinaev PII: DOI: Reference:

S0009-2541(17)30377-7 doi: 10.1016/j.chemgeo.2017.06.019 CHEMGE 18376

To appear in:

Chemical Geology

Received date: Revised date: Accepted date:

18 February 2017 9 June 2017 15 June 2017

Please cite this article as: Daniel Wiemer, Charlotte M. Allen, David T. Murphy, Irina Kinaev , Effects of thermal annealing and chemical abrasion on ca. 3.5Ga metamict zircon and evidence for natural reverse discordance: Insights for U-Pb LA-ICP-MS dating, Chemical Geology (2017), doi: 10.1016/j.chemgeo.2017.06.019

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ACCEPTED MANUSCRIPT Effects of thermal annealing and chemical abrasion on ca. 3.5 Ga metamict zircon and evidence for natural reverse discordance: insights for U-Pb LAICP-MS dating

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Daniel Wiemer1*, Charlotte M. Allen1,2, David T. Murphy1 and Irina Kinaev2

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Queensland University of Technology, 2 George St, QLD 4000, Brisbane, Australia

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Central Analytical Research Facility, 2 George St, QLD 4000, Brisbane, Australia

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*Corresponding author, e-mail: [email protected]

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Abstract

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We present a microstructural and U-Pb systematics study comparing pristine, thermally annealed (TA) and chemically abraded (CA) ~3500 Ma zircon from a quartz-dioritic gneiss,

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with the aim to improve pre-analytical workflows for more accurate and precise LA-ICP-MS U-Pb dating of ancient zircon. Four zircon domains are identified: i) low- to medium-U

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concentric-oscillatory zoned cores, ii-iii) two porous high-U outer alteration domains, and iv)

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low-U narrow inward growing recrystallization rims. Raman spectroscopy on the pristine zircon reveals a positive correlation between increasing structural damage and U content. The porous high-U outer domains show a drastic increase in non-formula Ca above estimated amorphous fractions of ~0.8, which we ascribe to hydrothermal alteration of high-damage zircon characterized by percolating networks of amorphous areas. Upon treatment (TA, CA), hyper-spectral CL and Raman spectroscopy suggest structural recovery of point defects, but not full repair of high-damage amorphous areas. Calculated Raman radiation damage ages

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ACCEPTED MANUSCRIPT suggest that natural annealing affected all domains at ~500 Ma, consistent with negligible laser ablation matrix effects comparing pristine and treated cores. We show that reverse discordance in pristine and TA cores is not an analytical artifact. HighU alteration domains are normal discordant and were partially reset at ca. 3410 Ma. Upon CA, U-Pb discordance and scatter are reduced in the cores, yielding intercepts of 3503 ± 14

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Ma and 560 ± 550 Ma (MSWD=1.0). The lower intercept matches the timing of Pb-loss recorded in the alteration domains and the Raman radiation damage age. We argue that short distance Pb redistribution within the cores, which led to reverse discordance, was controlled by U-Pb systematics recording both the crystallization age and the timing of Pb

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redistribution. Some excess Pb was redistributed from the partially re-set alteration domains into the cores, leading to mixed U-Pb systematics and further reverse discordance. Removal

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of the latter excess Pb upon CA suggests that radiogenic Pb from the alteration domains accumulated within distinct damage sites accessible for CA leaching.

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We conclude that standardization of pre-analytical treatment is not recommended; we propose to investigate the structural state of unknown specimen by means of Raman

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spectroscopy, in combination with commonly applied CL imaging. During LA-ICP-MS

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analyses, co-measurement of non-formula and selected trace elements is deemed highly

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valuable in detecting ablations for use in reliable U-Pb age determination.

Keywords: U-Pb LA-ICP-MS dating; metamict Archean zircon; thermal annealing; chemical abrasion; reverse discordance;

1. Introduction

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ACCEPTED MANUSCRIPT Uranium-lead geochronology of zircon represents a reliable, and widely used method to determine the absolute timing of magmatic crystallization and metamorphic events recorded in rocks (e.g. Compston et al., 1992; Davis et al., 2003). Zircon provenance studies from sedimentary rocks, and the increasing global zircon record have greatly improved our understanding of secular continental crustal growth (e.g. Campbell and Allen, 2008; Condie

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and Aster, 2009; Belousova et al., 2010; Cawood et al., 2013; Condie, 2014), and as the oldest physically preserved material on Earth (Jack Hills; e.g. Froude et al., 1983; Compston and Pidgeon, 1986; Wilde et al., 2001), zircon has become the key to insights into early Earth crustal formation and evolution, and ambient planetary surface conditions (e.g. Wilde et al.,

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2001; Valley et al., 2002, 2014; Bell et al., 2011; Bizzarro et al., 2012).

However, accurate and precise age determination of zircon remains biased particularly by

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natural discordance of the 238U-206Pb and 235U-207Pb systems through partial isotopic resetting usually ascribed to loss of Pb, resulting in normal discordance (e.g. Mattinson et al.,

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1996; Metzger and Krogstad, 1997). The fundamental cause of elemental and isotopic redistribution in zircon is based on the physical modification of its crystal structure induced

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mainly by the alpha decay of 238U (e.g. Davis and Krogh, 2000; Marsellos and Garver, 2010).

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Physically damaged zircon is characterized by a reduced chemo-physical durability, subject to diffusion, leaching and recrystallization processes, even under low-temperature conditions

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(e.g. Mezger and Krogstad, 1997; Davis and Krogh, 2000; Geisler et al., 2001a; Geisler, 2002), far below the closure temperature for crystalline zircon (>900 °C; e.g. Cherniak and Watson, 2000). More rarely, excess in radiogenic Pb relative to parental U is observed. Such reverse discordance has been ascribed to either analytical artifacts, or natural intra-grain redistribution of Pb from high- to low-U zircon zones (Mattinson et al., 1996; Carson et al., 2002; Zhao et al., 2014). Recent analytical advances attest for radiation damage related Pb mobility on the nano-scale, forming isolated clusters of high 207Pb/206Pb ratios, while on the

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ACCEPTED MANUSCRIPT µm-scale of commonly used U-Pb analytical spots closed-system behavior is observed (Valley et al., 2014; Kusiak et al., 2015; Peterman et al., 2016). The extent of intra-grain Pb redistribution in affecting common U-Pb analyses, and potentially resulting in reverse discordance, remains poorly understood. The cause of both normal and reverse U-Pb isotopic discordance is seemingly case-specific,

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though often not investigated in detail, and rather treated as a common phenomenon. Elimination of discordance through pre-analytical thermal annealing, and mechanical and chemical abrasion techniques to recover the crystal structure and to remove affected portions of zircon grains have been successfully applied to ID-TIMS (isotope dilution - thermal

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ionization mass spectrometry) analysis (e.g. Krogh and Davis, 1974; 1975; Krogh, 1982; 1994; Mattinson, 1997; 2005). For the faster and more cost-efficient SHRIMP (sensitive

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high-resolution ion microprobe) and LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) techniques, in which only small volumes of the zircon are targeted for U-

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Pb, pre-analytical workflows are debated. Mattinson (2005) proposed that thermal annealing alone could sufficiently reduce discordance to improve precision and accuracy in LA-ICP-

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MS apparent ages. In fact, Allen and Campbell (2012) demonstrated that the structural

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recovery upon thermal annealing reduced a radiation-damage induced matrix effect during laser ablation. Chen et al. (2002) performed step-wise chemical abrasion on thermally

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annealed metamict zircon, and found no improvement, until affected discordant domains were completely removed. In contrast, Crowley et al. (2014) recently argued that CA should be applied. In the present contribution, we investigate the effects of TA and CA on the U-Pb systematics and the microstructure of metamict Archean zircon to improve LA-ICP-MS pre-analytical workflow. Pristine and experimental grains are structurally and chemically characterized and compared using CL imaging, Raman spectroscopy, and LA-ICP-MS chemistry and U-Pb

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ACCEPTED MANUSCRIPT isotopic composition. The aim of the study is to constrain the effect of pre-analytical treatment, and its usefulness in LA-ICP-MS age determination of particularly ancient zircon. In addition, we aim to investigate the mode and cause of discordance.

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2. Analytical and experimental details

2.1 Sample background

In the present experiment, we use zircon from a quartz-dioritic gneiss (DW-370-1-14) from

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the southwestern margin of the multiphase Archean Muccan Granitic Complex, East Pilbara Terrane (Western Australia). The rock sample was collected ca. 200 m south of the Newman-

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Tabba-Tabba Road (138), east of 1 Mile Creek (20°52’31.98” S, 119°43’56.67” E; Supplementary Fig. S-1), and belongs to one of the ‘older’ gneiss components described in

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Wiemer et al. (2016). The quartz-dioritic gneiss comprises a mineral assemblage of hornblende, plagioclase, quartz, biotite, minor epidote, titanite and opaque phases, and

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accessory zircon and apatite. We selected this sample for the present experiment due to its

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morphologically homogeneous population of relatively large (~400 µm) zircon.

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2.2 Zircon separation, sample preparation, and experimental conditions

Zircon grains were extracted from bulk-rock at the University of Queensland (UQ) using standard techniques of mechanical, magnetic and density separation. From 240 selected zircon grains, about 100 grains were left untreated to characterize pristine zircon, and about 140 grains were selected for pre-analytical thermal annealing (TA).

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ACCEPTED MANUSCRIPT TA was performed in ceramic crucibles with lid at 850 ºC for 60 hours in a Ar-atmosphere furnace to prevent oxidation. The samples were cooled within the oven over a few hours. Due to previously observed changes in laser ablation behavior and down-hole fractionation between untreated and treated zircon (Allen and Campbell, 2012; Crowley et al., 2014), fractions from the reference materials (Plešovice, 337.1 Ma, 465-3084 ppm U; Sláma et al.,

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2008, and Temora2, 417 Ma; 82-320 ppm U; Black et al., 2004) were thermally annealed under the same conditions to create a set of physically matching standards (Crowley et al., 2014). Archean zircon reference material OG1 (207Pb/206Pb age of 3465 ± 0.6 Ma for chemically abraded, almost concordant thermal ionization mass spectrometry analyses; Stern

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et al., 2009) was analyzed in a pristine state.

A total of 40 of the thermally annealed grains of unknown sample 370-1 were selected for

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chemical abrasion under two experimental conditions (CA-I and CA-II, ~20 grains each). In both experiments the grains were put into Savillex Teflon vials, and partial dissolution was

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achieved within a diluted hydrofluoric acid solution of 0.2 ml HF and 1.8 ml 1:1 HNO3 + H2O, using a Milestone bench-top Ultra-wave single reaction chamber microwave digestion

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system, with an external chamber microwave absorbing base solution of 25 ºC and 40 bar.

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The experiments were performed at 1500 W microwave power under following internal chamber conditions: CA-I, 2 steps (10 min) at 175 ºC and 60 ºC, respectively, both steps at

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120 bar; CA-II, 2 steps (15 min) at 200 ºC and 70 ºC, respectively, both steps at 110 bar. The samples were then cleaned through repeated i) adding of 6ml MilliQ-H2O, ii) centrifuging for 15min, and iii) pipetting off of ~7ml. After repeated cleaning, the residues were evaporated dry on a hot plate at 100 ºC. After thermal annealing (TA) and chemical abrasion (CA-I, CA-II), both pristine and treated grains were mounted in epoxy, aligned with their crystallographic c-axis parallel to horizontal. Prior to polishing, reflected and transmitted light images were acquired. For

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ACCEPTED MANUSCRIPT electron beam excited imaging (scanning electron microprobe/SEM, electron microprobe/EMPA), polished mounts were carbon-coated. The C-coating was removed prior to Raman spectroscopic and LA-ICP-MS analyses.

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2.3 Instrumentation

For reflected and transmitted light imaging of the entire zircon mount, a ZEISS Axio Imager M2m microscope system was used. Cathodoluminescence (CL) imaging was performed with a ZEISS Sigma field emission scanning electron microscope (SEM) in variable pressure (VP)

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operation mode at 20 kV and 1.2 nA. Backscattered electron (BSE), hyper-spectral CL, and x-ray (EDS) mapping of selected elements were acquired with a JEOL JXA 8530F

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Hyperprobe field emission electron microprobe (EMPA), equipped with both a standard and high-resolution xCLent spectroscopic CL detectors. For the spectral CL imaging a dwell time

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of 40 ms were allowed and operating conditions held at 20 kV and 20 nA. The spectral CL data were evaluated with the xCLent V software.

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Inelastic Raman scattering was stimulated at room temperature using a Renishaw inVia

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Raman microscope fitted with a green 530 nm excitation laser, edge filter, orthogonal polarization, and 1800 l/mm spectrographic gratings, and focused beam size of 5 µm. Spectra

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were acquired perpendicular to the zircon c-axis, for Raman shift emissions from 600 to 1400 cm-1 with 16 seconds exposure and 4 accumulations. Raman data were treated with the GRAMS/AI Thermo Scientific spectroscopic data processing software, including background and multipoint baseline correction routine. Band position, height, and width were determined using an iterative Gaussian peak fitting above a calculated linear baseline with minimization of the reduced Chi2 value, giving a ‘goodness-of-fit’.

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ACCEPTED MANUSCRIPT Isotopic concentrations were determined with LA-ICP-MS. Selected analytical 30 µmdiameter spots were ablated using a 193 nm excimer laser. Ablated material was transported in He carrier gas and analyzed with an Agilent 8800 triple quadrupole ICP-MS. Zircon standards Temora2 and Plešovice were used as primary and secondary reference material, respectively. The NIST 610 glass (Jochum et al., 2011) was used as a primary standard for

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trace element composition. LA-ICP-MS raw data counts were imported to the Igor Pro-based Iolite software (WaveMetrics) for data de-convolution and evaluation, including baseline correction, integrations for U-Pb primary reference standards, down-hole fractionation correction and propagated error calculation (Paton et al., 2010). The LA-ICP-MS data were

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reduced relative to two different reference materials, pristine and annealed Temora2 in order to investigate the effects of treatment on the reference materials as well as the Archean

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unknown. All other parameters in the data reduction routine (Iolite) were held constant. U-Pb

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3. Results and discussion

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Concordia diagrams were plotted in the Isoplot Excel add-in.

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In the following sections analytical results are presented and discussed to structurally and chemically characterize the zircon and quantify radiation-induced structural damage and

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recovery thereof upon experimental TA and CA. Isotopic U-Pb systematics of pristine and experimental zircon reference standards and unknowns are investigated to distinguish potential laser ablation analytical artifacts from radiation-induced isotopic disturbance that can be directly associated with the zircon evolution. Morphological and optical characteristics are provided as Supplementary Material S-2 in the online version.

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ACCEPTED MANUSCRIPT 3.1 Identification and characterization of distinct CL zircon domains and effects on CL upon annealing

The CL imaging reveals grain interiors (core domain) characterized by mostly concentric zoning of alternating medium to light CL bands, parallel to the crystal faces, consistent with

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magmatic derivation. Rare sector zoning and convolute zoning are observed. Most grains display four distinct domains best observed in treated zircons, as within pristine grains the CL contrast is extremely faint using the scanning electron microscope (Fig. 1a-e): An inner core with moderate CL response,

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A domain of high CL response (alt-1) that forms a concentric band around the core, and

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

distribution of CL response, 

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showing a mostly sharp outer boundary, and is characterized by a heterogeneous

A domain of low CL response (alt-2), marking the outermost concentric band of the

A thin outermost zircon shell (rim domain) of medium-CL that is similar in intensity to the core domain.

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

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grains, and is characterized by the lowest CL intensity observed,

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The heterogeneity in CL intensity within domain alt-1 can be ascribed to two features: i) some regions are characterized by a spongy texture, where more or less homogeneously

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distributed dark CL (no CL response?) holes of up to 1 µm size are observed within overall high CL (Fig. 1c), and ii) regions, in which CL intensity gradually increases away from either boundary parallel, or perpendicular dark CL/no CL fractures, occasionally resulting in an overall convoluting texture formed by varying transitional CL intensities, apparently controlled by the fracture network (Fig. 1d). Many of the low-CL perpendicular fractures affecting the inward core domain, seem to terminate in domain alt-1 (Fig. 1c). Other, in- and outward extending fractures display the same high-CL intensity as domain alt-1. These

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ACCEPTED MANUSCRIPT fractures extend into low-CL core domains and/or into fractures or zones parallel to the concentric zoning bands of the cores, connecting low-CL zones within the zircon (Fig. 1b, d). Similar to domain alt-1, domain alt-2 comprises regions of a spongy heterogeneous CL texture. Here, however, the latter texture is much more developed, showing tight

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accumulations of “CL holes” that are confined to very restricted regions and/or domainparallel bands and zones (Fig. 1e). Towards the outer extent of domain alt-2, some larger (~5-10 µm) irregular-shaped areas of absence of material, connected to outward-penetrating fractures, occasionally obscure the band and cannot be correlated with any primary crystal-

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specific features (Fig. 1e). The latter outer zircon domains alt-1 and alt-2 are mostly present in grains that overall show darker CL, where both domains reach width of up to 20 µm, while

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in light CL zircon grains the domains are much thinner (<5 µm), or absent. The rim domain defines the well-developed crystal faces at its outward limits, but displays a

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curved inner boundary that penetrates into the interior of the zircon (Fig. 1e). Overall, the rim domain shows relatively consistent widths of ~5-10 µm, but very rarely it reaches into the

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zircon’s interior where it replaces larger portions of affected grains. The rim domain shows

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intensive radial fracturing. Most of the fractures are restricted to the rim, or terminate in the adjacent alt-2 domain (Fig. 1).

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The back-scattering electron (BSE) response correlates negatively with CL intensity in most instances, with high BSE intensity in zones of low CL intensity, and vice versa (e.g. Nasdala et al., 2002; Fig. 2). However, this seems not to be true for the alt-2 domain within the pristine grains, where low-CL corresponds to low BSE, or absence thereof (Fig. 2, top row). Only upon annealing do the dark CL alt-2 domains display expected bright BSE intensity. The alt-1 domain shows lowest BSE, even in the pristine grains, where its high CL intensity relative to the cores, as visible in the TA grains, is not too conspicuous.

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3.1.1 Change in CL upon annealing

The drastic increase in the integral panchromatic CL intensity upon annealing (TA) was investigated further using hyper-spectral CL. Spectral intensities were acquired for a photon

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energy range of 1.7 to 3.5 eV. Representative spectra from all domains (core, alt-1, alt-2, rim) are compared in Figure 3 from one pristine and one TA grain, highlighting the overall increase in CL intensity of almost up to an order of magnitude within rims and alt-1, and ca. five times higher intensities in cores and alt-2 compare to pristine grains. The spectra are

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dominated by the presence of very broad asymmetric emission bands, covering almost the entire spectral range, and additional narrow emission lines are superimposed on the broad

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bands of the alt-1, alt-2, and core domains of the TA grains (Fig. 3). The luminescence centers responsible for the here observed asymmetric broad band emissions (Fig. 3) could not

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be resolved unambiguously through Gaussian curve fitting due to multiple solutions. However, in a simple, 2-component solution, two overlapping broad band centers, one in the

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yellow-green range (A in Fig. 3) for the pristine zircon, the other in the blue range (B in Fig.

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3) for the thermally annealed zircon, are identified. Following previous studies, we propose that the broad blue CL band (Fig. 3) is of intrinsic nature, and its increasing intensity upon

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TA is the result of point defect recovery associated with SiO4 groups and overall reestablishment of the crystal field (e.g. Kempe et al., 2000; Nasdala et al., 2003). The yellowgreen broad band that particularly dominates the spectral range of the pristine cores (Fig. 3) possibly represents CL emission from electron transitions associated with additional radiogenic point defects, which healed during the thermal annealing (Gaft, 1992; Kempe et al., 2000; Tsuchiya et al., 2013, abstr.).

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ACCEPTED MANUSCRIPT A striking change upon TA is the appearance of narrow high intensity emission lines, particularly in the alt-1 and alt-2 domains relative to the broad band emissions. The narrow bands can be ascribed to the presence of various extrinsic trivalent rare earth element (REE) impurities, based on their specific spectral peak positions (e.g. Götze and Kempe, 2009). The following extrinsic REE3+ luminescence centers are identified based on their peak position in

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the alt-1 and alt-2 domains of the TA grains: Er3+ duplet at ~405 nm, Dy3+ triplet at ~475 nm, an undefined duplet at ~485 nm, Er3+ multiplet at ~525 nm, Tb3+ at ~557 nm, Dy3+ multiplet at ~580 nm, and Sm3+ at 617 nm (Fig. 3). The superposition of these peaks on the broad emissions is not observed in the rims and any of the pristine grain domains. The annealed

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cores show minor peaks of the 475 nm, 485 nm, 557 nm, and 580 nm REE3+ emission lines. In both the TA alt-1 and alt-2 domains the relative intensities between the REE3+ emissions

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are approximately constant, while the relative intensities between the REE3+ peaks show a slightly different pattern in the TA cores (Fig. 3), possibly due to varying concentration

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levels of REE.

Previous CL and photoluminescence studies suggested that a decrease in REE emissions

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could be the result of incorporation of non-formula elements, and/or increasing radiation

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damage (e.g. Lenz and Nasdala, 2015). In the present study, semi-quantitative X-ray maps of selected elements (Fig. 4a) and corresponding grain profiles (Fig. 4b) demonstrate the

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presence of elevated non-formula element content (Fe and Ca) within both alt-1 and alt-2 domains. The alteration domains show the most intensive REE CL emission peaks upon TA, compared to the cores and rims that do not show significant contents of non-formula elements (Fig. 4). Therefore, it is more likely that the suppression of the REE emissions in the pristine grains is dominated by radiation-induced structural damage (Nasdala et al., 2002).

3.2 Microstructural state and recovery revealed by Raman spectroscopy

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Raman vibrational spectra on each of the four identified domains from each experiment have been acquired, in order to determine the degree of structural damage and test the proposed recovery thereof, upon TA. Representative Raman spectra of each domain from both a pristine (pr04) and a TA grain (TA94) are displayed in Fig. 5a-c for comparison. A list of

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identified peaks for each spot is given in the Supplementary Table S-1.

The Raman spectra (Fig. 5a) clearly show an increase in the sharpness and intensity of two dominant peaks, situated at a spectral frequency of around 970 and 1000 cm-1, respectively, in the TA grain domains compared to the pristine zircon. The relative increase in intensity of

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the two peaks is different, with the higher frequency (~1000 cm-1) peak displaying drastic intensity increase, while the peak situated at ~970 cm-1 shows relatively minor increase.

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Within the pristine grains, the two peaks show similar intensities and are well developed within the rims, and cores, but show poor intensity and increased broadness within alt-1 and

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alt-2. The increasing sharpness and intensity from pristine to thermally annealed is most drastic in the alt-1 domain, where the peak at ~1000 cm-1 reaches the highest intensity

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observed. Another important observation is the systematic shift towards higher frequencies

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with increasing intensity and sharpness in the TA domains. Within the pristine grains, the two dominant peaks are situated at 962-972 cm-1 and 996-1004 cm-1, respectively, with the

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highest values in the rims and cores, shifting towards 973-979 cm-1 and 1004-1008 cm-1, respectively, with the highest frequencies in the rims, cores and alt-1 in both the TA and CAI/CA-II grains. We focus on the ~1000 cm-1 peak (Table 1) that has been well characterized in previous studies (internal stretching mode of Si-O bonds, v3 SiO4, e.g. Dawson et al., 1971). In crystalline synthetic zircon, this v3 SiO4 mode shows intensive Raman scattering of a phonon frequency at 1008 cm-1 characterized by a very narrow full-width at half-maximum

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ACCEPTED MANUSCRIPT (FWHM~1) peak (e.g. Geisler et al., 2001b). In contrast, the v3 SiO4 peak of metamict zircon is less intense, broader, and situated at lower phonon frequencies (e.g. Geisler et al., 2001b). The destruction of the zircon crystal structure during metamictization is caused by simultaneous accumulation of both point defects, and formation of amorphous domains (e.g. Murakami et al., 1991; Weber et al., 1994; Nasdala et al., 1995; Geisler et al., 2001b;

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Trachenko et al., 2001; Devanathan et al., 2006). Geisler et al. (2001b) compared the Raman response of variously naturally damaged zircon with metamict zircon annealed at different temperatures and duration. The results are displayed as the radiation damage trend and the annealing trend in the frequency versus line-width (FWHM) plot of the v3 SiO4 internal

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stretching mode (Fig. 6). In combination with XRD powder diffraction, the authors demonstrated that the structural recovery upon annealing is a two-stage process that differs

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from the process of metamictization. This means that annealing radiation damage in zircon does not directly reverse the process of damage accumulation. During the first stage, at

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annealing temperatures up to ~1000 K, Geisler et al. (2001b) observed significant decrease in unit-cell volume (XRD) with minor decrease in FWHM, while during the second stage

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(>1000 K), growth of crystalline domains corresponded to drastic decrease in FWHM. Due to

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the fact that the volume swelling is caused by overlap of damaged regions characterized by point defects, and not a result of the structure of amorphous domains (e.g. Trachenko et al.,

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2001), the slightly decreasing FWHM and increasing frequency during stage 1 is interpreted as the recovery of point defects in crystalline domains, while during stage 2 amorphous domains recrystallize (e.g. Geisler and Pidgeon, 2002). The pristine domains follow the radiation damage trend (Fig. 6). However, zircons from the highly damaged alt-1 and alt-2 domains plot above the damage trend, and at correspondingly high FWHM values (Fig 6). This may indicate some degree of natural annealing of the finer structure point defects (e.g. Geisler et al., 2003a,b,c). All of the annealed domains plot at high

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ACCEPTED MANUSCRIPT frequencies (>1004 cm-1; Table 1; Fig. 6), indicating significant recovery of short-range point defects, but still displaying a scatter in the FWHM. Hence, amorphous domains are likely still present. Because different grains are used for each experiment, and due to the fact that the initial damage state determines the annealing process (Zhang et al., 2000), it remains unclear, if amorphous domains recrystallized at all at the used annealing conditions (850 °C,

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60 h). For example, some of the annealed cores plot at higher FWHM than others. This hypothesis agrees with our CL images, in which some cores show darker CL in the TA grains, possibly reflecting higher initial damage. This means that some of the higher frequency, low FWHM annealed domains did not necessarily experience the second

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annealing stage, but started off from lower initial damage, somewhere on the damage trend line (Fig. 6). The alt-1 domains must have reached a critical level of amorphization, and the

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amorphous regions could not be healed under the experimental annealing conditions. The presented evidence for increased structural damage within the alt-1 and alt-2 domains

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can explain the development of some of the observed fractures (Fig. 1). The distribution and alignment of radial micro-cracks within the outermost rims, compared to rather concentric

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fractures along the inward boundaries of alt-1 and alt-2, fit numerical models by Lee and

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Tromp (1995) based on predicted internal stress distribution due to the macroscopic volume

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swelling of highly damaged domains (up to 18%; Weber, 1993).

3.3 Insights on structural damage from zircon chemistry

In this section zircon chemical data of selected elements are presented to further estimate the radiation damage based on actinide content and to chemically characterize the domains. Quantification of non-formula element incorporation is discussed in the light of chemical

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ACCEPTED MANUSCRIPT disturbance and potential alteration mechanisms in correlation to the microstructural damage. LA-ICP-MS zircon chemical data is given in the Supplementary Table S-2.

3.3.1 Actinide content and alpha-dose

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Uranium concentrations range widely in the studied zircons, but can be correlated with the CL domains. For the pristine grains, average U concentrations are 141 ± 79 ppm for core, 437 ± 212 ppm for alt-1, 2,545 ± 1,211 ppm for alt-2, and 140 ± 77 ppm for rim domains. Thorium concentrations correlate positively with U content, but overall show minor

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variations. In Figure 7a, Th versus U concentrations are plotted for pristine grains only, but treated grains show similar trends. Cores plot along relatively consistent “magmatic” Th/U

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values (~0.37; e.g. Belousova et al., 2002), while the rims and the highly damaged alt-1 and alt-2 domains plot at lower Th/U values (~0.1). Secondary loss of Th or gain of U relative to

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each other is rather unlikely in this context, as both elements should behave relatively similar during alteration (e.g. Geisler et al., 2002). Rather, the low Th/U ratios within the alt-1 and

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alt-2 domains are probably features related to late-stage magma composition, after Th was

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exhausted, or incorporated preferentially, for example, in competing, co-existing apatite that is found abundant in the host rock and often shows intergrowth with zircon.

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In order to correlate the U and Th contents to the structural damage revealed by Raman spectroscopy, alpha-doses (D) were estimated from the equation given by Holland and Gottfried (1955):

𝐷 = 8 238𝑈[𝑒𝑥𝑝(λ238 ∗t) − 1] + 7 235𝑈[𝑒𝑥𝑝(λ235 ∗t) − 1] + 6 232𝑇ℎ[𝑒𝑥𝑝(λ232 ∗t) − 1] [1]

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ACCEPTED MANUSCRIPT with λ, decay constants for 238U, 235U, and 232Th, respectively (Steiger and Jäger, 1977); 238U, 235

U, and 232Th, actinide content in atoms/mg. An age (t) of 3.50 Ga was used, based on

preliminary U-Pb age results (Section 3.5). The range in estimated alpha-doses for each domain are (average values in brackets): cores 0.7-13.3 1015α/mg (3.6), alt-1 3.1-34.4 1015α/mg (14.4), alt-2 4.5-93.4 1015α/mg (35.6), and rims 0.8-5.8 1015α/mg (3.2),

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respectively.

Previous X-ray diffraction studies have shown that the alpha-dose dependent progressive metamictization advances in three stages (Murakami et al., 1991; Pidgeon, 2014). In the first stage, up to D ~3*1015 a/mg, dominantly point defects accumulate, resulting in the unit cell

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expansion; in the second stage, up to D ~8*1015 a/mg (corresponding to the percolation point observed in the changing Raman response; Fig. 6), progressive distortion of crystalline

α /mg, zircon is fully amorphous.

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remnants and increase of amorphous regions occurs; and in the third stage, above D ~8*1015

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The amount of the amorphous fraction (p) can be estimated based on the received alpha-dose

[2]

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𝑝 = 1 − 𝑒𝑥𝑝−𝐵(𝐷−𝐷𝑖𝑛𝑐)

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by the equation (Geisler et al., 2003a):

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with B = 0.28*10-18 as the amorphous mass (g) produced per α-decay, D as the α-dose, and Dinc = 0.47*1018 (α/g) as the incubation dose accounting for natural annealing during the geological history (e.g. Geisler et al., 2003a). In Fig. 7b, the plot of the amorphous fraction p versus the received alpha-dose D for each domain of the pristine grains demonstrates that the identified domains overall represent the full range of possible radiation damage, from crystalline in the cores to almost completely amorphous within the alt-1 and alt-2 domains, if natural annealing has not occurred.

17

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3.3.2 Non-formula and rare earth element incorporation

In agreement with the X-ray maps (Fig. 4), LA-ICP-MS analyses reveal non-formula Ca concentrations of ~1,600 ppm in alt-1, and ~4,650 ppm with values >10,000 ppm, within alt-

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2. Such high concentrations of Ca must be of secondary origin, likely supplied by alteration fluids into the metamict zircon domains. This is consistent with the proposed natural annealing indicated in the Raman frequency versus line-width plot (Fig. 6). The observed drastic increase in Ca above an amorphous fraction of p~0.8 (Fig. 7c) corresponds to the

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critical alpha-dose value of D ~8*1015 α/mg that marks the transition between metamictization stage two and three (Murakami et al., 1991). Based on studies using

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percolation-type theory (e.g. Salje et al., 1999; Ríos et al., 2000), the critical dose marks the second percolation point characterized by a transition between a damage state in which

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amorphous material starts to form percolating networks, and a damage state in which crystalline material is not connected any more and amorphous material dominates. Such a

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structure of connected amorphous domains provides low-temperature diffusion pathways,

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promoting elemental redistribution (e.g. Meldrum et al., 1998; Geisler et al., 2001a; Balan et al., 2003; Utsunomiya et al., 2004). The drastic increase in Ca above an amorphous fraction

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of p~0.8 fits well with the existence of the second percolation threshold value, previously experimentally determined at p~0.7 (Geisler et al., 2001b; 2003a). The second percolation point at p~0.7 is associated with a dramatic increase in diffusion distance, interpreted as the appearance of high-diffusivity paths of reduced density in the damaged structure due to regions of depleted matter, as a result of alpha-radiation-induced atomic displacement cascades (Geisler et al., 2001a,b; 2003a; Trachenko etal., 2004).

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ACCEPTED MANUSCRIPT As shown in Figure 7d, the increasing Ca content in alt-1 and alt-2 positively correlates with increasing La (LREE) incorporation. Although progressive substitution of REE with decreasing ionic radii is facilitated during magmatic growth (e.g. Belousova et al., 2002), the coupled incorporation of La (i.e. LREE) and non-formula Ca in the more damaged alt-1 and alt-2 domains likely indicates addition of LREE into the damaged structure during an

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alteration event. This is in agreement with our observation of dark/no CL holes (Fig. 1) indicating porous textures in the alteration domains (e.g. Hay and Dampster, 2009). Solidstate recrystallization of the chemically weakened metamict material through lowtemperature hydrothermal fluid dissolution-reprecipitation likely resulted in the formation of

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porous domains (e.g Hay and Dampster, 2009). Besides significant Ca and LREE content, the composition and origin of the alteration fluids is unclear, but given the presence of altered

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feldspar in the host rock, a proximal source for the elemental redistribution may be inferred. Such fluid alteration process could have also caused some degree of natural annealing, as

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3.4 Radiation damage ages

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indicated in the Raman frequency versus line-width plot (Fig. 6).

As indicated in Figure 6, the pristine grains that plot above the un-annealed radiation damage

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trend likely experienced natural re-heating during their geological history, partially recovering the zircon structure. The quantification of the degree of structural damage in regards to known alpha flux determined by actinide content can provide constraints on the conditions and timing of natural annealing of zircon (e.g. Holland and Gottfried, 1955; Pidgeon et al., 1998; Pidgeon, 2014). The dependence of the v3 SiO4 line-width (i.e. FWHM) on the α-dose has been used to establish a calibration curve for un-annealed Sri Lankan zircon (Nasdala et al., 2001), where

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ACCEPTED MANUSCRIPT an exponential relationship of damage accumulation determined by Raman spectroscopy is given as (Palenik et al., 2003):

𝐹𝑊𝐻𝑀 = 𝐴[1 − 𝑒𝑥𝑝(−𝐵𝐹𝑊𝐻𝑀 𝐷𝑒𝑑 ) ]

[3]

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with A, asymptotically approached FWHM value of 35.64; B, related to mass of material damaged per alpha decay event (computed at ~5.49 x 10-19/g); Ded, equivalent damage dose determined from U and Th contents and age of radiation damage (375 Ma) for calibration using Sri Lankan zircon. Solving equation [3] for the equivalent damage dose Ded allows the

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calculation of radiation damage ages of unknown specimen, using U and Th concentrations representative of the Raman point analyses (Pidgeon, 2014).

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The relationship between the v3 SiO4 FWHM and D for a selection of representative pristine zircon is shown in Figure 8, which confirms that at a given alpha-dose (D) the broadness of

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the v3 SiO4 band lies below the calibration curve (Ded), hence natural annealing occurred (e.g.

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Nasdala et al., 2001). Calculated radiation damage ages yield relatively consistent values for all pristine zircon domains (Table 2), with an average damage accumulation age of 504 ± 19

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Ma (n=9). Note that doses calculated from the Raman relationship yield slightly higher values than the estimated total alpha dose for low damage core analyses (spot 01-2, 04-3).

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This is ascribed to the fact that the calibration curve does not take into account a minimum FWHM (1.8 cm-1) at zero alpha-dose (Nasdala et al., 2001; Pidgeon, 2014). Even the highly damaged alteration domains alt-1 and alt-2 show some degree of natural annealing. It is expected that at natural low-temperature conditions, primarily point defects recovered along a frequency – line-width vector similar to the annealing trend in Figure 6 (Geisler et al., 2001b). Both the state of structural distortion prior to ~504 Ma, and the degree of recovery (i.e. vector length in Fig. 6), remain speculative. Renewed accumulation of

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ACCEPTED MANUSCRIPT radiation damage after natural annealing would have shifted the frequency – line-width relationship to its present position, along a vector defined by the radiation damage trend. Because the degree of long-range order defect recovery, if occurring at all during initial (i.e. low-temperature) annealing, is poorly understood, the impact of natural annealing on the present amorphous fraction remains unclear. Our observations of the second percolation point

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for enhanced non-formula element diffusivity above an amorphous fraction of p~0.8, may approach the value (p~0.7) determined by Geisler et al. (2001b; 2003a), when accounting for possible long-range order recovery during natural annealing.

Radiation damage ages provide reliable last cooling ages that can be interpreted in a similar

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way to Ar-Ar or Rb-Sr cooling ages (Nasdala et al., 2001; Pidgeon, 2014). The radiation damage ages calculated here for the East Pilbara quartz-diorite gneiss from the Muccan

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Granitic Complex are in agreement with radiation damage ages of 420 ± 110 Ma calculated for zircon from the Darling Ranges, Yilgarn Craton, Western Australia (Pidgeon, 2014). The

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~420 Ma ages confirm the conditions of natural zircon annealing, evident in Rb-Sr age isopleths for biotite (400-500 Ma; Nemchin and Pidgeon, 1999; Pidgeon, 2014).

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It should be noted that natural annealing at ~500 Ma does not exclude the occurrence of prior

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annealing events. Similarly, the full extent of non-formula and LREE incorporation through hydrothermal fluids cannot be ascribed to the Phanerozoic event alone. As evident in the

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estimated total α-doses (D), it is expected that at least the high-U zircon domains were already highly damaged prior to ~500 Ma. Multiple alteration and annealing events may have contributed to the present chemical and structural state of the zircon.

3.5 U-Pb isotopic systematics

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ACCEPTED MANUSCRIPT Below, we systematically investigate the effects of pre-analytical treatment on the U-Pb systematics measured by LA-ICP-MS. First, the effects on annealed and pristine standards Temora2, Plešovice and OG1 are examined relative to their accepted TIMS U-Pb values. Then, we will compare the effects of using annealed and pristine reference standard Temora2 as applied to our unknown pristine zircon cores. Pristine grains are used to distinguish

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between analytical artifacts (i.e. ablation matrix effects) and natural characteristics in regards to the nature and degree of U-Pb discordance. Finally, we will assess the U-Pb data of pristine and experimental grains, integrating the physico-chemical state of the studied unknown zircon domains, towards the reliability and accuracy of recorded apparent ages. U-

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Pb data are given in the Supplementary Table S-3.1 and S-3.2 using annealed and pristine

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Temora2, respectively, as the primary reference material.

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3.5.1 Effects on U-Pb systematics of reference material

known TIMS results

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a) Matching of matrices among reference zircons in LA-ICP-MS analyses compared to

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Table 3 demonstrates several features well understood about the precision of ICP-MS measurement of zircon ages using excimer lasers, especially U-Pb fractionation (Allen and

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Campbell, 2012; Marillo-Sialer et al., 2014; Marillo-Sialer et al., 2016). For Temora2 and Plešovice zircons for both pristine and annealed types, Table 3 gives the TIMS (“accepted”) 206

Pb/238U, 207Pb/235U and 207Pb/206Pb values, and fractionation factors, defined here as the

accepted divided by the average measured ratios. These zircon samples are both Paleozoic or “middle aged” reference materials that are old enough to have built up measurable radiogenic Pb, but young enough to not be overly damaged by radioactive decay products. Note that although Plešovice is younger (337.1 Ma; Sláma et al., 2008), it has a greater U+Th content

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ACCEPTED MANUSCRIPT and therefore has a higher calculated radiation dose, which is about twice that of Temora2 (Black et al., 2004). Down-hole variation of Pb-U ratios is observed, as is instrument drift, but to emphasize the effects of relative standardization, what is reported here is the average ablation, then averaged as a group (n=12 to 15; Table 3). Once instrumental settings are fixed, most fractionation is caused at the ablation site and is controlled by laser-substrate

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interactions, which depend on structural integrity and trace element content (Marillo-Sialer et al., 2014; Marillo-Sialer et al., 2016). More damaged grains should appear older because they liberate their Pb more easily with every ablation pulse (sampled volume) compared to U. During ablation, energy from the laser radiation causes the breakdown of zircon to crystalline

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and amorphous ZrO2 and amorphous SiO2, which deposit around the ablation crater (Košler et al., 2005). Preferred incorporation of both U and Pb into the newly formed ZrO2 and SiO2

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deposits, results in elemental fractionation characterized by a decoupling of radiogenic Pb that escapes and is measured in higher Pb/U ratios, yielding older apparent ages (Košler et

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al., 2005; Allen and Campbell, 2012). Within highly damaged zircon, this effect is enhanced due to the physically and chemically weakened zircon being ablated more readily, as evident

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in deeper ablation pits (e.g. Crowley et al., 2014; Marillo-Sialer et al., 2016). As reviewed

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earlier, annealing improves crystallinity but does not fully repair it; radiation damage is a non-reversible process.

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What can be gleaned from Table 3 is that for this analytical session, in all cases, the measured Pb/U ratios are less than the accepted values and thus the fractionation factors to correct the measured ratio to known values are greater than 1. For annealed Temora2, the fractionation factors for both 206Pb/238U and 207Pb/235U are similar (~1.11) meaning that the correction factor for 207Pb/206Pb is unity, which is expected given that intra-element mass fractionation is not expected, except for mass bias associated with instrument tuning. If annealed Temora2 is used to treat the pristine Temora2, the resulting 206Pb/238U ages are

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ACCEPTED MANUSCRIPT about 2% too old, and this stems from the fact that after annealing, the reference material is physically different and thus has a different fractionation factor. The annealed grains from Plešovice should be well matched to annealed Temora2 but the calculated 206Pb/238U age is about 3% too old. The most extreme physical mismatch is between annealed Temora2 and pristine Plešovice. Using the tabulated bulk ages, the calculated age for pristine Plešovice is a

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full 8% (28 Ma) too old. Note that the uncertainties on fractionation factors are 1 to 2%, which limits the ultimate quoted accuracy for an individual ablation age even when standards and samples are well matched. The effect of mismatch is shown in Figure 9, in which the individual ablations are plotted having used annealed Temora2 as the reference material. The

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TIMS values are shown (orange dots in Fig. 9). Annealed Temora2 plots concordantly where dictated given that it is the reference. Pristine Temora2 appears somewhat older than

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accepted and is more scattered. Annealed Plešovice is somewhat older than its accepted age and pristine Plešovice is significantly older than accepted, is scattered, and is reversely

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discordant. The bulk 207Pb/206Pb fractionation factor for pristine Plešovice is 1.025 ± 0.025 (Table 3) making the 207Pb/206Pb fractionation factor “1” within calculated uncertainty. This

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test demonstrates potential fractionation factor complications due to matrix effects for

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Paleozoic zircons, but what of the effects on the Archean zircons? To test this, we analyzed pristine grains of the 3465 ± 0.6 Ma reference zircon OG1, which is slightly normally

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discordant after chemical abrasion, according to TIMS results (Stern et al., 2009; orange dot, inlet in Fig. 9). The pristine OG1 zircons are relatively concordant, when reduced using physically matched pristine Temora2 (Fig. 9, inlet). We obtain a mean 207Pb/206Pb age of 3445 ± 42 Ma for pristine OG1 (5 analyses), or an upper Concordia intercept of 3458 ± 36 Ma (MSWD=0.09). Figure 9 (inlet) shows that using the physically mismatched annealed Temora2 (upper intercept of 3475 ± 92; MSWD=0.12), introduces reverse discordance for the pristine OG1 grains.

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ACCEPTED MANUSCRIPT In synthesis, we confirm the notion of a radiation-damage induced LA-ICP-MS matrix effect with up to 8% difference in age compared to the known TIMS age for pristine Plešovice using the physically mismatched TA Temora2. This highlights the importance of preanalytical treatment of standards along with unknowns.

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b) The effects on U-Pb systematics of reference zircons and their physical state as applied to pristine Archean unknowns

The effects of switching from pristine to annealed reference Temora2 on the studied unknowns is examined in Figure 10a, showing a Concordia plot of the 21 pristine cores. The

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dots represent results when pristine Temora2 is applied as the reference (no uncertainty shown for clarity). The ellipses represent the application of annealed Temora2 as the

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reference to these pristine unknowns. Only one of the 21 points is normally discordant. The upper intercept age is the same within uncertainty no matter the reference (3507 ± 48 Ma;

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lower intercept: -564 ± 3100 Ma; MSWD=0.10 for pristine reference; 3498 ± 23 Ma; lower intercept: -226 ± 980 Ma; MSWD=0.82 for annealed reference; Fig. 10a). For the pristine

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unknowns, the matrix effect of using annealed Temora2 shows some shift towards reverse

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discordance, but is overall insignificant. Instead, the physical match between pristine standard and unknown clearly demonstrates that in this case, reverse discordance is not an

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artifact of mismatch between standards and unknowns; reverse discordance is a genuine characteristic of the studied cores. As demonstrated in Section 3.4, the studied zircons experienced natural annealing at ~500 Ma. The accepted TIMS age for Temora2 is ~417 Ma, and U concentrations are similar to the core domains in zircon from this study (Black et al., 2004). We suggest that the little effect observed between standard match and mismatch is grounded in a similar matrix effect during ablation between our (naturally annealed) unknowns and Temora2. Previous studies, in which zircons of different age and damage state

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ACCEPTED MANUSCRIPT without reported natural annealing, have been compared, matrix effects due to greater mismatch were more significant (e.g. Crowley et al., 2014; Marillo-Sialer et al., 2016). This is consistent with our observation that effects of standard mismatch are greater for the more radiation damaged Plešovice than for our unknowns (Fig. 9, Fig. 10a).

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3.5.2 U-Pb systematics of unknowns and the effects of thermal annealing and chemical abrasion

Pre-analytical treatment workflows that are derived based on the study of high-quality

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international reference standard zircon should not be readily justified for application to unknowns (Schaltegger et al., 2015). In fact, we will demonstrate below that the U-Pb

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systematics of the here studied unknowns are tied to a unique, complex zircon history, which requires detailed examination. In order to assess the nature and intensity of pre-analytical

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treatment effects, it is necessary to integrate observations on the natural characteristics of observed U-Pb systematics and discuss processes responsible for isotopic disturbance and U-

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Pb discordance.

a) Natural reverse discordance and Pb redistribution

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As demonstrated in Section 3.5.1b, reverse discordance of unknown core analyses is genuine, and not an analytical artifact. In contrast to normal discordance that is mostly attributed to Pb loss, reverse discordance implies the presence of excess, unsupported radiogenic Pb (or U loss). Concordia evaluation, on first hand, reveals co-linearity of U-Pb systematics within distinct structurally and chemically identified zircon domains: i) slightly normal to mostly reversely discordant cores (Fig. 10a-c), ii) slightly reversely discordant to medium normal discordant rims (Fig. 10d), and iii) rare reversely discordant/concordant to mostly highly

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ACCEPTED MANUSCRIPT discordant alt-1 and alt-2 domains (Fig. 10e,f). As exemplified for our TA cores in Figure 10b, reversely to normal discordant clusters of co-linear Discordia-fit generally correlate with U content. Reversely discordant spots are low in U concentration, while normal discordant spots are characterized by elevated U concentrations. Following previous studies, we argue that radiogenic Pb was mobilized and redistributed from high- to low-U domains, as the

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result of either event-diffusion, or alpha-recoil implantation (e.g. Williams et al., 1984; Mattinson et al., 1996; Carson et al., 2002; Valley et al., 2014; Kusiak et al., 2015; Peterman et al., 2016). Due to the similar behavior of 238U and 235U during diffusion, and/or the similar total decay chain energies of both U-Pb systems, isotopic fractionation is thought to be

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negligible for both diffusion and (at least instantaneous) alpha-recoil implantation (Mattinson et al., 1996). Therefore, Discordia chords can be used as in normal discordant “one-event”

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systems, providing geological meaning for either event-related (i.e. diffusion), or zero (i.e. instantaneous alpha-recoil implantation) lower intercept ages. However, we observe negative

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lower intercept apparent ages for the pristine core domains (Fig. 10a). According to Mattinson et al. (1996), this indicates increasing alpha-recoil implantation affecting

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increasingly damaged zircon, over time. The existence of this process cannot be excluded,

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nor conclusively confirmed, in the present study. Below, we discuss an alternative process, in which event-related diffusion caused a spatially associated co-existence of different excess

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Pb components in the cores, sourced from different domains, resulting in steeper Discordia slopes and negative lower intercepts through analytical mixing.

b) Analytical mixing of U-Pb components and comments on their apparent ages For the TA cores, Figure 10b shows a Discordia yielding an upper intercept of 3472 ± 17 Ma, and a lower intercept of 786 ± 450 Ma (N=26; MSWD=2.7). Both the slightly wider data dispersion along Discordia and the younger upper intercept are somewhat conspicuous, in

27

ACCEPTED MANUSCRIPT comparison to the pristine cores (Fig. 10a), and cannot be readily explained by thermal annealing effects. Closer examination of the data reveals that the normally discordant “cluster” defined by four analyses is characterized by highest U concentrations (Fig. 10b), as expected, and also highest (>200 ppm) non-formula Ca concentrations, which are in the range of Ca concentrations in the alteration domains. The U-Pb ratios and ages of the normally

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discordant “cluster” spots are significantly lower/younger than the concordant to reversely discordant analyses of the cores. In our view, the TA core Discordia (Fig. 10b) represents an example of mixing of different U-Pb components. In fact, by discarding the four spots of the normal discordant cluster, a Discordia defined by the remaining 22 spots yields upper and

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lower intercepts of 3498 ± 13 Ma and -521 ± 1200 Ma (MSWD=1.6), respectively, which are indistinguishable from the pristine cores (Fig. 10a). The inclusion of the high-U cluster data

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lowered the Discordia slope and shifted the upper intercept towards too young, and the lower intercept towards negative (i.e. future) ages (Fig. 10b).

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Here we relate the high-U normally discordant cluster in the TA cores to the alteration domains (alt-1, alt-2) that are strongly normally discordant to concordant, and rarely

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reversely discordant. For convenience we plot U/Pb data for both alt-1 and alt-2 domains

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from each experiment together (Fig. 10e,f). For both domains, high dispersion of data spots along Discordia is observed, without obvious systematic difference between the experiments.

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Although poorly fit, both alteration domains give similar intercept ages of 3413 ± 62 Ma and 599 ± 180 Ma (MSDW=33; Fig. 10e) for alt-1, and 3409 ± 110 Ma and 614 ± 210 Ma (MSDW=140; Fig. 10f) for alt-2, respectively. The lower intercepts are roughly in agreement with the ~500 Ma radiation damage age, and the upper intercepts (~3410 Ma) make sense in that they are younger than the core ages and are defined by discordant data with U-Pb systematics similar to the high-U TA core cluster spots. We propose that the alteration domains and the high-U core components experienced partial U-Pb isotopic re-setting,

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ACCEPTED MANUSCRIPT possibly close to ~3410 Ma. The latter resetting is likely related to a tectono-magmatic event that culminated in the study area around 3460-3420 Ma (Wiemer et al., 2016). Major Pb loss (lower intercepts) occurred contemporaneous to the ~500 Ma event recorded in the radiation damage ages. Revisiting the reversely discordant cores, we argue for a process, in which radiogenic Pb

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(slightly younger than the cores) was redistributed from the partially re-set high-U domains to explain the observed negative lower intercepts in pristine and TA cores. As indicated in Figure 10c, CA treated cores do not include the excess Pb component that results in negative lower intercepts. Instead, the lower intercept (560 ± 550 Ma) coincides with those of the

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alteration domains and with the Raman radiation damage age. Slight reversely discordant data persisting in the CA cores, is well matched (MSWD=1.0) along the Discordia that

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defines the upper (3503 ± 14 Ma) and lower intercepts.

Structural recovery during natural thermal annealing results in rejection of Pb, progressing

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from low- to higher damage portions (Kusiak et al., 2015). If higher damage sites become isolated and inaccessible for leaching, Pb will be trapped. Recent studies using high-

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resolution scanning transmission electron microscopy (Kusiak et al., 2015) and atom probe

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tomography (Valley et al., 2014; Peterman et al., 2016), have demonstrated the existence of randomly distributed nm-scale Pb clusters, particularly in ancient zircon, accumulated and

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trapped during metamorphism that lead to Pb loss. Outside these clusters new radiogenic Pb accumulated after annealing from low Pb zircon. This results in discordance characterized by discrete Pb reservoirs recording different isotopic compositions (Peterman et al., 2016). Our interpretation of co-existing Pb components is similar to these models, with the difference that, in the present case, Pb mobilization was related to a single event, but distinct reservoirs were sourced from distinct domains, one having experienced prior isotopic disturbance. One may argue that the younger excess radiogenic Pb component may have been

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ACCEPTED MANUSCRIPT accumulated from recent self-irradiation, after natural annealing. This is inconsistent with our data, in which highest relative Pb excess is associated with low-U domains. Recent (leachable) radiogenic Pb accumulation should show higher relative excess in proportion to higher U content. We argue that both short distance Pb redistribution within the cores, and Pb diffusion from adjacent high-U domains into the cores occurred contemporaneously during

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major Pb loss at ~500 Ma. In agreement with expected longer diffusion distances, and the fact that only the radiogenic Pb that was sourced from the high-U alteration domains was leached upon CA, it is argued that the latter Pb accumulated within distinct damage sites. Accumulated damage controls dissolution and leaching behavior in zircon (Mattinson, 1994;

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Davis and Krogh, 2000; Romer, 2003). The observed extensive radial and fir-tree fracture network observed in CL (Fig. 1) likely formed as a result of volume swelling due to

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radiation-induced point defect accumulation (Trachenko et al., 2003) in the alteration domains, and provided enhanced diffusion pathways during hydrothermal alteration,

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connecting alteration and core domains (Fig. 1c). There is no evidence for any hightemperature metamorphic event in the studied host quartz-dioritic gneiss, or anywhere else

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within the East Pilbara Terrane, that can be correlated with the 500 Ma natural annealing.

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Therefore, significant volume diffusion of Pb particularly within the low-U, low-damage cores is unlikely (Cherniak and Watson, 2000; Valley et al., 2014). Instead, it is proposed

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that micro-fracture networks and possibly short distance alpha-recoil damage-related pathways allowed limited fluid-enhanced Pb diffusion redistribution. Longer distance diffusion required to redistribute Pb from the alteration domains into the cores must have occurred along distinct enhanced diffusion pathways. X-ray mapping shows that elevated Ca and Fe concentrations in cores mostly focus along fractures (Fig. 4a), and hence these are likely the sites of excess radiogenic Pb concentrations derived from the adjacent high-U alteration domains. In contrast, radiogenic Pb concentrations in fracture-absent portions

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ACCEPTED MANUSCRIPT would have rather formed from Pb that accumulated within the cores themselves (see Kusiak et al., 2015). The fractures also provided accessibility for experimental CA leaching, while in-between the fractures, zircon cores were left unaffected by CA. In conclusion, we accept the upper intercept of 3503 ± 14 Ma of the CA cores (Fig. 10c) as the timing of magmatic core crystallization.

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Finally, there are seven discrete rim ablations, which are either pristine (n=4), TA (n=2) or CA (N=1), ranging from reversely to normally discordant, and give an upper intercept age of 3469 ± 68 Ma and a lower intercept of 445 ± 1200 Ma (MSWD=2.9; Fig. 10d).

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c) Constraints on the timing of alteration and the validity of the lower intercept apparent ages

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All high-U normal discordant spot analyses in the alteration and core domains are characterized by elevated non-formula Ca concentrations, and these domains experienced

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alteration fluid interaction after significant radiation damage occurred. We observe that increased normal discordance correlates with actinide contents in excess of ca. 400 ppm U

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and 100 ppm Th. Using these minimum concentrations to define high-U domains

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characterized by Pb loss, we can approximate minimum α-doses to constrain the time required to produce sufficient damage: in order to reach the first and second percolation point

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(3.0*1015 α-mg-1 and 8.0*1015 α-mg-1; e.g. Murakami et al., 1991), the ~3000 Myr before the natural annealing event (~500 Ma) are required, yielding a minimum alpha-dose of 5.99*1015 α-mg-1 for the high-U domains, compared to 0.72*1015 α-mg-1 for 500 Myr of recent damage accumulation (note: full dose calculates at 7.71*1015 α-mg-1 for 400 ppm U and 100 ppm Th, and 3500 Myr). Similarly, post-crystallization damage accumulation until partial re-setting of the alteration domains occurred (~3410 Ma) would not have allowed significant non-formula Ca incorporation. Re-calculation of the amorphous fractions for 3000 Myr-damage

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ACCEPTED MANUSCRIPT accumulation, shifts the second percolation point, here defined as the drastic increase in nonformula Ca in pristine cores (Fig. 7c), to the expected value of p~0.7, experimentally determined by Geisler et al (2001b). This confirms that most damage accumulated in the ~3000 Myr prior to the ~500 Ma event, and that the most extensive alteration and associated Pb loss occurred during the ~500Ma event, as indicated by the lower intercept ages (Fig. 10c-

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f).

d) Synthesis of the effects of thermal annealing and chemical abrasion on the U-Pb systematics of unknown DW-370-1-14

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Previous studies have suggested that thermal annealing removes ablation artifacts in LA-ICPMS (Mattinson, 2005; Allen and Campbell, 2012). In our study little effects are observed

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between pristine and TA grains using matching pristine and annealed standard Temora2. Importantly, the studied zircons experienced natural annealing at ~500 Ma. We argue that the

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coincidence of the natural annealing age of the studied grains with the crystallization age of Temora2, in combination with similar U concentrations in both our cores and Temora2,

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resulted in a physical match between unknowns and standard (i.e. similar α-dose and damage

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state). The upper intercept ages of both pristine (3507 ± 48 Ma) and TA (3498 ± 13 Ma, excluding the high-U cluster) cores, are in good agreement. The only difference is observed

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in the smaller errors for TA cores compared to the pristine ones. A reduced U-Pb data dispersion has been reported upon experimental TA in previous studies (Solari et al., 2015). In general, two processes may cause the reduced dispersion: i) improved physical integrity at the ablation site (e.g. Allen and Campbell, 2012), and ii) rejection of Pb due to the structural recovery upon TA (e.g. Kusiak et al., 2015). As a compromise of our study in using different grains for each experiment, an arbitrary error reduction due to a sampling bias cannot be excluded. However, as demonstrated in the present study, structural recovery of point defects

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ACCEPTED MANUSCRIPT that likely accumulated since the natural annealing, upon experimental TA, is relatively significant. This may imply that improved zircon density and associated smaller ablation volume in fact resulted in higher precision (reduced error). Single ablations show a slight reduction in internal 2-σ errors for 206Pb/207Pb ages between pristine (average ± 51 Ma) and TA (± 45 Ma) cores. The upper intercept age error of ± 48 Ma is reduced to ± 23 Ma upon

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introduction of annealed Temora2 for reduction of pristine cores, and further reduced to ± 13 Ma after TA of the unknowns. Both upper and lower intercept ages are statistically indistinguishable, and no difference in the range of discordant data are observed, excluding a process in which significant Pb was removed during experimental TA structural recovery. In

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synthesis, the full potential of thermal annealing alone could not be explored in regards to Archean zircon, due to the fact that natural annealing occurred. Thermal annealing of the

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more recent (~500 Ma to present) damage shows no effect on the general U-Pb systematics, but precision is somewhat improved, which we ascribe to the change in physical parameters

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influencing laser ablation.

The results of our CA treatment, on the other hand, not only show improved accuracy and

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precision of the U-Pb zircon age, but allow us to reconstruct a model of the case-specific U-

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Pb systematics history of the studied unknowns as discussed in Section 3.5.2b. Importantly, the favored Discordia (Fig. 10c) is most strongly supported by integrating the Raman

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spectroscopic and chemical zircon data, providing constraints and validation of the lower intercept (Section 3.5.2c). Previous HF dissolution experiments have demonstrated that U and other trace elements (radiogenic Pb) are preferentially and selectively removed from high-U domains, and/or that high-U domains affected by Pb loss are completely dissolved (Mattinson, 1994; Davis and Krogh, 2000; von Quadt et al., 2014; Crowley et al., 2014). Davis and Krogh (2000) found that threshold α-doses slightly in excess of 1.5*1015 α-mg-1 are required for experimental HF dissolution of U-rich zircon. In our experiment, distinct

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ACCEPTED MANUSCRIPT dissolution textures are not observed upon experimental CA, and we argue that no significant physical volume was dissolved/removed, not even from the high-U alteration zones. Instead, porous dissolution textures are observed in all experiments (Fig. 1, Fig. 2) within the high-U alteration domains. As stated in Section 3.5.2c the minimum α-dose for recent damage accumulation in the high-U domains is estimated at 0.72*1015 α-mg-1, hence below the

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threshold for dissolution proposed by Davis and Krogh (2000). However, these domains clearly reached a damage state susceptible for partial hydrothermal fluid dissolution – reprecipitation prior to the ~500 Ma natural alteration-annealing event, evident in the incorporation of non-formula elements. Based on the observations from reflected light

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imaging (Supplementary Material S-1), the introduction of milky-white areas that we ascribe to residue from the CA experiment, focuses most significantly along high-damage

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fracture networks (Supplementary Fig. S-2a). We predict that the fracture network promoted redistribution of radiogenic Pb from the partially reset alteration domains into the

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core domains during overall Pb loss at ~500 Ma, leading to reverse discordance (in the cores). Similarly, experimental HF exposure would have allowed preferential removal of the

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excess radiogenic Pb from these sites (i.e. fractures), while in between the fractures

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substantial natural annealing resulted in chemical resistance to CA under the used conditions, particularly in the low-U cores.

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The physically matched pristine cores - pristine Temora2 show larger scatter in 206Pb/238U ages (713 Ma) compared to 207Pb/235U ages (244 Ma). Upon CA (using matched annealed Temora2 standard), the scatter in 207Pb/235U remains the same (reduced by <1 %, i.e., negligible), while the scatter in 206Pb/238U ages is reduced by >30 %. Furthermore, it is particularly the high 206Pb/238U end-member data spots that “disappear”, and overall the latter ages are observed to approach the corresponding 207Pb/235U values/ages, hence improving concordance. Using weighted mean ages of only the most concordant (± 1.5 % discordance)

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ACCEPTED MANUSCRIPT analyses, yielding 3510 (± 9) and 3538 (± 21) Ma for 207Pb/235U and 206Pb/238U ages, respectively, for pristine (N=8), and 3511 (± 35) and 3522 (± 61) Ma, respectively, for CA cores (N=10), shows that the 206Pb/238U weighted ages approach the 207Pb/235U ages, which are basically identical for both experiments. The 207Pb/235U ages are more consistent with the accepted upper intercept of 3503 ± 14 Ma for CA cores. In our view, this demonstrates a

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preferential “removal” of particularly high excess 206Pb, where excess 206Pb was sourced from the adjacent partially reset alteration domains, redistributed during the ~500 Ma event.

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4. Conclusions

We present a microstructural and U-Pb systematics study comparing pristine, thermally

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annealed (TA) and chemically abraded (CA) early Archean zircon from a quartz-dioritic gneiss, in order to improve LA-ICP-MS U-Pb zircon dating workflows. Overall, we find that

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most reliable (i.e. accurate and precise) U-Pb ages are ultimately achieved through CA treatment, which includes annealing of radiation damage and chemical abrasion of high-U

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discordant portions. However, our study demonstrates following critical new insights into the

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use of pre-analytical treatment of zircon, which leads us to conclude that standardization of pre-analytical treatment is not recommended:

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Firstly, we show that the effects of pre-analytical treatment are case-specific and controlled by the initial state of the zircon or zircon domain. In general, this means that treatment conditions must be adjusted according to the initial damage state of unknowns, highlighting the importance of characterizing pristine grains in terms of microstructure and U-Pb systematics, first. In the present case, the core domains record part of the primary U-Pb signature, but they also comprise a secondary U-Pb component likely as the result of Pb redistribution from adjacent high-U alteration domains that experienced partial isotopic

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ACCEPTED MANUSCRIPT resetting. Only upon CA, the latter U-Pb component was removed, indicating that radiogenic Pb moved into specific damage sites, accessible for CA leaching. Without the prior characterization of pristine grains, identification of the secondary excess Pb component would not have been possible. Here, most fundamental information comes from Raman spectroscopy, revealing that the zircons experienced natural annealing during a Pb loss event,

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which constrains the true slope of the Discordia and its lower intercept. In addition, our data supports the notion that radiation damage is irreversible, and more data from natural samples are needed to establish reference frameworks that would correlate initial damage state with most plausible treatment conditions.

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Secondly, we demonstrate that physical mismatch between treated and pristine standardunknown-pairs results in up to 8% age difference compared to known TIMS values (here for

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pristine Plešovice using TA Temora2 as the primary reference standard), attesting for the existence of LA-ICP-MS matrix effects. However, despite their antiquity, matrix effects are

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negligible for our unknown cores, which experienced natural annealing at a time close to the crystallization age of the used reference standard Temora-2 characterized by similar U

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content. Therefore, matrix effects are reduced through natural or experimental TA-induced

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structural recovery. Standard material should be treated in the same way as unknowns, and standards should be selected carefully to comply with both U content and the age of

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crystallization, or the timing of the most recent natural annealing experienced by the unknowns. We propose to establish a Raman spectroscopic database for commonly used reference zircons in order to readily select physically matching reference standards for LAICP-MS data reduction of well-characterized unknowns. Furthermore, we demonstrate that reverse discordance in our zircons is not an analytical artifact, but a genuine U-Pb systematics characteristic, highlighting the impact of intra-grain Pb redistribution on the µm-scale of common LA-ICP-MS analytical spot sizes. We show

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ACCEPTED MANUSCRIPT that some excess radiogenic Pb within our cores could not be removed through CA treatment, and records primary U-Pb systematics. In combination with supporting validation of lower Discordia intercepts from Raman radiation damage ages, the reverse discordant analyses, which are not removed during CA, represent more reliable primary U-Pb systematics than the normal discordant analyses from high-U ablations.

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Finally, we observe an increasing coupled incorporation of non-formula elements (e.g. Ca, Fe) and LREE (e.g. La) with increasing U content and structural damage. This suggests that high LREE signatures in zircon are at least partially the result of hydrothermal alteration, and not of primary magmatic origin.

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In synthesis, we recommend a relatively time-efficient LA-ICP-MS U-Pb zircon workflow, in which a subset of pristine grains are characterized along with TA and CA grains by means of

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CL imaging and Raman spectroscopy. During LA-ICP-MS analyses, co-measurement of nonformula elements is recommended. Zircon standard material should be treated under the same

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5. Acknowledgements

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conditions as unknowns.

The Geological Survey of Western Australia, in particular A. H. Hickman, is acknowledged

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for logistical support of the fieldwork, during which the sample DW-370-1-14 was collected. D. Wiemer acknowledges fruitful input during the U-Th-Pb LA-ICP-MS network workshop at the Goldschmidt Conference, Prague, 2015. H. Cathey (CARF) is acknowledged for assistance with electron microprobe analyses, L. Rintoul (QUT) for assistance with Raman spectroscopy, C. East for assistance with VP-SEM-CL, and K. H. Moromizato for assistance with LA-ICP-MS analyses. M. DeBruyn (CARF) performed thermal annealing, and D. McAuley (CARF) assisted mount preparation and polishing. G. Fraser (Geoscience

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ACCEPTED MANUSCRIPT Australia) kindly supplied OG1 standard material. The authors thank two anonymous reviewers for improving the final paper.

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ACCEPTED MANUSCRIPT Weber, W.J. 1993: Alpha-Decay-Induced Amorphization in Complex Silicate Structures. Journal of the American Ceramic Society, 76, 7, 1729-1738 Weber, W.J., Ewing, R.C., and Wang, L.-M., 1994: The radiation-induced crystalline-toamorphous transition in zircon. J. Mater. Res., 9, 3, 688-698 Wiemer, D., Schrank, C.E., Murphy, D.T., and Hickman, A.H., 2016: Lithostratigraphy and

Western Australia. Prec. Res., 282, 121-138

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structure of the early Archaean Doolena Gap greenstone belt, East Pilbara Terrane,

Wilde, S.A., Valley, J.W., Peck, W.H., and Graham, C.M., 2001: Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature,

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409, 175-178

Williams, I.S., Compston, W., Black, L.P., Ireland, T.R., and Foster, J.J., 1984: Unsupported

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radiogenic Pb in zircon: a cause of anomalously high Pb-Pb, U-Pb and Th-Pb ages. Contrib Mineral Petrol, 88, 322-327

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Zhang, M., Salje, E.K.H., Capitani, G.C., Leroux, H., Clark, A.M., Schlüter, J., and Ewing, R.C., 2000: Annealing of α-decay damage in zircon: a Raman spectroscopic study. J.

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Phys.: Condens. Matter, 12, 3131-3148

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Zhao, K.-D., Jiang, S.-Y., Ling, H.-F., and Palmer, M.R., 2014: Reliability of LA-ICP-MS UPb dating of zircons with high U concentrations: A case study from the U-bearing

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Douzhashan Granite in South China. Chemical Geology, 389, 110-121

Figure captions

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ACCEPTED MANUSCRIPT Fig. 1: Selected VP-SEM CL images showing characteristic features of the distinct zircon domains; a) part of a pristine grain displaying very faint oscillatory zoning towards the outer cores; CL intensity of the alt-1 domain is hardly distinguished from the core, while domain alt-2 shows lower CL intensity; The rim domain shows the brightest CL and features radial fractures; b) annealed grain showing good contrast between the domains; fracture networks

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of bright CL extend from the alt-1 domain into adjacent domains and fractures aligned perpendicular to the concentric zoning; c) detail of annealed grain showing dark “CL holes” (arrow a) in bright CL alt-1 domain and dark CL fractures terminating in alt-1 with gradual CL increase at terminations (arrow b); d) detail of annealed grain displaying the convolute

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heterogeneous CL intensity distribution within alt-1 domain; e) detail of annealed grain featuring spongy textured zones (arrows d and c) within the dark CL alt-2 domain, and the

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inward curvature boundary of the rim domain.

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Fig. 2: Selected SEM-CL, integral EMPA spectral CL and BSE images (rows, from left to right), comparing a pristine (pr01, uppermost row), a thermally annealed (TA94, center row),

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and a chemically abraded (CA-II 17, lowermost row) grain; refer to text for description.

Fig. 3: Representative CL spectra comparing each of the four domains from a pristine grain

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(pr01) with an annealed grain (TA94); Note the dominant yellow-green broad band (A) in the pristine cores compared to the dominant blue broad band (B) in the annealed domains, and the appearance of superimposed narrow REE3+ emission lines in the alt-1, alt-2 and to a minor extent in the core domains, upon annealing; refer to text for further explanation.

Fig. 4: a) Representative EMPA x-ray mineral maps of selected elements of an annealed zircon, highlighting the incorporation of non-formula Ca and Fe in the alt-1 and alt-2

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Fig. 5: a) Representative Raman spectra comparing each domain from a pristine with an

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annealed zircon, highlighting the increased intensity and sharpening of the dominant peaks at ~970 and ~1000 cm-1; the v3 SiO4 peaks at ~1000 cm-1 are shown in detail for a pristine (b) and an annealed (c) grain; Note the increasing frequency/wavenumber upon annealing; a1,

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alt-1; a2, alt-2; refer to text for description.

Fig. 6: Raman frequency versus line-widths plot showing the radiation damage trend and the

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annealing trend of Raman response to the structural disorder (after Geisler et al., 2001b); the line-width is given as the full-width at half-maximum (FWHM); note that some of the

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pristine data domains plot above the radiation damage trend, indicating that natural annealing

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occurred; refer to text for description.

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Fig. 7: a) Th versus U plot, showing cores at common magmatic Th/U values, while alt-1, alt-2 and rim domains plot at relatively low Th/U; b) amorphous fraction p as a function of

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alpha-dose D; the critical value of D~8*1015 a/mg is indicated, demonstrating advanced metamictization of the alt-1 and alt-2 domains; c) non-formula Ca as a function of the amorphous fraction p; note the drastic increase in Ca above an amorphous fraction of p~0.8; the latter is interpreted as the second percolation point after Geisler et al. (2001b); d) La versus Ca concentration plot, suggesting coupled incorporation of non-formula Ca and LREE during alteration.

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ACCEPTED MANUSCRIPT Fig. 8: Raman FWHM (v3 SiO4 peak) versus alpha-dose plot showing the position of pristine zircon domains; using the total 3.5 Ga alpha-doses, data plot below the calibration curve for Sri Lankan zircon (Palenik et al., 2003), indicating the occurrence of natural annealing; recalculation of Raman equivalent alpha doses (squares) yield consistent radiation damage ages

Fig. 9: Concordia

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Pb/238U versus

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of ~0.5 Ga for all domains.

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Pb/235U (black curve) plot showing the effects of

annealing on the U-Pb systems of Temora2 and Plešovice standards compared to their known TIMS values using annealed Temora2 as the primary reference for LA-ICP-MS data

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reduction; inlet shows the effects of mismatching annealed Temora2 for reduction of pristine OG1 standard data; note the introduction of reverse discordance; refer to text for further

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description.

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Fig. 10: U-Pb Concordia diagrams (a-f); refer to text for description.

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ACCEPTED MANUSCRIPT Tables

Table 1: Asymmetric Si-O stretching mode v3 at ca. 1000 cm-1 grain spot position width 1008.15 1007.64 1007.33 1006.94 1006.53 1005.77 1005.62 1005.25 1005.13 1004.85

10.14 11.20 7.73 8.37 16.38 16.35 19.79 17.39 17.00 10.96

pr2rim pr3core pr2core pr4rim pr3rim pr2alt11 pr1core pr2alt2 pr3alt2 pr4core pr2alt1

1004.04 1001.95 1001.55 1001.35 1000.64 998.90 998.62 998.34 998.10 997.67 996.43

9.54 12.75 12.65 10.77 15.92 21.05 17.54 21.96 25.59 14.73 19.54

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ca1-2alt1 TA50rim 94alt1 94rim ca1-2alt2 TA50core 94alt2 ca1-2core TA50alt1 94core

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Annealed CAI02 TA50 TA94 TA94 CAI02 TA50 TA94 CAI02 TA50 TA94 Pristine pr04 pr01 pr04 pr72 pr01 pr04 pr07 pr04 pr01 pr72 pr04

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-1

cm

12.75 12.65 17.54 14.73 9.54 10.77 21.05 19.54 25.59

a-mg *10 0.81 0.80 1.23 0.97 0.57 0.66 1.63 1.45 2.31

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a-mg *10

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0.65 0.70 2.11 2.43 0.82 5.10 7.30 7.30 51.17

RadAGE

U

Th

Ma

ppm

ppm

527 526 512 506 518 488 496 494 470

33 36 108 121 44 257 392 392 2720

11 13 36 65 2 113 9 9 246

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01-2 04-3 07-1 72-1 04-1 72-2 04-4 04-4 01-4

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core core core core rim rim alt-1 alt-1 alt-2

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Table 2: Calculation of radiation ages for pristine zircon Domain Spot FWHM Deq Dcalc

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Table 3: LA-ICP-MS U-Pb data for zircon standards compared to accepted TIMS values Temora annealed ± pristine ± annealed

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Plešovice pristine 15 337.1 8.88 0.0537 0.0527 1.0196 0.3937 0.3766 1.0455 0.0532 0.0008 0.0153 365 28.1 356 18.4 292 -45.1

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N 13 12 14 TIMS age [Ma]* 416.9 416.9 337.1 Calc. alpha dose [a/mg 1015] 4.41 4.37 8.68 Accepted 206Pb/238U 0.0668 0.0668 0.0537 Measured 206Pb/238U 0.0603 0.0006 0.0618 0.0005 0.0499 0.0003 Fractionation factor 206Pb/238U 1.1079 1.0817 1.0750 Accepted 207Pb/235U 0.5077 0.5077 0.3937 Measured 207Pb/235U 0.4560 0.0090 0.4690 0.0120 0.3607 0.0030 Fractionation factor 207Pb/235U 1.1134 1.0825 1.0916 Accepted 207Pb/206Pb 0.0551 0.0551 0.0532 Measured 207Pb/206Pb 0.0548 0.0012 0.0551 0.0014 0.0523 0.0008 0.0518 Fractionation factor 207Pb/206Pb 1.0062 0.0220 1.0007 0.0254 1.0172 0.0150 1.0270 Calc. 206Pb/238U age [Ma]** 417 427 347 Delta age ref std 9.9 10.1 Calc. 207Pb/235U age [Ma]** 417 5.9 427 8.4 343 2.5 Delta age ref std 9.7 5.7 Calc. 207Pb/206Pb age [Ma]** 418 8.0 431 14.5 314 8.4 Delta age standard 14.1 -23.1 +these data are not drift corrected and no down hole correction is applied, for demonstration purposes *Black et al. (2004); **calculated ages using the fractonation factors derived from annealed Temora

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Graphical abstract

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