In situ measurement of hafnium isotopes in rutile by LA–MC-ICPMS: Protocol and applications

Chemical Geology 281 (2011) 72–82

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Chemical Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c h e m g e o

Research paper

In situ measurement of hafnium isotopes in rutile by LA–MC-ICPMS: Protocol and applications T.A. Ewing ⁎, D. Rubatto, S.M. Eggins, J. Hermann Research School of Earth Sciences, ANU, Canberra, ACT, Australia

a r t i c l e

i n f o

Article history: Received 24 March 2010 Received in revised form 30 November 2010 Accepted 30 November 2010 Available online 8 December 2010 Editor: R.L. Rudnick Keywords: Hafnium isotopes Rutile LA–MC-ICPMS Mass bias Garnet peridotite

a b s t r a c t This work presents a rigorous assessment of the accuracy and precision with which 176Hf/177Hf can be measured in rutile by laser ablation (LA) MC-ICPMS. For rutile with ≥ 40 ppm Hf, we demonstrate that 176Hf/ 177 Hf can be measured accurately and reproducibly with adequate precision for application to petrological problems. We present an analytical and data reduction protocol tailored to the specific challenges of measuring Hf isotope ratios in this low-Hf mineral. Precision and accuracy are optimised by interpolating between baselines measured every ~ 10 analyses. For many rutiles, the advantages of determining the Hf mass bias coefficient, βHf, during analysis are negated by the low precision with which it can be measured at low Hf contents. Hf mass bias is therefore monitored by regular analysis of a synthetic rutile doped with ~ 5000 ppm Hf (SR-2) to facilitate an external mass bias correction. Because rutile contains negligible Yb, the Yb mass bias coefficient βYb must be inferred from βYb/βHf measured on zircon in the same session. To account for a systematic instrument bias of 0.5–1 εHf units, rutile analyses are normalised to SR-2, for which 176Hf/177Hf has been determined by solution MC-ICPMS. In situ 176Hf/177Hf measurements for two natural rutile samples with 40–50 ppm Hf are in excellent agreement with solution MC-ICPMS values. Ti/Hf exerts no influence on the accuracy of 176Hf/177Hf measurements on our LA–MC-ICPMS, as illustrated by indistinguishable 176Hf/177Hf measurements for a series of synthetic rutiles doped with varying amounts of Hf. Rutile and zircon from the Duria mantle peridotite (Central Alps), which formed at different parts of the P–T–time path, record contrasting Hf isotope signatures and emphasise the complementary nature of Hf isotope analysis of rutile and zircon. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The measurement of hafnium (Hf) isotope ratios in zircon is well established, with 176Hf/177Hf providing information on mantle source reservoirs and crustal input (e.g. Patchett, 1983; Amelin et al., 1999; Woodhead et al., 2001). The application of this method has been greatly expanded since the development of in situ analysis using laser ablation multi-collector inductively coupled mass spectrometry (LA–MC-ICPMS) (Thirlwall and Walder, 1995). This has allowed individual isotopic measurements to be made on different growth zones within zircons that would otherwise be averaged by bulk analysis methods (e.g. Harrison et al., 2005; Hawkesworth and Kemp, 2006; Kemp et al., 2006). Recently, in situ analysis of Hf isotopes has been extended to rutile (Choukroun et al., 2005; Aulbach et al., 2008), which occurs as an accessory mineral in many metamorphic and sedimentary lithologies, and more rarely in igneous settings. Rutile is an appealing target for Hf isotope analysis as its formation can readily be linked to metamorphic reactions, providing a ⁎ Corresponding author. RSES, Bldg 61, Mills Rd Acton 0200, Canberra, ACT, Australia. Tel.: +61 2 6125 5472; fax: +61 2 6125 0941. E-mail address: [email protected] (T.A. Ewing). 0009-2541/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2010.11.029

tectonometamorphic context for isotopic results. Furthermore, it has been demonstrated that in situ determinations of U–Pb or Pb–Pb age are possible for this mineral when it contains sufficient U and/or radiogenic Pb (e.g. Clark et al., 2000; Vry and Baker, 2006; Birch et al., 2007). The recently-developed Zr-in-rutile thermometer allows estimation of the temperature of formation of rutile that grew in the presence of zircon and quartz (Zack et al., 2004a; Watson et al., 2006; Tomkins et al., 2007; Ferry and Watson, 2007). The Hf isotope composition of rutile and zircon in the same rock cannot be expected to always be identical. These minerals may form at different times, and should have different closure temperatures for Hf (Cherniak et al., 2007). Hf isotope analysis of rutile and zircon can therefore provide complementary information for samples with a complex history. The two studies that pioneered in situ Hf isotope analysis of rutile (Choukroun et al., 2005; Aulbach et al., 2008) both employed essentially the same protocol as for zircon. However, rutile contains relatively low levels of Hf (b300 ppm, and usually b50 ppm) compared to zircon (~ 1–2 wt.% HfO2), and therefore presents different analytical challenges. Furthermore, Münker et al. (2001) reported that for solution MC-ICPMS measurements, the measured 176 Hf/177Hf deviated increasingly from the true value with increasing

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Ti/Hf. Investigation of whether LA–MC-ICPMS measurements are also subject to this effect is critical for in situ analysis of rutile, which is dominantly TiO2. A rigorous assessment of the accuracy with which Hf isotopes can be measured in situ for rutile is warranted to allow this technique to be applied with confidence. We demonstrate that Hf isotopes can be measured in rutile by LA–MC-ICPMS with accuracy and sufficient precision to resolve petrological problems. We describe a protocol for making accurate measurements, in light of the challenges specific to rutile, and present a case study applying the technique. Fig. 1. CL image of typical R1 zircon. Scale bar is 100 μm.

2. Samples 2.1. Natural rutiles Rutiles from three rock samples were used in the development and assessment of the technique. Average TiO2, Hf, Yb and Lu contents for each sample are given in Table 1, along with information on source lithology. The methods used for trace element measurements by LA– ICPMS and electron microprobe are outlined in Electronic Appendix A. Prior to analysis rutiles were subject to back-scattered electron (BSE) imaging using a Cambridge S360 SEM (ANU Electron Microscopy Unit), operated with 15 kV accelerating voltage, 3 nA current, and a working distance of 15–18 mm. Compositional zoning and/or multiple generations of growth were seldom observed in rutile. However, exsolution needles or blebs of ilmenite are common features that need to be avoided during analyses, and are readily apparent in BSE images (Fig. 2). Zircons were imaged in cathodoluminescence on a Hitachi S2250-N SEM (ANU Electron Microscopy Unit) with 15 kV accelerating voltage, ~60 μA current, and a working distance of ~20 mm. Sample R10 is a ~1 mm × 0.5 mm fragment of the centimetre-scale rutile analysed by Luvizotto et al. (2009) for Hf isotopes by isotope dilution MC-ICPMS. Our LA–ICPMS trace element data (Table 1) agree well with their more precise isotope dilution MC-ICPMS results of 38.9 ± 0.4 ppm Hf and 0.03 ± 0.01 ppm Lu. The fragment was large enough for three LA–MC-ICPMS Hf isotope analyses. Sample R1 comes from a trondhjemite pod in New Caledonia. R1 rutile has an average Hf content of 49 ppm, and was analysed by both solution and laser ablation MC-ICPMS as part of this study. Zircon from R1 was also analysed for Hf isotopes by LA–MC-ICPMS, for comparison with rutile. R1 zircons are euhedral and show the oscillatory zoning typical of magmatic zircons (Fig. 1). The Duria sample is from a 10 m by 15 m lens of mantle peridotite at Monte Duria in the Central Alps, northern Italy. Hermann et al. (2006) undertook a detailed petrological, trace element and geochronological study of the same sample. They demonstrated that rutile in this sample formed as part of the garnet peridotite peak metamorphic assemblage. Two generations of zircon formed later in the spinel peridotite field, after the metamorphic peak (Hermann et al., 2006). Both generations of zircon grew during the influx of crustal fluids, based on trace element geochemistry and the suite of inclusions present (Hermann et al., 2006). We analysed both rutile and zircon from the Duria peridotite for Hf isotopes by LA–MC-ICPMS.

The average Hf contents for rutile samples in this study range between 35 and 49 ppm, while Yb and Lu contents are low and often below detection limits (Table 1). This is in keeping with Hf being slightly compatible in rutile, whereas Yb and Lu are both highly incompatible (Foley et al., 2000; Klemme et al., 2005). Trace element concentrations for all elements are generally within the range of values reported for rutile by other workers (Zack et al., 2002, 2004b). The significance of rutile trace element chemistry is not discussed here, as its main relevance to this study is to characterise samples selected for Hf isotope analysis. Full trace element analyses are given in Electronic Appendix B.

2.2. Synthetic rutiles A series of synthetic rutiles doped with varying amounts of Hf were made at the RSES to test the effect of varying Ti/Hf on measured 176 Hf/177Hf. This also served to create an in-house reference material for rutile analysis. Synthetic rutiles were made by combining Unilab N98% pure TiO2 powder and Aldrich N98% pure HfO2 powder. Five mixtures were made by progressive dilution of an initial mix containing 4.3 wt.% HfO2 with TiO2 in order to obtain rutiles with Hf concentration ranging from ~43,000 to 4 ppm. By doping TiO2 with Hf (rather than doping HfO2 with Ti) we best simulate the matrix we intend to analyse. The powders were ground together in a mortar and pestle under acetone for ~30 min to achieve thorough homogenisation of each mix. Aliquots of two of the mixes were fluxed with 30% and 10% respectively of anorthite–diopside eutectic composition powder, with the aim of creating larger rutile grains. Aliquots of each mix and both end-members (pure HfO2 and TiO2 powders) were pressed into pellets and run at 1500 °C in air for 3 days in a 1 atm furnace. Both melt-free and melt-bearing experiments produced large rutile grains (100–400 μm). BSE imaging showed that no Hf was left over from the starting material, and there were no zones with elevated BSE emission indicative of abnormally high Hf concentrations in rutile. The homogeneous distribution of Hf was confirmed by electron microprobe traverses across synthetic rutiles SR1–3A. Concentrations of Ti and Hf were measured by electron microprobe for the pure HfO2 and synthetic rutiles (SR) 1–3A. Hf was below the detection limits of electron microprobe for pure TiO2, SR-4 and SR5, and was measured by LA–ICPMS. Hf concentrations range from

Table 1 Selected trace element concentrations for rutiles analysed in this study, measured by LA–ICPMS. Analyses that were below detection limit are shown in italics. Note that only one R1 analysis had Yb above the detection limit of 0.02 ppm. TiO2 values are those that gave totals for all elements of approximately 100 wt.% when used as an internal standard. n signifies the number of analyses. 1C. Pirard, pers. comm.; 2cm-scale rutile from Gjerstad, Norway (Luvizotto et al., 2009), see also their data; 3Hermann et al. (2006), who also supplied analyses for Duria rutile.

Sample

Source lithology

R1 R10 Duria

Trondhjemite1 Unknown2 Garnet peridotite3

TiO2 int'l std (wt.%) 99 99 99

Hf content (ppm)

Yb content (ppm)

Lu content (ppm)

Mean

Std dev.

Mean

Std dev.

Mean

Std dev.

n

49 35 46

9 2 6

0.005 b0.04 0.246

0.015 – 0.037

0.0015 b 0.01 0.0006

0.0029 – 0.0005

8 3 4

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Fig. 2. BSE images showing common features of rutile. White scale bar represents 100 μm in all images. (A) Homogeneous rutile from R1. (B) R1 rutile with extensive ilmenite exsolution/replacement (bright white features). (C) Rutile from the Ivrea–Verbano Zone, Italy, showing zones of replacement by ilmenite and another phase.

11 ppm to 4.3 wt.% (Table 2). Yb and Lu contents for all samples were measured by LA–ICPMS, and were below detection limits (b0.04 ppm Yb and b0.01 ppm Lu) for all synthetic samples except the pure HfO2, which contained 0.02 ppm Lu (Table 2). The TiO2 powder was found to contain ~ 5 ppm Hf of a different isotopic composition from the HfO2 dopant. The 176Hf/177Hf of the synthetic rutiles therefore represents mixing in various proportions between the two different 176Hf/177Hf signatures of the HfO2 and TiO2 powders. For SR-4 and SR-5, the TiO2 powder contributed ~10% and ~ 50% respectively of their total Hf. These samples were not used in the assessment of the effect of Ti/Hf on accuracy. For SR-1, SR-2, SR-2B, SR-3 and SR-3A the Hf contributed by the TiO2 powder represents b1% of total Hf and makes negligible difference to the Hf isotope composition. 176Hf/177Hf was measured by LA–MC-ICPMS for these samples and compared to the 176Hf/177Hf measured for the pure HfO2 pellet. 3. Analytical protocol for Hf isotope analysis 3.1. Instrumental setup Hf isotope analyses were carried out using an ANU 193 nm ArF ‘HelEx’ laser ablation system coupled to a ThermoFinnigan Neptune MC-ICPMS. Hf isotope results for zircon measured using this instrumentation have been reported by Wang et al. (2009) and Hiess et al. (2009). Earlier results reported by Harrison et al. (2005) were obtained using the same instrumentation, but with a significantly different setup and data reduction process. Here we provide a brief review of the fundamentals of instrument operation and data reduction, as well as outlining some adaptations for analysis of rutile. Laser ablation sampling was conducted in a He atmosphere to suppress surface deposition (Eggins et al., 1998), and a small amount of N2 (~ 2 cm3/min) was introduced downstream of the ablation cell to increase sensitivity (Durrant, 1994). The laser was pulsed at 5 Hz, with an applied laser fluence at the target of ~5 J/cm2. Zircon analyses used spots with diameters of 47, 62 and 81 μm, whereas a 233 μm spot

was employed for rutile to maximise the analyte signal from this lowHf mineral. Synthetic rutiles were analysed with spot sizes between 28 μm and 233 μm, according to their Hf content. Nine masses were measured in static mode: 171Yb, 173Yb, (174Yb + 174 Hf), 175Lu, (176Hf + 176Yb + 176Lu), 177Hf, 178Hf, 179Hf, and 181Ta. Although 181Ta is not used in the data reduction process, changes in the Hf/Ta ratio can be helpful in identifying where inclusions have been intersected during an analysis. Detection is by nine faraday cups, with cup efficiencies set to unity. Amplifiers are calibrated for gain at the start of each analytical session. The instrument is tuned for sensitivity and peak shape on NIST-610 synthetic glass. In an early session, peak shapes and positions (relative to the central cup) were confirmed to be identical for NIST-610, a synthetic rutile doped with ~4 wt.% Hf, and Mudtank zircon, validating the use of a different matrix for instrument tuning. For zircon analyses, 100 cycles of data are generally collected with a total analysis time of approximately 104 s. For rutile, a longer analysis time of 120 cycles (~125 s) is adopted to improve precision. Baselines are measured after approximately every ten analyses, by acquiring data without ablating material. Sensitivity for rutile analyses is typically 4–6 mV/ppm total Hf, this range reflecting runto-run variation in instrument sensitivity. Rutiles with 40–50 ppm Hf give 0.1–0.3 V total Hf signal at the start of an analysis, which drops to 10–30% of this value over the course of the 120 s measurement (Fig. 3). Higher total Hf signals are measured for rutiles with higher Hf contents, e.g. 0.5 V total Hf for 100 ppm Hf, and 1.8 V for 300 ppm Hf. Raw data is time-resolved, enabling selective integration of parts of the analysis to avoid inclusions or heterogeneities, or if the grain is drilled through. Data is corrected for amplifier response factors,

Table 2 Mean Hf concentrations for synthetic rutiles. Hf concentration was measured by 1 electron microprobe or 2LA–ICPMS, and includes a small contribution of Hf from the TiO2 powder. ± is the standard deviation of a number (n) of analyses. Synthetic sample

Comment

Hf (ppm)

±

n

HfO2 SR-1 SR-2 SR-2B SR-3 SR-3A SR-4 SR-5 TiO2

Pure HfO2 powder

839,0001 42,5001 39901 27901 3881 4161 492 112 4.62

241 710 280 81 45 45 19 4 0.1

3 25 14 30 17 15 6 6 6

Fluxed (30%) Fluxed (10%)

Pure TiO2 powder

Fig. 3. Typical Hf signal intensity evolution over the course of an analysis for rutile with 40–50 ppm Hf. One cycle equates to 1.04 s. Representative analyses from R10 (grey symbols) and R1 (black symbols) have been chosen. The sharper drop in intensity for R1rut_17.1 is probably due to incorporation of some epoxy by drilling through at grain edges.

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Amplifier response factors (ARFs) have been empirically determined for each of the Neptune's nine amplifiers, and are applied to all data (irrespective of sample type) to correct for the slightly different response times of each amplifier. The relatively large rates of signal intensity change during laser ablation make this correction critical, although the effects are minimised by pairing amplifiers with similar response times to measure critical isotope pairs (e.g. 179Hf/177Hf and 176 Hf/177Hf). ARFs have been found to be very stable over many years on the RSES instrument.

always within error of the usual baselines measured after every ~10 analyses (Fig. 5). Data were processed using baselines determined by interpolating between (1) long baselines measured every 10 analyses; (2) 120 cycle baselines measured immediately before and after each analysis; and (3) long baselines measured every ~5 analyses. The weighted mean 176Hf/177Hf does not differ significantly between the three methods of baseline correction and is always within error of the solution MC-ICPMS value for R1 (Fig. 6). The lower precision on 120 cycle baselines is insufficient to resolve short term variability in baseline levels, but introduces additional scatter into baseline measurements (Figs. 5 and 6). The measurement of long baselines every ~5 analyses improves the reproducibility of the 176Hf/177Hf only marginally (Fig. 6), and adds significantly to the analysis time. This experiment demonstrates that long baseline measurements every ~10 analyses give superior results to shorter baselines measured more frequently.

3.3. Baselines

3.4. Isobaric interferences

Accurate and precise determination of baselines is particularly critical for rutile because of its low Hf content. Relatively long baseline measurements of 300 cycles (~ 5 min) have been adopted as part of the protocol for rutile, in order to improve counting statistics and determine baselines more precisely. Baselines are measured after every ~10 analyses. Whereas zircon data are commonly corrected by subtracting the nearest baseline value, for rutile interpolating between baseline measurements yields improved accuracy and precision on 176Hf/177Hf ratios. Fig. 4 shows a block of ten analyses of SR-4 (43 ppm Hf), reduced both using the closest baseline measurement, and with interpolation between the bracketing baselines. When individual baseline measurements are used, there is a clear step in measured 176Hf/177Hf at the point where the closest baseline switches from an earlier measurement to a later one. In contrast, if interpolation between the two baselines is used no such step occurs, and there is marked improvement in precision on the weighted mean. This reflects the fact that baselines are not necessarily stable during an analytical session. However, measuring baselines more often than every ~10 analyses does not yield a significant improvement in reproducibility, suggesting that this frequency is sufficient to constrain variations in baseline. This was tested over a four hour session by analysing R1 rutile with 300 cycle baselines measured every 5 instead of 10 analyses, and with every second unknown replaced by a 120 cycle baseline. Additional baselines are

Isobaric interferences on 176Hf arise from 176Yb and 176Lu, and are corrected for as described by Hiess et al. (2009). Interferences were stripped using a 176Yb/173Yb value of 0.786956 and a 176Lu/175Lu value of 0.02645 (Thirlwall and Anczkiewicz, 2004). Unlike zircon, rutile contains little or no Yb or Lu (see Section 2). However, given the relatively small amount of Hf in rutile, even the very low levels of Yb and/or Lu that are sometimes found in rutile (typically ≪ 0.1 ppm, and always b1 ppm) require correction for their contribution to mass 176. This correction is therefore applied to all rutile analyses.

baselines, isobaric interferences, and mass bias. Each of these corrections is outlined in detail below, with particular reference to any adaptations made for rutile as compared to the standard data reduction method used for zircon. 3.2. Amplifier response factors

Fig. 4. Ten analyses of synthetic rutile SR-4 (43 ppm Hf), showing the improved reproducibility achieved by interpolating between baseline measurements. The dashed line indicates the change from using an earlier baseline to a later one, where individual baselines are used without interpolation. Weighted means for the two data reduction methods are shown on the right; note much improved precision on the weighted mean when baseline interpolation is used. βHf was measured internally.

Fig. 5. Measured baselines for (A) mass 176 and (B) mass 177 across a four hour period. Error bars are 2 SE. Baselines for these masses have the most impact on results for rutile, but all other masses show similar behaviour. The black lines indicate the interpolated values between 300 cycle baselines measured every 10 analyses.

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Fig. 6. 176Hf/177Hf for 22 analyses of R1 rutile, processed with three different methods of baseline correction. Weighted mean 176Hf/177Hf, MSWD and standard deviation (σ) of 176Hf/177Hf are given for each method. Weighted means for each session are also plotted at the right. Grey field shows the solution MC-ICPMS value for this sample (2 SE).

174

Hf/177Hf has a much larger Yb interference than 176Hf/177Hf and is monitored to check the accuracy of this correction. Measured 174 Hf/177Hf values are reported in Electronic Appendices C–E and are usually within error of 0.008658, the value reported for this ratio by Thirlwall and Anczkiewicz (2004).

Mass bias is corrected using an exponential law (Russell et al., 1978). The mass bias coefficient for Hf, βHf, is calculated using 179 Hf/177Hf = 0.7325 (Patchett and Tatsumoto, 1980). For zircon, the Yb mass bias coefficient βYb is calculated from the measured 173 Yb and 171Yb, using 173Yb/171Yb = 1.123456 (Thirlwall and Anczkiewicz, 2004, TIMS value). It is not possible to independently determine a mass bias coefficient for Lu, hence βYb is used to correct mass bias for this element (e.g. Woodhead et al., 2004). For rutile analyses, it is not possible to measure βYb given the very low Yb contents. Accordingly, βYb is determined from βHf and a βYb/βHf value measured on zircon several times in the same analytical session. Measurements on zircon show that over the course of an analytical session, both βHf and βYb change in a regular, quasi-linear fashion (Fig. 7). As a consequence, βYb/βHf shows little variation and no systematic change (Fig. 7).

Iizuka and Hirata (2005) note that βYb/βHf is matrix-dependent, but the corrections for Yb and Lu are small in magnitude for rutile, and any inaccuracy introduced by measuring βYb/βHf on zircon will have minimal effect on 176Hf/177Hf. Iizuka and Hirata (2005) report ~ 5% difference in βYb/βHf between zircon and NIST glass (see their Fig. 5a). Changes of up to 50% in βYb make less than 70 ppm difference to the calculated 176Hf/177Hf for representative analyses of R1, Duria and R10 rutiles (Fig. 8). These analyses cover the approximate range of REE/Hf observed in rutiles in this study, as measured by the 176Lu/177Hf (see Electronic Appendix D). βYb/βHf was measured on Mudtank zircon using a 233 μm spot to avoid possible bias arising from different ablation yield and plasma loading effects, noting that βYb/βHf variation is much less sensitive to spot size than either βYb or βHf. This method of inferring βYb is also sometimes used for low-Yb zircons (e.g. Mudtank) when measured with relatively small spot sizes, and for which a correlation between calculated βYb and 176Hf/177Hf is observed. In this case βYb/βHf is taken from analyses of high-Yb zircons measured with the same spot size. The measurement of βHf can be problematic for rutile. Measured βHf varies much more for rutile than for zircon in the same analytical session. Analyses of natural rutile with ~ 49 ppm Hf (sample R1) bracketed by synthetic rutile with ~ 5000 ppm Hf (SR-2) demonstrate that the apparent variation in βHf for rutile arises from the low

Fig. 7. Percentage change in the Hf and Yb mass bias coefficients, βHf and βYb, and their ratio, βYb/βHf, over a five hour session. Note the steady increase in both βHf and βYb, while βYb/βHf remains relatively constant. 2 SE error bars for βHf are smaller than symbols. All analyses are on R1 zircon, which contains ~ 580 ppm Yb, using an 81 μm spot.

Fig. 8. Effect of changes of up to 50% in βYb on the calculated 176Hf/177Hf for representative analyses of three rutile samples. The difference in 176Hf/177Hf is measured in parts per 10,000, which is approximately equivalent to εHf units. The analyses used were R10-1.1, R1rut-17.1, and DURrut-4.

3.5. Mass bias

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precision with which it can be determined (Fig. 9). If the measured βHf is used to correct for mass bias in rutile, the apparent variation induced by poor precision will introduce unnecessary scatter into the measured 176Hf/177Hf. A more accurate estimate of βHf can be obtained from the nearest SR-2 measurement. This external correction for mass bias is justified given that the measured βHf for R1 is always within error of the value interpolated between bracketing synthetic rutile analyses (Fig. 9). An important feature of our protocol for Hf isotope analysis of rutile is therefore to analyse SR-2 with a 233 μm spot after every ~10 analyses of unknowns, facilitating an external Hf mass bias correction. In practice, external determination of βHf makes a relatively small difference to the calculated 176Hf/177Hf. For the example shown in Fig. 9, using the βHf interpolated from SR-2 analyses made 0.2–2.2 εHf units difference to individual analyses, and 0.4 εHf units difference to the weighted mean for this population. These differences are well within the analytical uncertainty, indicating that although an external mass bias correction will yield more accurate results, acceptable data can be obtained using an internal mass bias correction. A greater effect will inevitably occur for rutile with lower Hf contents, for which even bigger variations in βHf are expected. For example, variations in βHf of up to 0.25 have been recorded for consecutive analyses of rutile with 30 ppm Hf over ~1.5 h (cf Fig. 9, in which the entire range of the y axis is 0.18). With increasing Hf content and thus improved precision on βHf measurements, real analysis-to-analysis variations in βHf can be resolved (e.g. SR-2 analyses in Fig. 9). As a result, for high-Hf rutiles the βHf measured on the sample is not always within error of the value interpolated from bracketing SR-2 measurements. In these cases, inferring βHf from SR-2 is not justified and the measured βHf is used instead. The non-radiogenic 178Hf/177Hf ratio is typically used to monitor the accuracy of the mass bias correction, and is reported in Electronic Appendices C–E. 178Hf/177Hf for individual analyses is generally within error of the multidynamic solution MC-ICPMS value of Thirlwall and Anczkiewicz (2004) but falls systematically at the low end of the range defined by their data. However, estimates of the true value of this ratio vary substantially in the literature and typically have large errors (e.g. Kleinhanns et al., 2002; Chu et al., 2002; Thirlwall and Anczkiewicz, 2004, and see Electronic Appendix C). Our measured values for 178Hf/177Hf agree well with the value quoted by (Kleinhanns et al., 2002), for instance. Given the lack of consensus on

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the correct value of 178Hf/177Hf, it is difficult to assess the significance of these inter-laboratory differences in measured 178Hf/177Hf values. 3.6. Standardisation Previous studies on the ANU Neptune have measured 176Hf/177Hf for a range of zircon standards, and if necessary have normalised data for unknowns to these standards (Wang et al., 2009; Hiess et al., 2009). Wang et al. (2009) observed a small bias (−0.7 εHf units) in 176 Hf/177Hf for zircon standards with a range of Yb/Hf and Lu/Hf ratios, as well as for JMC 475 Hf solution. These results indicate a systematic bias that is not matrix-dependent. In the course of this study, we have observed subtle variation between analytical sessions in the magnitude by which 176Hf/177Hf values are offset. For this reason we prefer to normalise laser ablation analyses to mineral standards measured in the same session, rather than normalising to a long-term average or a value for JMC 475 obtained in a solution session. Although the bias appears to be independent of matrix, we consider it prudent to normalise measurements of 176Hf/177Hf of rutile to the synthetic rutile SR-2, which is matrix-matched and is already analysed to monitor mass bias. Because SR-2 contains ~5000 ppm Hf and is measured with a 233 μm spot, the precision obtained is comparable to or better than that for zircon standards measured with typical spot sizes. Rutile unknowns are normalised to the weighted mean of all SR-2 analyses from the same session. The 176Hf/177Hf for SR-2 has been determined as 0.281888 ± 0.000007 by solution MC-ICPMS at the Polish Academy of Sciences, Krakow (see Electronic Appendix A.3). Rutile samples analysed in earlier sessions were normalised to Mudtank zircon instead of SR-2, and where possible to Mudtank analyses with the same spot size as unknowns. 176 Hf/177Hf for zircon analyses is normalised to the average offset of the zircon standards Mudtank, 91500 and Temora from the solution values reported by Woodhead and Hergt (2005). The only exception is zircon from the Duria peridotite, which was normalised to Mudtank zircon. The 95% confidence error on the weighted mean of standard analyses is propagated in quadrature onto the calculated errors for each individual analysis of unknowns. Where several different zircon standards are used for normalisation in one session, their 95% confidence errors are combined by averaging as relative errors (i.e. 95% confidence error/weighted mean). In cases where the propagated error is less than two standard deviations (2σ) for the relevant standard in that session, sample error is forced to equal the 2σ for the standard. This is true for both individual analyses and weighted means. All non-radiogenic isotope ratios are reported without normalisation to standards. 4. Results All errors are reported at the 2 SE level. Weighted means were calculated using Isoplot 3 (Ludwig, 2003) and are quoted with 95% confidence errors; these take account of sample scatter by incorporating the MSWD of analyses as well as tσ. εHf is calculated using the λLu of Soderlund et al. (2004) and the CHUR reference isotope ratios of Bouvier et al. (2008). Zircon analyses used the same analytical setup as for rutile, but with smaller spot sizes and typical zircon data reduction protocol (as in Hiess et al., 2009). Full data tables for all standards and samples are presented in Electronic Appendices C–E.

Fig. 9. Hafnium mass bias coefficient, βHf, measured on natural rutiles from sample R1 (average Hf content 49 ppm), and on synthetic rutile SR-2 (~5000 ppm Hf). The change in βHf with time is generally, but not perfectly, linear. However, the apparent variation in βHf as measured on natural rutile is much greater than the real variation seen for SR-2, and arises from the poor precision with which βHf can be measured for natural rutile with relatively low Hf content. 2 SE errors for SR-2 are approximately the size of the symbols. The black line shows the best fit linear trend to SR-2 data.

4.1. R10 Three LA–MC-ICPMS analyses of R10 have a weighted mean 176Hf/ Hf of 0.28220 ± 0.00028 (Fig. 10) and a weighted mean 176Lu/177Hf of 0.0000291 ± 0.0000120. The measured 176Hf/177Hf is in agreement with the value of 0.282178 ± 0.000012 determined by isotope dilution 177

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Fig. 10. LA–MC-ICPMS results for three analyses of rutile R10, with the solution MCICPMS results of Luvizotto et al. (2009) for comparison. Error bars for the solution analysis are much smaller than the symbol. βHf was determined externally.

for this sample by Luvizotto et al. (2009) (Fig. 10). The precision on the weighted mean 176Hf/177Hf of the LA–MC-ICPMS analyses is considerably lower than normal because of the restricted number of analyses possible for this sample. The three analyses are highly reproducible with a low MSWD of 0.3.

4.2. R1 One mg of R1 rutile separate was analysed by solution MC-ICPMS at the Polish Academy of Sciences (Electronic Appendix A.3), and gave 176 Hf/177Hf of 0.283097 ± 0.000008. 95 LA–MC-ICPMS analyses from four sessions over three years gave 176Hf/177Hf measurements that are reproducible both between and within sessions (Fig. 11). One analysis has been excluded from each of the first two sessions because of inclusions intersected during analysis. The weighted mean 176Hf/177Hf for each session is in agreement with the solution MC-ICPMS value (Fig. 11). 176Lu/177Hf for R1 rutile ranges from −0.000007 ± 0.000007 to 0.000094 ± 0.000017. Ten analyses of zircon from R1 gave reproducible 176Hf/177Hf measurements with a weighted mean of 0.28310 ± 0.00003. This is indistinguishable from the 176Hf/177Hf of R1 rutile as determined by

Fig. 12. 176Hf/177Hf for individual analyses (unfilled diamonds) and weighted mean (filled square) of R1 zircons. The grey shaded area denotes the 2 SE limits of the 176Hf/ 177 Hf of R1 rutile measured by solution MC-ICPMS.

both solution and laser ablation MC-ICPMS (Fig. 12). 176Lu/177Hf for R1 zircons ranges from 0.000444 ± 0.000008 to 0.001500 ± 0.000012. 4.3. Synthetic rutiles The HfO2 pellet gave a weighted mean 176Hf/177Hf of 0.281876 ± 0.000009. Measured 176Hf/177Hf for SR-1, SR-2, SR-2B, SR-3 and SR-3A is always in agreement with this value, both for individual analyses and for weighted means (Fig. 13). There is no evidence for a systematic deviation in measured 176Hf/177Hf with increasing Ti/Hf. 4.4. Duria peridotite The few rutiles that were able to be separated from the Duria peridotite contained on average 46 ppm Hf (Hermann et al., 2006) and allowed four LA–MC-ICPMS analyses that yielded reproducible 176Hf/ 177 Hf ratios with a weighted mean of 0.28311 ± 0.00028 (Fig. 14A). 176 Lu/177Hf for the Duria rutiles varies from −0.0000025 ± 0.0000074 to 0.0011116± 0.0000589. Hermann et al. (2006) demonstrate that rutile in this sample formed prior to the oldest generation of zircon, which they dated at 34.2± 0.2 Ma. The Duria rutile population has an initial εHf of +12± 10 for an age of 35 Ma, the likely age of peak HP

Fig. 11. 176Hf/177Hf measurements for R1 rutile across four sessions. Weighted mean 176Hf/177Hf is given for each session and is also plotted at the right. The long term weighted mean (all sessions) is plotted as a black bar. The grey shaded rectangle indicates the 2 SE limits of the 176Hf/177Hf of R1 rutile determined by solution MC-ICPMS.

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Fig. 13. A. Individual 176Hf/177Hf measurements for the series of synthetic rutiles and HfO2. The black line indicates the weighted mean of HfO2 analyses, which is taken as the reference value for all synthetic rutiles shown. Note excellent agreement of all analyses with the mean HfO2 value, and that scatter is both above and below this value. Data have been normalised to Mudtank zircon. Errors have not been forced to the 2σ of standards because the interest is in relative, not absolute, 176Hf/177Hf. B. Weighted mean 176Hf/177Hf of each sample plotted in terms of Ti/Hf ratio. The grey box highlights the 95% confidence error range of the HfO2 weighted mean. Note lack of any kind of systematic trend.

metamorphism in this region. It is noted that the initial εHf values for both rutile and zircon are insensitive to changes in age of several Ma, and to variations of up to two orders of magnitude in the Lu/Hf ratio. We targeted the older of the two generations of zircon, which is dark and oscillatory zoned in cathodoluminescence (Fig. 15) and records the strongest crustal influence according to Hermann et al. (2006). This generation of zircon has an age of 34.2 ± 0.2 Ma (Hermann et al., 2006). Seven LA–MC-ICPMS analyses had very reproducible 176Hf/177Hf with a weighted mean of 0.28258 ± 0.00002 (Fig. 14B). 176Lu/177Hf varied from 0.0000235 ± 0.0000005 to 0.0001301 ± 0.0000057. The zircon population has an initial εHf of −6.6 ± 0.6 for an age of 34 Ma. This is distinguished from the initial

εHf of the rutile population (Fig. 14C), in spite of the fact that the precision on the weighted mean for rutile is poor because of the small number of rutile grains suitable for analysis. 5. Discussion 5.1. Accuracy The agreement of 176Hf/177Hf measured by LA–MC-ICPMS with solution MC-ICPMS values for R10 and R1 rutiles demonstrates that Hf isotopes can be measured accurately for rutile with ≥40 ppm Hf using the technique presented here. Excellent reproducibility both between

Fig. 14. Hf isotope analyses of Duria mantle peridotite. A. Rutile analyses. B. Zircon analyses. C. Weighted means for rutile and zircon. In A. and B. weighted mean 176Hf/177Hf is given in text and shown as a grey line.

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Fig. 15. P–T–time path of the Duria peridotite, modified after Hermann et al. (2006), with images of rutile (in thin section, transmitted light) and zircon (cathodoluminescence). Numbers refer to main stages of crystallisation, and stars indicate crustal fluid influx. The circle on the zircon image indicates a single LA–MC-ICPMS analysis for Hf isotopes.

and within sessions is demonstrated by 95 analyses of R1 rutile over four sessions that span more than three years. The lack of any systematic bias in measured 176Hf/177Hf with increasing Ti/Hf for a series of synthetic rutiles doped with Hf demonstrates that the accuracy of 176Hf/177Hf measurements by LA– MC-ICPMS is in no way compromised by high Ti/Hf. Although the synthetic rutiles used to assess the effect of Ti/Hf do not bracket natural rutiles in terms of Hf concentration (the lowest being 390 ppm), they do approach natural concentrations (30–300 ppm) and cover a similar range of Ti/Hf values to the study of Münker et al. (2001). From Fig. 1 of Münker et al. (2001) it can be calculated that they observed offsets of approximately 1.5–2.3 εHf units for four of the five analyses which have a Ti/Hf of 1250 (very similar to our highest Ti/Hf values). If we had observed the same offset, it would correspond to 176Hf/177Hf values of around 0.28180–0.28183 for SR-3 and SR-3A, which would lie just off the scale of our Fig. 13B. It can be demonstrated that ablation of small amounts of epoxy during analysis (e.g. if the edge of the grain is drilled through) does not compromise the accuracy of results. Several analyses of epoxy confirmed that it contains no Hf, Yb or Lu, with these analyses essentially identical to baseline measurements. As a test, the synthetic rutile SR-2 was also analysed in three configurations: (1) with the spot wholly on the rutile; (2) with the spot mostly on the rutile, but on the epoxy at the edge; and (3) with the spot one-third on the epoxy and two-thirds on the rutile. There was no measurable difference in 176 Hf/177Hf between these analyses, nor any apparent effect on mass bias coefficients. 5.2. Precision Individual measurements of 176Hf/177Hf have rather large uncertainties because of the low Hf content of rutile (typically ±10–12 εHf units for 40–50 ppm Hf). Nonetheless, 176Hf/177Hf measurements are well reproduced between analyses for a given sample, and accordingly the uncertainty on the weighted mean of 10–15 analyses is much lower. A precision of ±4 εHf units can be achieved for the weighted mean of populations with 40–50 ppm Hf (e.g. Fig. 11). The precision attained for rutile is poorer than for zircon, but is sufficient to distinguish 176Hf/177Hf values of different rutile samples, and in examples like the Duria peridotite, of rutile and zircon in the same

rock. This demonstrates the achievement of a useful level of precision to address geological problems. Importantly, errors for rutile analyses are small in comparison to the very large range of 176Hf/177Hf observed for this mineral — for example, the 155 εHf unit range reported by Choukroun et al. (2005). For rutile with unusually high Hf concentrations (200–300 ppm), much better precision can be achieved: ±2–3 εHf units on individual analyses, and ±0.8 εHf units on the weighted mean of 15 analyses. 5.3. Minimum Hf concentration The reliability of 176Hf/177Hf measurements is compromised at very low levels of Hf. For two synthetic rutiles with 5 ppm and 11 ppm Hf, adopting interpolation between baseline values changed the weighted mean of ten 176Hf/177Hf measurements by 75 εHf units and 26 εHf units respectively. This large difference in response to a small change in baseline values indicates that this level of Hf is insufficient to obtain reliable 176Hf/177Hf measurements. In contrast, for rutile with 40–50 ppm Hf the choice of baseline correction method changes the weighted mean of populations by at most 1–1.5 εHf units. An important observation is that for rutile with N40 ppm Hf, a much greater effect of baseline choice can be seen on individual 176Hf/ 177 Hf measurements than on the weighted means of 10–15 analyses. While the weighted means are relatively robust to choice of baseline, individual analyses can change by as much as 10 εHf units (although usually less than this). This emphasises the importance of making multiple analyses, preferably 10–15, for each sample and suggests that individual analyses should not necessarily be relied on as “standalone” analyses for low-Hf rutiles. 5.4. Application to igneous and metamorphic samples Sample R1 is an unmetamorphosed igneous rock. Rutile and zircon from this plutonic rock are expected to have formed at the same time and with identical Hf isotope compositions. The indistinguishable 176 Hf/177Hf measurements for zircon and rutile confirm this to be the case. The polyphase metamorphic history of the Duria peridotite presents a more complex case study (Fig. 15), in which rutile and zircon cannot be expected to record the same Hf isotope signature.

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The negative initial εHf (−6.6 ± 0.6) of zircon from the Duria peridotite indicates input of crustal material. In contrast, the positive initial εHf (+12 ± 10) of the Duria rutile population indicates a depleted mantle origin. Positive εHf may also be generated by the release of Hf during the breakdown of garnet, which can evolve strongly radiogenic Hf isotope compositions in a relatively short time because of its high Lu/Hf (Slama et al., 2007; Zheng et al., 2005). However, garnet breakdown cannot be invoked to explain the Hf isotope composition of the Duria rutile. Rutile occurs as inclusions in garnet, and grew in equilibrium with this mineral in the garnet peridotite stability field (Hermann et al., 2006, and Fig. 15). The simultaneous growth of garnet and rutile precludes garnet breakdown as a source of radiogenic Hf for rutile. Garnet breakdown did occur subsequently, but this reaction was associated with the formation of ilmenite, not rutile (Hermann et al., 2006). The depleted mantle Hf isotope signature of rutile from the Duria peridotite is consistent with the conclusion of Hermann et al. (2006) that the peak metamorphic assemblage to which rutile belongs had bulk rock chemistry very similar to depleted MORB mantle. Similarly, the Hf isotope evidence for crustal influence in zircons from the Duria peridotite is consistent with the evidence presented by Hermann et al. (2006) that zircon growth occurred in the presence of crustal fluids. The Hf isotope results for rutile and zircon record information about different parts of the metamorphic history (Fig. 15), emphasising the complementary nature of Hf isotope analysis of these two minerals in rocks that underwent a polyphase evolution. Although in situ 176Hf/ 177 Hf measurements for rutile have lower precision than those for zircon, they can provide information additional to that recorded by zircon. 6. Outlook In situ analysis of Hf isotopes in rutile by LA–MC-ICPMS offers significant benefits. It is rapid, cost-effective, allows single-grain analysis, and still yields sufficient precision to resolve many geological problems. Measurement of 176Hf/177Hf of individual rutile grains allows the characterisation of multiple generations of rutile in their textural context for thin sections with large rutile crystals. Better precision could in principle be achieved by solution MC-ICPMS, but would generally require bulk analysis of multiple rutile grains. The greater volume of material required is due to the loss of a significant proportion of the analyte in the spray chamber during solution analysis. For rutile with ≤100 ppm Hf, multiple grains would be required to obtain sufficient Hf for a single high-precision solution analysis, except in the case of unusually large (N400 μm) grains. Dissolution of less material is possible, but the resulting loss of precision begins to erode the advantages of the technique. An important advantage of in-situ Hf isotope analysis is the ability to avoid or exclude inclusions or areas of alteration. Alteration of rutile is common, but usually only affects a small volume fraction of the grain. Small areas of alteration can be difficult to detect during handpicking, especially in large dark grains. For solution MC-ICPMS, microdrilling is necessary to avoid these areas. However, micro-drilling can be laborious and does not guarantee exclusion of all altered domains. With LA–MC-ICPMS all or parts of analyses that show signs of contamination can be rejected prior to pooling data. Furthermore, the greater volume of material required for a high precision solution analysis would necessitate micro-drilling of more than the 10–15 rutiles required for LA–MC-ICPMS. Provenance studies of detrital rutile will be an important application of Hf isotope analysis of rutile, for which single-grain analysis is essential. The most common source of detrital rutile is medium- to high-grade metamorphic rocks (Force, 1980; Zack et al., 2004b), and the ability of detrital rutile to fingerprint source areas that are not recorded by detrital zircons has already been recognised (Zack et al., 2004b) and exploited through trace element and U–Pb age

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studies (e.g. Zack et al., 2004b; Triebold et al., 2007; Allen and Campbell, 2007; Meinhold et al., 2008; Morton and Chenery, 2009). Hf isotope measurements of detrital rutiles will improve the ability to distinguish between different sediment sources, and provide a more detailed characterisation of sources than trace element measurements alone. The ability to measure the 176Hf/177Hf of rutile is a valuable addition to the metamorphic petrologist's toolbox. Rutile participates in major metamorphic reactions that can be linked to specific parts of the metamorphic history more clearly than for other accessory minerals (e.g. Liu et al., 1996; Ernst and Liu, 1998; Zack et al., 2002; Zack et al., 2004a). The example of the Duria peridotite demonstrates that Hf isotope analysis of rutile can provide information not recorded by zircon when the two minerals grew at different stages in the metamorphic evolution of a sample. Hf isotope analysis of rutile will also be valuable for samples in which growth of metamorphic zircon is absent or limited to narrow overgrowths that may be too small to analyse for Hf isotopes by LA–MC-ICPMS. Metamorphic rutile tends to grow to much larger grain sizes than zircon, and almost never preserves multiple zones, and is thus an ideal target for LA–MCICPMS. Eclogitic and mantle xenoliths are obvious examples of lithologies in which rutile is common but zircon is unusual, and for which Hf isotopes are of interest (Choukroun et al., 2005; Aulbach et al., 2008). 7. Conclusions 1. The 176Hf/177Hf of rutile with ≥40 ppm Hf can be accurately measured in situ by laser ablation MC-ICPMS in spite of the relatively low Hf content, provided care is taken with the analytical protocol and data reduction process. 2. Accuracy is demonstrated by agreement of our laser ablation MCICPMS results with solution MC-ICPMS measurements for a large rutile crystal (R10, Luvizotto et al., 2009) and for rutile from a trondhjemite (R1). Robustness of the technique is demonstrated by 95 LA–MC-ICPMS analyses of R1 rutile across four sessions and ~3 years, which show that 176Hf/177Hf is measured reproducibly both between and within sessions. 3. Analysis of a series of synthetic rutiles doped with Hf in concentrations ranging from 390 ppm to 4.3 wt.% indicates no influence of Ti/Hf on the accuracy 176Hf/177Hf measured by LA–MC-ICPMS. 4. Precision is typically on the order of ±10–12 εHf units for individual analyses of rutile with 40–50 ppm Hf, and can be as good as ±4 εHf units for the weighted mean of a population of 10–15 analyses on one sample. Although lower than the precision possible for zircon, this permits many useful applications to petrological problems. 5. Rutile and zircon from the Duria garnet peridotite have different Hf isotope compositions that record different parts of the sample's metamorphic history. 6. Key features of our analytical protocol to maximise precision and accuracy of Hf isotope measurements on rutile are: (a) Baselines are determined by interpolating between long baseline measurements every ~10 analyses. (b) Isobaric interference corrections are applied to all analyses; in spite of the incompatibility of Lu and Yb in rutile, these elements can be present at levels that produce significant isobaric interferences on 176Hf. (c) An external correction for Hf mass bias is applied in cases where the low precision with which the Hf mass bias coefficient βHf can be measured negates the advantages of an internal correction for mass bias. βHf for the external correction is inferred from regular analyses of a synthetic rutile (SR-2) with ~ 5000 ppm Hf. (d) The Yb mass bias coefficient, βYb, for rutile is calculated from βYb/βHf measured on zircon with a 233 μm spot several times during a session.

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Acknowledgements We thank the ANU Electron Microscopy Unit for technical assistance and access to SEM facilities. L. Kinsley and C. Allen are thanked for their expert technical support and constructive discussions on mass spectrometry. We are grateful to T. Zack for supplying a chip of rutile R10, and to C. Pirard for donating zircon and rutile separates from R1. We are indebted to R. Anczkiewicz for undertaking solution MC-ICPMS analyses and providing constructive comments on the manuscript. Two anonymous reviewers are thanked for thorough reviews that significantly improved the manuscript. R. Rudnick is thanked for editorial handling. This research was financially supported by the Research School of Earth Sciences and the Australian Research Council (DP0556700 to D. Rubatto and J. Hermann). Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10.1016/j.chemgeo.2010.11.029. References Allen, C.M., Campbell, I.H., 2007. Spot dating of detrital rutile by LA–Q–ICP–MS: a powerful provenance tool: Geological Society of America Abstracts with Programs, Vol. 39, p. 527. Amelin, Y., Lee, D.C., Halliday, A.N., Pidgeon, R.T., 1999. Nature of the Earth's earliest crust from hafnium isotopes in single detrital zircons. Nature 399, 252–255. Aulbach, S., O'Reilly, S.Y., Griffin, W.L., Pearson, N.J., 2008. Subcontinental lithospheric mantle origin of high niobium/tantalum ratios in eclogites. Nature Geoscience 1, 468–472. Birch, W.D., Barron, L.M., Magee, C., Sutherland, F.L., 2007. Gold- and diamond-bearing White Hills Gravel, St Arnaud district, Victoria: age and provenance based on U–Pb dating of zircon and rutile. Australian Journal of Earth Sciences 54, 609–628. Bouvier, A., Vervoort, J.D., Patchett, P.J., 2008. The Lu–Hf and Sm–Nd isotopic composition of CHUR: constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets. Earth and Planetary Science Letters 273, 48–57. Cherniak, D., Manchester, J., Watson, E., 2007. Zr and Hf diffusion in rutile. Earth and Planetary Science Letters 261, 267–279. Choukroun, M., O'Reilly, S.Y., Griffin, W.L., Pearson, N.J., Dawson, J.B., 2005. Hf isotopes of MARID (mica–amphibole–rutile–ilmenite–diopside) rutile trace metasomatic processes in the lithospheric mantle. Geology 33, 45–48. Chu, N.C., Taylor, R.N., Chavagnac, V., Nesbitt, R.W., Boella, R.M., Milton, J.A., German, C.R., Bayon, G., Burton, K., 2002. Hf isotope ratio analysis using multi-collector inductively coupled plasma mass spectrometry: an evaluation of isobaric interference corrections. Journal of Analytical Atomic Spectrometry 17, 1567–1574. Clark, D.J., Hensen, B.J., Kinny, P.D., 2000. Geochronological constraints for a two-stage history of the Albany-Fraser Orogen, Western Australia. Precambrian Research 102, 155–183. Durrant, S.F., 1994. Feasibility of improvement in analytical performance in laserablation inductively-coupled plasma-mass spectrometry (LA–ICP–MS) by addition of nitrogen to the argon plasma. Fresenius Journal of Analytical Chemistry 349, 768–771. Eggins, S.M., Kinsley, L.P.J., Shelley, J.M.G., 1998. Deposition and element fractionation processes during atmospheric pressure laser sampling for analysis by ICP–MS. Applied Surface Science 129, 278–286. Ernst, W.G., Liu, J., 1998. Experimental phase-equilibrium study of Al- and Ti-contents of calcic amphibole in MORB — a semiquantitative thermobarometer. American Mineralogist 83, 952–969. Ferry, J.M., Watson, E.B., 2007. New thermodynamic models and revised calibrations for the Ti-in-zircon and Zr-in-rutile thermometers. Contributions to Mineralogy and Petrology 154, 429–437. Foley, S.F., Barth, M.G., Jenner, G.A., 2000. Rutile/melt partition coefficients for trace elements and an assessment of the influence of rutile on the trace element characteristics of subduction zone magmas. Geochimica et Cosmochimica Acta 64, 933–938. Force, E.R., 1980. The provenance of rutile. Journal of Sedimentary Petrology 50, 485–488. Harrison, T.M., Blichert-Toft, J., Muller, W., Albarede, F., Holden, P., Mojzsis, S.J., 2005. Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 Ga. Science 310, 1947–1950. Hawkesworth, C.J., Kemp, A.I.S., 2006. Using hafnium and oxygen isotopes in zircons to unravel the record of crustal evolution. Chemical Geology 226, 144–162. Hermann, J., Rubatto, D., Trommsdorff, V., 2006. Sub-solidus Oligocene zircon formation in garnet peridotite during fast decompression and fluid infiltration (Duria, Central Alps). Mineralogy and Petrology 88, 181–206.

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