Chemical Geology 281 (2011) 72–82

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Chemical Geology

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Research paper

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

T.A. Ewing ⁎, D. Rubatto, S.M. Eggins, J. HermannResearch School of Earth Sciences, ANU, Canberra, ACT, Australia

⁎ Corresponding author. RSES, Bldg 61, Mills Rd ActonTel.: +61 2 6125 5472; fax: +61 2 6125 0941.

E-mail address: tanya.ewing@anu.edu.au (T.A. Ewin

0009-2541/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.chemgeo.2010.11.029

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 March 2010Received in revised form 30 November 2010Accepted 30 November 2010Available online 8 December 2010

Editor: R.L. Rudnick

Keywords:Hafnium isotopesRutileLA–MC-ICPMSMass biasGarnet peridotite

This work presents a rigorous assessment of the accuracy and precision with which 176Hf/177Hf can bemeasured in rutile by laser ablation (LA) MC-ICPMS. For rutile with ≥40 ppm Hf, we demonstrate that 176Hf/177Hf can be measured accurately and reproducibly with adequate precision for application to petrologicalproblems. We present an analytical and data reduction protocol tailored to the specific challenges ofmeasuring Hf isotope ratios in this low-Hf mineral. Precision and accuracy are optimised by interpolatingbetween baselines measured every ~10 analyses. For many rutiles, the advantages of determining the Hf massbias coefficient, βHf, during analysis are negated by the low precision with which it can be measured at low Hfcontents. Hf mass bias is therefore monitored by regular analysis of a synthetic rutile doped with ~5000 ppmHf (SR-2) to facilitate an external mass bias correction. Because rutile contains negligible Yb, the Yb mass biascoefficient βYb must be inferred from βYb/βHf measured on zircon in the same session. To account for asystematic instrument bias of 0.5–1 εHf units, rutile analyses are normalised to SR-2, for which 176Hf/177Hf hasbeen determined by solutionMC-ICPMS. In situ 176Hf/177Hf measurements for two natural rutile samples with40–50 ppm Hf are in excellent agreement with solution MC-ICPMS values. Ti/Hf exerts no influence on theaccuracy of 176Hf/177Hf measurements on our LA–MC-ICPMS, as illustrated by indistinguishable 176Hf/177Hfmeasurements for a series of synthetic rutiles doped with varying amounts of Hf. Rutile and zircon from theDuria mantle peridotite (Central Alps), which formed at different parts of the P–T–time path, recordcontrasting Hf isotope signatures and emphasise the complementary nature of Hf isotope analysis of rutileand zircon.

0200, Canberra, ACT, Australia.

g).

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

The measurement of hafnium (Hf) isotope ratios in zircon is wellestablished, with 176Hf/177Hf providing information on mantle sourcereservoirs and crustal input (e.g. Patchett, 1983; Amelin et al., 1999;Woodhead et al., 2001). The application of thismethod has been greatlyexpanded since the development of in situ analysis using laser ablationmulti-collector inductively coupledmass spectrometry (LA–MC-ICPMS)(Thirlwall and Walder, 1995). This has allowed individual isotopicmeasurements to bemade ondifferentgrowthzoneswithin zircons thatwould otherwise be averaged by bulk analysis methods (e.g. Harrisonet al., 2005;Hawkesworth andKemp, 2006;Kempet al., 2006).Recently,in situ analysis of Hf isotopes has been extended to rutile (Choukrounet al., 2005; Aulbach et al., 2008), which occurs as an accessory mineralin many metamorphic and sedimentary lithologies, and more rarely inigneous settings.

Rutile is an appealing target for Hf isotope analysis as its for-mation can readily be linked to metamorphic reactions, providing a

tectonometamorphic context for isotopic results. Furthermore, it hasbeen demonstrated that in situ determinations of U–Pb or Pb–Pb ageare possible for this mineral when it contains sufficient U and/orradiogenic Pb (e.g. Clark et al., 2000; Vry and Baker, 2006; Birch et al.,2007). The recently-developed Zr-in-rutile thermometer allowsestimation of the temperature of formation of rutile that grew inthe presence of zircon and quartz (Zack et al., 2004a; Watson et al.,2006; Tomkins et al., 2007; Ferry and Watson, 2007). The Hf isotopecomposition of rutile and zircon in the same rock cannot be expectedto always be identical. These minerals may form at different times,and should have different closure temperatures for Hf (Cherniaket al., 2007). Hf isotope analysis of rutile and zircon can thereforeprovide complementary information for samples with a complexhistory.

The two studies that pioneered in situ Hf isotope analysis of rutile(Choukroun et al., 2005; Aulbach et al., 2008) both employedessentially the same protocol as for zircon. However, rutile containsrelatively low levels of Hf (b300 ppm, and usually b50 ppm)compared to zircon (~1–2 wt.% HfO2), and therefore presentsdifferent analytical challenges. Furthermore, Münker et al. (2001)reported that for solution MC-ICPMS measurements, the measured176Hf/177Hf deviated increasingly from the true value with increasing

73T.A. Ewing et al. / Chemical Geology 281 (2011) 72–82

Ti/Hf. Investigation of whether LA–MC-ICPMS measurements are alsosubject to this effect is critical for in situ analysis of rutile, which isdominantly TiO2. A rigorous assessment of the accuracywith which Hfisotopes can be measured in situ for rutile is warranted to allow thistechnique to be applied with confidence. We demonstrate that Hfisotopes can bemeasured in rutile by LA–MC-ICPMSwith accuracy andsufficient precision to resolve petrological problems. We describe aprotocol for making accurate measurements, in light of the challengesspecific 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 andassessment of the technique. Average TiO2, Hf, Yb and Lu contents foreach sample are given in Table 1, along with information on sourcelithology. The methods used for trace element measurements by LA–ICPMS and electronmicroprobe 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 Micros-copy Unit), operated with 15 kV accelerating voltage, 3 nA current,and a working distance of 15–18 mm. Compositional zoning and/ormultiple generations of growth were seldom observed in rutile.However, exsolution needles or blebs of ilmenite are commonfeatures that need to be avoided during analyses, and are readilyapparent in BSE images (Fig. 2). Zircons were imaged in cathodolu-minescence on a Hitachi S2250-N SEM (ANU Electron MicroscopyUnit) with 15 kV accelerating voltage, ~60 μA current, and a workingdistance of ~20 mm.

Sample R10 is a ~1 mm×0.5 mm fragment of the centimetre-scalerutile analysed by Luvizotto et al. (2009) for Hf isotopes by isotopedilution MC-ICPMS. Our LA–ICPMS trace element data (Table 1) agreewell with their more precise isotope dilution MC-ICPMS results of38.9±0.4 ppm Hf and 0.03±0.01 ppm Lu. The fragment was largeenough for three LA–MC-ICPMS Hf isotope analyses.

Sample R1 comes from a trondhjemite pod in New Caledonia. R1rutile has an average Hf content of 49 ppm, and was analysed by bothsolution and laser ablation MC-ICPMS as part of this study. Zirconfrom R1 was also analysed for Hf isotopes by LA–MC-ICPMS, forcomparison with rutile. R1 zircons are euhedral and show theoscillatory zoning typical of magmatic zircons (Fig. 1).

The Duria sample is from a 10 m by 15 m lens of mantle peridotiteat Monte Duria in the Central Alps, northern Italy. Hermann et al.(2006) undertook a detailed petrological, trace element and geochro-nological study of the same sample. They demonstrated that rutile inthis sample formed as part of the garnet peridotite peak metamorphicassemblage. Two generations of zircon formed later in the spinelperidotite 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 inclusionspresent (Hermann et al., 2006). We analysed both rutile and zirconfrom the Duria peridotite for Hf isotopes by LA–MC-ICPMS.

Table 1Selected trace element concentrations for rutiles analysed in this study, measured by LA–ICPanalysis had Yb above the detection limit of 0.02 ppm. TiO2 values are those that gave totals fthe number of analyses. 1C. Pirard, pers. comm.; 2cm-scale rutile from Gjerstad, Norway (analyses for Duria rutile.

TiO2 int'l std Hf content (ppm)

Sample Source lithology(wt.%)

Mean Std de

R1 Trondhjemite1 99 49 9R10 Unknown2 99 35 2Duria Garnet peridotite3 99 46 6

The average Hf contents for rutile samples in this study rangebetween 35 and 49 ppm, while Yb and Lu contents are low and oftenbelow detection limits (Table 1). This is in keeping with Hf beingslightly compatible in rutile, whereas Yb and Lu are both highlyincompatible (Foley et al., 2000; Klemme et al., 2005). Trace elementconcentrations for all elements are generally within the range ofvalues reported for rutile by other workers (Zack et al., 2002, 2004b).The significance of rutile trace element chemistry is not discussedhere, as its main relevance to this study is to characterise samplesselected for Hf isotope analysis. Full trace element analyses are givenin Electronic Appendix B.

2.2. Synthetic rutiles

A series of synthetic rutiles doped with varying amounts of Hfwere made at the RSES to test the effect of varying Ti/Hf on measured176Hf/177Hf. This also served to create an in-house reference materialfor rutile analysis. Synthetic rutiles were made by combining UnilabN98% pure TiO2 powder and Aldrich N98% pure HfO2 powder. Fivemixtures were made by progressive dilution of an initial mixcontaining 4.3 wt.% HfO2 with TiO2 in order to obtain rutiles with Hfconcentration ranging from ~43,000 to 4 ppm. By doping TiO2 with Hf(rather than doping HfO2 with Ti) we best simulate the matrix weintend to analyse. The powders were ground together in a mortar andpestle under acetone for ~30 min to achieve thorough homogenisa-tion of each mix. Aliquots of two of the mixes were fluxed with 30%and 10% respectively of anorthite–diopside eutectic compositionpowder, with the aim of creating larger rutile grains. Aliquots of eachmix and both end-members (pure HfO2 and TiO2 powders) werepressed into pellets and run at 1500 °C in air for 3 days in a 1 atmfurnace. Both melt-free andmelt-bearing experiments produced largerutile grains (100–400 μm). BSE imaging showed that no Hf was leftover from the starting material, and there were no zones withelevated BSE emission indicative of abnormally high Hf concentra-tions in rutile. The homogeneous distribution of Hf was confirmed byelectron microprobe traverses across synthetic rutiles SR1–3A.

Concentrations of Ti and Hf were measured by electron micro-probe for the pure HfO2 and synthetic rutiles (SR) 1–3A. Hf was belowthe detection limits of electronmicroprobe for pure TiO2, SR-4 and SR-5, and was measured by LA–ICPMS. Hf concentrations range from

MS. Analyses that were below detection limit are shown in italics. Note that only one R1or all elements of approximately 100 wt.% when used as an internal standard. n signifiesLuvizotto et al., 2009), see also their data; 3Hermann et al. (2006), who also supplied

Yb content (ppm) Lu content (ppm)

v. Mean Std dev. Mean Std dev. n

0.005 0.015 0.0015 0.0029 8b0.04 – b0.01 – 30.246 0.037 0.0006 0.0005 4

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 ilmeniteexsolution/replacement (bright white features). (C) Rutile from the Ivrea–Verbano Zone, Italy, showing zones of replacement by ilmenite and another phase.

74 T.A. Ewing et al. / Chemical Geology 281 (2011) 72–82

11 ppm to 4.3 wt.% (Table 2). Yb and Lu contents for all samples weremeasured by LA–ICPMS, and were below detection limits (b0.04 ppmYb 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 differentisotopic composition from the HfO2 dopant. The 176Hf/177Hf of thesynthetic rutiles therefore represents mixing in various proportionsbetween 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 theassessment 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 isotopecomposition. 176Hf/177Hf was measured by LA–MC-ICPMS for thesesamples 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 NeptuneMC-ICPMS. Hf isotope results for zircon measured using thisinstrumentation have been reported by Wang et al. (2009) andHiess et al. (2009). Earlier results reported by Harrison et al. (2005)were obtained using the same instrumentation, but with a signif-icantly different setup and data reduction process. Here we provide abrief review of the fundamentals of instrument operation and datareduction, as well as outlining some adaptations for analysis of rutile.

Laser ablation sampling was conducted in a He atmosphere tosuppress surface deposition (Eggins et al., 1998), and a small amountof N2 (~2 cm3/min)was introduced downstream of the ablation cell toincrease sensitivity (Durrant, 1994). The laser was pulsed at 5 Hz,with an applied laser fluence at the target of ~5 J/cm2. Zircon analysesused spots with diameters of 47, 62 and 81 μm,whereas a 233 μm spot

Table 2Mean Hf concentrations for synthetic rutiles. Hf concentration was measured by1electron microprobe or 2LA–ICPMS, and includes a small contribution of Hf from theTiO2 powder. ± is the standard deviation of a number (n) of analyses.

Synthetic sample Comment Hf (ppm) ± n

HfO2 Pure HfO2 powder 839,0001 241 3SR-1 42,5001 710 25SR-2 39901 280 14SR-2B Fluxed (30%) 27901 81 30SR-3 3881 45 17SR-3A Fluxed (10%) 4161 45 15SR-4 492 19 6SR-5 112 4 6TiO2 Pure TiO2 powder 4.62 0.1 6

was employed for rutile to maximise the analyte signal from this low-Hf mineral. Synthetic rutiles were analysed with spot sizes between28 μm and 233 μm, according to their Hf content.

Nine masses were measured in static mode: 171Yb, 173Yb, (174Yb+174Hf), 175Lu, (176Hf+176Yb+176Lu), 177Hf, 178Hf, 179Hf, and 181Ta.Although 181Ta is not used in the data reduction process, changes inthe Hf/Ta ratio can be helpful in identifying where inclusions havebeen intersected during an analysis. Detection is by nine faraday cups,with cup efficiencies set to unity. Amplifiers are calibrated for gain atthe start of each analytical session. The instrument is tuned forsensitivity and peak shape on NIST-610 synthetic glass. In an earlysession, peak shapes and positions (relative to the central cup) wereconfirmed to be identical for NIST-610, a synthetic rutile doped with~4 wt.% Hf, and Mudtank zircon, validating the use of a differentmatrix for instrument tuning.

For zircon analyses, 100 cycles of data are generally collected witha total analysis time of approximately 104 s. For rutile, a longeranalysis time of 120 cycles (~125 s) is adopted to improve precision.Baselines are measured after approximately every ten analyses, byacquiring data without ablating material. Sensitivity for rutileanalyses is typically 4–6 mV/ppm total Hf, this range reflecting run-to-run variation in instrument sensitivity. Rutiles with 40–50 ppm Hfgive 0.1–0.3 V total Hf signal at the start of an analysis, which drops to10–30% of this value over the course of the 120 s measurement(Fig. 3). Higher total Hf signals are measured for rutiles with higher Hfcontents, 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 partsof the analysis to avoid inclusions or heterogeneities, or if the grain isdrilled through. Data is corrected for amplifier response factors,

Fig. 3. Typical Hf signal intensity evolution over the course of an analysis for rutile with40–50 ppm Hf. One cycle equates to 1.04 s. Representative analyses from R10 (greysymbols) and R1 (black symbols) have been chosen. The sharper drop in intensity forR1rut_17.1 is probably due to incorporation of some epoxy by drilling through at grainedges.

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baselines, isobaric interferences, and mass bias. Each of thesecorrections is outlined in detail below, with particular reference toany adaptations made for rutile as compared to the standard datareduction method used for zircon.

3.2. Amplifier response factors

Amplifier response factors (ARFs) have been empirically deter-mined for each of the Neptune's nine amplifiers, and are applied to alldata (irrespective of sample type) to correct for the slightly differentresponse times of each amplifier. The relatively large rates of signalintensity change during laser ablation make this correction critical,although the effects are minimised by pairing amplifiers with similarresponse times to measure critical isotope pairs (e.g. 179Hf/177Hf and176Hf/177Hf). ARFs have been found to be very stable over many yearson the RSES instrument.

3.3. Baselines

Accurate and precise determination of baselines is particularlycritical for rutile because of its low Hf content. Relatively long baselinemeasurements of 300 cycles (~5 min) have been adopted as part ofthe protocol for rutile, in order to improve counting statistics anddetermine baselines more precisely. Baselines are measured afterevery ~10 analyses. Whereas zircon data are commonly corrected bysubtracting the nearest baseline value, for rutile interpolatingbetween baseline measurements yields improved accuracy andprecision on 176Hf/177Hf ratios. Fig. 4 shows a block of ten analysesof SR-4 (43 ppm Hf), reduced both using the closest baselinemeasurement, and with interpolation between the bracketing base-lines. When individual baseline measurements are used, there is aclear step in measured 176Hf/177Hf at the point where the closestbaseline switches from an earlier measurement to a later one. Incontrast, if interpolation between the two baselines is used no suchstep occurs, and there is marked improvement in precision on theweightedmean. This reflects the fact that baselines are not necessarilystable during an analytical session. However, measuring baselinesmore often than every ~10 analyses does not yield a significantimprovement in reproducibility, suggesting that this frequency issufficient to constrain variations in baseline. This was tested over afour hour session by analysing R1 rutile with 300 cycle baselinesmeasured every 5 instead of 10 analyses, and with every secondunknown replaced by a 120 cycle baseline. Additional baselines are

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

always within error of the usual baselines measured after every ~10analyses (Fig. 5). Data were processed using baselines determined byinterpolating between (1) long baselines measured every 10 analyses;(2) 120 cycle baselines measured immediately before and after eachanalysis; and (3) long baselines measured every ~5 analyses. Theweighted mean 176Hf/177Hf does not differ significantly between thethree methods of baseline correction and is always within error of thesolution MC-ICPMS value for R1 (Fig. 6). The lower precision on120 cycle baselines is insufficient to resolve short term variability inbaseline levels, but introduces additional scatter into baselinemeasurements (Figs. 5 and 6). The measurement of long baselinesevery ~5 analyses improves the reproducibility of the 176Hf/177Hf onlymarginally (Fig. 6), and adds significantly to the analysis time. Thisexperiment demonstrates that long baselinemeasurements every ~10analyses give superior results to shorter baselines measured morefrequently.

3.4. Isobaric interferences

Isobaric interferences on 176Hf arise from 176Yb and 176Lu, and arecorrected for as described by Hiess et al. (2009). Interferences werestripped using a 176Yb/173Yb value of 0.786956 and a 176Lu/175Lu valueof 0.02645 (Thirlwall and Anczkiewicz, 2004). Unlike zircon, rutilecontains little or no Yb or Lu (see Section 2). However, given therelatively small amount of Hf in rutile, even the very low levels of Yband/or Lu that are sometimes found in rutile (typically ≪0.1 ppm,and always b1 ppm) require correction for their contribution to mass176. This correction is therefore applied to all rutile analyses.

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

Fig. 6. 176Hf/177Hf for 22 analyses of R1 rutile, processedwith three differentmethods of baseline correction.Weightedmean 176Hf/177Hf,MSWDand standard deviation (σ) of 176Hf/177Hfare 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).

76 T.A. Ewing et al. / Chemical Geology 281 (2011) 72–82

174Hf/177Hf has a much larger Yb interference than 176Hf/177Hfand is monitored to check the accuracy of this correction. Measured174Hf/177Hf values are reported in Electronic Appendices C–E and areusually within error of 0.008658, the value reported for this ratio byThirlwall and Anczkiewicz (2004).

3.5. Mass bias

Mass bias is corrected using an exponential law (Russell et al.,1978). The mass bias coefficient for Hf, βHf, is calculated using179Hf/177Hf=0.7325 (Patchett and Tatsumoto, 1980). For zircon,the Yb mass bias coefficient βYb is calculated from the measured173Yb and 171Yb, using 173Yb/171Yb=1.123456 (Thirlwall andAnczkiewicz, 2004, TIMS value). It is not possible to independentlydetermine a mass bias coefficient for Lu, hence βYb is used to correctmass bias for this element (e.g. Woodhead et al., 2004). For rutileanalyses, it is not possible to measure βYb given the very low Ybcontents. Accordingly, βYb is determined from βHf and a βYb/βHf

value measured on zircon several times in the same analyticalsession. Measurements on zircon show that over the course of ananalytical session, both βHf and βYb change in a regular, quasi-linearfashion (Fig. 7). As a consequence, βYb/βHf shows little variationand no systematic change (Fig. 7).

Fig. 7. Percentage change in the Hf and Yb mass bias coefficients, βHf and βYb, and theirratio, 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 thansymbols. All analyses are on R1 zircon, which contains ~580 ppm Yb, using an 81 μmspot.

Iizuka and Hirata (2005) note that βYb/βHf is matrix-dependent,but the corrections for Yb and Lu are small in magnitude for rutile, andany inaccuracy introduced by measuring βYb/βHf on zircon will haveminimal 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 thecalculated 176Hf/177Hf for representative analyses of R1, Duria and R10rutiles (Fig. 8). These analyses cover the approximate range of REE/Hfobserved in rutiles in this study, as measured by the 176Lu/177Hf (seeElectronic Appendix D).

βYb/βHf was measured on Mudtank zircon using a 233 μm spot toavoid possible bias arising from different ablation yield and plasmaloading effects, noting that βYb/βHf variation is much less sensitive tospot size than either βYb or βHf. This method of inferring βYb is alsosometimes used for low-Yb zircons (e.g. Mudtank) when measuredwith relatively small spot sizes, and for which a correlation betweencalculated βYb and 176Hf/177Hf is observed. In this case βYb/βHf is takenfrom analyses of high-Yb zircons measured with the same spot size.

The measurement of βHf can be problematic for rutile. MeasuredβHf varies much more for rutile than for zircon in the same analyticalsession. Analyses of natural rutile with ~49 ppm Hf (sample R1)bracketed by synthetic rutile with ~5000 ppm Hf (SR-2) demonstratethat the apparent variation in βHf for rutile arises from the low

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

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precisionwith which it can be determined (Fig. 9). If themeasured βHf

is used to correct for mass bias in rutile, the apparent variationinduced by poor precision will introduce unnecessary scatter into themeasured 176Hf/177Hf. A more accurate estimate of βHf can beobtained from the nearest SR-2 measurement. This external correc-tion for mass bias is justified given that the measured βHf for R1 isalways within error of the value interpolated between bracketingsynthetic rutile analyses (Fig. 9). An important feature of our protocolfor Hf isotope analysis of rutile is therefore to analyse SR-2 with a233 μm spot after every ~10 analyses of unknowns, facilitating anexternal Hf mass bias correction.

In practice, external determination of βHf makes a relatively smalldifference to the calculated 176Hf/177Hf. For the example shown inFig. 9, using the βHf interpolated from SR-2 analyses made 0.2–2.2 εHfunits difference to individual analyses, and 0.4 εHf units difference tothe weighted mean for this population. These differences are wellwithin the analytical uncertainty, indicating that although an externalmass bias correction will yield more accurate results, acceptable datacan be obtained using an internal mass bias correction. A greater effectwill inevitably occur for rutile with lower Hf contents, for which evenbigger variations in βHf are expected. For example, variations in βHf ofup to 0.25 have been recorded for consecutive analyses of rutile with30 ppmHf over ~1.5 h (cf Fig. 9, in which the entire range of the y axisis 0.18).

With increasing Hf content and thus improved precision on βHf

measurements, real analysis-to-analysis variations in βHf can beresolved (e.g. SR-2 analyses in Fig. 9). As a result, for high-Hf rutilesthe βHf measured on the sample is not always within error of the valueinterpolated from bracketing SR-2 measurements. In these cases,inferring βHf from SR-2 is not justified and the measured βHf is usedinstead.

The non-radiogenic 178Hf/177Hf ratio is typically used to monitorthe accuracy of the mass bias correction, and is reported in ElectronicAppendices C–E. 178Hf/177Hf for individual analyses is generallywithin error of the multidynamic solution MC-ICPMS value ofThirlwall and Anczkiewicz (2004) but falls systematically at the lowend of the range defined by their data. However, estimates of the truevalue of this ratio vary substantially in the literature and typicallyhave large errors (e.g. Kleinhanns et al., 2002; Chu et al., 2002;Thirlwall and Anczkiewicz, 2004, and see Electronic Appendix C). Ourmeasured values for 178Hf/177Hf agree well with the value quoted by(Kleinhanns et al., 2002), for instance. Given the lack of consensus on

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 changein βHfwith time is generally, but not perfectly, linear. However, the apparent variation inβHf 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 rutilewith relatively low Hf content. 2 SE errors for SR-2 are approximately the size of thesymbols. The black line shows the best fit linear trend to SR-2 data.

the correct value of 178Hf/177Hf, it is difficult to assess the significanceof these inter-laboratory differences in measured 178Hf/177Hf values.

3.6. Standardisation

Previous studies on the ANU Neptune have measured 176Hf/177Hffor a range of zircon standards, and if necessary have normalised datafor unknowns to these standards (Wang et al., 2009; Hiess et al.,2009). Wang et al. (2009) observed a small bias (−0.7 εHf units) in176Hf/177Hf for zircon standardswith a range of Yb/Hf and Lu/Hf ratios,as well as for JMC 475 Hf solution. These results indicate a systematicbias that is not matrix-dependent.

In the course of this study, we have observed subtle variationbetween analytical sessions in the magnitude by which 176Hf/177Hfvalues are offset. For this reason we prefer to normalise laser ablationanalyses to mineral standards measured in the same session, ratherthan normalising to a long-term average or a value for JMC 475obtained in a solution session. Although the bias appears to beindependent of matrix, we consider it prudent to normalisemeasurements of 176Hf/177Hf of rutile to the synthetic rutile SR-2,which is matrix-matched and is already analysed to monitor massbias. Because SR-2 contains ~5000 ppm Hf and is measured with a233 μm spot, the precision obtained is comparable to or better thanthat for zircon standards measured with typical spot sizes. Rutileunknowns are normalised to the weighted mean of all SR-2 analysesfrom the same session. The 176Hf/177Hf for SR-2 has been determinedas 0.281888±0.000007 by solution MC-ICPMS at the Polish Academyof Sciences, Krakow (see Electronic Appendix A.3). Rutile samplesanalysed in earlier sessions were normalised to Mudtank zirconinstead of SR-2, and where possible to Mudtank analyses with thesame spot size as unknowns.

176Hf/177Hf for zircon analyses is normalised to the average offsetof the zircon standardsMudtank, 91500 and Temora from the solutionvalues reported byWoodhead and Hergt (2005). The only exception iszircon from the Duria peridotite, which was normalised to Mudtankzircon.

The 95% confidence error on the weighted mean of standardanalyses is propagated in quadrature onto the calculated errors foreach individual analysis of unknowns. Where several different zirconstandards 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 propagatederror is less than two standard deviations (2σ) for the relevantstandard in that session, sample error is forced to equal the 2σ for thestandard. This is true for both individual analyses and weightedmeans. All non-radiogenic isotope ratios are reported withoutnormalisation to standards.

4. Results

All errors are reported at the 2 SE level. Weighted means werecalculated using Isoplot 3 (Ludwig, 2003) and are quoted with 95%confidence errors; these take account of sample scatter by incorpo-rating the MSWD of analyses as well as tσ. εHf is calculated using theλLu of Soderlund et al. (2004) and the CHUR reference isotope ratios ofBouvier et al. (2008). Zircon analyses used the same analytical setupas for rutile, but with smaller spot sizes and typical zircon datareduction protocol (as in Hiess et al., 2009). Full data tables for allstandards and samples are presented in Electronic Appendices C–E.

4.1. R10

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

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

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/177Hf of R1 rutile measured by solution MC-ICPMS.

78 T.A. Ewing et al. / Chemical Geology 281 (2011) 72–82

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

4.2. R1

One mg of R1 rutile separate was analysed by solution MC-ICPMSat the Polish Academy of Sciences (Electronic Appendix A.3), and gave176Hf/177Hf of 0.283097±0.000008. 95 LA–MC-ICPMS analyses fromfour sessions over three years gave 176Hf/177Hfmeasurements that arereproducible both between and within sessions (Fig. 11). One analysishas been excluded from each of the first two sessions because ofinclusions intersected during analysis. Theweightedmean 176Hf/177Hffor each session is in agreement with the solution MC-ICPMS value(Fig. 11). 176Lu/177Hf for R1 rutile ranges from−0.000007±0.000007to 0.000094±0.000017.

Ten analyses of zircon from R1 gave reproducible 176Hf/177Hfmeasurements with a weighted mean of 0.28310±0.00003. This isindistinguishable from the 176Hf/177Hf of R1 rutile as determined by

Fig. 11. 176Hf/177Hf measurements for R1 rutile across four sessions. Weighted mean 176Hf/mean (all sessions) is plotted as a black bar. The grey shaded rectangle indicates the 2 SE l

both solution and laser ablation MC-ICPMS (Fig. 12). 176Lu/177Hf forR1 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-3Ais always in agreement with this value, both for individual analysesand for weighted means (Fig. 13). There is no evidence for a sys-tematic deviation in measured 176Hf/177Hf with increasing Ti/Hf.

4.4. Duria peridotite

The few rutiles that were able to be separated from the Duriaperidotite contained on average 46 ppm Hf (Hermann et al., 2006) andallowed four LA–MC-ICPMS analyses that yielded reproducible 176Hf/177Hf ratios with a weighted mean of 0.28311±0.00028 (Fig. 14A).176Lu/177Hf for theDuria rutiles varies from−0.0000025±0.0000074 to0.0011116±0.0000589. Hermann et al. (2006) demonstrate that rutilein this sample formed prior to the oldest generation of zircon, whichthey dated at 34.2±0.2 Ma. The Duria rutile population has an initialεHf of +12±10 for an age of 35 Ma, the likely age of peak HP

177Hf is given for each session and is also plotted at the right. The long term weightedimits of the 176Hf/177Hf of R1 rutile determined by solution MC-ICPMS.

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 thereference value for all synthetic rutiles shown. Note excellent agreement of all analyses with themean HfO2 value, and that scatter is both above and below this value. Data have beennormalised 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 eachsample 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.

79T.A. Ewing et al. / Chemical Geology 281 (2011) 72–82

metamorphism in this region. It is noted that the initial εHf values forboth rutile and zircon are insensitive to changes in ageof severalMa, andto variations of up to two orders of magnitude in the Lu/Hf ratio.

We targeted the older of the two generations of zircon, which isdark and oscillatory zoned in cathodoluminescence (Fig. 15) andrecords 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 veryreproducible 176Hf/177Hf with a weighted mean of 0.28258±0.00002(Fig. 14B). 176Lu/177Hf varied from 0.0000235±0.0000005 to0.0001301±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

Fig. 14. Hf isotope analyses of Duria mantle peridotite. A. Rutile analyses. B. Zircon analyses. Cin text and shown as a grey line.

εHf of the rutile population (Fig. 14C), in spite of the fact that theprecision on the weighted mean for rutile is poor because of the smallnumber of rutile grains suitable for analysis.

5. Discussion

5.1. Accuracy

The agreement of 176Hf/177Hf measured by LA–MC-ICPMS withsolutionMC-ICPMS values for R10 and R1 rutiles demonstrates that Hfisotopes can bemeasured accurately for rutile with≥40 ppmHf usingthe technique presented here. Excellent reproducibility both between

. Weighted means for rutile and zircon. In A. and B. weighted mean 176Hf/177Hf is given

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.

80 T.A. Ewing et al. / Chemical Geology 281 (2011) 72–82

and within sessions is demonstrated by 95 analyses of R1 rutile overfour sessions that span more than three years.

The lack of any systematic bias in measured 176Hf/177Hf withincreasing Ti/Hf for a series of synthetic rutiles doped with Hfdemonstrates that the accuracy of 176Hf/177Hf measurements by LA–MC-ICPMS is in no way compromised by high Ti/Hf. Although thesynthetic rutiles used to assess the effect of Ti/Hf do not bracketnatural rutiles in terms of Hf concentration (the lowest being390 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 thatthey observed offsets of approximately 1.5–2.3 εHf units for four of thefive analyses which have a Ti/Hf of 1250 (very similar to our highestTi/Hf values). If we had observed the same offset, it would correspondto 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 epoxyduring analysis (e.g. if the edge of the grain is drilled through) doesnot compromise the accuracy of results. Several analyses of epoxyconfirmed that it contains no Hf, Yb or Lu, with these analysesessentially identical to baselinemeasurements. As a test, the syntheticrutile SR-2 was also analysed in three configurations: (1) with thespot wholly on the rutile; (2)with the spotmostly on the rutile, but onthe epoxy at the edge; and (3) with the spot one-third on the epoxyand two-thirds on the rutile. There was no measurable difference in176Hf/177Hf between these analyses, nor any apparent effect on massbias coefficients.

5.2. Precision

Individual measurements of 176Hf/177Hf have rather large uncer-tainties because of the low Hf content of rutile (typically ±10–12 εHfunits for 40–50 ppm Hf). Nonetheless, 176Hf/177Hf measurements arewell reproduced between analyses for a given sample, and accord-ingly the uncertainty on the weighted mean of 10–15 analyses ismuch lower. A precision of ±4 εHf units can be achieved for theweighted mean of populations with 40–50 ppm Hf (e.g. Fig. 11). Theprecision attained for rutile is poorer than for zircon, but is sufficientto distinguish 176Hf/177Hf values of different rutile samples, and inexamples like the Duria peridotite, of rutile and zircon in the same

rock. This demonstrates the achievement of a useful level of precisionto address geological problems. Importantly, errors for rutile analysesare small in comparison to the very large range of 176Hf/177Hfobserved for this mineral — for example, the 155 εHf unit rangereported by Choukroun et al. (2005). For rutile with unusually high Hfconcentrations (200–300 ppm), much better precision can beachieved: ±2–3 εHf units on individual analyses, and ±0.8 εHf unitson the weighted mean of 15 analyses.

5.3. Minimum Hf concentration

The reliability of 176Hf/177Hf measurements is compromised atvery low levels of Hf. For two synthetic rutiles with 5 ppm and 11 ppmHf, adopting interpolation between baseline values changed theweighted mean of ten 176Hf/177Hf measurements by 75 εHf units and26 εHf units respectively. This large difference in response to a smallchange in baseline values indicates that this level of Hf is insufficientto obtain reliable 176Hf/177Hf measurements. In contrast, for rutilewith 40–50 ppm Hf the choice of baseline correction method changesthe weighted mean of populations by at most 1–1.5 εHf units.

An important observation is that for rutile with N40 ppm Hf, amuch greater effect of baseline choice can be seen on individual 176Hf/177Hf 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 (althoughusually less than this). This emphasises the importance of makingmultiple analyses, preferably 10–15, for each sample and suggeststhat individual analyses should not necessarily be relied on as “stand-alone” analyses for low-Hf rutiles.

5.4. Application to igneous and metamorphic samples

Sample R1 is an unmetamorphosed igneous rock. Rutile and zirconfrom this plutonic rock are expected to have formed at the same timeand with identical Hf isotope compositions. The indistinguishable176Hf/177Hf measurements for zircon and rutile confirm this to be thecase.

The polyphase metamorphic history of the Duria peridotitepresents a more complex case study (Fig. 15), in which rutile andzircon 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 Duriaperidotite indicates input of crustal material. In contrast, the positiveinitial εHf (+12±10) of the Duria rutile population indicates adepleted mantle origin. Positive εHf may also be generated by therelease of Hf during the breakdown of garnet, which can evolvestrongly radiogenic Hf isotope compositions in a relatively short timebecause of its high Lu/Hf (Slama et al., 2007; Zheng et al., 2005).However, garnet breakdown cannot be invoked to explain the Hfisotope composition of the Duria rutile. Rutile occurs as inclusions ingarnet, and grew in equilibrium with this mineral in the garnetperidotite stability field (Hermann et al., 2006, and Fig. 15). Thesimultaneous growth of garnet and rutile precludes garnet break-down as a source of radiogenic Hf for rutile. Garnet breakdown didoccur subsequently, but this reaction was associated with theformation of ilmenite, not rutile (Hermann et al., 2006).

The depleted mantle Hf isotope signature of rutile from the Duriaperidotite is consistent with the conclusion of Hermann et al. (2006)that the peak metamorphic assemblage to which rutile belongs hadbulk rock chemistry very similar to depleted MORB mantle. Similarly,the Hf isotope evidence for crustal influence in zircons from the Duriaperidotite 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 aboutdifferent parts of the metamorphic history (Fig. 15), emphasising thecomplementary nature of Hf isotope analysis of these two minerals inrocks that underwent a polyphase evolution. Although in situ 176Hf/177Hf measurements for rutile have lower precision than those forzircon, they can provide information additional to that recorded byzircon.

6. Outlook

In situ analysis of Hf isotopes in rutile by LA–MC-ICPMS offerssignificant benefits. It is rapid, cost-effective, allows single-grainanalysis, and still yields sufficient precision to resolvemany geologicalproblems. Measurement of 176Hf/177Hf of individual rutile grainsallows the characterisation of multiple generations of rutile in theirtextural context for thin sections with large rutile crystals. Betterprecision could in principle be achieved by solution MC-ICPMS, butwould generally require bulk analysis of multiple rutile grains. Thegreater volume of material required is due to the loss of a significantproportion of the analyte in the spray chamber during solutionanalysis. For rutile with ≤100 ppm Hf, multiple grains would berequired to obtain sufficient Hf for a single high-precision solutionanalysis, except in the case of unusually large (N400 μm) grains.Dissolution of less material is possible, but the resulting loss ofprecision begins to erode the advantages of the technique.

An important advantage of in-situ Hf isotope analysis is the abilityto avoid or exclude inclusions or areas of alteration. Alteration of rutileis common, but usually only affects a small volume fraction of thegrain. Small areas of alteration can be difficult to detect during hand-picking, especially in large dark grains. For solutionMC-ICPMS, micro-drilling is necessary to avoid these areas. However, micro-drilling canbe laborious and does not guarantee exclusion of all altered domains.With LA–MC-ICPMS all or parts of analyses that show signs ofcontamination can be rejected prior to pooling data. Furthermore, thegreater volume of material required for a high precision solutionanalysis would necessitate micro-drilling of more than the 10–15rutiles required for LA–MC-ICPMS.

Provenance studies of detrital rutile will be an importantapplication of Hf isotope analysis of rutile, for which single-grainanalysis is essential. The most common source of detrital rutile ismedium- to high-grade metamorphic rocks (Force, 1980; Zack et al.,2004b), and the ability of detrital rutile to fingerprint source areasthat are not recorded by detrital zircons has already been recognised(Zack et al., 2004b) and exploited through trace element and U–Pb age

studies (e.g. Zack et al., 2004b; Triebold et al., 2007; Allen andCampbell, 2007;Meinhold et al., 2008;Morton and Chenery, 2009). Hfisotope measurements of detrital rutiles will improve the ability todistinguish between different sediment sources, and provide a moredetailed characterisation of sources than trace elementmeasurementsalone.

The ability to measure the 176Hf/177Hf of rutile is a valuableaddition to the metamorphic petrologist's toolbox. Rutile participatesin major metamorphic reactions that can be linked to specific parts ofthe metamorphic history more clearly than for other accessoryminerals (e.g. Liu et al., 1996; Ernst and Liu, 1998; Zack et al., 2002;Zack et al., 2004a). The example of the Duria peridotite demonstratesthat Hf isotope analysis of rutile can provide information not recordedby zircon when the two minerals grew at different stages in themetamorphic evolution of a sample. Hf isotope analysis of rutile willalso be valuable for samples inwhich growth of metamorphic zircon isabsent or limited to narrow overgrowths that may be too small toanalyse for Hf isotopes by LA–MC-ICPMS. Metamorphic rutile tends togrow to much larger grain sizes than zircon, and almost neverpreserves multiple zones, and is thus an ideal target for LA–MC-ICPMS. Eclogitic and mantle xenoliths are obvious examples oflithologies in which rutile is common but zircon is unusual, and forwhich Hf isotopes are of interest (Choukroun et al., 2005; Aulbachet al., 2008).

7. Conclusions

1. The 176Hf/177Hf of rutile with ≥40 ppm Hf can be accuratelymeasured in situ by laser ablation MC-ICPMS in spite of therelatively low Hf content, provided care is taken with the analyticalprotocol and data reduction process.

2. Accuracy is demonstrated by agreement of our laser ablation MC-ICPMS results with solution MC-ICPMS measurements for a largerutile crystal (R10, Luvizotto et al., 2009) and for rutile from atrondhjemite (R1). Robustness of the technique is demonstrated by95 LA–MC-ICPMS analyses of R1 rutile across four sessions and~3 years, which show that 176Hf/177Hf is measured reproduciblyboth between and within sessions.

3. Analysis of a series of synthetic rutiles doped with Hf in con-centrations ranging from 390 ppm to 4.3 wt.% indicates no influenceof 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 individualanalyses of rutile with 40–50 ppm Hf, and can be as good as ±4 εHfunits for the weighted mean of a population of 10–15 analyses onone 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 Hfisotope compositions that record different parts of the sample'smetamorphic history.

6. Key features of our analytical protocol to maximise precision andaccuracy 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, theseelements can be present at levels that produce significantisobaric interferences on 176Hf.

(c) An external correction for Hf mass bias is applied in caseswhere the low precision with which the Hf mass biascoefficient βHf can be measured negates the advantages of aninternal correction for mass bias. βHf for the externalcorrection is inferred from regular analyses of a syntheticrutile (SR-2) with ~5000 ppm Hf.

(d) The Yb mass bias coefficient, βYb, for rutile is calculated fromβYb/βHf measured on zircon with a 233 μm spot several timesduring a session.

82 T.A. Ewing et al. / Chemical Geology 281 (2011) 72–82

Acknowledgements

We thank the ANU Electron Microscopy Unit for technicalassistance and access to SEM facilities. L. Kinsley and C. Allen arethanked for their expert technical support and constructive discus-sions on mass spectrometry. We are grateful to T. Zack for supplying achip of rutile R10, and to C. Pirard for donating zircon and rutileseparates from R1. We are indebted to R. Anczkiewicz for undertakingsolutionMC-ICPMS analyses and providing constructive comments onthe manuscript. Two anonymous reviewers are thanked for thoroughreviews that significantly improved the manuscript. R. Rudnick isthanked for editorial handling. This research was financially sup-ported by the Research School of Earth Sciences and the AustralianResearch Council (DP0556700 to D. Rubatto and J. Hermann).

Appendix A. Supplementary data

Supplementary data to this article can be found online atdoi:10.1016/j.chemgeo.2010.11.029.

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