Mathieu Rospabé
email: mathieu.rospabe@get.omp.eu

Mathieu Benoit

Frédéric Candaudap

Determination of Trace Element Mass Fractions in Ultramafic Rocks by HR-ICP-MS: A Combined Approach Using a Direct Digestion/Dilution Method and Pre-concentration by Co-precipitation

Keywords: trace elements, ultramafic rocks, reference materials, thulium spike, coprecipitation, HR-ICP-MS

published or not. The documents may come     Determination of Trace Element Mass Fractions in Ultramafic Rocks by HR-ICP-MS: A Combined Approach Using a Direct Digestion/Dilution Method and Preconcentration by Coprecipitation

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Ultramafic rocks are an important component of the Earth, constituting more than 80% of its volume. Exposed in response to tectonic processes (ophiolites, orogenic massifs or abyssal peridotites) or as xenoliths in lavas, mantle peridotites are viewed as residues of partial melting. Undoubtedly linked to the petrogenesis of basaltic magmas, to their transfer as well as to their chemical signature, peridotites have, up to the present, been mostly investigated through the chemistry of their pyroxenes, either after mineral separation or with in situ measurement techniques (i.e., SIMS and LA-ICP-MS). In contrast with the wealth of data acquired for pyroxene, olivine remains poorly studied for its trace element contents, despite making up more than half of the modal content of peridotites. In fact, olivine reveals significant differences in mass fractions that can reach ten orders of magnitude between major Mg and Fe cations (approx. MgO ~ 50 g/100 g and FeO ~ 10 g/100 g) and trace elements such as REE or HFSE (sub ng g -1 ) (e.g., Rampone et al. 2016, Stead et al. 2016). Attempts to investigate the chemical content of olivine and/or bulk olivine-rich lithologies (dunites, orthopyroxene-poor harzburgites) commonly used ICP-MS after dissolution followed by simple dilution. In rock analysis by ICP-MS, the high mass fraction of major elements in the rock solution can lead to a reduction of trace element signal intensities or to a partial blockage of the ICP-MS orifice [START_REF] Beauchemin | Study of the effects of concomitant elements in inductively coupled plasma-mass spectrometry[END_REF], Kawaguchi et al. 1987[START_REF] Makishima | Suppression of matrix effects in ICP-MS by high power operation of ICP: Application to precise determination of Rb, Sr, Y, Cs, Ba, REE, Pb, Th and U at ng g -1 levels in milligram silicate samples[END_REF]. Consequently the dilution factor is critical in order to investigate trace element mass fractions in peridotites: when the dilution factor is high, most of the trace elements occur below the detection limit; when low, matrix effects become a critical issue (Makishima andNakamura 1997, 2006).

To overcome the matrix effects caused by high major element contents and to minimise the dilution factor, separating trace elements from the matrix and pre-concentrating them may be performed either by extraction chromatography or by co-precipitation process. The coprecipitation method was first applied to seawater and soil analyses (e.g., Goldberg et al. 1963[START_REF] Akagi | Simultaneous multi-element determination of trace metals in seawater by inductively coupled plasma-atomic emission spectrometry after co-precipitation with gallium[END_REF], Greaves et al. 1989, Taicheng et al. 2002, Shaw et al. 2003, Jeandel et al. 2011). It consists of co-precipitating trace elements with an induced precipitation of iron, magnesium, titanium or gallium hydroxides after Fe, Mg, Ti or Ga addition. This method was later adapted to ultramafic rocks to investigate their elemental and isotopic chemistry, using the sample's natural Mg and Fe contents, and does not therefore need addition of other cations (e.g., Sharma et al. 1995, Sharma and Wasserburg 1996, Bizzarro et al. 2003[START_REF] Qi | Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 co-precipitation[END_REF][START_REF] Bayon | Determination of rare earth elements, Sc, Y, Zr, Ba, Hf and Th in geological samples by ICP-MS after Tm addition and alkaline fusion[END_REF]. A pH higher than 10 triggers the precipitation of Mg(OH) 2 , co-precipitating the trace elements and leaving iron in solution; however, a pH of

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This article is protected by copyright. All rights reserved. ca. 9 leads to the precipitation of Fe(OH) 3 and leaves the magnesium in the supernatant (Sharma andWasserburg, 1996, Qi et al. 2005). Recently, the Tm addition method [START_REF] Barrat | Determination of rare earth elements in sixteen silicate reference samples by ICP-MS after Tm addition and ion exchange separation[END_REF] was introduced in co-precipitation procedures allowing (1) the calculation of the trace element compositions using the Tm positive anomaly in the REE pattern and (2) correction for any loss of analyte during preparation [START_REF] Bayon | Determination of rare earth elements, Sc, Y, Zr, Ba, Hf and Th in geological samples by ICP-MS after Tm addition and alkaline fusion[END_REF], Freslon et al. 2011).

During our experiments, the aim was to combine and extend previous methods in order to measure the mass fractions of a large set of trace elements that may potentially give information on the magmatic history of ultra-depleted residual peridotites, such as harzburgite or dunite. Following the Tm addition [START_REF] Barrat | Determination of rare earth elements in sixteen silicate reference samples by ICP-MS after Tm addition and ion exchange separation[END_REF][START_REF] Bayon | Determination of rare earth elements, Sc, Y, Zr, Ba, Hf and Th in geological samples by ICP-MS after Tm addition and alkaline fusion[END_REF], we associated the sample digestion/dilution method from Yokoyama et al. (1999) with an adapted preconcentration procedure combining specific aspects of different co-precipitation methods (Bizzarro et al. 2003[START_REF] Qi | Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 co-precipitation[END_REF]. The procedure of [START_REF] Qi | Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 co-precipitation[END_REF] is a way to avoid any potential matrix effect caused by the high Mg content in peridotites, using two successive Mg(OH) 2 and Fe(OH) 3 co-precipitations for the determination of REE + Y. In their study Bizzarro et al. (2003) showed the efficiency of the co-precipitation method to determine the mass fraction of other HFSE such as Zr and Hf. Our approach, combining the direct method after sample digestion and dilution and an adapted co-precipitation method, allowed us to determine thirty-seven minor and trace elements including Li, LILE (Rb, Sr, Cs, Ba), HFSE (Y, Zr, Nb, REE, Hf, Ta, Th, U), Pb, other transition elements (Sc, Ti, V, Cr, Mn Co, Ni, Zn) and Ga in four ultramafic reference materials ranging for example from 10 -2 to two times the chondritic values in REE (DTS-2B, JP-1, MGL-GAS, UB-N).

Experimental procedure

Reagents and reference materials

During all experiments we used de-ionised water (18.2 M cm) purified with a Milli-Q system (Millipore ® ), hydrochloric (~ 37 g/100 g, Fisher Scientific ® , Loughborough, England) and nitric (68 g/100 g, GPR RECTAPUR ® , VWR International S.A.S., Fontenay-sous-Bois, France; VWR BDH Prolabo ® ) acids cleaned by sub-boiling distillation, 49 g/100 g hydrofluoric acid for ultra trace analysis (TraceSELECT ® Ultra, Fluka® analytical, SIGMA-ALDRICH ® , St. Louis, USA) and 69-72 g/100 g certified ACS reagent grade perchloric acid (Fluka ® analytical, SIGMA-ALDRICH ® , St. Louis, USA) without further purification, and a high purity thulium solution (custom grade, 62N-1, AccuStandard ® ).

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To trigger co-precipitation during the pre-concentration procedure, 25 g/100 g ammonia (Suprapur ® , Merck KGaA, Darmstadt, Germany) and 7.5 mol l -1 sodium hydroxide solutions were used. This latter was prepared by dissolving about 300 g of anhydrous NaOH pellets (purity > 99%, AnalaR NORMAPUR ® , VWR International BVBA, Leuven, Belgium; VWR BDH Prolabo ® ) in one litre of high purity water.

To test and to validate the measurement precision of our procedure we investigated four ultramafic reference materials (RMs) including the serpentinite UB-N (ANRT), the peridotite JP-1 (Geological Survey of Japan, GSJ), the serpentinite MGL-GAS (GeoPT12; International Association of Geoanalysts, IAG) and the dunite DTS-2B (United States Geological Survey, USGS). BHVO-2 (USGS) was analysed as the calibrator during our measurement sessions.

Sample preparation

Our experimental procedure combined further aspects of previous studies: (1) the Tm addition of [START_REF] Barrat | Determination of rare earth elements in sixteen silicate reference samples by ICP-MS after Tm addition and ion exchange separation[END_REF], later adapted by [START_REF] Bayon | Determination of rare earth elements, Sc, Y, Zr, Ba, Hf and Th in geological samples by ICP-MS after Tm addition and alkaline fusion[END_REF]; (2) the sample digestion presented by Yokoyama et al. (1999); (3) a pre-concentration of trace elements in ultramafic samples especially following the double Mg(OH) 2 -Fe(OH) 3 co-precipitation method developed by [START_REF] Qi | Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 co-precipitation[END_REF]. About 0.200 g of sample powder were accurately weighed and transferred to 22 ml Savillex ® PFA beakers. From about 1.5 to 15 ng of Tm (in solution) were added to each sampleexcept to the blank and to the reference material BHVO-2 -depending on the expected bulkrock trace element content: about 1.5, 3, 6 and 15 ng for DTS-2B, JP-1, MGL-GAS and UB-N respectively.

Our sample digestion procedure largely followed method C of Yokoyama et al. (1999), following three distinctive stages. The first stage consisted to add 1 ml 72 g/100 g HClO 4 + 1 ml 49 g/100 g HF to each sample; then capped beakers were placed on a hot plate at 130 °C for 24 h. After cooling acids were evaporated following three dryness steps: 8 h at 130 °C, 12 h at 165 °C and 1-2 h at 195 °C. Due to the high evaporation temperature of perchloric acid, these steps allowed us to remove silica without forming fluorides which are known to coprecipitate trace elements and to be insoluble (Langmyhr and Sveen 1965, Croudace 1980, Boer et al. 1993, Yokoyama et al. 1999). The second stage comprised 1 ml 72 g/100 g HClO 4 ,

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This article is protected by copyright. All rights reserved. a heating step of 10-12 h at 140 °C, and a replication of the three previous dryness steps.

Because the complete dissolution of spinel in ultramafic rocks is particularly difficult to achieve (Jain et al. 2000, Nakamura and[START_REF] Nakamura | Precise determination of ultra-low (sub-ng g -1 ) level rare earth elements in ultramafic rocks by quadrupole ICP-MS[END_REF], this second stage required repeated heating and treatments in ultrasonic bath steps for DTS-2B, which contains a higher amount of spinel than the three other investigated RMs. For the same reason, the duration of the heating step needed to be extended to 24 h for DTS-2B. The third and last stage consisted of the addition of 1 ml HCl (6 mol l -1 ) and evaporation to dryness at 110 °C. The 6 mol l -1 HCl was previously prepared from our in-house produced doubly distilled HCl.

The solid residue was dissolved in 1 ml HNO 3 (15 mol l -1 ) and diluted with 4 ml of high purity water. The solution was transferred to a 50 ml centrifuge tube and the volume adjusted to 40 ml with high purity water to reach a solution concentration of 0.37 mol l -1 HNO 3 . At this stage a fraction of 0.8 ml was collected from this "mother solution", transferred to a 50 ml centrifuge tube and adjusted to 40 ml with 0.37 mol l -1 HNO 3 to give a dilution factor of 10000. This final solution was dedicated to the direct measurement of trace element content, i.e., without pre-concentration, and is referenced for the following as the fraction being analysed through the direct method. BHVO-2 and blanks were analysed only following this procedure with dilution factors of 6000 and 1000 respectively.

The remnant mother solution was retained in order to be processed through a double coprecipitation reaction, mainly adapted from [START_REF] Qi | Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 co-precipitation[END_REF]. First, 400 l of a 7.5 mol l -1 sodium hydroxide solution were added to raise pH above 12, leading to Mg(OH) 2 precipitation from the sample's natural Mg content, while the main iron content remained in solution; a white to yellowish precipitate containing a small amount of iron was formed at this stage. After centrifugation for 5 min at 3600 rpm, the supernatant was discarded. Two successive rinses were made, adding 40 ml high purity water followed by further centrifugation and supernatant separation steps. Following this first Mg co-precipitation stage, the precipitates were dissolved with three drops of concentrated bi-distilled HNO 3 (15 mol l -1 ) and diluted again in 40 ml of high purity water. Following Sharma et al. (1995) andBizzarro et al. (2003), we added 1 ml 25 g/100 g ammonia solution leading to the formation of iron hydroxides from the small Fe amount trapped with previous Mg(OH) 2 , which gave a rusty coloured precipitate. With a pH of about 9 (ammonia solution) most of the magnesium remains in solution (Sharma andWasserburg 1996, Qi et al. 2005). The supernatant containing magnesium was removed by re-applying the centrifugation and the two rinse steps.

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The solid precipitate was then dissolved in 0.5 ml HNO 3 (15 mol l -1 ) and adjusted to 20 ml with high purity water. At this stage 1 ml was collected and adjusted to 9.8 ml with 0.37 mol l -1 HNO 3 to obtain a final solution with a dilution factor of 1000. The procedure that allowed to the determination of trace element mass fractions following co-precipitation is referred to as the co-precipitated method.

Instrumentation and data acquisition

An ELEMENT XR TM (Thermo Scientific TM ) HR-ICP-MS at the Géosciences Environnement Toulouse laboratory, Université Paul Sabatier Toulouse III (Observatoire Midi-Pyrénées, Toulouse) was used for trace element determinations. The operating conditions are summarised in Table 1. Sample solutions (0.37 mol l -1 HNO 3 ) were introduced with a peristaltic pump through the nebuliser and transported via Ar gas to the quartz plasma torch and to the interface made of Ni sampling and skimmer cones. The nebuliser and the cones were dedicated to the analysis of ultra-depleted samples as those reported here, and were used only during our measurement sessions to avoid contamination.

Oxide or hydroxide recombination in the Ar plasma may generate interferences on measured intensity of a given element, especially barium oxide on europium and LREE oxides on HREE (e.g., Ionov et al. 1992). Daily analytical conditions showed low element recombination with oxide rates (BaO + /Ba + , CeO + /Ce + ) mainly below 1%. In Table 1 oxide rate and doubly charged for Ba are given as information values applicable to the operating conditions before the start of a measurement session. To monitor interference effects, we measured calibration solutions of high purity water, Sm + Eu + Gd + Tb, Pr + Nd and Ba + Ce at the beginning of each measurement session to estimate the oxide and hydroxide contribution on REE measurements (Ionov et al. 1992[START_REF] Barrat | Determination of rare earth elements in sixteen silicate reference samples by ICP-MS after Tm addition and ion exchange separation[END_REF], and to correct the interferences that may occur on 169 Tm in particular.

Measurement sequences consisted of the analysis of a 0.37 mol l -1 HNO 3 blank, a sample procedural blank corresponding to the sample digestion procedure, BHVO-2 as a calibrator, and test portions of the selected reference materials. Thanks to its precisely known composition (http://georem.mpch-mainz.gwdg.de; [START_REF] Barrat | Geochemistry of CI chondrites: Major and trace elements, and Cu and Zn isotopes[END_REF], BHVO-2 was measured both as a calibrator to calculate the trace element content of samples and to correct measurements for instrumental drift. As part of this protocol four reference materials were analysed between two of the calibrator BHVO-2. We also measured blanks for the NH 3

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This article is protected by copyright. All rights reserved. ultrapur reagent as well as for the NaOH solution used to perform the double-co-precipitation process. They were prepared following the same procedure applied to co-precipitate trace elements including the successive dilution/evaporation steps. As explained below the RMs were corrected from the total co-precipitation blank contribution following two steps, first raw data obtained on the procedural blank (0.37 mol l -1 HNO 3 blank + contribution following the sample digestion) is subtracted from the samples measurements (i.e., taking into account the different dilution factor between the samples and the blanks), and then the calculated content in added NaOH and NH 3 fractions is subtracted after the samples concentrations calculation. Samples prepared following the direct method were thus corrected from the procedural blank only (related to the samples digestion and dilution), while co-precipitated ones were corrected from the total co-precipitation blank (i.e., including the blank contribution following the NaOH and NH 3 solutions using). Because sodium highly affects the stability of the plasma during acquisition by ICP-MS, we decided to analyse only one Tmspiked NaOH fraction at the beginning of our experiment rather than measure it at each measurement session. In this way we determined a blank for our NaOH base solution, as well as for NH 3 to facilitate the correction from the both buffer/basic solution blanks contribution.

Each measurement performed on pre-concentrated samples was corrected from this blank's contribution assuming that this contribution was constant from an added fraction to the other.

Data reduction

First, the raw data were corrected for instrumental drift, then for the procedural blank contribution and finally for the oxide/hydroxide interferences. The drift of the signal intensity through time was constrained by bracketing using BHVO-2. We analysed four reference materials between two BHVO-2. To calculate the mass fraction of a given element X in each RM, we used an interpolation of the BHVO-2 signal intensity (counts per second or cps), assuming that the instrumental drift was linear between two BHVO-2 analyses. Raw data obtained following both the direct and pre-concentration methods were corrected for the procedural blank (i.e., contribution from the 0.37 mol l -1 HNO 3 and from the sample digestion) by subtracting blank signal intensities. Measured intensities were then corrected for oxide/hydroxide interferences following the method of [START_REF] Barrat | Determination of rare earth elements in sixteen silicate reference samples by ICP-MS after Tm addition and ion exchange separation[END_REF]. The blank and oxide/hydroxide corrections were made both for BHVO-2 and for the reference materials.

The following variables are used in the equations:

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-X sample solution : cps for the element X in the sample solution after blank subtraction, and REE corrected for oxide/hydroxide interferences.

-X BHVO-2 : cps for the element X in the BHVO-2 solution after blank subtraction, and REE corrected for oxide/hydroxide interferences.

-[X] BHVO-2 : reference mass fraction for the element X ( g g -1 ) in BHVO-2 following [START_REF] Barrat | Geochemistry of CI chondrites: Major and trace elements, and Cu and Zn isotopes[END_REF] (see also GeoRem values).

-[X] sample solution : mass fraction of the element X ( g g -1 ) in the sample solution.

-[Tm*]: Tm mass fraction in the sample solution with no spike contribution, interpolated from [Er] sample solution and [Yb] sample solution ( g g -1 ).

- [Tm] spike : Tm mass fraction in the spike solution ( g g -1 ).

-M sample : sample mass (g).

-M Tm spike : spike mass (g).

-[X] sample : mass fraction of the element X in the sample ( g g -1 ).

The trace element content of BHVO-2 (GeoRem; [START_REF] Barrat | Geochemistry of CI chondrites: Major and trace elements, and Cu and Zn isotopes[END_REF]) was used to estimate all [X] mass fractions in the sample solution following:

X sample solution = Xsample solution × X BHVO-2 X BHVO-2 (1)
This mass fraction is an approximation, uncorrected from the Tm spike/sample ratio. The sample mass fraction was calculated using this ratio following the description of [START_REF] Barrat | Determination of rare earth elements in sixteen silicate reference samples by ICP-MS after Tm addition and ion exchange separation[END_REF]

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This article is protected by copyright. All rights reserved. Following (3), the final mass fractions for co-precipitated fractions are obtained after NH 3 and NaOH blank subtractions.

Results and discussion

Detection limits and total procedural blanks

The procedural blank (Table 2) includes the 0.37 mol l -1 HNO 3 blank in which samples were diluted and the sample blank prepared using the digestion method. Its intensity (cps) was subtracted from the data for the direct method and is given here as concentration (ng g -1 ). The total co-precipitation blank combines the procedural blank and blanks measured for NaOH and NH 3 solutions, and was therefore subtracted from all samples processed through the preconcentration method (see Table 3 for details).

In several cases the NaOH has to be purified (e.g., [START_REF] Qi | Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 co-precipitation[END_REF]. A NaOH purification was tested adding a little amount of an ultrapure Mg solution to NaOH, in order to co-precipitate trace elements together with Mg(OH) 2 . This led to the purification of some trace elements by a factor of 20 to 50% but also increased Y, LREE, MREE and Hf contents in the NaOH solution. Furthermore the total co-precipitation blank, which included blanks of unpurified NaOH and NH 3 , was about one to two times the procedural blank only, especially in Y, Nb, La, Ce, Pr, Tb, Ho and Th. This highlights that the NaOH and NH 3 blank contributions were low enough to hardly affect the results, and that these reagents could be used without any purification.

The total blank contribution was, with few exceptions, within the 1% of the RMs trace element mass fraction. The exceptions concern elements in very low mass fraction including Nb and Ta. The blank contribution on Nb signal intensity was about 4.7% and 4,4 % for UB-N and MGL-GAS and 6.7 and 7.9% for DTS-2B and JP-1 respectively. Concerning Ta, the blank contribution was low for UB-N (2.7%) and reached 14.4% in JP-1. Total blanks in REE were below 1% of the UB-N and MGL-GAS content, below 4% and 5% in JP-1 and DTS-2B respectively. Exceptions were the blank contributions on Eu in JP-1 (5.5%) and on MREE in DTS-2B (Eu: 10.7%, Gd: 4.3%, Tb: 8%).

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Direct method versus co-precipitation method

In their paper [START_REF] Qi | Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 co-precipitation[END_REF] show that the Mg-Fe double-co-precipitation allows the determination of REE and Y mass fractions in ultramafic rock samples with low measurement uncertainties. To investigate which additional trace elements may be determined using this method, we focussed on UB-N, a widely studied rock reference material (serpentinite) for which the literature offers mass fractions for a large set of elements.

Figure 1 shows our averaged results obtained on five replicates of UB-N following both the direct method and our adapted co-precipitation procedure. In regard to working values proposed by previous authors, we can see that REE and Y have co-precipitated as expected while mobile elements (Li, Rb, Sr, Cs, Ba) remained in the supernatant. The pre-concentration method also co-precipitated a much larger set of elements -Sc, Cr, Mn, Co, Zr, Ta, Pb, Th and U -for which determined concentrations are very similar to (1) the results following the direct method and (2) the recommended or proposed values. Some differences may further be noticed between both methods. Ti, V, Nb and Hf are systematically lower following the coprecipitation method but are nearly similar to compiled literature data, leading us to conclude that the direct method overestimates the mass fraction of these four elements. On the contrary, Ni, Cu, Zn and Ga, display lower values in the pre-concentration batch, therefore appearing to be not fully co-precipitated. The mass fraction calculated for the five co-precipitated replicates shows a huge variability for these elements, which implies that the bias is not systematic. Following these observations the co-precipitation procedure appears to be an efficient method to catch a larger set of trace elements than previously investigated [START_REF] Qi | Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 co-precipitation[END_REF][START_REF] Bayon | Determination of rare earth elements, Sc, Y, Zr, Ba, Hf and Th in geological samples by ICP-MS after Tm addition and alkaline fusion[END_REF]) including REE and Y, HFSE, U, Th, Pb and some of the transition elements (Sc, Ti, V, Cr, Mn, Co). On the contrary, LILE and other transition elements (Ni, Cu, Zn) and Ga seem to be well determinable only following the simpler direct digestion/dissolution method.

Measurement precision

The mass fraction of each element in the four investigated RMs is given in Table 3 following the method that displays the best accuracy. The associated standard deviation s ( g g -1 ) and relative standard deviation (RSD in %) are given as reflecting the intermediate precision of the averaged composition. Note that our results obtained for the RM DTS-2B are compared with both DTS-2 and DTS-2B due to the lack of data concerning DTS-2B excepting the large set proposed by Robin-Popieul et al. (2012).

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The REE mass fractions are determined following the co-precipitation procedure and display an intermediate precision expressed as RSD lower than 5% for UB-N and lower than 10% for MGL-GAS (GeoPT12), JP-1 and DTS-2B (excepting Gd in MGL-GAS ~ 11%) (Table 3). In comparison the direct method allows to a slightly lower intermediate precision on HREE (10 < RSD < 15% for MGL-GAS (GeoPT12) and DTS-2B) and the precision is particularly worse for MREE (Sm, Eu and Gd) with RSDs between 10 and 35%. Measured mass fractions are in good agreement with previously proposed values and REE patterns are particularly smoothed (Figure 2). JP-1 and MGL-GAS exhibit a well-defined negative Eu anomaly in agreement with [START_REF] Bayon | Determination of rare earth elements, Sc, Y, Zr, Ba, Hf and Th in geological samples by ICP-MS after Tm addition and alkaline fusion[END_REF] and [START_REF] Qi | Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 co-precipitation[END_REF] respectively. On the contrary, the direct method leads to an overestimation of MREE and especially of Eu that displays a strong positive anomaly in patterns for the most depleted ultramafic RMs JP-1 and DTS-2B. This may be attributed to an under-estimation of the Ba oxide interferences during the direct method procedure leading to an overestimation of the Eu mass fraction. This is supported by the fact that the most affected reference materials are those with the highest barium mass fraction relative to middle rare earth elements (i.e., Ba/Sm = 1000-4000 in JP-1 and DTS-2B, less than 300 in UB-N and MGL-GAS). On the contrary, Ba is lost during the co-precipitation procedure and therefore barium oxide interferences are drastically lowered. The preconcentrated samples expose a Eu mass fraction similar to working values; accordingly, the co-precipitation method is well adapted for the determination of REE, and especially of MREE, in ultra depleted ultramafic rocks.

Other (ultra-)trace elements (Y, Zr, Nb, Hf, Ta, Pb, Th and U) were measured using the two procedures. The intermediate precision after the direct method is systematically worse than after the pre-concentration process and is not specifically discussed here. Following the coprecipitation procedure the intermediate precision for Y, Pb, Th and U is, with few exceptions, better than 10%, being slightly higher for the most depleted RM DTS-2B (U: 12.3%) (Table 3). Measured mass fractions are in good agreement with published results on UB-N, MGL-GAS and JP-1. For DTS-2B, Y, Pb and U mass fractions are in good agreement with the values proposed by Robin-Popieul et al. (2012) while the Th content is slightly higher (Table 3). 3). Results for Zr and Hf in JP-1 show a good

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This article is protected by copyright. All rights reserved. precision (< 3%) while Nb (14.2%) and Ta (28.8%) show a much lower intermediate precision, which we attribute to their very low contents (30 and 3 ng g -1 respectively). Results for Hf, Nb and Ta are in good agreement with working values and compiled values from GeoReM (http://georem.mpch-mainz.gwdg.de) while determined concentration for Zr is slightly higher (Table 3). The intermediate precision of Zr and Nb in DTS-2B remains below 20% but reaches 38.3% for Hf, and is therefore indicated as an information value only (Table 3). The values we present are higher in each Zr, Nb and Hf than previously proposed (Robin-Popieul et al. 2012), but they are calculated from ten replicate analyses, on the contrary to those proposed by Robin-Popieul et al. (2012). Similarly to JP-1, measurements on MGL-GAS display a good precision for Zr and Hf (< 6%), being much variable for Nb (17%). The result for Hf is in good agreement with the value proposed by [START_REF] Potts | GeoPT12 -An international proficiency test for analytical geochemistry laboratories -Report on round 12[END_REF]. Because they were never proposed we present new values for Zr and Nb in MGL-GAS but not for Ta, owing to its poor precision (Table 3). Scandium, Ti, V, Cr, Mn and Co are fully co-precipitated, as shown in Figure 1. Results obtained for Sc, Ti and V are similar between direct and pre-concentration methods. The intermediate precision is better after co-precipitation in particular for Ti. RSD values for these elements were systematically lower than 10.9%. On the contrary, the intermediate precision for Cr, Mn and Co is better following the direct method, especially for UB-N and DTS-2B (< 8.3%). Mass fractions are generally in good agreement with previous studies. Titanium and V contents are slightly higher for MGL-GAS and JP-1 than values proposed by [START_REF] Potts | GeoPT12 -An international proficiency test for analytical geochemistry laboratories -Report on round 12[END_REF] (MGL-GAS) and [START_REF] Makishima | Determination of major, minor and trace elements in silicate samples by ICP-QMS and ICP-SFMS applying isotope dilution-internal standardisation (ID-IS) and multi-stage internal standardisation[END_REF] and [START_REF] Barrat | Geochemistry of diogenites: Still more diversity in their parental melts[END_REF] (JP-1) but the values for UB-N and DTS-2B are very similar to compiled values (Figure 3). Other transition elements (Ni, Cu, Zn) and Ga were not fully co-precipitated through the preconcentration procedure (Figure 1) and were only accurately determined by the direct method.

With the exception of Zn in UB-N (10.6%), the RSD values for Ni, Cu, Zn, Ga for UB-N and MGL-GAS were systematically lower than 7.5%. For JP-1 and DTS-2B, the RSD values of Ni, Cu, Zn, Ga were slightly higher, but, with the exception of Zn in both materials and Cu in JP-1, they remain below 9%. Despite the variability, our results are close to the published values (Figure 3 and Table 3).

Lithium and LILE (Rb, Sr, Cs, Ba) were totally discarded during the pre-concentration procedure and thus were determined following the direct method. The intermediate precision is lower than 6% for UB-N and MGL-GAS (excepting Cs in UB-N with a RSD = 9.5%) and

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This article is protected by copyright. All rights reserved. lower than 9% for JP-1. The precision was more variable for DTS-2B. RSD values were lower than 5.2% for Sr and Ba, but worse for Li (11.3%), Rb (13.2%) and particularly for Cs (21.8%). In the case of Cs this can be linked to the high blank contribution in regard to its very low mass fraction in DTS-2B (1.8 ng g -1 ).

Conclusions

To investigate trace element contents in natural peridotite samples, the simple direct digestion/dilution method enables the measurement of a large set of elements and to determine trace element mass fractions as low as 0.1 g g -1 . A pre-concentration method is however needed to determine with a good intermediate precision elements in lower mass fraction. Pre-concentration methods using co-precipitation are efficient to separate REE from major elements, avoiding matrix effect and displaying a intermediate precision generally better than 10% (e.g., [START_REF] Qi | Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 co-precipitation[END_REF][START_REF] Bayon | Determination of rare earth elements, Sc, Y, Zr, Ba, Hf and Th in geological samples by ICP-MS after Tm addition and alkaline fusion[END_REF]. Adapting the double Mg(OH) 2 and Fe(OH) 3 co-precipitation procedure from [START_REF] Qi | Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 co-precipitation[END_REF], we show that other trace elements such as HFSE and transition elements can be determined following this method. Combined with the Tm addition of [START_REF] Barrat | Determination of rare earth elements in sixteen silicate reference samples by ICP-MS after Tm addition and ion exchange separation[END_REF], we show reproducible results for UB-N, JP-1, MGL-GAS and DTS-2B, similar to proposed values in the literature, and with good precision.

Concerning DTS-2B for which trace element compositions were rarely proposed due to their very low content, we propose HFSE values that slightly differ with the ones published by Robin-Popieul et al. (2012). Due to the good intermediate precision obtained on other ultra depleted elements such as MREE, we propose to better refer about our new proposed values.

We also provide new HFSE data for MGL-GAS. Considering the extremely depleted character of olivine and the difficulty in reaching mass fractions as low as 0.5-1 ng g -1 , we envision that this procedure can be applied to determine trace element contents in separated olivine or olivine-rich peridotite lithologies and to facilitate pre-concentration in the perspective of isotope studies. (2012). For ease of readability, the normalised compositions of Ni on the one hand and of Li, Cs and Ba in the other are divided by 1000 and 10 respectively. The grey shaded area highlights the elements that were efficiently co-precipitated following our procedure adapted from [START_REF] Qi | Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 co-precipitation[END_REF].

Figure captions

Figure 2. Chondrite-normalised [START_REF] Barrat | Geochemistry of CI chondrites: Major and trace elements, and Cu and Zn isotopes[END_REF]) REE patterns obtained for UB-N, MGL-GAS, JP-1 and DTS-2B following both the direct and co-precipitation procedures. Results are

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compared with literature values from Govindaraju (1995), [START_REF] Raczek | High-precision trace element data for the USGS reference materials BCR-1, BCR-2, BHVO-1, BHVO-2, AGV-1, AGV-2, DTS-1, DTS-2, GSP-1 and GSP-2 by ID-TIMS and MIC-SSMS[END_REF]), Carignan et al. (2001), [START_REF] Potts | GeoPT12 -An international proficiency test for analytical geochemistry laboratories -Report on round 12[END_REF], [START_REF] Qi | Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 co-precipitation[END_REF], [START_REF] Makishima | Determination of major, minor and trace elements in silicate samples by ICP-QMS and ICP-SFMS applying isotope dilution-internal standardisation (ID-IS) and multi-stage internal standardisation[END_REF], Nakamura andChang (2007), Godard et al. (2008), [START_REF] Barrat | Geochemistry of diogenites: Still more diversity in their parental melts[END_REF], [START_REF] Bayon | Determination of rare earth elements, Sc, Y, Zr, Ba, Hf and Th in geological samples by ICP-MS after Tm addition and alkaline fusion[END_REF]), Chauvel et al. (2011), Ulrich et al. (2012), and Sun et al. (2013).

Figure 3. Chondrite-normalised [START_REF] Barrat | Geochemistry of CI chondrites: Major and trace elements, and Cu and Zn isotopes[END_REF] trace element patterns obtained for UB-N, MGL-GAS, JP-1 and DTS-2B compared with previous work. Mass fractions were compiled following the direct method (regular font) and the co-precipitation method (bold) (Table 3).

Results are compared with literature values from Govindaraju (1995) together with the SARM certificate for UB-N, [START_REF] Raczek | High-precision trace element data for the USGS reference materials BCR-1, BCR-2, BHVO-1, BHVO-2, AGV-1, AGV-2, DTS-1, DTS-2, GSP-1 and GSP-2 by ID-TIMS and MIC-SSMS[END_REF]), Carignan et al. (2001), [START_REF] Potts | GeoPT12 -An international proficiency test for analytical geochemistry laboratories -Report on round 12[END_REF], [START_REF] Qi | Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 co-precipitation[END_REF], [START_REF] Makishima | Determination of major, minor and trace elements in silicate samples by ICP-QMS and ICP-SFMS applying isotope dilution-internal standardisation (ID-IS) and multi-stage internal standardisation[END_REF], Nakamura andChang (2007), Godard et al. (2008), [START_REF] Barrat | Geochemistry of diogenites: Still more diversity in their parental melts[END_REF], [START_REF] Bayon | Determination of rare earth elements, Sc, Y, Zr, Ba, Hf and Th in geological samples by ICP-MS after Tm addition and alkaline fusion[END_REF]), Chauvel et al. (2011), Gao and Casey (2012)

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  . The measured Tm signal comprises both the signal brought by the spike and the one due to the natural Tm mass fraction in the sample. The sample [Tm*] is calculated using the sample solution [Er] and [Yb] mass fractions, following: gives the mass fraction of each element X ([X] sample ) in the sample, knowing the mass fraction X and Tm in the sample solution ([X] sample solution and [Tm] sample solution ) calculated from (1), [Tm*] calculated from (2), and M sample , M Tm spike and [Tm] Tm spike : [X] sample = [X] sample solution × MTm spike × [Tm]spike Msample × ( Tm sample solution -Tm* ) (3)

  With regard to other HFSE (Zr, Nb, Hf, Ta), measured mass fractions are close to available published data for UB-N. The measurement results for Zr and Nb in UB-N display a intermediate precision slightly higher (< 12%) than recent precise results from Godard et al. (2008) and Chauvel et al. (2011) (Table

Figure 1 .

 1 Figure 1. Chondrite-normalised (Barrat et al. 2012) minor and trace elements pattern for an average of five replicates of UB-N. To focus on the selectivity of the elements co-precipitated during the pre-concentration procedure, no distinction is made between the patterns from the literature. A more detailed comparison is shown in Figure 3. Compositions are taken from Govindaraju (1995), Carignan et al. (2001), Godard et al. (2008), Bayon et al. (2009), Chauvel et al. (2011) and Robin-Popieul et al. (2012). Chondrite values are from Barrat et al.

  , Ulrich et al. (2012), Zeng et al. (2012), Sun et al. (2013) and from the USGS certificate for DTS-2B.

Table 1 . Instrum ental operating conditions

 1 Medium resolution mode (M 45 Sc, 47 Ti, 51 V, 52 Cr, 55 Mn,59 Co, 60 Ni, 63 Cu, 66 Zn,69,71 Ga Li, 85 Rb, 88 Sr, 89 Y, 90 Zr, 93 Nb, 133 Cs, 135 Ba, 139 La, 140 Ce, 141 Pr, 143,146 Nd, 147,149 Sm, 151 Eu, 157 Gd, 159 Tb, 163 Dy, 165 Ho, 167 Er,

	Instrument	HR-ICP-MS ELEMENT XR
	Plasma RF pow er	1200 W
	Torch	Quartz glass torch w ith 2.2 mm quartz injector
	Plasma Ar gas flow rate 15 l min -1
	Auxiliary Ar gas flow rate 1 l min -1
	Nebuliser Ar gas flow rate1.17 l min -1
	Nebuliser type	Quartz MicroMist ™ 100 μl
	Spray chamber	Dual cyclone-Scott quartz glass spray chamber
	Sampler/skimmer cones Nickel (H)
	Low resolution (LR)
	Acquisition mode	E-scan
	Detection mode	Triple
	Run	6 (LR) -6 (MR)
	Oxide BaO/Ba	0.4%
	Doubly charged Ba 2+ /Ba 3.2%
	Sample time	10 ms per mass (LR) -40 ms per mass (MR)
	Sample per peak	30 (LR) -20 (MR)
	Sample uptake rate	100 μl min -1
	Total data acquisition time 148 s
	Wash time	120 s (2% v/v HNO 3 )
	Internal standard element 169 Tm

7

Table 2 . Instrum ental detection lim its, analytical and total procedural blanks

 2

		pg g -1 Instrum ental detection lim it	ng g -1 Procedural blank	ng g -1 Total co-precipitation
	Li	nd	20.6	48.6
	Sc	nd	1.05	1.24
	Ti	0.969	630	642
	V	0.278	19.6	20.2
	Cr	1.56	1249	1252
	Mn	2.90	76.0	82.0
	Co	0.113	17.5	18.0
	Ni	22.2	199	214
	Cu	2.02	17.7	18.7
	Zn	74.5	281	301
	Ga	0.216	1.06	1.11
	Rb	0.433	3.89	6.28
	Sr	5.32	4.28	7.41
	Y	nd	0.093	0.171
	Zr	1.38	4.61	5.38
	Nb	1.71	1.83	2.37
	Cs	0.100	1.13	1.28
	B a	2 0 . 1	7 . 8 4	8 . 3 5
	La	0.121	0.115	0.307
	Ce	0.151	0.289	0.606
	Pr	0.040	0.022	0.070
	Nd	0.084	0.077	0.100
	Sm	0.078	0.061	0.065
	Eu	0.065	0.081	0.090
	Gd	0.259	0.123	0.134
	Tb	nd	0.018	0.047
	Dy	0.079	0.041	0.045
	Ho	0.025	0.008	0.018
	Er	0.033	0.070	0.073
	Yb	0.051	0.022	0.029
	Lu	0.013	0.014	0.021
	Hf	1.21	0.225	0.240
	Ta	0.701	0.313	0.413
	Pb	0.245	7.05	7.97
	Th	0.247	0.035	0.073
	U	0.036	0.064	0.076

Table 3 . Trace element mass fractions (μg g -1 ) for UB-N, JP-1, GAS (GeoPT 12) and DTS-2B BHVO-2

 3

							UB-N
		Barrat et al.	This study		Govindaraju	Carignan et	Godard et	Bayon et al.	Chauvel et	Robin-
	μg g -1	(2012)	Mean (n = 5	s	% RSD	(1995)	al. (2001)	al. (2008)	(2009)	al. (2011)	Popieul et
	Li	4.7	28.4	0.893	3.1	27		26.2		28.4	31.6
	Sc	32.3	11.8	0.523	4.4	13		10.7	14.7	12.0	13.7
	Ti	16364	706	77.0	10.9			677		542	606
	V	317	75.8	7.80	10.3	75	67			59.4	67.2
	Cr	280	2107	154	7.3	2300				2090	2280
	Mn	1290	902	48.0	5.3
	Co	45	95.9	4.00	4.2	100	102	106		95.8	102
	Ni	121	1880	122	6.5	2000	1901			1740	1930
	Cu	123	22.0	1.33	6.1	28	26.5	27		21.8	25.1
	Zn	101	73.8	7.80	10.6	85	88			79.4	90.3
	Ga	20.6	2.35	0.172	7.3	3	2 . 9 4
	Rb	9.08	3.30	0.102	3.1	4	3.5	3.49		2.54	2.83
	Sr	396	7.61	0.201	2.6	9	8	8.48		7.47	8.01
	Y	27.6	2.66	0.122	4.6	2.5	2.56	2.57	2.76	2.5	2.69
	Zr	164.9	3.58	0.351	9.8	4	4.7	4.06	4.7	3.65	3.96
	Nb	16.82	0.0509	0.00522 10.3	0.05	0.087	0.0676		0.0463	0.0516
	Cs	0.096	10.5	0.995	9.5	10	12	11.3		11.3	11.9
	Ba	131	26.3	1.10	4.2	27	24	26.4	26	26.4	28.4
	La	15.2	0.307	0.0106	3.5	0.35	0.351	0.321	0.29	0.315	0.34
	Ce	37.5	0.778	0.0343	4.4	0.8	0.86	0.814	0.77	0.788	0.846
	Pr	5.31	0.112	0.00517	4.6	0.12	0.12	0.119	0.118	0.117	0.126
	Nd	24.5	0.582	0.0192	3.3	0.6	0.62	0.629	0.613	0.603	0.636
	Sm	6.07	0.208	0.00621	3	0.2	0.22	0.223	0.222	0.215	0.234
	Eu	2.07	0.0792	0.00247	3.1	0.08	0.083	0.0843	0.087	0.081	0.0863
	Gd	6.24	0.302	0.00572	1.9	0.3	0.309	0.338	0.32	0.319	0.345
	Tb	0.94	0.0571	0.000506 0.9	0.06	0.055	0.0628	0.063	0.0594	0.0635
	Dy	5.31	0.416	0.00653	1.6	0.38	0.38	0.462	0.434	0.423	0.455
	Ho	1	0 . 0 9 4 8	0.00177	1.9	0.09	0.088	0.103	0.099	0.0941	0.1
	Er	2.54	0.288	0.00549	1.9	0.28	0.25	0.306	0.299	0.288	0.308
	Yb	2	0 . 2 9 1	0.00592	2	0.28	0.282	0.305	0.299	0.293	0.317
	Lu	0.27	0.0450	0.00142	3.2	0.045	0.046	0.052	0.047	0.0455	0.0491
	Hf	4.474	0.145	0.0173	11.9	0.1	0.141	0.139	0.15	0.13	0.141
	Ta	1.1	0.0151	0.000796 5.3	0.02	0.02	0.019		0.0132	0.0137
	Pb	1.51	12.8	0.859	6.7	13	12.9	13.4			15.3
	Th	1.21	0.0724	0.00792 10.9	0.07	0.09	0.074	0.063		0.0799
	U	0.41	0.0576	0.00349	6.1	0.07	0.06	0.0578			0.0638

Qi et al. (2005) JP-1 Makishima and Barrat et al. (2008) Bayon et al. (2009) This study

	Mean (n = 5	s	% RSD
	μg g -1
	Li	1.68	0.138	8.2		1.75
	Sc	7.08	0.206	2.9		7.34	7.25	9
	Ti	47.8	2.90	6.1			19.9
	V	40.0	4.25	10.6		24.9	21.65
	Cr	2959	212	7.2
	Mn	919	52.7	5.7			860
	Co	113	6.71	5.9		121	104
	Ni	2443	138	5.7		2501	2211
	Cu	3.84	0.518	13.5		4.25	3.63
	Zn	45.1	4.57	10.1		46.7	37.66
	Ga	0.518	0.0444	8.6		0.467	0.47
	Rb	0.348	0.0309	8.9		0.288
	Sr	0.675	0.0605	9		0.546	0.527
	Y	0.100	0.00280	2.8	0.0882	0.0823	0.1	0.098
	Zr	8.83	0.242	2.7			5.39	5.7
	Nb	0.0298	0.00424 14.2			0.04
	Cs	0.0370	0.00216	5.8		0.036
	Ba	8.98	0.502	5.6		9.5	10.04	8.8
	La	0.0264	0.00144	5.5	0.0308	0.0268	0.0271	0.0278
	Ce	0.0576	0.00201	3.5	0.0536	0.0597	0.0597	0.062
	Pr	0.00719	0.000244 3.4	0.00765	0.0074	0.00716	0.0081
	Nd	0.0296	0.00164	5.5	0.0297	0.0318	0.0298	0.031
	Sm	0.00739	0.000471 6.4	0.00769	0.0084	0.00726	0.0079
	Eu	0.00162	0.000134 8.2	0.00109	0.0024	0.00385	0.0016
	Gd	0.00827	0.000948 11.5	0.00644	0.0097	0.0085	0.0101
	Tb	0.00164	0.000105 6.4	0.00174	0.0019	0.00166	0.00195
	Dy	0.0132	0.000737 5.6	0.013	0.0146	0.0135	0.0135
	Ho	0.00329	0.000209 6.4	0.00312	0.0036	0.00316	0.0034
	Er	0.0115	0.000502 4.4	0.0111	0.0123	0.0116	0.012
	Yb	0.0198	0.000559 2.8	0.0188	0.0209	0.0194	0.0206
	Lu	0.00371	0.000144 3.9	0.00338	0.0042	0.00352	0.004
	Hf	0.193	0.00581	3			0.113	0.112
	Ta	0.00345	0.000991 28.8
	Pb	0.0987	0.00633	6.4		0.0765
	Th	0.0108	0.000869	8		0.013	0.0122	0.009
	U	0.0106	0.000386 3.6		0.0123

This study Raczek et al . (2001) Robin- Popieul et eng et al . (201

		©
		MGL-GAS [GeoPT 12]
	Accepted Article	Mean (n = 5 μg g -1 Li 2.25 Sc 6.78 Ti 134 V 53.4 Cr 2880 Mn 647 Co 106 Ni 2398 Cu 6.84 Zn 39.0 Ga 0.869 Rb 0.253 Sr 7.00 Y 0.384 Zr 0.987 Nb 0.0535 Cs 0.0273 Ba 7.29 La 0.133 Ce 0.254 Pr 0.0309 Nd 0.126 Sm 0.0307 Eu 0.00664 Gd 0.0393 Tb 0.00723 Dy 0.0513 Ho 0.0127 Er 0.0403 Yb 0.0521 Lu 0.00820 Hf 0.0215 Ta Pb 2.54 Th 0.028 U 0.785 This study s 0.126 0.119 2.79 2.29 147 22.9 4.23 88.3 0.506 1.83 0.0607 0.00949 0.298 0.00561 0.0281 0.00910 0.00148 0.320 0.0117 0.0238 0.00215 0.00827 0.00168 0.000343 5.2 % RSD 5.6 1.8 2.1 4.3 5.1 3.5 4 3.7 7.4 4.7 7 3.8 4.3 1.5 2.8 17 5.4 4.4 8.9 9.4 7 6.6 5.5 0.00165 4.2 0.000248 3.4 0.00256 5 0.000265 2.1 0.00136 3.4 0.00171 3.3 0.000263 3.2 0.00127 5.9 0.132 5.2 0.00170 6.1 0.0178 2.3 Li 1.67 0.188 11.3 Sc 2.99 0.385 12.9 2.72 Ti 55.8 4.37 7.8 V 25.0 1.75 7 17.6 Cr 16659 1376 8.3 13004 Mn 752 61.0 8.1 Co 124 9.17 7.4 115 Ni 3480 298 8.6 3923 Cu 2.29 0.185 8.1 2.91 Zn 44.3 4.78 10.8 40 Ga 0.94 0.0597 6.4 Rb 0.123 0.0162 13.2 1.53 Sr 0.538 0.0277 5.1 Y 0.0384 0.00301 7.8 Zr 0.471 0.0846 18 Nb 0.0353 0.00671 19 Cs 0.00180 0.000393 21.8 Ba 11.0 0.502 4.6 La 0.0130 0.00104 8 Ce 0.0264 0.00219 8.3 Pr 0.00316 0.000264 8.4 Nd 0.0131 0.00118 9 Sm 0.00281 0.000160 5.7 Eu 0.000843 0.0000486 5.8 Gd 0.00309 0.000166 5.4 Tb 0.000596 0.0000213 3.6 Dy 0.00421 0.000165 3.9 Ho 0.00121 0.0000797 6.6 Er 0.00494 0.000425 8.6 Yb 0.0101 0.000708 7 Lu 0.00199 0.000112 5.6 Hf 0.00839 0.00322 38.3 Ta Pb 3.53 0.354 10 Th 0.00412 0.000451 11 U 0.00192 0.000236 12.3 DTS-2B Mean (n = 10 s % RSD μg g -1	2.28 6.77 32.91 2788 106.4 2240 38.84 7.65 0.4 0.03 9.22 0.15 0.279 0.0308 0.136 0.037 0.0095 0.042 0.01 0.057 0.012 0.0421 0.05 0.0092 0.02 0.03 0.464 Potts et al. (2003) 1.91 3.04 46.7 21.5 14300 130 3240 3.33 50.2 0.0379 0.538 0.0353 0.15 0.019 0.000948 11.2 0.0112 0.0237 0.00314 0.0153 0.00253 0.00113 0.00472 0.00112 0.00547 0.00115 0.0046 0.00856 0.00201 0.00555 0.00107 4.04 0.00287 0.0017	0.403 0.157 0.291 0.0326 0.134 0.0322 0.00638 0.0286 0.00692 0.0539 0.0127 0.0392 0.0495 0.00876 Qi et al. (2005) 31 8046 128 4152 47 Certificate 3 22 15500 830 120 3780 3 45 2 16 4 Bologne and USGS Duchesne	0.389 0.145 0.275 0.0305 0.13 0.0313 0.0106 0.0441 0.00831 0.0513 0.0127 0.0394 0.0502 0.00926 0.0318 Sun et al. (2013) 0.0123 0.534 11.9 0.0127 0.0254 0.0032 0.0131 0.00302 0.00087 0.00304 0.00419 0.00465 0.00963 0.002 DTS-2 0.0132 0.0263 0.00326 0.0136 0.00332 0.00088 0.0038 0.00063 0.00469 0.0013 0.00553 0.0107 0.00237 Nakam ura and Chang	0.0124 0.0252 0.003 0.0132 0.0027 0.0008 0.0037 0.0006 0.0044 0.0012 0.0051 0.01 0.0021 Ulrich et al. (2009)	1.81 Gao and Casey (2012)

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Acknowledgements

Technical assistance in the cleanroom laboratory provided by M. Henry and J. Prunier was greatly appreciated. Financial support for this study was provided by the Centre National de la Recherche Scientifique under the programme "Syster". We are grateful to the two anonymous reviewers for their constructive comments that significantly improved this paper. This work

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Bizzarro M., Baker J.A. and Ulfbeck D. (2003)

A new digestion and chemical separation technique for rapid and highly reproducible determination of Lu/Hf and Hf isotope ratios in geological materials by MC-ICP-MS.

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This article is protected by copyright. All rights reserved. also greatly benefited from the editorial handling provided by T. Meisel and, together with Ed Williams, from their numerous comments and advice.