Chemical Geology 267 (2009) 175–184

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Chemical Geology

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Routine isotopic analysis of by HR-MC-ICPMS: How precise and how accurate?

Nicolas Dauphas a,b,⁎, Ali Pourmand a,b, Fang-Zhen Teng a,b,1 a Origins Laboratory, Department of the Geophysical Sciences, the University of Chicago, 5734 South Ellis Avenue, Chicago IL 60637, USA b Enrico Fermi Institute, the University of Chicago, 5640 South Ellis Avenue, Chicago IL 60637, USA article info abstract

Article history: High-temperature isotopic variations documented in natural materials span a narrow range and call for Received 14 June 2008 precise and accurate isotopic analyses. Precisions better than ~0.03‰ (95% confidence interval) for δ56Fe Received in revised form 16 November 2008 have been obtained by High-Resolution Multi-Collector Inductively Coupled Plasma Mass Spectrometry (HR- Accepted 5 December 2008 MC-ICPMS). Whether these uncertainties encompass all sources of error has not been carefully evaluated. The current study examines all parameters that can affect accuracy and precision of Fe isotopic Keywords: measurements by HR-MC-ICPMS. It is demonstrated that accurate δ56Fe measurements can be routinely Iron isotopes HR-MC-ICPMS achieved within the quoted uncertainties. Chromatography © 2008 Elsevier B.V. All rights reserved. Precision Accuracy

1. Introduction have variable compositions averaging ~0‰ (Williams et al., 2005; Schoenberg and von Blanckenburg, 2006; Weyer and Ionov, 2007). The field of Fe isotope geochemistry has seen important develop- Although increasing the number of repeats (n)inHighResolution ments in methodology and scope since the advent of Multi-Collector (HR-) MC-ICPMS reduces the uncertainty relatedp toffiffiffi instability in Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) (see recent instrumental mass bias and counting statistics (~1 = n), many other reviews by Anbar, 2004; Beard and Johnson, 2004; Johnson et al., 2004; parameters can affect the precision and accuracy of Fe isotopic analyses Dauphas and Rouxel, 2006). There is now increasing evidence that Fe (Malinovsky et al., 2003; Schoenberg and von Blanckenburg, 2005). isotopic variations are not confined to the realm of biology or low These include mass fractionation during column chromatography, temperature aqueous geochemistry but can also occur at high incomplete separation of Fe from matrix elements that can cause temperature by equilibrium fractionation between minerals and melts instrumental mass bias to vary, presence of direct isobars from 54Cr+ on (Polyakov and Mineev, 2000; Polyakov et al., 2007; Dauphas et al., 2007; 54Fe+ and 58Ni+ on 58Fe+ and imperfect matching in acid molarity or Fe Schuessler et al., 2007; Shahar et al., 2008), by diffusion in solid concentration between samples and bracketing standards. (Mullen, 1961; Roskosz et al., 2006; Dauphas, 2007 and references A major challenge in making precise and accurate measurements therein) and silicate melts (Richter et al., 2009), by evaporation/ of Fe isotopes arises from incomplete mass separation between Fe and condensation (Wang et al., 1994; Alexander and Hewins, 2004; Dauphas argides, which include interferences of 40Ar14N+, 40Ar16O+, and et al., 2004; Tachibana et al., 2007; Zipfel and Weyer, 2007), and by Soret 40Ar16O1H+ on 54Fe+, 56Fe+, and 57Fe+, respectively. Several strategies diffusion (Huang et al., in press; Richter et al., 2009). Considering that the have been employed to address this analytical difficulty and are briefly Fe isotopic compositions of most geomaterials formed at high tempera- discussed below. (i) The first method employed for isotopic analysis of ture span a narrow range, important unsettled questions require high Fe by MC-ICPMS relies on desolvating nebulization to increase the precision and accurate analyses. For instance, the Fe isotopic composition ratio of Fe/argide (Belshaw et al., 2000; Anbar et al., 2000; Zhu et al., of the silicate Earth is still uncertain. While basalts have almost 2002, de Jong et al., 2007). Nevertheless, this brute-force method is homogeneous δ56Fe around +0.1‰ (Beard et al., 2003; Schoenberg and incapable of eliminating all interferences and careful matching von Blanckenburg, 2006; Weyer and Ionov, 2007), mantle peridotites between standard and sample ion intensities during bracketing is essential. (ii) The GV IsoProble MC-ICPMS is equipped with a hexapole collision cell, which is primarily used to thermalize the incoming ions ⁎ Corresponding author. Origins Laboratory, Department of the Geophysical Sciences, before introduction into the magnetic sector. It also serves the purpose the University of Chicago, 5734 South Ellis Avenue, Chicago IL 60637, USA. of breaking down molecules such as argides through interaction with E-mail address: [email protected] (N. Dauphas). 1 Present address: Isotope Laboratory, Department of Geosciences and Arkansas the collision gas, a mixture of Ar and H2 for Fe isotopes (Beard et al., Center for Space and Planetary Sciences, University of Arkansas, 113 Ozark Hall, 2003; Rouxel et al., 2003; Dauphas et al., 2004). (iii) Operating the MC- Fayetteville, AR 72701, USA. ICPMS at reduced power (cold plasma) can also drastically reduce

0009-2541/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2008.12.011 176 N. Dauphas et al. / Chemical Geology 267 (2009) 175–184 argide interferences (Kehm et al., 2003). The main drawback of this be a problem for certain matrices, such as sulfides. To mitigate this technique, however, is that it promotes formation of CaOH+, which problem, Schoenberg and von Blanckenburg (2005) introduced an iron 57 + can interfere with Fe peak. (iv) Medium to high mass-resolving hydroxide precipitation step by basification with NH4(OH). power on the Thermo Scientific Neptune, Nu-Plasma HR, and Nu Plasma 1700 are used to separate argide interferences from Fe isotopes 3. Mass spectrometry (Malinovsky et al., 2003; Weyer and Schwieters, 2003; Arnold et al., 2004; Poitrasson and Freydier, 2005; Schoenberg and von Blancken- Iron isotopic analyses were performed with a Thermo Scientific burg, 2005; Williams et al., 2005; Borrok et al., 2007). Precisions of Neptune HR-MC-ICPMS (see Wieser and Schwieters, 2005 for a better than 0.04‰ are achievable with this method. description of the instrument). The mass-resolution of the instrument In the current study, we evaluate the variables that can affect the is controlled with a set of three switchable slits (250 μm, 30 μm, and accuracy of Fe isotope measurements using HR-MC-ICPMS. Error bars 16 μm width) located before the electrostatic analyzer. Measurements in Fe isotope geochemistry have been traditionally ascribed based on were performed in both medium-resolution (MR) and high-resolution either the long-term external reproducibility of geostandards or the (HR) modes. In these two modes, the Fe+ peaks are resolved from argide dispersion of replicate analyses of a solution during a session of interferences as flat-topped shoulders on the low mass side of argide measurements. In the following, we present a method, which is also peaks (Weyer and Schwieters, 2003). The mass resolution is defined as applicable to other isotope systems, to take into account both sources m/(m0.95 −m0.05), where m0.95 and m0.05 are the masses measured at of analytical uncertainties. We demonstrate that δ56Fe can be 5% and 95% intensity peak heights (see Weyer and Schwieters, 2003, measured accurately at a precision of ~0.03‰ in rocks and minerals. for details). Mass resolutions are equal to ~8500 and ~11,000 for MR and HR, respectively. The ion transmission efficiency of the instru- 2. Chemical separation of iron ment for Fe is reduced by a factor of ~8 from LR to MR and another factor i i 54 of ~2 from MR to HR. The δ notation (in ‰), δ Fe=[( Fe/ Fe)sample / i 54 3 All chemical protocols used for Fe isotopic analyses rely on the ( Fe/ Fe)IRMM-014 −1]×10 , is used throughout the current study. partition behavior of Fe with anion-exchange resin in HCl medium IRMM-014 is a pure Fe metal reference material (Taylor et al., 1992)that (Strelow, 1980). Matrix and interfering elements are eluted with has a Fe isotopic composition identical to chondrites (Dauphas and concentrated HCl while Fe, which forms strong anion chloride- Rouxel, 2006). Unless otherwise noted, the pair of isotopes reported is complexes, is retained on the resin. Iron is subsequently eluted at 56Fe/54Fe (i=56). In the course of this study, the stock solution of IRMM- lower HCl molarity. Protocols using AG MP-1 anion exchange resin can 014 used during sample-standard bracketing was renewed. The δ56Fe be found in Maréchal et al. (1999), Archer and Vance (2004), Brantley et and δ57Fe of the new batch of IRMM-014 normalized to the old batch are al. (2004), Borrok et al. (2007) and de Jong et al. (2007). Protocols using −0.004±0.009‰ and 0.001±0.015‰ (95% ci), respectively. This AG1-X3, -X4 or -X8 are described in Rouxel et al. (2003), Dauphas et al. confirms the conclusion of Dauphas et al. (2004) that IRMM-014 has (2004), Poitrasson and Freydier (2005), Schoenberg and von Blancken- homogeneous isotopic composition at the 0.01‰ level. burg (2005), Dideriksen et al. (2006),andThompson et al. (2007). Except for a series of tests aimed at estimating the influence of acid

Here we used the separation protocol described by Dauphas et al. molarity on isotopic ratios, all analyzed solutions were in 0.3 M HNO3. (2004, 2007).Typically2–10 mg of powdered sample is subjected to the An enclosed SC-2 sample changer installation (autosampler) sequence of acid addition and heating and evaporations presented in Table equipped with an ULPA filter unit was used throughout this study. 1.Thepurification procedure of column chromatography is repeated twice Most analyses were performed with the Thermo Scientific Stable to ensure complete elimination of the matrix. After elution of Fe from the Introduction System (SIS), which combines quartz cyclonic and Scott- second column, the solution is dried down under a heat lamp, taken up in type spray chambers. A few tests were also performed with the Apex-

~100 μLof15.3MHNO3 and evaporated to dryness to eliminate chloride Q+Spiro TMD desolvating inlet system from Elemental Scientific Inc. complexes. The dried Fe residue is finally dissolved in 10 mL of 0.3 M HNO3 operated with addition of N2 (~10 mL/min). Introduction of N2 (an ultrasonic bath is often used at this step to ascertain complete increases the sensitivity of the Apex+Spiro inlet system but it also 54 + 40 14 + dissolution). The 0.3 M HNO3 comes from a large batch that also serves for increases the level of interferences on Fe from Ar N . The preparation of IRMM-014 and blanks used in standard bracketing and on temperatures of cooler (multipass condenser) and heater (cyclonic peak zero correction. This ensures perfect matching in acid molarity spray chamber) were set at 2 °C and 100 °C, respectively. A 50–100 μL/ between all analyzed solutions, which is critical for accurate isotopic min PFA Teflon self-aspirating micronebulizer was used with both SIS analyses. At this point, Fe is ready for isotopic measurement by MC-ICPMS. and Apex-Q. The virtue of the Apex-Q+Spiro relative to SIS is that it It must be noted that the chromatographic procedure described herein drastically reduces oxides and increases Fe sensitivity. However, does not allow for quantitative separation of Cu and Zn from Fe, which can instrumental mass fractionation is less stable and achieving optimal tuning conditions is more daunting with the Apex-Q+Spiro than with Table 1 the SIS. Therefore, using the SIS is recommended unless low quantities Sample dissolution and Fe column chemistry protocols. of Fe are analyzed. The sampler and skimmer cones are made of Ni, yet the contribution of 58Ni from the cones on 58Fe is negligible (Dauphas • Addition of 1 mL of 28.8 M HF+0.5 mL 15.3 M HNO3 +1 drop 11.3 M HClO4 • Heating in close beakers at 100 °C for 5 h et al., 2004). Two cone geometries are available for the Neptune; the

• Evaporation, addition of 0.75 mL of 10.2 M HCl+0.25 mL 15.3 M HNO3 +1 drop standard (H) and high-sensitivity (X). X-cones can increase transmis- 11.3 M HClO4 sion by a factor of 3 relative to H-cones (Weyer and Schwieters, 2003), • Heating and evaporation which can be important for samples containing little Fe. H-cones were • Addition of 1.0 mL of 10.2 M HCl+0.5 mL 15.3 M HNO3 +1 drop 11.3 M HClO4 • Heating and evaporation. Addition of 500 μL 6 M HCl. exclusively used in this study. • Disposable Bio-Rad Poly-Prep polyethylene columns filled with 1 mL of AG1-X8 200- Instrumental mass fractionation is large in MC-ICPMS (between 400 mesh Cl-form anion exchange resin (resin bed length 2.0 cm, diameter 0.8 cm) 3.5 and 5.5%/amu for Fe, see Fig. 2 of Dauphas and Rouxel, 2006). • The resin is conditioned with 10 mL of H2O, 5 mL of 1 M HNO3,10mLofH2O, 10 mL Excluding double spike (Johnson and Beard, 1999; Balci et al., 2006; of 0.4 M HCl, 5 mL of H O, and 2 mL of 6 M HCl 2 Dideriksen et al., 2006; Fantle and Bullen, 2009), three strategies have • The samples in ~250 μ L 6 M HCl are loaded on the column • Most matrix and interfering elements are eliminated with 8 mL (0.5+0.5+1+2+4) been developed for correcting instrumental mass fractionation during of 6 M HCl. MC-ICPMS analysis: (i) Ni-doping, (ii) Cu-doping, and (iii) standard • Iron is recovered in a Teflon beaker by passing 9 mL (0.5+0.5+1+3+4) of 0.4 M HCl. bracketing (Belshaw et al., 2000, Malinovsky et al., 2003; Arnold et al., • The solution is dried down and the column procedure is repeated a second time with 2004, Dauphas et al., 2004; Schoenberg and von Blanckenburg, 2005; new resin. also see Albarède and Beard, 2004, for a detailed review of this topic). N. Dauphas et al. / Chemical Geology 267 (2009) 175–184 177

Instrumental mass bias in MC-ICPMS is adequately described by the 1011 Ω, and one 1010 Ω amplifiers. With the standard-bracketing exponential law (Maréchal et al., 1999), which relates the measured method, 54Fe, 56Fe, 57Fe, and 58Fe were measured on L2, C, H1, and H2 isotopic ratio (r) to the true ratio (R) and the relative mass difference Faraday collectors. Possible isobaric interferences from Cr and Ni were of the isotopes (△m/m): monitored at masses 53Cr and 60Ni on L4 and H4 Faraday collectors, respectively. All measurements were performed in medium and high β r = RðÞ1+Δm=m ; ð1Þ mass resolution. Isobaric interferences of 40Ar14N, 40Ar16O, 40Ar16O1H, and 40Ar18Oon54Fe, 56Fe, 57Fe, and 58Fe, were resolved by measuring where β represents the instrumental mass bias and is a free iron isotopes as flat-topped peak shoulders. The ion intensities of 53Cr parameter. In standard-bracketing with element doping, a fixed and 60Ni were measured at the center of the peaks. High-intensity amount of Ni or Cu of known isotopic composition is added to the measurements of minor isotopes became possible by measuring the sample and the standard solutions. β is calculated for Ni or Cu using high-abundance 56Fe+ with a 1010 or 1011 Ω amplifier. The low Eq. (1). The fractionation corrected ratio for Fe is subsequently intensity 60Ni+ signal, on the other hand, was measured with a 1012 Ω computed from the measured ratio assuming that βFe =βNi,Cu.In amplifier (settling time ~3 s) to reduce the Johnson noise/signal ratio β ≠β reality Fe Ni,Cu (e.g., Maréchal et al., 1999; Woodhead, 2002; (Wieser and Schwieters, 2005). The washout time with 0.45 M HNO3 Archer and Vance, 2004) and the exponential law may not be between samples and standards was 90 s. The take-up time was 60 s. completely adequate in describing instrumental mass fractionation The mass spectrometer baseline was measured with the beam (e.g., Vance and Thirlwall, 2002; Wombacher and Rehkämper, 2003). defocused for 30 cycles of 1.05 s each. Before a sequence of analysis,

It is therefore critical to bracket sample analyses by measurements of a clean 0.3 M HNO3 solution was measured under conditions identical the IRMM-014 Fe reference material doped with the same amount of to sample measurements (e.g., same number of cycles and integration Ni or Cu and reduced in the same manner. In standard bracketing time). The ion intensities of this solution were subtracted from all without element doping, the instrumental mass bias is computed for sample/standard measurements (On Peak Zero, OPZ). The idle time 56 54 the two reference standard Fe/ Fe analyses (βSTD1 and βSTD2 using was set to 3 s. Iron isotopes were acquired in a single block consisting R=15.6986 in Eq. (1); Taylor et al., 1992) that bracket the sample. The of 25 cycles of 8.389 s each. The Fe concentrations of samples and average of these two values is used to correct the sample analysis for standards ranged between 2 to 5 ppm at a sensitivity of 5 to 13 V/ppm 56 instrumental mass fractionation (βSAMPLE =0.5βSTD1 +0.5βSTD2). In in MR on Fe with a 50–100 μL/m nebulizer. both cases, the isotopic ratios are then reported in δ-notation relative All measurements for the Cu-dopping tests were made in HR. The to IRMM-014. same amounts of pure Cu were added to the sample and standard The advantage of the Ni-doping method is that both Fe and Ni (60 solutions to reach a constant concentration of ~5 ppm, with a and 61) isotopes can be measured at the same time, i.e., no peak sensitivity of 2 V/ppm on 63Cu. The sample δFe values were computed jumping is involved. In addition, Ni is easily separated from Fe during relative to bracketing IRMM-014 also doped with the same level of Cu. the column chemistry. Poitrasson and Freydier (2005) pointed out It is well documented that βFe ≠βCu so sample-standard bracketing is that keeping the 61Ni+ signal above 500 mV is essential for precise critical. The measurements were performed in 25 cycles of 2 analyses because uncertainty in the measurement of 61Ni (1.13% of sequences. The first sequence was identical to the standard-bracketing total Ni) propagates unfavorably to the corrected Fe isotope ratios. method and a 3 s idle time was used between each sequence. In the The main drawback of this method is that it generates a major second sequence, 63Cu and 65Cu were measured on C and H2 Faraday interference on 58Fe (0.2819 atom % of total iron) from 58Ni collectors. The integration times were 8.389 s for both Fe and Cu (68.27 atom % of total Ni). This is not a problem if the laws of mass- sequences. The relative mass dispersion between 65Cu and 63Cu was dependent fractionation solely govern variations of isotopic ratios. different from that of 58Fe and 56Fe, which was corrected by adjusting However, non-mass-dependent isotopic variations may arise from the dispersion lens to ~−75 V. To correct aberrations in the Cu peak photochemistry in aqueous systems or incomplete mixing of the shape, the focus lens was also adjusted to ~10 V, which resulted in products of in meteorites (Völkening and square peak shapes appropriate for Cu isotopic measurement. The Papanastassiou, 1989; Dauphas et al., 2004, 2008). Another difficulty lenses switched between Fe and Cu mode on a timescale of ~10 s. No is that while Fe and Ni isotopes can be measured at the same time, our peak centering was required during the analysis because hysteresis collector setting does not allow 53Cr+ to be measured simultaneously was not observed at any time. In other words, the center of peak with 61Ni+ and the contribution of 54Cr+ on 54Fe+ cannot be measurement consistently returned to the appropriate position on the controlled. For these reasons, this scheme was not utilized (Mal- flat-topped iron peak shoulder. inovsky et al., 2003, Schoenberg and von Blanckenburg, 2005; The standard-sample sequences were repeated n times (typically Poitrasson and Freydier, 2005, provide ample details on how to 5≤n≤9). Data processing was performed offline. In a sequence implement this correction scheme). consisting of standard (i−1), sample (i−1), standard (i), sample (i), On the other hand, Sharma et al. (2001), Kehm et al. (2003), and standard (i+1), sample (i+1), the isotopic composition of the ith sample

Arnold et al. (2004) advocated the use of Cu-doping. Nevertheless, the repeat is δFesample(i) =[Rsample (i)/(0.5⁎Rstandard (i) +0.5⁎Rstandard (i + 1))− mass-dispersion in the collector array of the Neptune is not wide 1]×103. The reported isotopic compositions are the averages of n repeat enough to allow analysis of the 4 stable isotopes of Fe and 2 isotopes of analyses,

Cu (63 anf 65) at the same time. Two sequences must therefore be Xn used, making peak jumping unavoidable. As is the case with the Ni δ δ = : ð Þ Fesample = FesampleðÞi n 2 doping scheme, isotopic analyses with Cu-doping are potentially less i =1 sensitive to matrix effects, which can improve accuracy. However, the duration of the measurements is significantly extended, more Fe is δFe values for standards were also computed by considering each consumed, and in Cu-rich samples, one must ascertain complete standard i as a sample bracketed by 2 standards (i − 1 and i+1). removal of Cu during the chemistry. Although tests using Cu-doping Because sample-standard bracketing is equivalent to a linear time were performed on selected samples to validate the quality of the interpolation, inaccurate δFe (≠0 for standards bracketed by measurements, the standard-bracketing method was preferred for standards) could arise from curvature in instrument drift with time, most tests and measurements (also see Schoenberg and von which was not observed. The dispersion of the standard-bracketed Blanckenburg, 2005). standard δFe was used to estimate the instrumental uncertainty (σ)of

The Neptune MC-ICPMS at the University of Chicago is equipped the sample measurements by assuming that σsample=σstandard.The with 9 Faraday collectors connected to an array of one 1012 Ω, seven main virtue of using standards rather than samples to quantify the 178 N. Dauphas et al. / Chemical Geology 267 (2009) 175–184

the number of standard brackets (m) is 44. The uncertainty on the sample δFe analysis is therefore:

Xm hi 1 2 σ 2 = δFe = m; ð3Þ MassSpec n standardðÞi i =1

where n represents the number of sample measurements, as we are interested in the uncertainty of the average of n.Forn=9 replicate

analyses of the sample solution, σMassSpec is the standard deviation of all δ thepffiffiffi standard Fe values measured in that session divided by 3 ( n). Instrumental uncertainty only represents a part of the total uncertainty, which must be assessed by examining the external reproducibility of the analyses.

4. Optimization of isotopic analyses

Several parameters such as the peak position, acid molarity, Fe concentration, presence of matrix elements or isobaric interferences can influence the quality of isotopic analyses. We explored these factors in order to establish a robust and optimized method that can be used routinely for high-precision Fe isotope measurements.

4.1. Shoulder peak plateau in medium-resolution (MR) mode

Malinovsky et al. (2003) tested how the flatness of the peak shoulder in HR mode can affect the precision and accuracy of isotopic analyses. They showed that highly reproducible results can be obtained over a mass range of ~0.01 provided the Faraday cups are properly aligned. Iron isotopic analyses can degrade the HR slit very rapidly by ion milling and thereby reduce the mass resolution. Because the MR slit blocks a smaller fraction of the incoming ions, it has a longer lifetime in terms of the number of analyses that can be performed. Weyer and Schwieters (2003) looked at the same issue in MR and showed that consistent δ56Fe within 0.06‰ (2σ) ratios can be obtained over a mass range of 0.005 (δm/m=100 ppm). In the following we improve the results reported by Weyer and Schwieters (2003) to higher precision δ56Fe (0.02 to 0.03‰,2σ). A 5 ppm solution of Fe was introduced into the mass spectrometer using the SIS. A sequence was established where the Fe isotopic composition was measured alternatively at the center and off-center of the Fe shoulder peak. The On Peak Zero (OPZ) used for all analyses was taken at the center (during routine analyses only one OPZ is measured at the beginning of a sequence). The center was selected visually, as would be done during a routine analysis. Six off-center measurements were performed, spanning a mass interval of 0.006 amu. We computed the δ56Fe, δ57Fe, and δ58Fe values for off-center analyses relative to the bracketing center measurements. The results are shown in Fig. 1. Expectedly, as the mass positions approach the argide peaks, tailing leads to erroneous δ56Fe and δ57Fe values. Uncertainties also tend to increase, as the position gets further away from the center, due to fluctuations in argide/Fe ratio and/or peak position. Overall, con- sistent measurements were achieved with minimum uncertainties of ~0.023‰ for δ56Fe, 0.043‰ for δ57Fe, and 0.187‰ for δ58Fe (2σ) over a Fig. 1. Test of the peak flatness and interference tailing on Fe isotope measurements in mass interval of 0.003 amu. These observations indicate that it is

MR using the SIS and 5 ppm Fe solutions in HNO3 0.3 M. The top panel (A) shows peak preferable to position the mass slightly to the left (~0.001 amu) of scans at masses 54, 56, 57, and 58 (on L2, C, H1, and H2). The voltages of some curves what visually appears to be the center of the Fe shoulder peak plateau. were multiplied by an arbitrary factor for clarity purposes (provided in the legend). The vertical lines represent the masses where isotopic measurements were made. All measurements were normalized by bracketing with measurements taken at mass 4.2. Concentration matching between samples and standards 55.730. The bottom panels (B) show the Fe isotopic compositions at all masses relative to the composition measured at mass 55.730. With the first generation of MC-ICPMS, where Fe isotopic analyses depended on the use of a desolvating nebulizer to reduce argide interferences, it was absolutely crucial to match the concentration of samples and standards (Belshaw et al., 2000; Anbar et al., 2000; Zhu instrumental uncertainty is that there are many more standard et al., 2002). In the Neptune, three strategies help eliminate the effect analyses than samples and the standard deviation is better known. For of argide interferences. Clearly the most important factor is the use of instance, in a session of 5 sample analyses repeated 9 times each, MR or HR modes. Subtraction of OPZ ion intensities can also help N. Dauphas et al. / Chemical Geology 267 (2009) 175–184 179

the concentration of the matrix element (this is easily converted to a ratio to iron by dividing by 1 to 5 ppm). We do not report δ56Fe for the Cr-doping test because there is a direct isobaric interference of 54Cr on

Fig. 2. Effect of concentration matching on the isotopic composition of Fe. Standard solutions with mismatched concentrations were analyzed by bracketing with the same standard at 5×10−11 A(filled circles), 25×10−11 A(filled squares), and 38×10−11 A(open circles) on 56Fe. The solutions were measured using the SIS and Apex-Q in both MR and HR. No clear correlation is observed between the Fe isotopic composition and concentration mismatch. Ion intensities of samples and standards are typically matched to within ±5%. reduce the influence of tailing from argide peaks, if present. Finally, matching the concentration between samples and standards can also play a role in improving accuracy. Schoenberg and von Blanckenburg (2005) studied this aspect by bracketing standard measurements by analyses of the same standard but with a Fe concentration that was deliberately mismatched. These authors concluded that δ56Fe was not affected by mismatch between the concentration of Fe in the standard and the sample as long as the difference was less than a factor of 2. Examination of Fig. 4 from Schoenberg and von Blanckenburg (2005) however reveals a statistically significant positive correlation between 56 δ Fe and Csample/Cstandard. We performed similar tests in both MR and HR modes using the SIS and Apex-Q at different ion intensities of the reference solution (5, 25 and 38× 10−11 Aon56Fe). The measurements reveal that indeed, individual sample analyses do not depart from the standard isotopic compositions when 0.5bC/Cstandard b2(Fig. 2). Nevertheless, there seems to be a negative correlation between δ56Fe and the degree of mismatch between the sample and standard. In MC-ICPMS, concentration matching within 5% is easily achieved, which is sufficient to obtain accurate analyses within ~0.03‰ on δ56Fe. Under these conditions, only one OPZ analysis needs to be made per session.

4.3. Influence of matrix elements on instrumental mass fractionation

Although our chemistry is highly efficient at eliminating most common rock forming elements, certain transition elements that are present in appreciable quantities in Fe-bearing rocks can persist after chromatography. This is true in particular for Cu, which follows Fe throughout the chemistry. Such elements can affect the accuracy of the isotopic analyses by modifying the value and the stability of Fig. 3. Influence of matrix elements and isobars (Cr and Ni) on the isotopic composition instrumental mass fractionation. Schoenberg and von Blanckenburg of Fe. The measurements were made using the SIS in both MR and HR at Fe (2006) examined the influence of Cu, Zn, Cd, and Co on Fe isotopic concentrations ranging from 1 to 5 ppm in HNO3 0.45 M. The top panel (A) shows the isobar-free Fe isotopic composition (because of an interference of 54Cr on 54Fe, the analyses on a Neptune MC-ICPMS. They found that even when the isotopic compositions for Cr-doping experiments are reported as 2 × δ57/56Fe). concentrations of these elements reach 50% of Fe concentration, no Instrumental mass bias for Fe is not affected by the presence of Co, Ni, Cr, and Cu up significant deviation of δ56Fe is observed within ~0.06‰. to concentrations of ~10 ppm. The bottom panels (B) show the effect of the presence of 54 58 54 58 We have extended the range to higher matrix concentrations. direct isobars on the isotopic composition of Fe ( Cr and Ni interfere on Fe and Fe, respectively). 54Cr and 58Ni interferences were monitored and corrected for using 53Cr Solutions of 1 to 5 ppm Fe were mixed with up to ~100 ppm Co, Ni, Cr, and 60Ni (see text for details). The corrections break down for 54Cr/54Fe and 58Ni/58Fe and Cu. These solutions were then analyzed by bracketing with pure ratios above ~0.01 to 0.1 (corrections of 10 to 100‰, measured with precisions of 0.1‰ 56 Fe solutions. The resulting δ Fe are plotted in Fig. 3A as a function of for δ56Fe and ~0.5‰ for δ58Fe). 180 N. Dauphas et al. / Chemical Geology 267 (2009) 175–184

anomalies on δ56/54Fe and δ58/54Fe, which is a practical way of

accounting for the fact that βFe ≠βCr,Ni. It is important to note that the threshold 54Cr/54Fe and 58Ni/58Fe are well above those obtained in natural samples after Fe separation chemistry. These results agree with the study of Schoenberg and von Blanckenburg (2005) who showed that Fe isotopic analyses are unaffected for 54Cr/54Fe and 58Ni/58Fe ratios up to 0.001 and 0.3, respectively. Dauphas et al. (2008) reported normal 58Fe/54Fe analyses in Ni-rich meteoritic materials with a precision of ±0.04‰ after correcting for 58Ni isobaric interference (mainly from memory in the Apex-Q+Spiro desolvating nebulizer) and internal normalization using the 57Fe/54Fe ratio.

4.5. Acid molarity

Malinovsky et al. (2003) showed that the molarity of the acid used to dilute the samples can affect instrumental mass fractionation for Fe. 56 They observed an increase of δ Fe with increasing HNO3 concentration. Fig. 4. Effect of acid concentration matching on the accuracy of Fe isotopic analyses. All Correcting mass fractionation by doping with Ni, however, eliminates measurements were bracketed by standard measurements in 0.3 mol/L HNO3 and were done in MR. The filled circles were measured using the Apex-Q/Spiro introduction the observed trend. The potential effect of acid molarity on the quality of system with 1 ppm Fe (~50×10− 11 Aon56Fe). The open circles were measured using Fe isotopic analyses was studied as follows. Standards in 0.15 to 0.9 M the SIS with 5 ppm Fe (also ~50×10− 11 Aon56Fe). As shown, during simple sample- HNO3 were bracketed with standards in 0.3 M HNO3.Testswere standard bracketing, it is crucial to precisely match the acid concentrations of the performed with both the SIS (5 ppm Fe) and the Apex-Q+Spiro (1 ppm sample and the standard (within ~1%), which can be achieved easily. Fe). In both cases, very large isotopic variations with changing acid molarity were measured (Fig. 4). Note that the uncertainty also seems to 54Fe. Instead, 2×δ57/56Fe is reported, which should be similar to δ56Fe increase with increasing acid molarity. The use of a desolvating nebulizer for mass-dependent fractionation. All doped solutions have normal does not seem to considerably improve this problem. It is therefore δ56Fe within 0.1‰ up to matrix concentrations of ~10 ppm but error critical to match the molarity of the sample and standard whenever the bars seem to increase with increasing matrix concentrations, reflect- standard bracketing method is applied. The best results are achieved by ing greater instability of instrumental mass bias. Because Cu is not using the same batch of acid to dilute all solutions, including standards easily separated during the chemistry, particular attention was paid to (IRMM-014) and blanks. Effort must be also directed towards eliminat- this element. Iron has normal isotopic composition within 0.035‰ ing evaporative loss during the course of sample preparation and (2σ) even when the Cu concentration reaches 10 ppm. These results, analyses, which can change the molarity of the remaining solution (e.g., together with the study of Schoenberg and von Blanckenburg (2005) use an ultrasonic bath rather than a hot plate to dissolve the iron indicate that conservatively, matrix elements should not affect the residue). accuracy of isotopic analyses, as long as their concentration is kept below ~10 ppm (Element/Feb2). 4.6. Cu-doping

4.4. Correction of isobaric interferences from 54Cr and 68Ni Details on how to implement Cu-doping method for instrumental mass fractionation correction are given by Sharma et al. (2001), Kehm Separation of Cr and Ni from Fe is straightforward with the chemistry presented in Section 2. However, trace amounts of these elements may remain after the chemistry, which can affect measurements of 54Fe and 58Fe. In particular, isotopic analyses are very sensitive to the presence of Ni because 58Fe is the least abundant isotope of Fe (0.2819 atom %, Taylor et al.,1992)while58Ni is the most abundant isotope of Ni (68.0769 atom %, Gramlich et al., 1989). Even in HR mode, Cr and Ni interferences cannot be resolved from Fe. However, one can correct these interferences by analyzing the ion intensities of 53Cr and 60Ni. The data reduction proceeds as follows. At first, β is calculated for the sample or standard based on the 57Fe/56Fe ratio. This β is then used to compute fractionated 54Cr/53Cr and 58 60 Ni/ Ni ratios assuming that βCr =βNi =βFe and using unfractionated ratios 54Cr/53Cr=0.24890 (Shields et al., 1966)and58Ni/60Ni=2.59607 (Gramlich et al., 1989). The fractionated 54Cr/53Cr and 58Ni/60Ni ratios are then combined with measured 53Cr and 60Ni intensities to estimate and subtract isobaric interferences on 54Fe and 58Fe. In order to test this correction scheme and evaluate its potential limitations, we have analyzed the isotopic composition of Fe mixed with various levels of Cr and Ni (Fig. 3B). The δ56/54Fe value is normal within 0.08‰ up to 54Cr/ 54Fe =0.04. This ratio corresponds to a correction of 40‰.Whenthe 54Cr/54Fe ratio reaches 0.7 (700‰), the correction begins to break down (δ56/54Fe ≈+0.6‰), most likely because the mass bias obtained from 57Fe/56Fe is not appropriate for Cr (and Ni). The δ58/54Fe value remains normal within uncertainties (±0.5‰)forNi,aslongasthe58Ni/58Fe ratio is kept below ~0.05 (50‰). Erroneous negative δ58/54Fe are observed for Fig. 5. Comparison between δ56Fe analyses by sample-standard bracketing and Cu- 58 58 fi higher Ni/ Fe ratios. Better corrections can be obtained by arti cially doping methods for basalts from Kilauea Iki lava lake (Teng et al., 2008). The error bars 54 53 58 60 modifying the Cr/ Cr and Ni/ Ni ratios to eliminate isotopic take into account the long-term external reproducibility (see Section 5 for details). N. Dauphas et al. / Chemical Geology 267 (2009) 175–184 181

dissolution (assuming that the powder is homogeneous) to isotopic measurement. This will help better estimate the external reproducibility. However, it is also desirable to incorporate the instrumental uncertainty when reporting results. This can be easily achieved by writing the standard deviation of the data as the quadratic sum of the standard deviation associated with instability

of the mass spectrometer (σMassSpec, Eq. (3)), the standard deviation corresponding to the distribution in δFe of the material

being analyzed (σNatural), and a standard deviation (σUnknown)that accounts for all other sources of variability associated with the analytical procedure (Humayun and Clayton, 1995; Rehkämper and Halliday, 1999): Fig. 6. Long-term (a year) external reproducibility of BHVO-1 δ56Fe measurements (n=23). Each data point represents an independent measurement (from weighing the powder to isotopic analysis). The small error bars represent 95% confidence intervals σ 2 σ 2 σ 2 σ 2 : ð Þ Data = MassSpec + Natural + Unknown 4 based on the reproducibility of IRMM-014 measurements during the session when 56 BHVO-1 was analyzed (σMassSpec, Eq. (3)). The large error bars include an additional component of uncertainty to yield a MSWD of ~1 (σ56 =0.011, Eq. (4)). The Unknown In order to estimate σUnknown, the Fe isotopic compositions of δ56 ‰ weighted average Fe of BHVO-1 is 0.111±0.006 . geostandard powders were analyzed. The starting materials have 56 homogeneous δ Fe with σNatural =0. For a set of k replicate analyses, et al. (2003), Arnold et al. (2004),andSchoenberg and von one can compute the Mean Squared Weighted Deviates (MSWD or Blanckenburg (2005). The advantage of this method is that it reduces reduced χ2), the uncertainty arising from instability in instrumental mass bias and ÀÁ it can also mitigate matrix effects that could be a source of inaccuracy. X − 2 1 xi X However the duration of each analysis conducted on the Neptune is MSWD = ; ð5Þ k − 1 σ 2 + σ 2 increased by a factor of ~2 (Cr, Fe, Ni, and Cu cannot be analyzed i MassSpec; i Unknown simultaneously and multi-dynamic mode must be used) and one must ― ascertain that Cu from the sample has been completely eliminated where X is the weighted average. Note that in this equation, σMassSpec during the chemical separation of Fe or that it is present at a depends on the sample while σUnknown is fixed. If the analytical sufficiently low level in the original sample not to affect Cu-doping. uncertainty has been properly estimated, the dispersion of the data

Considering the pros and cons, sample-standard bracketing was around the average should be explained by σMassSpec and the MSWD adopted for routine analyses. Cu-doping is still useful to evaluate the should be around unity. Otherwise, the measurements would call for a accuracy of the measurements on selected samples. To test the Cu- positive σUnknown. The isotopic compositions of 23 replicates of the doping method, we analyzed basalt samples from Kilauea Iki lava lake BHVO-1 geostandard were measured over a period of a year using the (Hawaii) measured previously by sample-standard bracketing (Teng SIS (Fig. 6). The MSWD associated with these 23 replicates is 2.52. This fi – χ2 et al., 2008). The Fe/Cu ratio in these basalts is ~700 (~12 wt.% FeOtotal is outside the 95% con dence interval of 0.5 1.7 given by statistics. 56 and ~130 ppm Cu; Helz and Wright, 1992). Although Cu was not The MSWD can be brought down to unity by adjusting σUnknown to 57 57 separated from Fe, the contribution of Cu already in the sample to total 0.011. Similarly, for δ Fe, one can compute σUnknown=0.012. For Cu after doping is negligible (~0.0014) and uncertainty on the isotopic composition of indigenous Cu does not affect δ56Fe measurements during mass discrimination correction. Unlike most sample-standard Table 2 bracketing measurements, which were done in MR mode, the Cu- Iron isotope results. doping tests were performed in HR mode because isotopic ratios 56 57 δ Fe σMassSpec 95% ci δ Fe σMassSpec 95% ci should be less sensitive to the peak position. As shown in Fig. 5, there Geostandard is perfect agreement between sample-standard bracketing and Cu- matrix+IRMM014 56 doping δ Fe analyses. Although this does not unambiguously Allende a 0.004 0.011 0.031 0.004 0.017 0.042 demonstrate accuracy, it shows that the measurements do not suffer Allende b –0.016 0.015 0.038 −0.025 0.028 0.062 from matrix effects. AC-E a −0.004 0.008 0.028 −0.004 0.015 0.039 AC-E b −0.010 0.011 0.031 −0.014 0.017 0.042 BCR-2 a −0.003 0.009 0.028 −0.008 0.013 0.035 5. Precision and accuracy BCR-2 b −0.011 0.008 0.028 −0.017 0.015 0.039 DTS-2 a 0.015 0.009 0.028 0.017 0.013 0.036 The uncertainties calculated using Eq. (3) only represent one DTS-2 b −0.002 0.009 0.028 −0.007 0.013 0.036 − − component of the total uncertainty, i.e., instrumental instability. IF-G a 0.017 0.011 0.031 0.016 0.018 0.044 IF-G b −0.007 0.009 0.028 −0.018 0.014 0.037 Indeed, other sources of uncertainty that were discussed in Sections 2and4couldinfluence the reproducibility of Fe isotopic analyses. Geostandards Given that one can arbitrarily reduce the uncertainty associated Allende 0.002 0.006 0.025 0.008 0.010 0.031 with the instrument by increasing the number of analyses (e.g., AC-E 0.322 0.006 0.025 0.461 0.010 0.031 BCR-2 0.091 0.006 0.025 0.133 0.010 0.031 precisions as low as 0.01‰ on δ56Fe have been reported in the DTS-2 0.017 0.006 0.025 0.023 0.010 0.031 literature), special care must be given to other sources of error. IF-G 0.647 0.006 0.025 0.939 0.010 0.031 There are two aspects that need to be addressed regarding the BHVO-1 (n=23) 0.111 0.006 0.164 0.009 overall uncertainty of the measurements: (i)Howcanthe Geostandard matrix+IRMM014 correspond to sample matrices mixed with IRMM-014 uncertainty arising from the whole procedure be quantified, iron. These samples have δFe=0, demonstrating that the measurements are accurate. considering that only the instrumental uncertainty is known for The geostandards agree with recommended values (Dauphas and Rouxel, 2006). σ most samples? (ii) Are the measurements accurate within the MassSpec corresponds to the standard deviation from the mass spectrometer (typically, replicate analyses obtained on a single day). The 95% ci (confidence interval) integrates uncertainties that are quoted? our knowledge of the long-term reproducibility of BHVO-1 (Section 5) a and b represent fi The proper way to address the rst question is to run replicate 2 aliquots of IRMM-014 mixed with sample matrices processed independently through analyses of the same sample, from loading on the column or the chemistry. 182 N. Dauphas et al. / Chemical Geology 267 (2009) 175–184

Over the past several years, important developments have been made in terms of analytical precision. Therefore, systematic biases that could not be detected before may be revealed as the precision improves. In order to test whether the measurements are accurate, we have proceeded in a similar manner as that described in Bermin et al. (2006) and Dauphas et al. (2007). Powdered geostandards (Allende CV3 chondrite, AC-E granite, BCR-2 basalt, DTS-2 dunite, and IF-G banded iron formation) with previously documented Fe isotopic compositions (see the compilation in Dauphas and Rouxel, 2006) were passed through a first column of anion exchange. Both Fe and the matrix were recovered. The matrix was passed through a second column to completely eliminate any Fe that may have persisted through the first column. The Fe-free matrix was doped with 50 μg of IRMM-014 and then split into 2 aliquots (a and b in Table 2 and Fig. 7). Each fraction was treated by 2 columns following the procedure described in Section 2. The sample Fe eluted from the first column was passed through a second column. To summarize, three Fe solutions were generated from the original geostandard solution. The first solution is Fe from the sample treated using our usual procedure. The remaining two solutions contained matrix elements doped with IRMM-014 and purified using the procedure described in Section 2. If the measurements are accurate and the analytical uncertainties properly estimated, the isotopic composition of the solutions of IRMM-014+matrix should have δ56Fe=0 within uncertainties. As shown in Fig. 7 (Table 2), this is indeed the case. The weighted average δ56Fe of all 10 measurements is −0.0045±0.0094‰ and the MSWD is 0.39, which falls within the 2-sided 95% confidence interval of 0.30– 2.11. The geostandard matrix+IRMM-014 tests indicate that the measurements are highly accurate. The results obtained on geostandards were also compared with those published in the literature (Fig. 7B). δ56Fe values of 0.647± 0.027, 0.091±0.027, 0.017±0.027, 0.002±0.027, and 0.322± 0.027‰ were measured for IF-G, BCR-2, DTS-2, Allende, and AC-E, respectively. This compares very well with published values of Fig. 7. Accuracy and precision of Fe isotopic analyses. A. Measurements of 50 μg IRMM- 014 mixed with sample matrices (a and b represent two aliquots processed 0.631±0.019, 0.064±0.034, 0.022±0.041, − 0.011±0.047, independently through the chemistry; see text for details). IRMM-014 purified by 0.289±0.024‰ (see compilation in Dauphas and Rouxel, 2006). column chromatography from these mixtures was analyzed by bracketing with pure Overall, the results from these tests demonstrate that that δ56Fe can be IRMM-014 not passed through column. Each measurement was repeated a large measured accurately with a precision of less than±0.03‰ (95% number of times to reduce the uncertainty arising from mass spectrometer instability fi and thus test the accuracy. Those uncertainties were compounded with the long-term con dence interval). external reproducibility characterized based on BHVO-1 replicate analyses (Fig. 6). All measurements are equal to 0 within uncertainties. The MSWD (reduced χ2) is 0.39, 6. Double-spike vs. sample-standard bracketing within the 2-sided 95% confidence interval of 0.30 to 2.11 for n−1=9 degrees of freedom. The weighted mean δ56Fe is −0.0045±0.0094‰ (95% confidence interval), Double-spike (58Fe–54Fe or 58Fe–57Fe have been used) is an demonstrating that the measurements are accurate within ~0.01‰ for δ56Fe. B. Comparison of geostandard measurements with recommended values (Dauphas approach that was not investigated in this study but has the and Rouxel, 2006). All data points fall on the line with the slope of unity within potential to yield precise and accurate Fe isotopic analyses by uncertainties. Note that a solid residue was left in the beaker after dissolution of DTS-2. TIMS (Johnson and Beard, 1999; Fantle and Bullen, 2009)orMC- ICPMS (Balci et al., 2006; Dideriksen et al., 2006, 2007, 2008) with a high sample throuhput. The main advantage of the double spike is that the instrumental mass fractionation can be efficiently δ58Fe, the precision does not allow resolving other possible sources of corrected. Dideriksen et al. (2006, 2007, 2008) reported a uncertainty than those from the mass spectrometer. So for this dispersion (2SD) of standard δ56Fe values of ±0.06‰.Thisis 56 isotope, σUnknown for δ Fe was simply scaled by a factor of 2, similar to what can be achieved by standard-sample bracketing corresponding to the slope between δ58Fe and δ56Fe for mass but the measurements of Dideriksen et al. were acquired with a 58 dependent fractionation (σUnknown=0.022). The values of σUnknown Thermoelemental Axiom instrument, which provides no intrinsic for all isotopes based on BHVO-1 external reproducibility will be used capability to resolve argide interferences. Balci et al. (2006) reported in all future studies and will be updated when more BHVO-1 external reproducibilities (2SD) of ±0.07‰ on a Neptune HR-MC-ICPMS. measurements become available. All error bars quoted hereafter Whether better precisions can be achieved with such instruments include σUnknown. It is worth mentioning that σUnknown may differ remains to be explored. One major drawback of the double-spike depending on the sample matrix, the chemistry used for Fe separation, technique is that the search for 58Fe anomalies in meteorites may be the inlet system (SIS or desolvating nebulizer) and the instrument hindered (Völkening and Papanastassiou, 1989; Dauphas et al., 2008). 60 type. Therefore, similar tests (long-term external reproducibility of Iron-58 was produced in massive together with Fe (t1/2 =1.49My) high-precision measurements) should be performed in each labora- by -capture reactions and as such, it represents a useful proxy to tory. If the long-term external reproducibility can be explained by the probe the distribution of 60Fe in the protoplanetary disk (Dauphas et al., instrumental instability (MSWD≈1 in Eq. (5)), σUnknown can be set to 2008). An additional limitation is that the double-spike method 0. The same approach can also be used for other isotopic systems. does not allow one to study the form of the mass-fractionation N. Dauphas et al. / Chemical Geology 267 (2009) 175–184 183 law, which may provide clues on the mechanisms underlying Anbar, A.D., Roe, J.E., Barling, J., Nealson, K.H., 2000. Nonbiological fractionation of iron isotopes. Science 288, 126–128. isotopic fractionation (Young et al., 2002). Archer, C., Vance, D., 2004. Mass discrimination correction in plasma source mass spec- trometry: an example using Cu and Zn isotopes. J. Anal. At. Spectrom. 19, 656–665. 7. Conclusion Arnold, G.L., Weyer, S., Anbar, A.D., 2004. Fe isotope variations in natural materials measured using high mass resolution multiple collector ICPMS. Anal. Chem. 76, 322–327. A protocol was developed for routine analyses of the Fe isotopic Balci, N., Bullen, T.D., Witte-Lien, K., Shanks, W.C., Motelica, M., Mandernack, K.W., 2006. compositions of common natural materials. Iron is purified using a Iron isotope fractionation during microbially stimulated Fe(II) oxidation and Fe(III) – two-step anion exchange column scheme. Iron isotopes are then precipitation. Geochim. Cosmochim. Acta 70, 622 639. Beard, B.L., Johnson, C.M., 2004. Fe isotope variations in the modern and ancient earth analyzed by HR-MC-ICPMS. A number of parameters that can affect and other planetary bodies. Rev. Mineral. Geochem. 55, 319–357. the accuracy and precision of δ56Fe measurements were identified and Beard, B.L., Johnson, C.M., Skulan, J.L., Nealson, K.H., Cox, L., Sun, H., 2003. Application of examined: Fe isotopes to tracing the geochemical and biological cycling of Fe. Chem. Geol. 195, 87–117. ′ 1. The samples were introduced into the mass spectrometer using the Belshaw, N.S., Zhu, X.K., Guo, Y., O Nions, R.K., 2000. High precision measurement of μ fl iron isotopes by plasma source mass spectrometry. Int. J. Mass Spectrom. 197, Standard Introduction System (a 50 L/min Te on nebulizer with a 191–195. cyclonic+Scott spray chamber). For samples with high Fe content, Bermin, J., Vance, D., Archer, C., Statham, P.J., 2006. The determination of the isotopic utilizing an Apex-Q+Spiro inlet system did not offer any composition of Cu and Zn in seawater. Chem. Geol. 226, 280–297. Borrok, D.M., Wanty, R.B., Ridley, W.I., Wolf, R., Lamothe, P.J., Adams, M., 2007. advantage over a conventional wet-introduction system. Medium Separation of copper, iron, and zinc from complex aqueous solutions for isotopic resolution and standard Ni H-cones provided adequate intensities measurement. Chem. Geol. 242, 400–414. for high-precision measurements with ~2 ppm Fe solutions. Brantley, S.L., Liermann, L.J., Guynn, R.L., Anbar, A., Icopini, G.A., Barling, J., 2004. Fe isotopic fractionation during mineral dissolution with and without bacteria. 2. The analyte concentrations of the sample and the reference Geochim. Cosmochim. Acta 68, 3189–3204. material (IRMM-014) were matched within 5%. Tests reveal that Dauphas, N., Janney, P.E., Mendybaev, R.A., Wadhwa, M., Richter, F.M., Davis, A.M., van concentration was not critical to acquiring accurate analyses. This Zuilen, M., Hines, R., Foley, C.N., 2004. Chromatographic separation and multi- disagrees with the results of Malinovsky et al. (2003) but is in collection-ICPMS analysis of iron. Investigating mass-dependent and independent isotope effects. Anal. Chem. 76, 5855–5863. agreement with Schoenberg and von Blanckenburg (2005). Dauphas, N., 2007. Diffusion-driven kinetic isotope effect of Fe and Ni during formation 3. Matching in acid molarity between samples and standards is of the Widmanstätten pattern, Meteoritics Planet. Sci. 42, 1597–1614. critical to achieving accurate measurements. Batches of blank and Dauphas, N., Rouxel, O., 2006. Mass spectrometry and natural variations of iron isotopes. Mass Spectrom. Rev. 25, 515–550. Erratum 25, 831–832. reference material were prepared together with samples with the Dauphas, N., van Zuilen, M., Busigny, V., Lepland, A., Wadhwa, M., Janney, P.E., 2007. Iron exact same molarity (HNO3 0.3 M). isotope, major and trace element characterization of early Archean supracrsutal fi 4. The measurements were not sensitive to the presence of other rocks from SW Greenland: protolith identi cation and metamorphic overprint. Geochim. Cosmochim. Acta 71, 4745–4770. transition elements in the solution, as long as their concentrations Dauphas, N., Cook, D.L., Sacarabany, A., Fröhlich, C., Davis, A.M., Wadhwa, M., Pourmand, A., remained below ~10 ppm (Element/Feb2). Rauscher, T., Gallino, R., 2008. Iron-60 evidence for early injection and efficient mixing 5. Measurement of each solution was repeated 5 to 9 times. This of stellar debris in the protosolar nebula. Astrophys. J. 686, 560–569. de Jong, J., Schoemann, V., Tison, J.-L., Becquevort, S., Masson, F., Lannuzel, D., Petit, J., allowed the uncertainty associated with instability in instrumental Chou, L., Weis, D., Mattielli, N., 2007. Precise measurement of Fe isotopes in marine 56 mass fractionation to be reduced to less than 0.02‰ for δ Fe. samples by multi-collector inductively coupled plasma mass spectrometry (MC- However, precision may not be limited by instrument instability per ICP-MS). Anal. Chim. Acta 589, 105–119. Dideriksen, K., Baker, J.A., Stipp, S.L.S., 2006. Iron isotopes in natural carbonate minerals se, but rather by the overall external reproducibility. Taking that into determined by MC-ICP-MS with a 58Fe-54Fe double spike. Geochim. Cosmochim. account, a precision of ~0.03‰ (95% confidence interval) can be Acta 70, 118–132. routinely obtained. Dideriksen, K., Christiansen, B.C., Baker, J.A., Frandsen, C., Balic-Zunic, T., Tullborg, E., fi 6. The accuracy of Fe isotopic analyses was tested by measuring Mørup, S., Stipp, S.L.S., 2007. Fe-oxide fracture llings as a paleo-redox indicator: Structure, crystal form and Fe isotope composition. Chem. Geol. 244, 330–343. IRMM-014 mixed with Fe-free matrix from geostandards. The Dideriksen, K., Baker, J.A., Stipp, S.L.S., 2008. Equilibrium Fe isotope fractionation average δ56Fe is −0.0045±0.0094‰, demonstrating that δ56Fe between inorganic aqueous Fe(III) and the siderophore complex, Fe(III)-desfer- – measurements are accurate within the quoted uncertainties. rioxamine B. Earth Planet. Sci. Lett. 269, 280 290. Fantle, M.S., Bullen, T.D., 2009. Essentials of iron, chromium, and calcium isotope This study complements the earlier efforts of Weyer and analysis of natural materials by thermal ionization mass spectrometry. Chem. Geol. 258, 50–64. Schwieters (2003), Malinovsky et al. (2003), Arnold et al. (2004), Gramlich, J.W., Machlan, L.A., Barnes, I.L., Paulsen, P.J., 1989. The absolute abundance Poitrasson and Freydier (2005) and Schoenberg and von Blancken- ratios and atomic weight of a reference sample of . J. Res. Natl. Inst. Stan. 94, burg (2005) by demonstrating that accurate isotopic analyses with 347–356. ‰ δ56 Helz, R.T., Wright, T.L., 1992. Differentiation and magma mixing on Kilaueas east rift precisions of less than 0.03 on Fe can be achieved routinely on zone—A further look at the eruptions of 1955 and 1960. Part I. The late 1955 lavas. HR MC-ICPMS. This opens up exciting avenues of research in high Bull. Volcanol. 54, 361–384. temperature Fe isotope geochemistry. Huang, F., Lundstrom, C.C., Glessner, J., Ianno, A., Boudreau, A., Li, J., Ferré, E.C., Marshak, S., DeFrates, J., 2009. Chemical and isotopic fractionation of wet andesite in a temperature gradient: experiments and models suggesting a new mechanism of Acknowledgments magma differentiation. Geochim. Cosmochim. Acta 73, 729–749. Humayun, M., Clayton, R.N., 1995. Precise determination of the isotopic composition of We thank Brian Lynch, Frank M. Richter, and Olivier J. Rouxel for potassium: application to terrestrial rocks and lunar soils. Geochim. Cosmochim. Acta 59, 2115–2130. support and discussions. Careful reviews by two anonymous referees Johnson, C.M., Beard, B.L., 1999. Correction of instrumentally produced mass fractiona- were greatly appreciated. This work was supported by a fellowship tion during isotopic analysis of Fe by thermal ionization mass spectrometry. Int. J. – from the Packard Foundation, the France Chicago Center, and NASA Mass Spectrom. 193, 87 99. Johnson, C.M., Beard, B.L., Roden, E.E., Newman, D.K., Nealson, K.H., 2004. Isotopic grant NNG06GG75G to N.D. constraints on biogeochemical cycling of Fe. Rev. Mineral. Geochem. 55, 359–408. Kehm, K., Hauri, E.H., Alexander, C.M.O′D., Carlson, R.W., 2003. High precision iron References isotope measurements of meteoritic material by cold plasma ICP-MS. Geochim. Cosmochim. Acta 67, 2879–2891. Albarède, F., Beard, B., 2004. Analytical methods for non-traditional isotopes. In: Malinovsky, D., Stenberg, A., Rodushkin, I., Andren, H., Ingri, J., Öhlander, N., Baxter, D.C., Johnson, C.M., Beard, B.L., Albarède, F. (Eds.), Geochemistry of Non-Traditional 2003. Performance of high resolution MC-ICP-MS for Fe isotope ratio measure- Isotopes, Reviews in Mineralogy and Geochemistry, vol. 55. Mineralogical Society of ments in sedimentary geological materials. J. Anal. At. Spectrom. 18, 687–695. America and Geochemical Society, Washington DC, pp. 113–152. Maréchal, C.N., Télouk, P., Albarède, F., 1999. Precise analysis of copper and zinc isotopic Alexander, C.M.O′D., Hewins, R.H., 2004. Mass fractionation of Fe and Ni isotopes in compositions by plasma-source mass spectrometry. Chem. Geol. 156, 251–273. metal in Hammadah Al Hamrah 237. Meteoritics Planet. Sci. 39, A13. Mullen, J.G., 1961. Isotope effect in intermetallic diffusion. Phys. Rev. 121, 1649–1658. Anbar, A.D., 2004. Iron stable isotopes: beyond biosignatures. Earth Planet. Sci. Lett. 217, Poitrasson, F., Freydier, R., 2005. Heavy iron isotope composition of granites determined 223–236. by high resolution MC-ICP-MS. Chem. Geol. 222, 132–147. 184 N. Dauphas et al. / Chemical Geology 267 (2009) 175–184

Polyakov, V.B., Mineev, S.D., 2000. The use of Mössbauer spectroscopy in stable isotope Taylor, P.D.P., Maeck, R., De Bièvre, P., 1992. Determination of the absolute isotopic geochemistry. Geochim. Cosmochim. Acta 64, 849–865. composition and atomic weight of a reference sample of natural iron. Int. J. Mass Polyakov, V.B., Clayton, R.N., Horita, J., Mineev, S.D., 2007. Equilibrium iron isotope Spectrom. Ion Processes. 121, 111–125. fractionation factors of minerals: reevaluation from the data of nuclear inelastic Teng, F.-Z., Dauphas, N., Helz, R.T., 2008. Iron isotope fractionation during magmatic resonant X-ray scattering and Mössbauer spectroscopy. Geochim. Cosmochim. Acta differentiation in Kilauea Iki lava lake. Science 320, 1620–1622. 71, 3833–3846. Thompson, A., Ruiz, J., Chadwick, O.A., Titus, M., Chorover, J., 2007. Rayleigh Rehkämper, M., Halliday, A.N., 1999. The precise measurement of Tl isotopic fractionation of iron isotopes during pedogenesis along a climate sequence of compositions by MC-ICPMS: application to the analysis of geological materials Hawaiian basalt. Chem. Geol. 238, 72–83. and meteorites. Geochim. Cosmochim. Acta 63, 935–944. Vance, D., Thirlwall, M., 2002. An assessment of mass discrimination in MC-ICPMS using Richter, F.M., Dauphas, N., Teng, F.-Z., 2009. Non-traditional fractionation of non- Nd isotopes. Chem. Geol. 185, 227–240. traditional isotopes: Evaporation, chemical diffusion and Soret diffusion. Chem. Völkening, J., Papanastassiou, D.A., 1989. Iron isotope anomalies. Astrophys. J. 347, Geol. 258, 92–103. L43–L46. Roskosz, M., Luais, B., Watson, H.C., Toplis, M.J., Alexander, C.M.O′D., Mysen, B.O., 2006. Wang, J., Davis, A.M., Clayton, R.N., Mayeda, T., 1994. Kinetic isotopic fractionation Experimental quantification of the fractionation of Fe isotopes during metal during the evaporation of the iron oxide from liquid state. Lunar Planet. Sci. XXV, segregation from a silicate melt. Earth Planet. Sci. Lett. 248, 851–867. 1459–1460. Rouxel, O., Dobbek, N., Ludden, J., Fouquet, Y., 2003. Iron isotope fractionation during Weyer, S., Schwieters, J.B., 2003. High precision Fe isotope measurements with high oceanic crust alteration. Chem. Geol. 202, 155–182. mass resolution MC-ICPMS. Int. J. Mass Spectrom. 226, 355–368. Schoenberg, R., von Blanckenburg, F., 2005. An assessment of the accuracy of stable Fe Weyer, S., Ionov, D.A., 2007. Partial melting and melt percolation in the mantle: the isotope ratio measurements on samples with organic and inorganic matrices by message from Fe isotopes. Earth Planet. Sci. Lett. 259, 119–133. high-resolution multicollector ICP-MS. Int. J. Mass Spectrom. 242, 257–272. Wieser, M.E., Schwieters, J.B., 2005. The development of multiple collector mass Schoenberg, R., von Blanckenburg, F., 2006. Modes of planetary-scale Fe isotope spectrometry for isotope ratio measurements. Int. J. Mass Spectrom. 242, 97–115. fractionation. Earth Planet. Sci. Lett. 252, 342–359. Williams, H.M., Peslier, A.H., McCammon, C., Halliday, A.N., Levasseur, S., Teutsch, N., Schuessler, J.A., Schoenberg, R., Behrens, H., von Blanckenburg, F., 2007. The Burg, J.-P., 2005. Systematic iron isotope variations in mantle rocks and minerals: experimental calibration of the iron isotope fractionation factor between pyrrhotite the effects of partial melting and oxygen fugacity. Earth Planet. Sci. Lett. 235, and peralkaline rhyolitic melt. Geochim. Cosmochim. Acta 71, 417–433. 435–452. Shahar, A., Young, E.D., Manning, C.E., 2008. Equilibrium high-temperature Fe isotope Wombacher, F., Rehkämper, M., 2003. Investigation of the mass discrimination of fractionation between fayalite and magnetite: an experimental calibration. Earth multiple collector ICP-MS using neodymium isotopes and the generalized power Planet. Sci. Lett. 268, 330–338. law. J. Anal. At. Spectrom. 18, 1371–1375. Sharma, M., Polizzotto, M., Anbar, A.D., 2001. Iron isotopes in hot springs along the Juan Woodhead, J., 2002. A simple method for obtaining highly accurate Pb isotope data by de Fuca Ridge. Earth Planet. Sci. Lett. 194, 39–51. MC-ICP-MS. J. Anal. At. Spectrom. 17, 1381–1385. Shields, W.R., Murphy, T.J., Catanzaro, E.J., Garner, E.L., 1966. Absolute isotopic abundance Young, E.D., Galy, A., Nagahara, H., 2002. Kinetic and equilibrium mass-dependent ratios and the atomic weight of a reference sample of chromium. J. Res. Natl. Bur. Stand. isotope fractionation laws in nature and their geochemical and cosmochemical 70A, 193–197. significance. Geochim. Cosmochim. Acta 66, 1095–1104. Strelow, F.W.E., 1980. Improved separation of iron from copper and other elements by Zipfel, J., Weyer, S., 2007. In situ analyses of Fe isotopes in zoned metal grains of anion-exchange chromatography on a 4% cross-linked resin with high concentra- Hammadad Al Hamra 237. Lunar Planet. Sci. XXXVIII #1927. tions of hydrochloric acid. Talanta 27, 727–732. Zhu, X.K., Guo, Y., Williams, R.J.P., O′Nions, R.K., Matthews, A., Belshaw, N.S., Canters, G.W., Tachibana, S., Nagahara, H., Ozawa, K., Yamada, M., 2007. Isotopic fractionation of iron de Waal, E.C., Weser, U., Burgess, B.K., Salvato, B., 2002. Mass fractionation processes of during kinetic evaporation of metallic iron. Meteoritics Planet. Sci. 42, A146. transition metal isotopes. Earth Planet. Sci Lett. 200, 47–62.