Nickel isotopic anomalies in troilite from iron ... - Wiley Online Library

GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L01203, doi:10.1029/2007GL032431, 2008

Nickel isotopic anomalies in troilite from iron meteorites David L. Cook,1,2,3,4 Robert N. Clayton,1,2,5 Meenakshi Wadhwa,1,2,3,6 Philip E. Janney,3,6 and Andrew M. Davis1,2,5 Received 22 October 2007; revised 30 November 2007; accepted 7 December 2007; published 12 January 2008.

[1] We measured nickel isotopes, via multicollector inductively coupled plasma mass spectrometry (MCICPMS), in troilite (FeS) from ten iron meteorites from both non-magmatic (IAB) and magmatic (IIAB, IIIAB, IVA) groups. These are the first reported measurements of 64 Ni (the least abundant Ni isotope) in meteoritic sulfides. No excesses of 60Ni from the decay of the short-lived radionuclide 60Fe (t1/2 = 1.49 My) were found in any of the samples. Resolvable deficits in 60Ni were observed in five samples. These deficits are inconsistent with the effects from 60Fe decay, given the Fe/Ni ratios in these troilites. Also, variations were found in 61Ni and 64Ni in several samples. The variations in Ni isotopic compositions of these samples may be due to the preservation of a component within the troilite that is characterized by the nucleosynthetic signature expected for Ni produced either by a type II supernova or by an AGB star. Citation: Cook, D. L., R. N. Clayton, M. Wadhwa, P. E. Janney, and A. M. Davis (2008), Nickel isotopic anomalies in troilite from iron meteorites, Geophys. Res. Lett., 35, L01203, doi:10.1029/2007GL032431.

1. Introduction [2] High-resolution chronometry based on the decay of the short-lived nuclide 60Fe (t1/2 = 1.49 My), which decays to stable 60Ni, has been applied to various meteoritic components. Recent analyses of various phases in unequilibrated enstatite chondrites [Guan et al., 2007] and ordinary chondrites [Tachibana and Huss, 2003; Mostefaoui et al., 2005; Tachibana et al., 2006] have revealed 60Ni excesses in sulfides, oxides, and silicates. Furthermore, deficits of 60 Ni have been reported in metal separates from several unequilibrated chondrites [Cook et al., 2006]. The deficits in metal and the excesses in non-metal phases in unequilibrated chondrites are consistent with formation from a reservoir containing live 60Fe and indicate that the solar system initial 60Fe/56Fe was 1 106 [Mostefaoui et al., 2005; Tachibana et al., 2006; Cook et al., 2006]. [3] In addition to providing chronometric information, isotopic measurements can be used to determine the scale of isotopic homogeneity in the solar system. Nucleosynthetic anomalies in Ni isotopes have been reported in some 1

Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, USA. 2 Chicago Center for Cosmochemistry, Chicago, Illinois, USA. 3 Department of Geology, Field Museum, Chicago, Illinois, USA. 4 Now at Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey, USA. 5 Enrico Fermi Institute, Chicago, Illinois, USA. 6 Now at Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA.

meteoritic components. For example, excesses in the neutron-rich isotopes of Ni (i.e., 62Ni, 64Ni) were reported in several Allende CAIs [Birck and Lugmair, 1988; Quitte´ et al., 2007]. Guan et al. [2003] reported that one sulfide grain in an unequilibrated enstatite chondrite exhibited a large excess of 62Ni in addition to an excess in 60Ni. They noted that these excesses were similar to the pattern observed in Allende CAIs by Birck and Lugmair [1988] and are consistent with a nucleosynthetic origin. A recent study indicated the presence of Ni isotope variations, consistent with a nucleosynthetic origin, in sulfides of several iron meteorites [Quitte´ et al., 2006]; these variations are rather enigmatic since none were detected in metal from the same iron meteorites. To clarify the question of whether or not variations of nucleosynthetic origin are indeed present in sulfides of iron meteorites and if these phases record any evidence of live 60Fe, we have analyzed the Ni isotopic compositions of troilite from ten iron meteorites.

2. Analytical Methods 2.1. Samples and Sample Preparation [4] Samples were chosen to represent a wide variety of iron meteorite groups including non-magmatic (IAB) and magmatic (IIAB, IIIAB, IVA) irons. Troilite separates were available from the Field Museum collection for three of the iron meteorites studied (Augustinovka, Bella Roca, Odessa). For the remaining seven samples, troilite nodules in iron meteorite slabs were sampled using stainless steel dental tools. Approximately 100 mg of troilite was obtained from a single nodule in each iron meteorite, except Mundrabilla, for which two separate nodules were sampled. Sampling of troilite nodules near the sulfide-metal contact was avoided because graphite and other accessory sulfides (e.g., ZnS) are common in this region [Buchwald, 1977]. We also analyzed three terrestrial Fe-Ni sulfides to verify that there were no analytical artifacts resulting from our chemical separation and mass spectrometry protocols; these samples included pentlandite ((Fe,Ni)9S8), pyrrhotite (Fe1-xS), and violarite (FeNi2S4). In addition to the natural samples, three aliquots of the NIST SRM 986 Ni standard were processed through the Ni separation chemistry. 2.2. Sample Digestion and Ni Separation [5] Sulfides were digested in Teflon beakers by treatment with reverse aqua regia (2:1 conc. HNO3 to conc. HCl) for 24 hours at 120°C and then evaporated to dryness. Next, samples were treated with concentrated HCl, evaporated to dryness again, and finally dissolved in 6 M HCl. This solution was split into two aliquots, one for chemical separation of Ni and another for Fe/Ni elemental ratio measurements.

Copyright 2008 by the American Geophysical Union. 0094-8276/08/2007GL032431

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COOK ET AL.: TROILITE NICKEL ISOTOPIC ANOMALIES

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Table 1. The Ni Isotopic Compositions of Chemically Processed Aliquots of the SRM 986 Ni Isotopic Standard and of Terrestrial Sulfidesa Sample

e60 ± 2SE

e61 ± 2SE

e64 ± 2SE

n

SRM #1 SRM #2 SRM #3 Pentlandite Pyrrhotite Violarite

0.09 ± 0.28 0.08 ± 0.19 0.06 ± 0.23 0.03 ± 0.29 0.04 ± 0.25 0.05 ± 0.18

0.29 ± 0.45 0.21 ± 1.54 0.34 ± 0.93 0.15 ± 0.84 0.44 ± 0.28 0.47 ± 0.48

0.0 ± 1.3 0.7 ± 2.5 0.2 ± 0.9 0.5 ± 1.8 0.3 ± 1.4 0.8 ± 2.1

5 5 5 5 5 5

a

SE, standard error; n, number of repeat measurements.

[6] Nickel was separated using a combination of anion and cation exchange chromatography by modifying the protocol described by Cook et al. [2006] for processing metallic samples. As much as a few hundred ppm of Ti may be present in troilite from iron meteorites [Buchwald, 1977], so all samples were processed using the three-column protocol described by Cook et al. [2006]. The sulfide daubreelite (FeCr2S4) commonly occurs as exsolution lamellae in troilite nodules, and Cr/Ni ratios up to 40 may occur [Buchwald, 1977]. Nickel is not separated from Cr using the above protocol; therefore, a fourth column was employed to separate Ni from Cr in sulfides. The total procedural blank (4.5 ng) is insignificant compared to the amount of Ni separated from the samples. 2.3. Isotopic Measurements [7] Nickel isotopic measurements were made on a Micromass IsoProbe multicollector ICPMS in the Isotope Geochemistry Laboratory located at the Field Museum. All Ni isotopes (58, 60, 61, 62, 64) were measured simultaneously, along with 57Fe and 66Zn, which were used to correct for isobaric interferences from 58Fe on 58Ni and from 64Zn on 64 Ni. Samples were measured via the standard-sample bracket technique using the NIST SRM 986 as the Ni isotope standard [Gramlich et al., 1989a]. Samples were corrected for mass bias assuming the exponential fractionation law and 62Ni/58Ni 0.053388 [Gramlich et al., 1989a]. A more detailed discussion of the measurement protocol for Ni isotopic analyses is provided by Cook et al. [2006]. 2.4. Iron/Nickel Elemental Ratio Measurements [8] The Fe/Ni ratios were determined in sample solutions using a Varian quadrupole ICPMS instrument at the Field Museum. A known amount of a Co concentration standard was added to the sample solution and served as an internal standard. The isotopes 57Fe, 59Co, and 60Ni were measured. The Fe and Ni concentrations were calculated using calibration curves obtained with external standards. Each aliquot was measured 4 times, and the uncertainty in the reported 56Fe/58Ni ratios represents the two standard deviations (2SD) of these four measurements. 2.5. Precision and Accuracy [9] All Ni isotope ratio data are reported relative to SRM 986 in e units given by: ei ¼

Rsample Rstandard =Rstandard 104

ð1Þ

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where R is the iNi/58Ni ratio (i = 60, 61, or 64). The internal precisions on the Ni isotopic compositions of individual samples, based on the number of repeat measurements, are represented by two standard errors of the mean (2SE). Repeated analyses of an Aesar Ni solution bracketed with the NIST SRM 986 standard over a 35-month period were used to determine the external precisions for e60, e61, and e64. A protocol consisting of 5 repeat measurements during a single analytical session yielded the following external precisions (2SD): e60 (±0.15 e), e61 (±0.83 e), and e64 (±1.5 e). A protocol consisting of 10 repeat measurements during a single analytical session resulted in an improvement of 40% in the external precision for e60 (±0.09 e). Improvements of 37% and 7% were also obtained for e61 (±0.52 e) and e64 (±1.4 e), respectively. All troilite samples were measured using the latter protocol due to the improved ability to resolve potential variations in the Ni isotopes. [10] All purified Ni solutions measured had Fe/Ni (1.6 103) and Zn/Ni (5.0 104) ratios well below the values for which the interference corrections become ineffective [Cook et al., 2006]. Additionally, all sample and standard signal intensities were matched adequately (within 6%) to provide accurate measurements of Ni isotopes [Cook et al., 2006]. Gramlich et al. [1989b] measured numerous terrestrial sulfides and showed that the mass-bias corrected Ni isotopic compositions of terrestrial samples are not resolvably different from those of SRM 986. The e60, e61, and e64 values for the terrestrial sulfides analyzed here and for the processed aliquots of SRM 986 are given in Table 1. As expected, the mean values for all ratios are identical to SRM 986 within uncertainty. Hence, these data demonstrate that our chemical separation and mass spectrometry procedures for sulfides are free from analytical artifacts.

3. Results and Discussion [11] Table 2 lists the Ni isotopic compositions of the troilite samples analyzed in this study. The 56Fe/58Ni ratios for each sample are also listed. In the following discussion, we will only consider those deviations in e60, e61, and e64 values to be ‘‘resolvable’’ if they are outside of both the 2SD external precision and the 3SE error. [12] We did not detect resolvable excesses in e60 from the decay of 60Fe in any of the troilites analyzed (Table 2) despite elevated 56Fe/58Ni ratios up to 8600. However, there are resolvable deficits in 5 of the 11 samples. Cook et al. [2006] discussed that deficits in e60 are expected in early-formed phases with 56Fe/58Ni ratios less than the chondritic value of 24 [Anders and Grevesse, 1989]. However, all of the troilites analyzed have superchondritic 56 Fe/58Ni ratios, and the observed deficits in e60 are inconsistent with the effects from the decay of 60Fe. Additionally, resolvable variations in e61 and e64 occur in several samples, although the positive e61 value for Chupaderos is only marginally outside of the 2SD external precision and is not considered unambiguous. Chen et al. [2007] suggested that apparent variations in Ni isotopes in troilite may be analytical artifacts caused by isobaric interferences. However, our troilite data are not consistent with the potential effects of the following interferences on the various isotopes of Ni: Cd++, Sn++, Te++, TiN, MgS, CaF, ScF, MgAr, NeAr, CaO, ScO, TiO, ArOH, CaOH, ScOH, TiOH, CaC, CrC,

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Fe/58Ni Ratios and Ni Isotopic Compositions of Troilites From Iron Meteoritesa Group

b

Canyon Diablo Tolucab Mundrabilla #1b Mundrabilla #2 Odessa Gressk Augustinovka Bella Rocab Chupaderos Grant Gibeonb

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IAB IAB IAB IAB IIAB IIIAB IIIAB IIIAB IIIAB IVA

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Fe/58Ni ± 2SD 2058 ± 32 200 ± 3 4473 ± 72 3924 ± 64 54.2 ± 0.8 1145 ± 34 4644 ± 136 2919 ± 47 1941 ± 42 8599 ± 115 242 ± 5

e60 ± 2SE

e61 ± 2SE

e64 ± 2SE

n

0.30 ± 0.06 0.01 ± 0.04 2.40 ± 0.21 0.07 ± 0.11 0.09 ± 0.06 0.35 ± 0.13 0.11 ± 0.10 0.42 ± 0.07 0.13 ± 0.12 0.19 ± 0.09 0.12 ± 0.18

0.04 ± 0.55 0.25 ± 0.31 2.02 ± 0.69 0.25 ± 0.53 0.24 ± 0.71 0.02 ± 0.40 0.19 ± 0.70 0.09 ± 0.41 0.58 ± 0.36 0.33 ± 0.66 0.64 ± 0.60

0.5 ± 1.1 2.1 ± 0.6 11 ± 1.0 0.4 ± 1.2 0.4 ± 0.8 0.8 ± 1.1 0.5 ± 0.6 1.3 ± 0.7 1.3 ± 0.7 2.2 ± 0.8 1.6 ± 1.2

9 10 10 10 9 10 9 10 10 10 10

a

SD, standard deviation; SE, standard error; n, number of repeat measurements. Samples previously investigated for non-mass-dependent effects in Ni isotopes in the metal phase [Cook et al., 2006].

b

ScC, TiC, and CoH. In addition, the observed anomalies are not due to an interference affecting only 62Ni. Such an interference could lead to apparent deficits in eNi because 62 Ni is used to correct the mass bias (see section 2.3). However, the resulting deficits would have relative magnitudes such that e61/e60 = 1.5 and e64/e60 = 2.9, which is not observed. Although we cannot rule out the possibility that our data reflect the influence of an as yet unidentified interference, this appears to be an unlikely explanation for the observed variations. [13] In order to investigate possible causes for the observed Ni variations, we used equations 1 to 3 from Dauphas et al. [2004] to calculate mixing lines between the isotopic composition of terrestrial Ni (i.e., SRM 986) and the isotopic composition of Ni produced by the s-process in an AGB star of two solar masses (R. Gallino, unpublished data, 2007), as well as for Ni made in the explosion of a type Ia supernova (SNIa) [Dominguez et al., 2006; I. Dominguez, unpublished data, 2006] and type II supernovae (SNII) of 15, 20, and 25 solar masses [Rauscher et al., 2002]. Because several hundred ppm of Cu is typically contained in troilite from iron meteorites [Buchwald, 1977], we also used the parameterization of Yashima et al. [2002] to calculate the potential effects of copper spallation by 230 MeV galactic cosmic ray protons on the isotopic composition of Ni with a starting composition equal to SRM 986. [14] Our troilite data, along with the mixing lines for an AGB star, a SNIa, a SNII (15 solar masses), and the effects from Cu spallation, are plotted in Figures 1 and 2. [15] Six of our troilite samples contain a clearly resolvable deviation in at least one of the eNi values. Of these, four troilites (i.e., Bella Roca, Canyon Diablo, Gressk, Mundrabilla #1) are most consistent with the trend expected for the 15 solar mass SNII (the trends for the 20 and 25 solar mass SNII, not shown, and the SNIa provide poorer fits to the data), and two troilites (i.e., Grant, Toluca) are consistent with the AGB trend. Conversely, all of the variations reported by Quitte´ et al. [2006] follow the AGB trend. Finally, none of our six anomalous samples is consistent with the effects of Cu spallation. [16] Based on compositional and textural characteristics, Wasson and Kallemeyn [2002] proposed that the nonmagmatic IAB irons formed from quickly solidified melts generated by impacts into carbonaceous chondrite asteroids.

Entrainment of a presolar grain(s) by the troilite melt, followed by rapid cooling and crystallization, could explain the preservation of the large deviations in eNi found in Mundrabilla #1. Quitte´ et al. [2006] remarked that the largest Ni variations tend to occur in IAB troilites. Potential effects in other samples may have been reduced or erased by diffusional equilibration with the surrounding Ni-rich metal during more prolonged cooling. Entrainment versus nonentrainment of presolar grains into melts could account for the distinct compositions of troilites from the same iron (e.g., Mundrabilla), an observation also noted by Quitte´ et al. [2006]. In contrast to IAB irons, magmatic irons formed as the cores of differentiated parent bodies via fractional crystallization of well-mixed metallic melts [e.g., Scott, 1972]. Given their origins, the preservation of Ni anomalies in troilites from magmatic irons would seem unlikely, yet our data and those of Quitte´ et al. [2006] indicate that such effects may survive.

Figure 1. e61 vs. e60 values for troilite in iron meteorites (triangles, magmatic irons; circles, non-magmatic irons). Mixing lines between terrestrial Ni and Ni expected from an AGB star and type Ia and type II supernovae are shown for comparison (see text for details). The effects of Cu spallation by galactic cosmic ray protons on Ni isotopes are also shown.

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of the superchondritic 56Fe/58Ni ratios in these troilites. In addition, resolvable variations were found in e61 and e64 in several samples. The Ni isotopic composition of four anomalous samples is similar to that expected for a mixing trend between terrestrial Ni and the Ni isotopic composition expected from nucleosynthesis in a 15 solar mass type II SN; two anomalous samples follow a mixing trend between terrestrial Ni and that expected from an AGB star. The preservation of the large effects in one of the Mundrabilla troilites is compatible with the Wasson and Kallemeyn [2002] model of group IAB irons forming from rapidly crystallized melts generated by impacts on the IAB parent body. Our data and those of Quitte´ et al. [2006] suggest that components with anomalous Ni isotopic compositions representing two disparate stellar sources may be preserved in troilite in iron meteorites.

Figure 2. e64 vs. e60 values for troilite in iron meteorites (triangles, magmatic irons; circles, non-magmatic irons). Mixing lines between terrestrial Ni and Ni expected from an AGB star and type Ia and type II supernovae are shown for comparison (see text for details). The effects of Cu spallation by galactic cosmic ray protons on Ni isotopes are also shown. [17] Gao and Thiemens [1991] analyzed sulfur isotopes in troilite from iron meteorites; no variations of nucleosynthetic origin were found. If the carrier of the anomalous Ni is a refractory phase (e.g., presolar grain), it would likely be highly depleted in S, because S is a volatile element, and would not make a significant contribution to the sulfur budget in troilite. Also, troilite is 35 wt.% S and any potential effects may be diluted beyond detection by isotopically normal S. Thus, the lack of anomalous S in troilite from iron meteorites sheds no light on the nature of the Ni variations. Isotopic analyses of other transition metals with low abundances in troilite (e.g., Cr, Ti) may help to constrain the origin of the Ni variations. [18] Four of the anomalous troilite samples are from iron meteorites for which we previously investigated the Ni isotopic composition of the metal phase [Cook et al., 2006]. Similar to the findings of Quitte´ et al. [2006], we did not find any Ni anomalies in the metal phase of irons for which deviations were observed in troilite. However, Bizzarro et al. [2007] recently reported uniform deficits in eNi in the metal phase from iron meteorites. The reported effects suggest that on a bulk scale the Ni isotopic composition of iron meteorites differs from terrestrial Ni (i.e., SRM 986). Bizzarro et al. [2007] suggested that the iron meteorite parent bodies accreted prior to the injection of 60 Fe into the early solar system to explain their observed deficits in e60 of 0.25. If this hypothesis is correct, we would expect to find similar uniform deficits in all troilites analyzed as well as uniform collateral effects in e61 and e64. Hence, our troilite data do not support the above hypothesis.

4. Conclusions [19] Resolvable deficits in e60 were detected in five of the eleven troilites analyzed in this study. These deficits are inconsistent with the effects from in-situ 60Fe decay in light

[20] Acknowledgments. We thank Laure Dussubieux for assistance with the quadrupole ICPMS analyses, Lawrence Grossman for providing advice and tools for separating troilite from iron meteorites, Inma Dominguez and Roberto Gallino for providing unpublished data from their respective nucleosynthesis calculations, and John Wasson and an anonymous reviewer for their comments which led to significant improvements in the manuscript. This work was supported in part by the National Aeronautics and Space Administration through grants to RNC, MW, and AMD.

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R. N. Clayton and A. M. Davis, Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL, 60637, USA. D. L. Cook, Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854, USA. (davecook@ rci.rutgers.edu) P. E. Janney and M. Wadhwa, Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Box 871404, Tempe, AZ 85287-1404, USA.

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