Planetary accretion and core formation inferred from Ni isotopes in enstatite meteorites
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Keywords: Ni stable isotopes, enstatite meteorites, aubrites, core formation, metal-sulfide segregation
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Abstract
Figures and Tables
 Figure 1 Ni stable isotope composition of solar system materials (Table S-1). Circles and triangles are chondrites and iron meteorites, respectively (Cameron et al., 2009; Steele et al., 2012; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021), while diamonds are enstatite achondrites and ureilites (Zhu et al., 2022). The uncertainty is mostly at 2 s.e. level, or 2 s.d. level when the 2 s.e. is not available in literature or there are averaged values of multiple data. The grey bar represents the Ni isotope composition of bulk silicate Earth (0.10 ± 0.07 ‰; Klaver et al., 2020; Wang et al., 2021). Enstatite achondrites (diamonds), representing their core composition for Ni, have δ60/58Ni values (0.14 ± 0.18 ‰, 2 s.d.; excluding Norton County) overlapping with ECs, suggesting the core formation does not effectively fractionate Ni isotopes. |  Figure 2 Ni stable isotope compositions of acid leaching phases of Indarch [EH4]. The light blue bar represents the average δ60/58Ni value of bulk ECs. Sulfide phases show similar δ60/58Ni values as bulk and other phases, suggesting the sulfide in chondrites does not cause any Ni isotope variation. |  Figure 3 Ni stable isotope compositions versus (a) Ni contents, (b) Mg#, (c) sulfur contents and (d) S/Ni ratios. The symbols are the same as in Figure 1. EC Ni isotope data are from this study and literature (Cameron et al., 2009; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021), while S content data are from Defouilloy et al. (2016). Lack of correlation between Ni isotopes and Ni contents and Mg# (a, b) indicates Ni isotope variation in enstatite achondrites is not caused by silicate differentiation. We also do not find a co-variation between S contents and Ni isotopes in ECs (c), suggesting sulfide does not control the Ni isotope variation in chondrites. A broad correlation between S/Ni ratio and Ni isotopes for enstatite achondrites is consistent with a mixing between sulfide and metal (d). |  Table 1 Ni stable isotopic compositions of enstatite meteorites. |
| Figure 1 | Figure 2 | Figure 3 | Table 1 |
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Introduction
Core formation is one of the first planetary differentiation processes that can be reconstructed by using the stable isotope composition of siderophile elements. Nickel (Ni) is one such siderophile element; it is also a major element (>1 wt. %) in chondrites, nearly refractory (Tc50 % of 1363 K), and occurs as Ni0 and Ni2+ in planetary materials (Wood et al., 2019
Wood, B.J., Smythe, D.J., Harrison, T. (2019) The condensation temperatures of the elements: A reappraisal. American Mineralogist 104, 844–856. https://doi.org/10.2138/am-2019-6852CCBY
). These properties suggest that Ni stable isotopes probably were not, or only little, fractionated by volatilisation and silicate differentiation processes (Klaver et al., 2020Klaver, M., Ionov, D.A., Takazawa, E., Elliott, T. (2020) The non-chondritic Ni isotope composition of Earth’s mantle. Geochimica et Cosmochimica Acta 268, 405–421. https://doi.org/10.1016/j.gca.2019.10.017
; Wang et al., 2021Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
; Zhu et al., 2022Zhu, K., Barrat, J.-A., Yamaguchi, A., Rouxel, O., Germain, Y., Langlade, J., Moynier, F. (2022) Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth’s Core Formation and Differentiation of the Ureilite Parent Body. Geophysical Research Letters 49, e2021GL095557. https://doi.org/10.1029/2021GL095557
). Core formations in the Earth and other differentiated planets/asteroids can be traced by comparing the bulk Ni stable isotope compositions (represented by chondritic Ni isotope compositions) and Ni isotope compositions in the silicate portion.Previous studies have investigated the Ni stable isotope composition (expressed as δ60/58Ni, the mass dependent deviation of 60Ni/58Ni ratios of samples relative to the ratio of NIST SRM 986, given in per mille) of chondrites with an average δ60/58Ni value of 0.23 ± 0.14 ‰ (2 s.d.) and of the bulk silicate Earth (BSE) with an average δ60/58Ni value of 0.10 ± 0.07 ‰ (2 s.d.) (Cameron et al., 2009
Cameron, V., Vance, D., Archer, C., House, C.H. (2009) A biomarker based on the stable isotopes of nickel. Proceedings of the National Academy of Sciences 106, 10944–10948. https://doi.org/10.1073/pnas.0900726106
; Gall et al., 2017Gall, L., Williams, H.M., Halliday, A.N., Kerr, A.C. (2017) Nickel isotopic composition of the mantle. Geochimica et Cosmochimica Acta 199, 196–209. https://doi.org/10.1016/j.gca.2016.11.016
; Klaver et al., 2020Klaver, M., Ionov, D.A., Takazawa, E., Elliott, T. (2020) The non-chondritic Ni isotope composition of Earth’s mantle. Geochimica et Cosmochimica Acta 268, 405–421. https://doi.org/10.1016/j.gca.2019.10.017
; Wang et al., 2021Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
; Zhu et al., 2022Zhu, K., Barrat, J.-A., Yamaguchi, A., Rouxel, O., Germain, Y., Langlade, J., Moynier, F. (2022) Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth’s Core Formation and Differentiation of the Ureilite Parent Body. Geophysical Research Letters 49, e2021GL095557. https://doi.org/10.1029/2021GL095557
). Klaver et al. (2020)Klaver, M., Ionov, D.A., Takazawa, E., Elliott, T. (2020) The non-chondritic Ni isotope composition of Earth’s mantle. Geochimica et Cosmochimica Acta 268, 405–421. https://doi.org/10.1016/j.gca.2019.10.017
were the first to resolve a difference of Ni stable isotope compositions between the chondrites and BSE (Δ60/58NiChondrites-BSE ≈ 0.13 ‰), which is, however, impossible to explain by single stage terrestrial core formation. Evidences include 1) Ni isotope similarity between chondrites and mantle of ureilite parent body (Zhu et al., 2022Zhu, K., Barrat, J.-A., Yamaguchi, A., Rouxel, O., Germain, Y., Langlade, J., Moynier, F. (2022) Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth’s Core Formation and Differentiation of the Ureilite Parent Body. Geophysical Research Letters 49, e2021GL095557. https://doi.org/10.1029/2021GL095557
), 2) ab initio calculations (Guignard et al., 2020Guignard, J., Quitté, G., Méheut, M., Toplis, M.J., Poitrasson, F., Connetable, D., Roskosz, M. (2020) Nickel isotope fractionation during metal-silicate differentiation of planetesimals: Experimental petrology and ab initio calculations. Geochimica et Cosmochimica Acta 269, 238–256. https://doi.org/10.1016/j.gca.2019.10.028
; Wang et al., 2021Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
), and 3) high pressure experiments (Lazar et al., 2012Lazar, C., Young, E.D., Manning, C.E. (2012) Experimental determination of equilibrium nickel isotope fractionation between metal and silicate from 500 °C to 950 °C. Geochimica et Cosmochimica Acta 86, 276–295. https://doi.org/10.1016/j.gca.2012.02.024
; Guignard et al., 2020Guignard, J., Quitté, G., Méheut, M., Toplis, M.J., Poitrasson, F., Connetable, D., Roskosz, M. (2020) Nickel isotope fractionation during metal-silicate differentiation of planetesimals: Experimental petrology and ab initio calculations. Geochimica et Cosmochimica Acta 269, 238–256. https://doi.org/10.1016/j.gca.2019.10.028
) which show that metal-silicate partitioning of Ni during core segregation at high temperatures and pressures would not induce measurable Ni stable isotope fractionation in Earth’s mantle. Two scenarios were considered for this issue: 1) chondrites cannot represent bulk Earth while chondrules are potential precursor material of Earth (Zhu et al., 2022Zhu, K., Barrat, J.-A., Yamaguchi, A., Rouxel, O., Germain, Y., Langlade, J., Moynier, F. (2022) Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth’s Core Formation and Differentiation of the Ureilite Parent Body. Geophysical Research Letters 49, e2021GL095557. https://doi.org/10.1029/2021GL095557
), and 2) Wang et al. (2021)Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
proposed a different model in which the object (Theia) that hit the Earth during the Moon-forming impact was Mercury-like, i.e. highly reduced, and its mantle may have been rich in sulfide with isotopically light Ni. According to this hypothesis, the impactor mantle merged into the Proto-Earth’s mantle and decreased the chondritic δ60/58Ni value of the proto-Earth’s mantle to the modern BSE value.Enstatite achondrites, which include aubrites (most of them are brecciated; Table 1; Keil, 2010
Keil, K. (2010) Enstatite achondrite meteorites (aubrites) and the histories of their asteroidal parent bodies. Geochemistry 70, 295–317. https://doi.org/10.1016/j.chemer.2010.02.002
), come from the silicate portions of multiple differentiated asteroids that have similar isotope compositions as the Earth-Moon system (Clayton et al., 1984Clayton, R.N., Mayeda, T.K., Rubin, A.E. (1984) Oxygen isotopic compositions of enstatite chondrites and aubrites. Journal of Geophysical Research: Solid Earth 89, C245–C249. https://doi.org/10.1029/JB089iS01p0C245
; Zhu et al., 2021Zhu, K., Moynier, F., Schiller, M., Becker, H., Barrat, J.A., Bizzarro, M. (2021) Tracing the origin and core formation of the enstatite achondrite parent bodies using Cr isotopes. Geochimica et Cosmochimica Acta 308, 256–272. https://doi.org/10.1016/j.gca.2021.05.053
). Depletion in highly siderophile elements (HSEs), supports the idea that their parent body (or bodies) have a core (van Acken et al., 2012avan Acken, D., Brandon, A.D., Lapen, T.J. (2012a) Highly siderophile element and osmium isotope evidence for postcore formation magmatic and impact processes on the aubrite parent body. Meteoritics and Planetary Science 47, 1606–1623. https://doi.org/10.1111/j.1945-5100.2012.01425.x
). Therefore, comparing the Ni isotope signatures between enstatite achondrites and enstatite chondrites (i.e. the potential precursors of enstatite achondrites), is important to understand the Ni isotope fractionation during core formation and the Ni isotope gap between chondrites and BSE (Klaver et al., 2020Klaver, M., Ionov, D.A., Takazawa, E., Elliott, T. (2020) The non-chondritic Ni isotope composition of Earth’s mantle. Geochimica et Cosmochimica Acta 268, 405–421. https://doi.org/10.1016/j.gca.2019.10.017
; Wang et al., 2021Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
; Zhu et al., 2022Zhu, K., Barrat, J.-A., Yamaguchi, A., Rouxel, O., Germain, Y., Langlade, J., Moynier, F. (2022) Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth’s Core Formation and Differentiation of the Ureilite Parent Body. Geophysical Research Letters 49, e2021GL095557. https://doi.org/10.1029/2021GL095557
). Enstatite achondrites are highly reduced (IW−2 to IW−6) like Mercury and rich in sulfide (Mittlefehldt et al., 1998Mittlefehldt, D.W., McCoy, T.J., Goodrich, C.A., Kracher, A. (1998) Non-chondritic meteorites from asteroidal bodies. In: Papike, J.J. (Ed.) Reviews in Mineralogy 36: Planetary Materials. De Gruyter, Berlin, 1–195. https://doi.org/10.1515/9781501508806-019
), so their Ni isotope compositions can also be used to constrain the role of metal-sulfide fractionation. On the other hand, the number of Ni isotope data of enstatite chondrites, are limited, and the Ni isotope variation in chondrites is poorly understood. Thus, we determined the Ni stable isotope compositions of both enstatite chondrites (ECs), including the leaching components, and enstatite achondrites for 1) surveying the Ni stable isotope reservoir of ECs, 2) understanding core formation in highly reduced differentiated asteroids, and 3) understanding the possible origin of the Ni stable isotope difference between the Earth and chondrites.Table 1 Ni stable isotopic compositions of enstatite meteorites.
S content data are from Defouilloy et al. (2016)Defouilloy, C., Cartigny, P., Assayag, N., Moynier, F., Barrat, J.-A. (2016) High-precision sulfur isotope composition of enstatite meteorites and implications of the formation and evolution of their parent bodies. Geochimica et Cosmochimica Acta 172, 393–409. https://doi.org/10.1016/j.gca.2015.10.009. All the bulk enstatite chondrites show average δ60/58Ni values of 0.24 ± 0.08 ‰ (2 s.d., n = 13), and the δ60/58Ni variation should not be caused by the S contents.
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Results
The sample information, elemental and Ni stable isotope data are shown in Table 1. Additional sample information, leaching and analytical methods (using the double spike technique) are provided in detail in the Supplementary Information. Bulk enstatite chondrites show variable δ60/58Ni values ranging from 0.18 ‰ to 0.35 ‰ with an average value of 0.24 ± 0.08 ‰ (2 s.d.) or ± 0.02 ‰ (2 s.e., n = 13; Table 1, Fig. 1; including data from the literature). The Ni isotopic variation is independent of the EH or EL grouping, meteorite fall or find and petrological (E3–E6) types. Bulk and leachates (including leachates of the magnetic sulfide-bearing fraction) of magnetically separated phase mixtures of Indarch (EH4) show similar Ni stable isotope compositions (Fig. 2), and no correlation has been found between S contents and δ60/58Ni values for bulk enstatite chondrites (Fig. 3c).
Figure 1 Ni stable isotope composition of solar system materials (Table S-1). Circles and triangles are chondrites and iron meteorites, respectively (Cameron et al., 2009
Cameron, V., Vance, D., Archer, C., House, C.H. (2009) A biomarker based on the stable isotopes of nickel. Proceedings of the National Academy of Sciences 106, 10944–10948. https://doi.org/10.1073/pnas.0900726106
; Steele et al., 2012Steele, R.C.J., Coath, C.D., Regelous, M., Russell, S., Elliott, T. (2012) Neutron-poor nickel isotope anomalies in meteorites. The Astrophysical Journal 758, 59. https://doi.org/10.1088/0004-637X/758/1/59
; Gall et al., 2017Gall, L., Williams, H.M., Halliday, A.N., Kerr, A.C. (2017) Nickel isotopic composition of the mantle. Geochimica et Cosmochimica Acta 199, 196–209. https://doi.org/10.1016/j.gca.2016.11.016
; Klaver et al., 2020Klaver, M., Ionov, D.A., Takazawa, E., Elliott, T. (2020) The non-chondritic Ni isotope composition of Earth’s mantle. Geochimica et Cosmochimica Acta 268, 405–421. https://doi.org/10.1016/j.gca.2019.10.017
; Wang et al., 2021Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
), while diamonds are enstatite achondrites and ureilites (Zhu et al., 2022Zhu, K., Barrat, J.-A., Yamaguchi, A., Rouxel, O., Germain, Y., Langlade, J., Moynier, F. (2022) Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth’s Core Formation and Differentiation of the Ureilite Parent Body. Geophysical Research Letters 49, e2021GL095557. https://doi.org/10.1029/2021GL095557
). The uncertainty is mostly at 2 s.e. level, or 2 s.d. level when the 2 s.e. is not available in literature or there are averaged values of multiple data. The grey bar represents the Ni isotope composition of bulk silicate Earth (0.10 ± 0.07 ‰; Klaver et al., 2020Klaver, M., Ionov, D.A., Takazawa, E., Elliott, T. (2020) The non-chondritic Ni isotope composition of Earth’s mantle. Geochimica et Cosmochimica Acta 268, 405–421. https://doi.org/10.1016/j.gca.2019.10.017
; Wang et al., 2021Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
). Enstatite achondrites (diamonds), representing their core composition for Ni, have δ60/58Ni values (0.14 ± 0.18 ‰, 2 s.d.; excluding Norton County) overlapping with ECs, suggesting the core formation does not effectively fractionate Ni isotopes.Figure 2 Ni stable isotope compositions of acid leaching phases of Indarch [EH4]. The light blue bar represents the average δ60/58Ni value of bulk ECs. Sulfide phases show similar δ60/58Ni values as bulk and other phases, suggesting the sulfide in chondrites does not cause any Ni isotope variation.
Figure 3 Ni stable isotope compositions versus (a) Ni contents, (b) Mg#, (c) sulfur contents and (d) S/Ni ratios. The symbols are the same as in Figure 1. EC Ni isotope data are from this study and literature (Cameron et al., 2009
Cameron, V., Vance, D., Archer, C., House, C.H. (2009) A biomarker based on the stable isotopes of nickel. Proceedings of the National Academy of Sciences 106, 10944–10948. https://doi.org/10.1073/pnas.0900726106
; Gall et al., 2017Gall, L., Williams, H.M., Halliday, A.N., Kerr, A.C. (2017) Nickel isotopic composition of the mantle. Geochimica et Cosmochimica Acta 199, 196–209. https://doi.org/10.1016/j.gca.2016.11.016
; Klaver et al., 2020Klaver, M., Ionov, D.A., Takazawa, E., Elliott, T. (2020) The non-chondritic Ni isotope composition of Earth’s mantle. Geochimica et Cosmochimica Acta 268, 405–421. https://doi.org/10.1016/j.gca.2019.10.017
; Wang et al., 2021Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
), while S content data are from Defouilloy et al. (2016)Defouilloy, C., Cartigny, P., Assayag, N., Moynier, F., Barrat, J.-A. (2016) High-precision sulfur isotope composition of enstatite meteorites and implications of the formation and evolution of their parent bodies. Geochimica et Cosmochimica Acta 172, 393–409. https://doi.org/10.1016/j.gca.2015.10.009
. Lack of correlation between Ni isotopes and Ni contents and Mg# (a, b) indicates Ni isotope variation in enstatite achondrites is not caused by silicate differentiation. We also do not find a co-variation between S contents and Ni isotopes in ECs (c), suggesting sulfide does not control the Ni isotope variation in chondrites. A broad correlation between S/Ni ratio and Ni isotopes for enstatite achondrites is consistent with a mixing between sulfide and metal (d).In contrast, Ni stable isotope compositions of enstatite achondrites vary more, ranging from −0.03 ± 0.03 ‰ (Pesyanoe) to 0.57 ± 0.06 ‰ (Norton County). δ60/58Ni values of enstatite achondrites do not correlate with Ni contents nor with Mg#, i.e. the molar Mg/(Mg + Fe) ratio (Fig. 3a,b), while their δ60/58Ni values broadly correlate with the S contents and S/Ni ratio (excluding Norton County; Fig. 3c,d). The mean δ60/58Ni values of enstatite achondrites (0.19 ± 0.28 ‰, 2 s.d., n = 14) overlap with ECs and BSE. Samples from the three enstatite achondrite parent bodies, i.e. main-group aubrites, Shallowater and Itqiy (Zhu et al., 2021
Zhu, K., Moynier, F., Schiller, M., Becker, H., Barrat, J.A., Bizzarro, M. (2021) Tracing the origin and core formation of the enstatite achondrite parent bodies using Cr isotopes. Geochimica et Cosmochimica Acta 308, 256–272. https://doi.org/10.1016/j.gca.2021.05.053
) have overlapping Ni isotope compositions.top
Discussion
Ni stable isotope composition of enstatite chondrites. No systematic Ni stable isotope variation exists between meteorite falls and finds, suggesting that terrestrial weathering had no, or just a limited, effect on modifying the Ni stable isotope compositions. The δ60/58Ni similarity of Indarch components suggests that Ni stable isotopes may not be fractionated to a measurable degree during nebular and parent body heating processes at this sampling scale, and nor also during the leaching processes (e.g., intermediate leaching phases might adsorb Ni which could induce kinetic isotope fractionation processes). Also considering the lack of correlation between S contents and Ni isotopes for ECs (Fig. 3c), it is difficult to envision that sulfides in chondrites may cause the δ60/58Ni variation in chondrites (Wang et al., 2021
Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
; Fig. 1). Considering that Ca-Al-rich inclusions (CAIs) are extremely rare in ECs and have rather low Ni contents, CAIs are unlikely to contribute to the Ni stable isotope variation in ECs. Some coarse grained CAIs have extremely heavy δ60/58Ni compositions (Render et al., 2018Render, J., Brennecka, G.A., Wang, S.-J., Wasylenki, L.E., Kleine, T. (2018) A Distinct Nucleosynthetic Heritage for Early Solar System Solids Recorded by Ni Isotope Signatures. The Astrophysical Journal 862, 26. https://doi.org/10.3847/1538-4357/aacb7e
); this implies that the limited δ60/58Ni variation among the ECs, and resolvable but small isotopic variation in other chondrites, could reflect heterogeneities and non-representative sampling of chondrules, matrix and refractory inclusions. Compared to the more equilibrated type 4–6 ECs (with δ60/58Ni = 0.21 ± 0.04 ‰, 2 s.d., n = 6), the unequilibrated ECs of type 3 possess more variable (larger 2 s.d. uncertainties) δ60/58Ni values (0.28 ± 0.12 ‰, 2 s.d., n = 6). This hints that parent body metamorphism equilibrates stable Ni isotopic compositions and thus yields more homogeneous bulk compositions. The average δ60/58Ni composition of ECs is 0.24 ± 0.08 ‰ (2 s.d., n = 13), very similar to the mean of ordinary chondrites (OCs; δ60/58Ni = 0.25 ± 0.18 ‰, 2 s.d., n =16) and carbonaceous chondrites (CCs; δ60/58Ni = 0.23 ± 0.12 ‰, 2 s.d., n = 13). This implies a relatively homogeneous δ60/58Ni of 0.24 ± 0.14 ‰ (2 s.d., n = 43) of diverse nebular reservoirs and this average composition can be used as a baseline for tracing planetary differentiation, despite the potential for small Ni isotope differences on the typical sampling scale.Mineralogical control on δ60/58Ni variations in enstatite achondrites. Enstatite achondrites show variable δ60/58Ni values ranging from −0.03 ± 0.03 ‰ to 0.57 ± 0.06 ‰; note, Norton County shows the heaviest δ60/58Ni value among all the enstatite achondrites, which could be caused by the presence of the Ni-rich mineral carletonmooreite (Ni3Si) (Garvie et al., 2021
Garvie, L.A.J., Ma, C., Ray, S., Domanik, K., Wittmann, A., Wadhwa, M. (2021) Carletonmooreite, Ni3Si, a new silicide from the Norton County aubrite meteorite. American Mineralogist 106, 1828–1834. https://doi.org/10.2138/am-2021-7645
). The impact of this phase on the Ni isotope composition of bulk Norton County is unknown, however. Lack of correlation between Ni contents and Mg# (Fig. 3b) suggests this variation is not controlled by a simple differentiation and partitioning process involving silicates. This is also consistent in that the enstatite contains little Ni, typically less than the detection limit of electron microprobe analyses (Watters and Prinz, 1979Watters, T.R., Prinz, M. (1979) Aubrites: Their origin and relationship to enstatite chondrites. Proceedings of the 10th Lunar and Planetary Science Conference, 19–23 March 1979, Houston, Texas, 1073–1093. https://articles.adsabs.harvard.edu/pdf/1979LPSC…10.1073W
). The Ni budget in enstatite achondrites is dominated by Fe-Ni metal and sulfide (e.g., troilite) with Ni contents of 4–80 wt. % and 0.03–0.9 wt. %, respectively (Casanova et al., 1993Casanova, I., Keil, K., Newsom, H.E. (1993) Composition of metal in aubrites: Constraints on core formation. Geochimica et Cosmochimica Acta 57, 675–682. https://doi.org/10.1016/0016-7037(93)90377-9
; van Acken et al., 2012bvan Acken, D., Humayun, M., Brandon, A.D., Peslier, A.H. (2012b) Siderophile trace elements in metals and sulfides in enstatite achondrites record planetary differentiation in an enstatite chondritic parent body. Geochimica et Cosmochimica Acta 83, 272–291. https://doi.org/10.1016/j.gca.2011.12.025
). Hence, the δ60/58Ni variation in aubrites may be caused by heterogeneities in the fractions of these minerals, since Ni should have different valance states in metal (Ni0) and sulfide, phosphides, and other non-metal phases (Ni2+). It is known that magmatic sulfides in terrestrial komatiites can possess isotopically light δ60/58Ni values down to approximately −1 ‰ (Hiebert et al., 2022Hiebert, R.S., Bekker, A., Houlé, M.G., Rouxel, O.J. (2022) Nickel isotope fractionation in komatiites and associated sulfides in the hart deposit, Late Archean Abitibi Greenstone Belt, Canada. Chemical Geology 603, 120912. https://doi.org/10.1016/j.chemgeo.2022.120912
), however, these rocks formed at very different fO2 compared to the highly reduced enstatite achondrites. The δ60/58Ni variations in ureilites may also be controlled by isotopically light sulfide, relative to metal (Zhu et al., 2022Zhu, K., Barrat, J.-A., Yamaguchi, A., Rouxel, O., Germain, Y., Langlade, J., Moynier, F. (2022) Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth’s Core Formation and Differentiation of the Ureilite Parent Body. Geophysical Research Letters 49, e2021GL095557. https://doi.org/10.1029/2021GL095557
). In the present data set on enstatite achondrites, a relationship between sulfides and the variation in δ60/58Ni is supported by the correlation between Ni/S ratios and δ60/58Ni (except for Norton County; Fig. 3d). δ60/58Ni variations in enstatite achondrites are both isotopically lighter and heavier compared to chondrites. From these observations we conclude that the variable δ60/58Ni values for the main-group aubrites reflects the mixing of sulfides (light) and metal (heavy) in these rocks, and the metal-sulfide fractionation factor can also be roughly estimated as Δ60/58Nimetal-sulfide ≈ 0.3 ‰ by the δ60/58Ni variation amongst enstatite aubrites. This is in accord with the variable δ60/58Ni values of ureilites and interpretation of metal-sulfide mixing (Zhu et al., 2022Zhu, K., Barrat, J.-A., Yamaguchi, A., Rouxel, O., Germain, Y., Langlade, J., Moynier, F. (2022) Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth’s Core Formation and Differentiation of the Ureilite Parent Body. Geophysical Research Letters 49, e2021GL095557. https://doi.org/10.1029/2021GL095557
). It is also comparable to the δ60/58Ni variation in iron meteorites (Fig. 3d, Table S-1), which potentially reflect asteroidal core crystallisation (e.g., Ni et al., 2020Ni, P., Chabot, N.L., Ryan, C.J., Shahar, A. (2020) Heavy iron isotope composition of iron meteorites explained by core crystallization. Nature Geoscience 13, 611–615. https://doi.org/10.1038/s41561-020-0617-y
). Magmatic sulfides in enstatite achondrites have a different petrogenesis compared to sulfides in chondrites which do not show light δ60/58Ni values. The sulfide minerals in enstatite achondrites are diverse; besides troilite, other Ni-rich minerals may contribute to their bulk Ni isotope compositions, e.g., schreibersite [(Fe,Ni)3P], djerfisherite [(K,Na)6(Cu,Fe,Ni)25S26Cl], lawrencite [(Fe,Ni)Cl2] (Weisberg and Kimura, 2012Weisberg, M.K., Kimura, M. (2012) The unequilibrated enstatite chondrites. Geochemistry 72, 101–115. https://doi.org/10.1016/j.chemer.2012.04.003
). In the next section we discuss the origin of the assemblage of Ni-bearing phases in enstatite achondrites and their significance for the difference of δ60/58Ni between chondrites and the bulk silicate Earth.δ60/58Ni compositions of enstatite achondrites and core formation of their parent bodies. Since Ni is very abundant in Fe-Ni metal, the measured δ60/58Ni values of the aubrites (except the acid leaching residue of Larned) should dominantly reflect the composition of the metal in the aubrites. Lack of appreciable fractionation in the trace element signature of the metal suggests that the latter may have been trapped during incomplete metal-silicate segregation in their parent body (Casanova et al., 1993
Casanova, I., Keil, K., Newsom, H.E. (1993) Composition of metal in aubrites: Constraints on core formation. Geochimica et Cosmochimica Acta 57, 675–682. https://doi.org/10.1016/0016-7037(93)90377-9
). Van Acken et al. (2012b)van Acken, D., Humayun, M., Brandon, A.D., Peslier, A.H. (2012b) Siderophile trace elements in metals and sulfides in enstatite achondrites record planetary differentiation in an enstatite chondritic parent body. Geochimica et Cosmochimica Acta 83, 272–291. https://doi.org/10.1016/j.gca.2011.12.025
proposed that the aubrite parent body(ies) experienced a complicated history, including break up and re-accretion by impact, which may have involved trapping of minor metal and sulfide from the cores of their parent bodies. Assuming the metal and sulfide in aubrites represent re-accreted core material, the Ni stable isotope composition of aubrites could be representative of the bulk isotopic composition of the core of the main-group aubrite parent body, i.e. δ60/58Ni = 0.14 ± 0.19 ‰ (2 s.d., n = 11; except for Norton County). This large δ60/58Ni variation of main -group aubrites and large uncertainty of the estimated core δ60/58Ni composition may reflect heterogeneous sulfide distribution in the core. On the other hand, the acid leaching residue of Larned mostly represents the composition of the silicate portion, and thus the mantle after core formation, of the main-group aubrite parent body. The δ60/58Ni value of the silicate portion of Larned, 0.13 ± 0.04 ‰ (2 s.e.), could suggest that both the core and mantle of the aubrite parent body have δ60/58Ni values overlapping those of ECs (0.24 ± 0.08 ‰) and the average of all the chondrites (0.24 ± 0.14 ‰). This suggests limited Ni isotope fraction during the segregation of sulfur-rich cores of the main-group aubrite parent body, i.e. S can be present as an FeS phase in the core (e.g., Wood et al., 2014Wood, B.J., Kiseeva, E.S., Mirolo, F.J. (2014) Accretion and core formation: The effects of sulfur on metal–silicate partition coefficients. Geochimica et Cosmochimica Acta 145, 248–267. https://doi.org/10.1016/j.gca.2014.09.002
). Shallowater and Itqiy contain more metal and show higher Ni contents relative to main-group aubrites (Keil et al., 1989Keil, K., Ntaflos, T., Taylor, G.J., Brearley, A.J., Newsom, H.E., Romig Jr., A.D. (1989) The Shallowater aubrite: Evidence for origin by planetesimal impacts. Geochimica et Cosmochimica Acta 53, 3291–3307. https://doi.org/10.1016/0016-7037(89)90108-7
; Patzer et al., 2001Patzer, A., Hill, D.H., Boynton, W.V. (2001) Itqiy: A metal‐rich enstatite meteorite with achondritic texture. Meteoritics and Planetary Science 36, 1495–1505. https://doi.org/10.1111/j.1945-5100.2001.tb01841.x
), so their bulk δ60/58Ni values may also reflect the isotopic composition of the core of their parent bodies. The δ60/58Ni values, 0.18 ± 0.02 ‰ for Shallowater and 0.31 ± 0.01 ‰ for Itqiy, fall into the average of chondrites too, which is consistent with the conclusion that metal-sulfide segregation processes at highly reduced conditions possibly do not effectively fractionate Ni isotopes. This interpretation of the aubrite data is also supported by the overlapping Ni stable isotope compositions of rocks derived from the mantle of the ureilite parent body and chondrites (Zhu et al., 2022Zhu, K., Barrat, J.-A., Yamaguchi, A., Rouxel, O., Germain, Y., Langlade, J., Moynier, F. (2022) Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth’s Core Formation and Differentiation of the Ureilite Parent Body. Geophysical Research Letters 49, e2021GL095557. https://doi.org/10.1029/2021GL095557
). The mantle of the ureilite parent body also underwent segregation of sulfide melt into its core (Warren et al., 2006Warren, P.H., Ulff-Møller, F., Huber, H., Kallemeyn, G.W. (2006) Siderophile geochemistry of ureilites: A record of early stages of planetesimal core formation. Geochimica et Cosmochimica Acta 70, 2104–2126. https://doi.org/10.1016/j.gca.2005.12.026
). However, this interpretation should be further tested, since 1) metal-sulfide-silicate segregation in aubrite parent bodies may have been incomplete (Casanova et al., 1993Casanova, I., Keil, K., Newsom, H.E. (1993) Composition of metal in aubrites: Constraints on core formation. Geochimica et Cosmochimica Acta 57, 675–682. https://doi.org/10.1016/0016-7037(93)90377-9
), and 2) the δ60/58Ni variation range for the aubrites is relatively large.Earth might be built from chondrules, not bulk chondrites. Since core formation cannot be caused by a Ni isotopic gap between chondrites and Earth (Klaver et al., 2020
Klaver, M., Ionov, D.A., Takazawa, E., Elliott, T. (2020) The non-chondritic Ni isotope composition of Earth’s mantle. Geochimica et Cosmochimica Acta 268, 405–421. https://doi.org/10.1016/j.gca.2019.10.017
), other possible scenarios include 1) pebble accretion (Zhu et al., 2022Zhu, K., Barrat, J.-A., Yamaguchi, A., Rouxel, O., Germain, Y., Langlade, J., Moynier, F. (2022) Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth’s Core Formation and Differentiation of the Ureilite Parent Body. Geophysical Research Letters 49, e2021GL095557. https://doi.org/10.1029/2021GL095557
), 2) a highly reduced Moon-forming impactor (Wang et al., 2021Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
), and 3) nebular fractionation (Morbidelli et al., 2020Morbidelli, A., Libourel, G., Palme, H., Jacobson, S.A., Rubie, D.C. (2020) Subsolar Al/Si and Mg/Si ratios of non-carbonaceous chondrites reveal planetesimal formation during early condensation in the protoplanetary disk. Earth and Planetary Science Letters 538, 116220. https://doi.org/10.1016/j.epsl.2020.116220
). Bulk chondrites are not the only candidates for the precursor material for Earth; instead, chondrules can be the main accretion material for Earth and other terrestrial planets (Johansen et al., 2015Johansen, A., Low, M.-M.M., Lacerda, P., Bizzarro, M. (2015) Growth of asteroids, planetary embryos, and Kuiper belt objects by chondrule accretion. Science Advances 1, e1500109. https://doi.org/10.1126/sciadv.1500109
), which could be tested by measuring Ni isotope compositions of chondrules, especially chondrules from ECs with Earth-like isotopic signatures (Zhu et al., 2020Zhu, K., Moynier, F., Schiller, M., Bizzarro, M. (2020) Dating and Tracing the Origin of Enstatite Chondrite Chondrules with Cr Isotopes. The Astrophysical Journal Letters 894, L26. https://doi.org/10.3847/2041-8213/ab8dca
).Wang et al. (2021)
Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
proposed a different model in which the object (Theia) that hit the Earth during the Moon-forming impact was like Mercury, i.e. highly reduced, with its mantle rich in sulfur and isotopically light Ni. The Ni isotope data of enstatite achondrites cannot be used to directly test this model because enstatite achondrites are mainly composed of sulfide minerals (Keil, 2010Keil, K. (2010) Enstatite achondrite meteorites (aubrites) and the histories of their asteroidal parent bodies. Geochemistry 70, 295–317. https://doi.org/10.1016/j.chemer.2010.02.002
), rather than S2− in silicates (e.g., in the Mercury mantle). This model could be more readily tested by measuring Ni stable isotope compositions of lunar samples and experimental samples of sulfur-rich metal-silicate segregation. Alternatively, this Ni stable isotope fractionation could have occurred during condensation and evaporation at the nebular stage, predating the accretion of the terrestrial planets (Morbidelli et al., 2020Morbidelli, A., Libourel, G., Palme, H., Jacobson, S.A., Rubie, D.C. (2020) Subsolar Al/Si and Mg/Si ratios of non-carbonaceous chondrites reveal planetesimal formation during early condensation in the protoplanetary disk. Earth and Planetary Science Letters 538, 116220. https://doi.org/10.1016/j.epsl.2020.116220
).top
Acknowledgements
KZ thanks an Alexander von Humboldt postdoc fellowship and a UK STFC grant (no. ST/V000888/1). We thank Jean-Alix Barrat and Fred Moynier for samples. Wei Dai is appreciated for sample preparation. Constructive comments from David van Acken and Paul Savage and editorial handling by Helen Williams are highly appreciated. Discussion from Jing-Ya Hu also improved this manuscript.
Editor: Helen Williams
top
References
Cameron, V., Vance, D., Archer, C., House, C.H. (2009) A biomarker based on the stable isotopes of nickel. Proceedings of the National Academy of Sciences 106, 10944–10948. https://doi.org/10.1073/pnas.0900726106
Show in context Previous studies have investigated the Ni stable isotope composition (expressed as δ60/58Ni, the mass dependent deviation of 60Ni/58Ni ratios of samples relative to the ratio of NIST SRM 986, given in per mille) of chondrites with an average δ60/58Ni value of 0.23 ± 0.14 ‰ (2 s.d.) and of the bulk silicate Earth (BSE) with an average δ60/58Ni value of 0.10 ± 0.07 ‰ (2 s.d.) (Cameron et al., 2009; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021; Zhu et al., 2022).
View in article
Cameron et al. (2009).
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Ni stable isotope composition of solar system materials (Table S-1). Circles and triangles are chondrites and iron meteorites, respectively (Cameron et al., 2009; Steele et al., 2012; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021), while diamonds are enstatite achondrites and ureilites (Zhu et al., 2022).
View in article
EC Ni isotope data are from this study and literature (Cameron et al., 2009; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021), while S content data are from Defouilloy et al. (2016).
View in article
Casanova, I., Keil, K., Newsom, H.E. (1993) Composition of metal in aubrites: Constraints on core formation. Geochimica et Cosmochimica Acta 57, 675–682. https://doi.org/10.1016/0016-7037(93)90377-9
Show in context The Ni budget in enstatite achondrites is dominated by Fe-Ni metal and sulfide (e.g., troilite) with Ni contents of 4–80 wt. % and 0.03–0.9 wt. %, respectively (Casanova et al., 1993; van Acken et al., 2012b).
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Lack of appreciable fractionation in the trace element signature of the metal suggests that the latter may have been trapped during incomplete metal-silicate segregation in their parent body (Casanova et al., 1993).
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However, this interpretation should be further tested, since 1) metal-sulfide-silicate segregation in aubrite parent bodies may have been incomplete (Casanova et al., 1993), and 2) the δ60/58Ni variation range for the aubrites is relatively large.
View in article
Clayton, R.N., Mayeda, T.K., Rubin, A.E. (1984) Oxygen isotopic compositions of enstatite chondrites and aubrites. Journal of Geophysical Research: Solid Earth 89, C245–C249. https://doi.org/10.1029/JB089iS01p0C245
Show in context Enstatite achondrites, which include aubrites (most of them are brecciated; Table 1; Keil, 2010), come from the silicate portions of multiple differentiated asteroids that have similar isotope compositions as the Earth-Moon system (Clayton et al., 1984; Zhu et al., 2021).
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Defouilloy, C., Cartigny, P., Assayag, N., Moynier, F., Barrat, J.-A. (2016) High-precision sulfur isotope composition of enstatite meteorites and implications of the formation and evolution of their parent bodies. Geochimica et Cosmochimica Acta 172, 393–409. https://doi.org/10.1016/j.gca.2015.10.009
Show in context S content data are from Defouilloy et al. (2016).
View in article
EC Ni isotope data are from this study and literature (Cameron et al., 2009; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021), while S content data are from Defouilloy et al. (2016).
View in article
Gall, L., Williams, H.M., Halliday, A.N., Kerr, A.C. (2017) Nickel isotopic composition of the mantle. Geochimica et Cosmochimica Acta 199, 196–209. https://doi.org/10.1016/j.gca.2016.11.016
Show in context Gall et al. (2017).
View in article
Gall et al. (2017).
View in article
Previous studies have investigated the Ni stable isotope composition (expressed as δ60/58Ni, the mass dependent deviation of 60Ni/58Ni ratios of samples relative to the ratio of NIST SRM 986, given in per mille) of chondrites with an average δ60/58Ni value of 0.23 ± 0.14 ‰ (2 s.d.) and of the bulk silicate Earth (BSE) with an average δ60/58Ni value of 0.10 ± 0.07 ‰ (2 s.d.) (Cameron et al., 2009; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021; Zhu et al., 2022).
View in article
Ni stable isotope composition of solar system materials (Table S-1). Circles and triangles are chondrites and iron meteorites, respectively (Cameron et al., 2009; Steele et al., 2012; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021), while diamonds are enstatite achondrites and ureilites (Zhu et al., 2022).
View in article
EC Ni isotope data are from this study and literature (Cameron et al., 2009; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021), while S content data are from Defouilloy et al. (2016).
View in article
Garvie, L.A.J., Ma, C., Ray, S., Domanik, K., Wittmann, A., Wadhwa, M. (2021) Carletonmooreite, Ni3Si, a new silicide from the Norton County aubrite meteorite. American Mineralogist 106, 1828–1834. https://doi.org/10.2138/am-2021-7645
Show in context Enstatite achondrites show variable δ60/58Ni values ranging from −0.03 ± 0.03 ‰ to 0.57 ± 0.06 ‰; note, Norton County shows the heaviest δ60/58Ni value among all the enstatite achondrites, which could be caused by the presence of the Ni-rich mineral carletonmooreite (Ni3Si) (Garvie et al., 2021).
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Guignard, J., Quitté, G., Méheut, M., Toplis, M.J., Poitrasson, F., Connetable, D., Roskosz, M. (2020) Nickel isotope fractionation during metal-silicate differentiation of planetesimals: Experimental petrology and ab initio calculations. Geochimica et Cosmochimica Acta 269, 238–256. https://doi.org/10.1016/j.gca.2019.10.028
Show in context Evidences include 1) Ni isotope similarity between chondrites and mantle of ureilite parent body (Zhu et al., 2022), 2) ab initio calculations (Guignard et al., 2020; Wang et al., 2021), and 3) high pressure experiments (Lazar et al., 2012; Guignard et al., 2020) which show that metal-silicate partitioning of Ni during core segregation at high temperatures and pressures would not induce measurable Ni stable isotope fractionation in Earth’s mantle.
View in article
Hiebert, R.S., Bekker, A., Houlé, M.G., Rouxel, O.J. (2022) Nickel isotope fractionation in komatiites and associated sulfides in the hart deposit, Late Archean Abitibi Greenstone Belt, Canada. Chemical Geology 603, 120912. https://doi.org/10.1016/j.chemgeo.2022.120912
Show in context It is known that magmatic sulfides in terrestrial komatiites can possess isotopically light δ60/58Ni values down to approximately −1 ‰ (Hiebert et al., 2022), however, these rocks formed at very different fO2 compared to the highly reduced enstatite achondrites.
View in article
Johansen, A., Low, M.-M.M., Lacerda, P., Bizzarro, M. (2015) Growth of asteroids, planetary embryos, and Kuiper belt objects by chondrule accretion. Science Advances 1, e1500109. https://doi.org/10.1126/sciadv.1500109
Show in context Bulk chondrites are not the only candidates for the precursor material for Earth; instead, chondrules can be the main accretion material for Earth and other terrestrial planets (Johansen et al., 2015), which could be tested by measuring Ni isotope compositions of chondrules, especially chondrules from ECs with Earth-like isotopic signatures (Zhu et al., 2020).
View in article
Keil, K. (2010) Enstatite achondrite meteorites (aubrites) and the histories of their asteroidal parent bodies. Geochemistry 70, 295–317. https://doi.org/10.1016/j.chemer.2010.02.002
Show in context Enstatite achondrites, which include aubrites (most of them are brecciated; Table 1; Keil, 2010), come from the silicate portions of multiple differentiated asteroids that have similar isotope compositions as the Earth-Moon system (Clayton et al., 1984; Zhu et al., 2021).
View in article
The Ni isotope data of enstatite achondrites cannot be used to directly test this model because enstatite achondrites are mainly composed of sulfide minerals (Keil, 2010), rather than S2− in silicates (e.g., in the Mercury mantle).
View in article
Keil, K., Ntaflos, T., Taylor, G.J., Brearley, A.J., Newsom, H.E., Romig Jr., A.D. (1989) The Shallowater aubrite: Evidence for origin by planetesimal impacts. Geochimica et Cosmochimica Acta 53, 3291–3307. https://doi.org/10.1016/0016-7037(89)90108-7
Show in context Shallowater and Itqiy contain more metal and show higher Ni contents relative to main-group aubrites (Keil et al., 1989; Patzer et al., 2001), so their bulk δ60/58Ni values may also reflect the isotopic composition of the core of their parent bodies.
View in article
Klaver, M., Ionov, D.A., Takazawa, E., Elliott, T. (2020) The non-chondritic Ni isotope composition of Earth’s mantle. Geochimica et Cosmochimica Acta 268, 405–421. https://doi.org/10.1016/j.gca.2019.10.017
Show in context These properties suggest that Ni stable isotopes probably were not, or only little, fractionated by volatilisation and silicate differentiation processes (Klaver et al., 2020; Wang et al., 2021; Zhu et al., 2022).
View in article
Previous studies have investigated the Ni stable isotope composition (expressed as δ60/58Ni, the mass dependent deviation of 60Ni/58Ni ratios of samples relative to the ratio of NIST SRM 986, given in per mille) of chondrites with an average δ60/58Ni value of 0.23 ± 0.14 ‰ (2 s.d.) and of the bulk silicate Earth (BSE) with an average δ60/58Ni value of 0.10 ± 0.07 ‰ (2 s.d.) (Cameron et al., 2009; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021; Zhu et al., 2022).
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Klaver et al. (2020) were the first to resolve a difference of Ni stable isotope compositions between the chondrites and BSE (Δ60/58NiChondrites-BSE ≈ 0.13 ‰), which is, however, impossible to explain by single stage terrestrial core formation.
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Therefore, comparing the Ni isotope signatures between enstatite achondrites and enstatite chondrites (i.e. the potential precursors of enstatite achondrites), is important to understand the Ni isotope fractionation during core formation and the Ni isotope gap between chondrites and BSE (Klaver et al., 2020; Wang et al., 2021; Zhu et al., 2022).
View in article
Klaver et al. (2020).
View in article
Klaver et al. (2020).
View in article
Klaver et al. (2020).
View in article
Klaver et al. (2020).
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Klaver et al. (2020).
View in article
Klaver et al. (2020).
View in article
Klaver et al. (2020).
View in article
Klaver et al. (2020).
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Ni stable isotope composition of solar system materials (Table S-1). Circles and triangles are chondrites and iron meteorites, respectively (Cameron et al., 2009; Steele et al., 2012; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021), while diamonds are enstatite achondrites and ureilites (Zhu et al., 2022).
View in article
The grey bar represents the Ni isotope composition of bulk silicate Earth (0.10 ± 0.07 ‰; Klaver et al., 2020; Wang et al., 2021).
View in article
EC Ni isotope data are from this study and literature (Cameron et al., 2009; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021), while S content data are from Defouilloy et al. (2016).
View in article
Since core formation cannot be caused by a Ni isotopic gap between chondrites and Earth (Klaver et al., 2020), other possible scenarios include 1) pebble accretion (Zhu et al., 2022), 2) a highly reduced Moon-forming impactor (Wang et al., 2021), and 3) nebular fractionation (Morbidelli et al., 2020).
View in article
Lazar, C., Young, E.D., Manning, C.E. (2012) Experimental determination of equilibrium nickel isotope fractionation between metal and silicate from 500 °C to 950 °C. Geochimica et Cosmochimica Acta 86, 276–295. https://doi.org/10.1016/j.gca.2012.02.024
Show in context Evidences include 1) Ni isotope similarity between chondrites and mantle of ureilite parent body (Zhu et al., 2022), 2) ab initio calculations (Guignard et al., 2020; Wang et al., 2021), and 3) high pressure experiments (Lazar et al., 2012; Guignard et al., 2020) which show that metal-silicate partitioning of Ni during core segregation at high temperatures and pressures would not induce measurable Ni stable isotope fractionation in Earth’s mantle.
View in article
Mittlefehldt, D.W., McCoy, T.J., Goodrich, C.A., Kracher, A. (1998) Non-chondritic meteorites from asteroidal bodies. In: Papike, J.J. (Ed.) Reviews in Mineralogy 36: Planetary Materials. De Gruyter, Berlin, 1–195. https://doi.org/10.1515/9781501508806-019
Show in context Enstatite achondrites are highly reduced (IW−2 to IW−6) like Mercury and rich in sulfide (Mittlefehldt et al., 1998), so their Ni isotope compositions can also be used to constrain the role of metal-sulfide fractionation.
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Morbidelli, A., Libourel, G., Palme, H., Jacobson, S.A., Rubie, D.C. (2020) Subsolar Al/Si and Mg/Si ratios of non-carbonaceous chondrites reveal planetesimal formation during early condensation in the protoplanetary disk. Earth and Planetary Science Letters 538, 116220. https://doi.org/10.1016/j.epsl.2020.116220
Show in context Since core formation cannot be caused by a Ni isotopic gap between chondrites and Earth (Klaver et al., 2020), other possible scenarios include 1) pebble accretion (Zhu et al., 2022), 2) a highly reduced Moon-forming impactor (Wang et al., 2021), and 3) nebular fractionation (Morbidelli et al., 2020).
View in article
Alternatively, this Ni stable isotope fractionation could have occurred during condensation and evaporation at the nebular stage, predating the accretion of the terrestrial planets (Morbidelli et al., 2020).
View in article
Ni, P., Chabot, N.L., Ryan, C.J., Shahar, A. (2020) Heavy iron isotope composition of iron meteorites explained by core crystallization. Nature Geoscience 13, 611–615. https://doi.org/10.1038/s41561-020-0617-y
Show in context It is also comparable to the δ60/58Ni variation in iron meteorites (Fig. 3d, Table S-1), which potentially reflect asteroidal core crystallisation (e.g., Ni et al., 2020).
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Patzer, A., Hill, D.H., Boynton, W.V. (2001) Itqiy: A metal‐rich enstatite meteorite with achondritic texture. Meteoritics and Planetary Science 36, 1495–1505. https://doi.org/10.1111/j.1945-5100.2001.tb01841.x
Show in context Shallowater and Itqiy contain more metal and show higher Ni contents relative to main-group aubrites (Keil et al., 1989; Patzer et al., 2001), so their bulk δ60/58Ni values may also reflect the isotopic composition of the core of their parent bodies.
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Render, J., Brennecka, G.A., Wang, S.-J., Wasylenki, L.E., Kleine, T. (2018) A Distinct Nucleosynthetic Heritage for Early Solar System Solids Recorded by Ni Isotope Signatures. The Astrophysical Journal 862, 26. https://doi.org/10.3847/1538-4357/aacb7e
Show in context Some coarse grained CAIs have extremely heavy δ60/58Ni compositions (Render et al., 2018); this implies that the limited δ60/58Ni variation among the ECs, and resolvable but small isotopic variation in other chondrites, could reflect heterogeneities and non-representative sampling of chondrules, matrix and refractory inclusions. Compared to the more equilibrated type 4–6 ECs (with δ60/58Ni = 0.21 ± 0.04 ‰, 2 s.d., n = 6), the unequilibrated ECs of type 3 possess more variable (larger 2 s.d. uncertainties) δ60/58Ni values (0.28 ± 0.12 ‰, 2 s.d., n = 6).
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Steele, R.C.J., Coath, C.D., Regelous, M., Russell, S., Elliott, T. (2012) Neutron-poor nickel isotope anomalies in meteorites. The Astrophysical Journal 758, 59. https://doi.org/10.1088/0004-637X/758/1/59
Show in context Ni stable isotope composition of solar system materials (Table S-1). Circles and triangles are chondrites and iron meteorites, respectively (Cameron et al., 2009; Steele et al., 2012; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021), while diamonds are enstatite achondrites and ureilites (Zhu et al., 2022).
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van Acken, D., Brandon, A.D., Lapen, T.J. (2012a) Highly siderophile element and osmium isotope evidence for postcore formation magmatic and impact processes on the aubrite parent body. Meteoritics and Planetary Science 47, 1606–1623. https://doi.org/10.1111/j.1945-5100.2012.01425.x
Show in context Depletion in highly siderophile elements (HSEs), supports the idea that their parent body (or bodies) have a core (van Acken et al., 2012a).
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van Acken, D., Humayun, M., Brandon, A.D., Peslier, A.H. (2012b) Siderophile trace elements in metals and sulfides in enstatite achondrites record planetary differentiation in an enstatite chondritic parent body. Geochimica et Cosmochimica Acta 83, 272–291. https://doi.org/10.1016/j.gca.2011.12.025
Show in context The Ni budget in enstatite achondrites is dominated by Fe-Ni metal and sulfide (e.g., troilite) with Ni contents of 4–80 wt. % and 0.03–0.9 wt. %, respectively (Casanova et al., 1993; van Acken et al., 2012b).
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Van Acken et al. (2012b) proposed that the aubrite parent body(ies) experienced a complicated history, including break up and re-accretion by impact, which may have involved trapping of minor metal and sulfide from the cores of their parent bodies.
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Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
Show in context These properties suggest that Ni stable isotopes probably were not, or only little, fractionated by volatilisation and silicate differentiation processes (Klaver et al., 2020; Wang et al., 2021; Zhu et al., 2022).
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Previous studies have investigated the Ni stable isotope composition (expressed as δ60/58Ni, the mass dependent deviation of 60Ni/58Ni ratios of samples relative to the ratio of NIST SRM 986, given in per mille) of chondrites with an average δ60/58Ni value of 0.23 ± 0.14 ‰ (2 s.d.) and of the bulk silicate Earth (BSE) with an average δ60/58Ni value of 0.10 ± 0.07 ‰ (2 s.d.) (Cameron et al., 2009; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021; Zhu et al., 2022).
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Evidences include 1) Ni isotope similarity between chondrites and mantle of ureilite parent body (Zhu et al., 2022), 2) ab initio calculations (Guignard et al., 2020; Wang et al., 2021), and 3) high pressure experiments (Lazar et al., 2012; Guignard et al., 2020) which show that metal-silicate partitioning of Ni during core segregation at high temperatures and pressures would not induce measurable Ni stable isotope fractionation in Earth’s mantle.
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Two scenarios were considered for this issue: 1) chondrites cannot represent bulk Earth while chondrules are potential precursor material of Earth (Zhu et al., 2022), and 2) Wang et al. (2021) proposed a different model in which the object (Theia) that hit the Earth during the Moon-forming impact was Mercury-like, i.e. highly reduced, and its mantle may have been rich in sulfide with isotopically light Ni.
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Therefore, comparing the Ni isotope signatures between enstatite achondrites and enstatite chondrites (i.e. the potential precursors of enstatite achondrites), is important to understand the Ni isotope fractionation during core formation and the Ni isotope gap between chondrites and BSE (Klaver et al., 2020; Wang et al., 2021; Zhu et al., 2022).
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Wang et al. (2021).
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Wang et al. (2021).
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Ni stable isotope composition of solar system materials (Table S-1). Circles and triangles are chondrites and iron meteorites, respectively (Cameron et al., 2009; Steele et al., 2012; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021), while diamonds are enstatite achondrites and ureilites (Zhu et al., 2022).
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The grey bar represents the Ni isotope composition of bulk silicate Earth (0.10 ± 0.07 ‰; Klaver et al., 2020; Wang et al., 2021).
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EC Ni isotope data are from this study and literature (Cameron et al., 2009; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021), while S content data are from Defouilloy et al. (2016).
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Also considering the lack of correlation between S contents and Ni isotopes for ECs (Fig. 3c), it is difficult to envision that sulfides in chondrites may cause the δ60/58Ni variation in chondrites (Wang et al., 2021; Fig. 1).
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Since core formation cannot be caused by a Ni isotopic gap between chondrites and Earth (Klaver et al., 2020), other possible scenarios include 1) pebble accretion (Zhu et al., 2022), 2) a highly reduced Moon-forming impactor (Wang et al., 2021), and 3) nebular fractionation (Morbidelli et al., 2020).
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Wang et al. (2021) proposed a different model in which the object (Theia) that hit the Earth during the Moon-forming impact was like Mercury, i.e. highly reduced, with its mantle rich in sulfur and isotopically light Ni.
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Warren, P.H., Ulff-Møller, F., Huber, H., Kallemeyn, G.W. (2006) Siderophile geochemistry of ureilites: A record of early stages of planetesimal core formation. Geochimica et Cosmochimica Acta 70, 2104–2126. https://doi.org/10.1016/j.gca.2005.12.026
Show in context The mantle of the ureilite parent body also underwent segregation of sulfide melt into its core (Warren et al., 2006).
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Watters, T.R., Prinz, M. (1979) Aubrites: Their origin and relationship to enstatite chondrites. Proceedings of the 10th Lunar and Planetary Science Conference, 19–23 March 1979, Houston, Texas, 1073–1093. https://articles.adsabs.harvard.edu/pdf/1979LPSC...10.1073W.
Show in context This is also consistent in that the enstatite contains little Ni, typically less than the detection limit of electron microprobe analyses (Watters and Prinz, 1979).
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Weisberg, M.K., Kimura, M. (2012) The unequilibrated enstatite chondrites. Geochemistry 72, 101–115. https://doi.org/10.1016/j.chemer.2012.04.003
Show in context The sulfide minerals in enstatite achondrites are diverse; besides troilite, other Ni-rich minerals may contribute to their bulk Ni isotope compositions, e.g., schreibersite [(Fe,Ni)3P], djerfisherite [(K,Na)6(Cu,Fe,Ni)25S26Cl], lawrencite [(Fe,Ni)Cl2] (Weisberg and Kimura, 2012).
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Wood, B.J., Kiseeva, E.S., Mirolo, F.J. (2014) Accretion and core formation: The effects of sulfur on metal–silicate partition coefficients. Geochimica et Cosmochimica Acta 145, 248–267. https://doi.org/10.1016/j.gca.2014.09.002
Show in context This suggests limited Ni isotope fraction during the segregation of sulfur-rich cores of the main-group aubrite parent body, i.e. S can be present as an FeS phase in the core (e.g., Wood et al., 2014).
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Wood, B.J., Smythe, D.J., Harrison, T. (2019) The condensation temperatures of the elements: A reappraisal. American Mineralogist 104, 844–856. https://doi.org/10.2138/am-2019-6852CCBY
Show in context Nickel (Ni) is one such siderophile element; it is also a major element (>1 wt. %) in chondrites, nearly refractory (Tc50 % of 1363 K), and occurs as Ni0 and Ni2+ in planetary materials (Wood et al., 2019).
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Zhu, K., Moynier, F., Schiller, M., Bizzarro, M. (2020) Dating and Tracing the Origin of Enstatite Chondrite Chondrules with Cr Isotopes. The Astrophysical Journal Letters 894, L26. https://doi.org/10.3847/2041-8213/ab8dca
Show in context Bulk chondrites are not the only candidates for the precursor material for Earth; instead, chondrules can be the main accretion material for Earth and other terrestrial planets (Johansen et al., 2015), which could be tested by measuring Ni isotope compositions of chondrules, especially chondrules from ECs with Earth-like isotopic signatures (Zhu et al., 2020).
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Zhu, K., Moynier, F., Schiller, M., Becker, H., Barrat, J.A., Bizzarro, M. (2021) Tracing the origin and core formation of the enstatite achondrite parent bodies using Cr isotopes. Geochimica et Cosmochimica Acta 308, 256–272. https://doi.org/10.1016/j.gca.2021.05.053
Show in context Enstatite achondrites, which include aubrites (most of them are brecciated; Table 1; Keil, 2010), come from the silicate portions of multiple differentiated asteroids that have similar isotope compositions as the Earth-Moon system (Clayton et al., 1984; Zhu et al., 2021).
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Samples from the three enstatite achondrite parent bodies, i.e. main-group aubrites, Shallowater and Itqiy (Zhu et al., 2021) have overlapping Ni isotope compositions.
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Zhu, K., Barrat, J.-A., Yamaguchi, A., Rouxel, O., Germain, Y., Langlade, J., Moynier, F. (2022) Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth’s Core Formation and Differentiation of the Ureilite Parent Body. Geophysical Research Letters 49, e2021GL095557. https://doi.org/10.1029/2021GL095557
Show in context These properties suggest that Ni stable isotopes probably were not, or only little, fractionated by volatilisation and silicate differentiation processes (Klaver et al., 2020; Wang et al., 2021; Zhu et al., 2022).
View in article
Previous studies have investigated the Ni stable isotope composition (expressed as δ60/58Ni, the mass dependent deviation of 60Ni/58Ni ratios of samples relative to the ratio of NIST SRM 986, given in per mille) of chondrites with an average δ60/58Ni value of 0.23 ± 0.14 ‰ (2 s.d.) and of the bulk silicate Earth (BSE) with an average δ60/58Ni value of 0.10 ± 0.07 ‰ (2 s.d.) (Cameron et al., 2009; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021; Zhu et al., 2022).
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Evidences include 1) Ni isotope similarity between chondrites and mantle of ureilite parent body (Zhu et al., 2022), 2) ab initio calculations (Guignard et al., 2020; Wang et al., 2021), and 3) high pressure experiments (Lazar et al., 2012; Guignard et al., 2020) which show that metal-silicate partitioning of Ni during core segregation at high temperatures and pressures would not induce measurable Ni stable isotope fractionation in Earth’s mantle.
View in article
Two scenarios were considered for this issue: 1) chondrites cannot represent bulk Earth while chondrules are potential precursor material of Earth (Zhu et al., 2022), and 2) Wang et al. (2021) proposed a different model in which the object (Theia) that hit the Earth during the Moon-forming impact was Mercury-like, i.e. highly reduced, and its mantle may have been rich in sulfide with isotopically light Ni.
View in article
Therefore, comparing the Ni isotope signatures between enstatite achondrites and enstatite chondrites (i.e. the potential precursors of enstatite achondrites), is important to understand the Ni isotope fractionation during core formation and the Ni isotope gap between chondrites and BSE (Klaver et al., 2020; Wang et al., 2021; Zhu et al., 2022).
View in article
Ni stable isotope composition of solar system materials (Table S-1). Circles and triangles are chondrites and iron meteorites, respectively (Cameron et al., 2009; Steele et al., 2012; Gall et al., 2017; Klaver et al., 2020; Wang et al., 2021), while diamonds are enstatite achondrites and ureilites (Zhu et al., 2022).
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The δ60/58Ni variations in ureilites may also be controlled by isotopically light sulfide, relative to metal (Zhu et al., 2022).
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This is in accord with the variable δ60/58Ni values of ureilites and interpretation of metal-sulfide mixing (Zhu et al., 2022).
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This interpretation of the aubrite data is also supported by the overlapping Ni stable isotope compositions of rocks derived from the mantle of the ureilite parent body and chondrites (Zhu et al., 2022).
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Since core formation cannot be caused by a Ni isotopic gap between chondrites and Earth (Klaver et al., 2020), other possible scenarios include 1) pebble accretion (Zhu et al., 2022), 2) a highly reduced Moon-forming impactor (Wang et al., 2021), and 3) nebular fractionation (Morbidelli et al., 2020).
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Figures
Figure 1 Ni stable isotope composition of solar system materials (Table S-1). Circles and triangles are chondrites and iron meteorites, respectively (Cameron et al., 2009
Cameron, V., Vance, D., Archer, C., House, C.H. (2009) A biomarker based on the stable isotopes of nickel. Proceedings of the National Academy of Sciences 106, 10944–10948. https://doi.org/10.1073/pnas.0900726106
; Steele et al., 2012Steele, R.C.J., Coath, C.D., Regelous, M., Russell, S., Elliott, T. (2012) Neutron-poor nickel isotope anomalies in meteorites. The Astrophysical Journal 758, 59. https://doi.org/10.1088/0004-637X/758/1/59
; Gall et al., 2017Gall, L., Williams, H.M., Halliday, A.N., Kerr, A.C. (2017) Nickel isotopic composition of the mantle. Geochimica et Cosmochimica Acta 199, 196–209. https://doi.org/10.1016/j.gca.2016.11.016
; Klaver et al., 2020Klaver, M., Ionov, D.A., Takazawa, E., Elliott, T. (2020) The non-chondritic Ni isotope composition of Earth’s mantle. Geochimica et Cosmochimica Acta 268, 405–421. https://doi.org/10.1016/j.gca.2019.10.017
; Wang et al., 2021Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
), while diamonds are enstatite achondrites and ureilites (Zhu et al., 2022Zhu, K., Barrat, J.-A., Yamaguchi, A., Rouxel, O., Germain, Y., Langlade, J., Moynier, F. (2022) Nickel and Chromium Stable Isotopic Composition of Ureilites: Implications for the Earth’s Core Formation and Differentiation of the Ureilite Parent Body. Geophysical Research Letters 49, e2021GL095557. https://doi.org/10.1029/2021GL095557
). The uncertainty is mostly at 2 s.e. level, or 2 s.d. level when the 2 s.e. is not available in literature or there are averaged values of multiple data. The grey bar represents the Ni isotope composition of bulk silicate Earth (0.10 ± 0.07 ‰; Klaver et al., 2020Klaver, M., Ionov, D.A., Takazawa, E., Elliott, T. (2020) The non-chondritic Ni isotope composition of Earth’s mantle. Geochimica et Cosmochimica Acta 268, 405–421. https://doi.org/10.1016/j.gca.2019.10.017
; Wang et al., 2021Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
). Enstatite achondrites (diamonds), representing their core composition for Ni, have δ60/58Ni values (0.14 ± 0.18 ‰, 2 s.d.; excluding Norton County) overlapping with ECs, suggesting the core formation does not effectively fractionate Ni isotopes.
Figure 2 Ni stable isotope compositions of acid leaching phases of Indarch [EH4]. The light blue bar represents the average δ60/58Ni value of bulk ECs. Sulfide phases show similar δ60/58Ni values as bulk and other phases, suggesting the sulfide in chondrites does not cause any Ni isotope variation.
Figure 3 Ni stable isotope compositions versus (a) Ni contents, (b) Mg#, (c) sulfur contents and (d) S/Ni ratios. The symbols are the same as in Figure 1. EC Ni isotope data are from this study and literature (Cameron et al., 2009
Cameron, V., Vance, D., Archer, C., House, C.H. (2009) A biomarker based on the stable isotopes of nickel. Proceedings of the National Academy of Sciences 106, 10944–10948. https://doi.org/10.1073/pnas.0900726106
; Gall et al., 2017Gall, L., Williams, H.M., Halliday, A.N., Kerr, A.C. (2017) Nickel isotopic composition of the mantle. Geochimica et Cosmochimica Acta 199, 196–209. https://doi.org/10.1016/j.gca.2016.11.016
; Klaver et al., 2020Klaver, M., Ionov, D.A., Takazawa, E., Elliott, T. (2020) The non-chondritic Ni isotope composition of Earth’s mantle. Geochimica et Cosmochimica Acta 268, 405–421. https://doi.org/10.1016/j.gca.2019.10.017
; Wang et al., 2021Wang, S.-J., Wang, W., Zhu, J.-M., Wu, Z., Liu, J., Han, G., Teng, F.-Z., Huang, S., Wu, H., Wang, Y., Wu, G., Li, W. (2021) Nickel isotopic evidence for late-stage accretion of Mercury-like differentiated planetary embryos. Nature Communications 12, 294. https://doi.org/10.1038/s41467-020-20525-1
), while S content data are from Defouilloy et al. (2016)Defouilloy, C., Cartigny, P., Assayag, N., Moynier, F., Barrat, J.-A. (2016) High-precision sulfur isotope composition of enstatite meteorites and implications of the formation and evolution of their parent bodies. Geochimica et Cosmochimica Acta 172, 393–409. https://doi.org/10.1016/j.gca.2015.10.009
. Lack of correlation between Ni isotopes and Ni contents and Mg# (a, b) indicates Ni isotope variation in enstatite achondrites is not caused by silicate differentiation. We also do not find a co-variation between S contents and Ni isotopes in ECs (c), suggesting sulfide does not control the Ni isotope variation in chondrites. A broad correlation between S/Ni ratio and Ni isotopes for enstatite achondrites is consistent with a mixing between sulfide and metal (d).