Metal differentiation on asteroids recorded in Zn and Fe isotopic signatures of ureilites
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Keywords: ureilite, stable isotopes, evaporation, core formation
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Abstract
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 Figure 1 (a) Plot of δ66/64Zn vs. Zn concentration in ureilites, earlier interpreted to result from evaporation (Moynier et al., 2010). The lines represent evaporative Rayleigh fractionation. (b) δ66/64Zn vs. bulk Ir/Pd ratio (tracer of solid-liquid metal fractionation). The fractional melting model line shows a trajectory calculated with DIr = 23.28 and DPd = 0.769 after Chabot and Jones (2003), and a hypothetical α(66Zn/64Zn)solid/liquid = 1.00045. The starting composition is defined by the δ66/64Zn of the UPB silicate and the lowest Ir/Pd found. The literature data are from Warren and Huber (2006), Rankenburg et al. (2008), Moynier et al. (2010), Goderis et al. (2015), Brugier et al. (2019). |  Figure 2 (a) Diagram of δ56/54Fe versus bulk Ir/Pd ratio. The model line shows fractional metal melting, calculated after Chabot and Jones (2003), using α(56Fe/54Fe)solid/liquid = 0.99994. (b) δ56/54Fe versus δ66/64Zn for ureilites. The metal fractions align with the model for fractional metal melting. |  Figure 3 Plot of δ57/54Femetal versus δ57/54Feolivine for ureilites and pallasites; the isotherms calculated using the NRIXS parameters for γ-iron and olivine (Dauphas et al., 2012). The unrealistically high temperatures hint towards silicate-metal disequilibrium. |  Figure 4 Profiles of δ56/54Fe and Fa# = Fe2+/(Fe2+ + Mg2+) across an olivine grain of the GRA 95205 ureilite, measured via fsLA-MC-ICP-MS and electron microprobe. In situ pyroxene data of GRA 95205 are shown using black crosses out of distance context. |
| Figure 1 | Figure 2 | Figure 3 | Figure 4 |
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Introduction
The chemical pathways of moderately volatile elements (MVEs) in the Solar System and, ultimately, the compositions of the terrestrial planets, strongly depend on the processes of evaporation and condensation. As kinetic processes under extreme heating regimes drive isotope fractionation, in the recent decade the isotope ratios of MVEs (e.g., K, Fe, Si) have been found to be a useful proxy to unravel the evaporation histories of the planetary materials (Pringle et al., 2014
Pringle, E.A., Moynier, F., Savage, P.S., Badro, J., Barrat, J.-A. (2014) Silicon isotopes in angrites and volatile loss in planetesimals. Proceedings of the National Academy of Sciences 111, 17029–17032. https://doi.org/10.1073/pnas.1418889111
; Dauphas et al., 2015Dauphas, N., Poitrasson, F., Burkhardt, C., Kobayashi, H., Kurosawa, K. (2015) Planetary and meteoritic Mg/Si and δ30Si variations inherited from solar nebula chemistry. Earth and Planetary Science Letters 427, 236–248. https://doi.org/10.1016/j.epsl.2015.07.008
; Wang and Jacobsen, 2016Wang, K., Jacobsen, S.B. (2016) Potassium isotopic evidence for a high-energy giant impact origin of the Moon. Nature 538, 487–490. https://doi.org/10.1038/nature19341
). Ureilites form a group of achondrites particularly interesting in this context due to their depletion in siderophile (e.g., Ag and Ge) and lithophile (K, Rb) volatile elements compared to the Solar System abundances. At the same time Zn, one of the most volatile elements among MVEs known for chalcophile and lithophile properties, is present in higher concentrations than that in chondrites (Goodrich, 1992Goodrich, C.A. (1992) Ureilites: A critical review. Meteoritics 27, 327–352. https://doi.org/10.1111/j.1945-5100.1992.tb00215.x
), and although the exact precursors and delivery mechanisms of Zn on the ureilite parent body (UPB) are not known, isotopically heavy Zn signatures have been interpreted to result from evaporation (Moynier et al., 2010Moynier, F., Beck, P., Yin, Q.-Z., Ferroir, T., Barrat, J.-A., Paniello, R., Telouk, P., Gillet, P. (2010) Volatilization induced by impacts recorded in Zn isotope composition of ureilites. Chemical Geology 276, 374–379. https://doi.org/10.1016/j.chemgeo.2010.07.005
; Brugier et al., 2019Brugier, Y.-A., Barrat, J.-A., Gueguen, B., Agranier, A., Yamaguchi, A., Bischoff, A. (2019) Zinc isotopic variations in ureilites. Geochimica et Cosmochimica Acta 246, 450–460. https://doi.org/10.1016/j.gca.2018.12.009
).The main group ureilites are coarse-grained, ultramafic olivine-pyroxene rocks with interstitial refractory components, including carbon-rich material, Fe-Ni metal and accessories (Mittlefehldt et al., 1998
Mittlefehldt, D.W., McCoy, T.J., Goodrich, C.A., Kracher, A. (1998) Non-chondritic meteorites from asteroidal bodies. In: Papike, J.J. (Ed.) Planetary Materials. Reviews in Mineralogy, 36, Degruyter, Berlin, 523–718. https://doi.org/10.1515/9781501508806-019
). Petrography and geochemistry of ureilites record high degree melting and melt extraction from an unknown lithology similar to a carbonaceous chondrite (Rankenburg et al., 2008Rankenburg, K., Humayun, M., Brandon, A.D., Herrin, J.S. (2008) Highly siderophile elements in ureilites. Geochimica et Cosmochimica Acta 72, 4642–4659. https://doi.org/10.1016/j.gca.2008.07.003
; Collinet and Grove, 2020Collinet, M., Grove, T.L. (2020) Incremental melting in the ureilite parent body: Initial composition, melting temperatures, and melt compositions. Meteoritics & Planetary Science 55, 832–856. https://doi.org/10.1111/maps.13471
). Ureilites are thought to have formed on a heterogeneously accreted planetesimal as mantle restites after extraction of metal and silicate partial melts at 1120–1280 °C (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.) Planetary Materials. Reviews in Mineralogy, 36, Degruyter, Berlin, 523–718. https://doi.org/10.1515/9781501508806-019
). A diversity among the nebular components that accreted into the heterogeneous UPB is supported by non-equilibrated oxygen isotopic signatures of ureilites, plotting along the carbonaceous chondrite anhydrous minerals line (Clayton and Mayeda, 1988Clayton, R.N., Mayeda, T.K. (1988) Formation of ureilites by nebular processes. Geochimica et Cosmochimica Acta 52, 1313–1318. https://doi.org/10.1016/0016-7037(88)90286-4
), and correlation of mineral and O-isotopic compositions (Rai et al., 2020Rai, N., Downes, H., Smith, C. (2020) Ureilite meteorites provide a new model of early planetesimal formation and destruction. Geochemical Perspectives Letters 14, 20–25. https://doi.org/10.7185/geochemlet.2018
). Non-equilibrated oxygen isotope ratios indicate that the UPB differentiation did not reach the magma ocean stage (Rai et al., 2020Rai, N., Downes, H., Smith, C. (2020) Ureilite meteorites provide a new model of early planetesimal formation and destruction. Geochemical Perspectives Letters 14, 20–25. https://doi.org/10.7185/geochemlet.2018
).Mass-dependent variations in the isotope ratios of “non-traditional” systems in meteorites record not only the evaporation/condensation processes, but have also been applied to study planetary differentiation in the early Solar System, such as: (i) the extraction of metal phases during early core formation (Barrat et al., 2015
Barrat, J.-A., Rouxel, O., Wang, K., Moynier, F., Yamaguchi, A., Bischoff, A., Langlade, J. (2015) Early stages of core segregation recorded by Fe isotopes in an asteroidal mantle. Earth and Planetary Science Letters 419, 93–100. https://doi.org/10.1016/j.epsl.2015.03.026
), (ii) fractionation between solid and liquid metal during crystallisation (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
), and the differentiation of silicate magmas during melting and crystallisation (Sossi and Moynier, 2017Sossi, P.A., Moynier, F. (2017) Chemical and isotopic kinship of iron in the Earth and Moon deduced from the lunar Mg-Suite. Earth and Planetary Science Letters 471, 125–135. https://doi.org/10.1016/j.epsl.2017.04.029
). As mantles can misrepresent the composition of PBs due to the complexity of differentiation and isotope fractionation, incomplete sampling of all the PB reservoirs further complicates the interpretation of the isotope fractionation patterns.The overall goal of this work is to constrain the nature of the peculiarly heavy isotopic signatures of Fe and Zn in main group ureilites. It focuses on the roles of evaporation, extraction and differentiation of metallic melts following the development of the incipient core, considering that Zn and Fe are chalcophile and siderophile elements. Although several studies on the Fe and Zn stable isotope ratios of ureilites exist, only whole rock analysis has been covered so far. The combined stable isotopic signatures of silicate and metal reservoirs, mineral separates and in situ analysis aids in deconvolving the processes taking place.
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Materials and Methods
Sixteen main group ureilites, both pigeonite and augite-bearing lithologies, with mean olivine compositions Fo78–97 were allocated (Table S-1). Metal and silicate fractions of the ureilites, and handpicked mineral separates were analysed along with the bulk meteorites. Fe and Zn isotope ratios were measured via MC-ICP-MS, following acid digestion and chromatographic isolation. Finally, lateral profiles of Fe isotopic signatures and major element compositions in mafic silicate minerals of selected ureilites were measured in situ via laser ablation MC-ICP-MS and EMPA, respectively. The description of the samples and methods are reported in the Supplementary Information.
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Results and Discussion
The isotopic compositions of Fe and Zn of the bulk samples, silicate and magnetic separates, and handpicked mineral fractions, as well as in situ fsLA-MC-ICP-MS isotopic profiles of Fe, are summarised in the Supplementary Information. The Fe and Zn isotope ratios correspond to mass-dependent fractionation and fall onto the terrestrial fractionation lines (Fig. S-3).
Fe and Zn isotopic signatures. As observed in previous studies, Zn is isotopically heavy in most bulk ureilites, and the δ66/64Zn value is negatively correlated with the concentration of Zn, suggesting a Rayleigh-like process of Zn removal from the ureilite parent material, associated with Zn isotopic fractionation. Moynier et al. (2010)
Moynier, F., Beck, P., Yin, Q.-Z., Ferroir, T., Barrat, J.-A., Paniello, R., Telouk, P., Gillet, P. (2010) Volatilization induced by impacts recorded in Zn isotope composition of ureilites. Chemical Geology 276, 374–379. https://doi.org/10.1016/j.chemgeo.2010.07.005
and Brugier et al. (2019)Brugier, Y.-A., Barrat, J.-A., Gueguen, B., Agranier, A., Yamaguchi, A., Bischoff, A. (2019) Zinc isotopic variations in ureilites. Geochimica et Cosmochimica Acta 246, 450–460. https://doi.org/10.1016/j.gca.2018.12.009
have previously suggested, based on bulk rock analyses, that such a process may be linked to evaporative loss of Zn from the UPB. While our bulk data agree with this observation, the Zn isotopic signatures measured in the metal and silicate separates define a more complex scenario. While δ66/64Zn values of the metal fraction show large variations (0.64 to 2.46 ‰), the fractionation recorded in the silicate fractions of ureilites is limited. This indicates that the Zn isotopic composition of silicates is significantly less affected by the event that fractionated the Zn isotopic composition of the metal of ureilites towards heavy values during a Rayleigh-like process (Fig. 1a).Figure 1 (a) Plot of δ66/64Zn vs. Zn concentration in ureilites, earlier interpreted to result from evaporation (Moynier et al., 2010
Moynier, F., Beck, P., Yin, Q.-Z., Ferroir, T., Barrat, J.-A., Paniello, R., Telouk, P., Gillet, P. (2010) Volatilization induced by impacts recorded in Zn isotope composition of ureilites. Chemical Geology 276, 374–379. https://doi.org/10.1016/j.chemgeo.2010.07.005
). The lines represent evaporative Rayleigh fractionation. (b) δ66/64Zn vs. bulk Ir/Pd ratio (tracer of solid-liquid metal fractionation). The fractional melting model line shows a trajectory calculated with DIr = 23.28 and DPd = 0.769 after Chabot and Jones (2003)Chabot, N.L., Jones, J.H. (2003) The parameterization of solid metal-liquid metal partitioning of siderophile elements. Meteoritics & Planetary Science 38, 1425–1436. https://doi.org/10.1111/j.1945-5100.2003.tb00248.x
, and a hypothetical α(66Zn/64Zn)solid/liquid = 1.00045. The starting composition is defined by the δ66/64Zn of the UPB silicate and the lowest Ir/Pd found. The literature data are from Warren and Huber (2006)Warren, P.H., Huber, H. (2006) Ureilite petrogenesis: A limited role for smelting during anatexis and catastrophic disruption. Meteoritics & Planetary Science 41, 835–849. https://doi.org/10.1111/j.1945-5100.2006.tb00489.x
, Rankenburg et al. (2008)Rankenburg, K., Humayun, M., Brandon, A.D., Herrin, J.S. (2008) Highly siderophile elements in ureilites. Geochimica et Cosmochimica Acta 72, 4642–4659. https://doi.org/10.1016/j.gca.2008.07.003
, Moynier et al. (2010)Moynier, F., Beck, P., Yin, Q.-Z., Ferroir, T., Barrat, J.-A., Paniello, R., Telouk, P., Gillet, P. (2010) Volatilization induced by impacts recorded in Zn isotope composition of ureilites. Chemical Geology 276, 374–379. https://doi.org/10.1016/j.chemgeo.2010.07.005
, Goderis et al. (2015)Goderis, S., Brandon, A.D., Mayer, B., Humayun, M. (2015) s-Process Os isotope enrichment in ureilites by planetary processing. Earth and Planetary Science Letters 431, 110–118. https://doi.org/10.1016/j.epsl.2015.09.021
, Brugier et al. (2019)Brugier, Y.-A., Barrat, J.-A., Gueguen, B., Agranier, A., Yamaguchi, A., Bischoff, A. (2019) Zinc isotopic variations in ureilites. Geochimica et Cosmochimica Acta 246, 450–460. https://doi.org/10.1016/j.gca.2018.12.009
.As the ratios of highly siderophile elements (HSEs) are largely controlled by the distribution between solid metal and progressively evolving Fe-FeS melts (Chabot and Jones, 2003
Chabot, N.L., Jones, J.H. (2003) The parameterization of solid metal-liquid metal partitioning of siderophile elements. Meteoritics & Planetary Science 38, 1425–1436. https://doi.org/10.1111/j.1945-5100.2003.tb00248.x
), HSEs are known to be fractionated in the process of metal melting during asteroid differentiation. The HSE patterns in ureilitic metal (e.g., the Ir/Pd ratios) are demonstrated to result from batch melting accompanied by the removal of a S-rich melt, followed by admixing of a metal component with near chondritic HSE pattern (Rankenburg et al., 2008Rankenburg, K., Humayun, M., Brandon, A.D., Herrin, J.S. (2008) Highly siderophile elements in ureilites. Geochimica et Cosmochimica Acta 72, 4642–4659. https://doi.org/10.1016/j.gca.2008.07.003
). When the Zn and Fe isotopic signatures of the ureilitic metal are compared to the Ir/Pd ratios of the bulk meteorites (Ir/Pd from 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
; Rankenburg et al., 2008Rankenburg, K., Humayun, M., Brandon, A.D., Herrin, J.S. (2008) Highly siderophile elements in ureilites. Geochimica et Cosmochimica Acta 72, 4642–4659. https://doi.org/10.1016/j.gca.2008.07.003
; Goderis et al., 2015Goderis, S., Brandon, A.D., Mayer, B., Humayun, M. (2015) s-Process Os isotope enrichment in ureilites by planetary processing. Earth and Planetary Science Letters 431, 110–118. https://doi.org/10.1016/j.epsl.2015.09.021
), clear correlations can be observed (Figs. 1b and 2a). Bulk Ir/Pd are virtually identical to that in the metal due to the depletion of HSEs in silicate. Although these trends are defined by only a few data points, they show that these ratios are likely fractionated by the same process. Partial melting and extraction of the metal phases in the UPB may have progressively shifted the Fe and Zn isotope values of ureilites to lower (δ56/54Fe = 0.01 ‰) and higher (δ66/64Zn = 1.27 ‰) values, respectively. Here, we model the evolution of the Fe and Zn isotopic composition of the remaining metal, following 0–99 % fractional melting of chondritic metal (dashed lines in Figs. 1b and 2a) using the Ir and Pd distribution coefficients from Chabot and Jones (2003)Chabot, N.L., Jones, J.H. (2003) The parameterization of solid metal-liquid metal partitioning of siderophile elements. Meteoritics & Planetary Science 38, 1425–1436. https://doi.org/10.1111/j.1945-5100.2003.tb00248.x
at the eutectic S content of the liquid phase. Although the solid/liquid fractionation factors α for 56Fe/54Fe and 66Zn/64Zn are not known for such processes, fractional melting with reasonable fractionation factors (α56/54 = 0.9994 and α66/64 = 1.00045) fits the measured data perfectly. Melting of the metal with such fractionation factors for Fe and Zn isotopes should result in a negative linear correlation of δ56/54Fe and δ66/64Zn, as confirmed by the slope of the measured values (Fig. 2b).Figure 2 (a) Diagram of δ56/54Fe versus bulk Ir/Pd ratio. The model line shows fractional metal melting, calculated after Chabot and Jones (2003)
Chabot, N.L., Jones, J.H. (2003) The parameterization of solid metal-liquid metal partitioning of siderophile elements. Meteoritics & Planetary Science 38, 1425–1436. https://doi.org/10.1111/j.1945-5100.2003.tb00248.x
, using α(56Fe/54Fe)solid/liquid = 0.99994. (b) δ56/54Fe versus δ66/64Zn for ureilites. The metal fractions align with the model for fractional metal melting.Formation of the ureilite interstitial metal by partial melting . The correlated Fe and Zn isotopic compositions and Ir/Pd ratios in the residual ureilite metal strongly indicate that these signatures developed from partitioning between solid and liquid metal phases during progressive partial melting. Similar Fe isotopic fractionation between solid and liquid metal phases, although during fractional crystallisation, has previously been suggested to take place during formation of the metal core(s) of pallasites and iron meteorites (Chernonozhkin et al., 2016
Chernonozhkin, S.M., Goderis, S., Costas-Rodríguez, M., Claeys, P., Vanhaecke, F. (2016) Effect of parent body evolution on equilibrium and kinetic isotope fractionation: a combined Ni and Fe isotope study of iron and stony-iron meteorites. Geochimica et Cosmochimica Acta 186, 168–188. https://doi.org/10.1016/j.gca.2016.04.050
; 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
). The observed direction of the Fe isotope vs. Ir/Pd trend (Fig. 2a) is opposite to that expected from thermodynamic properties, as iron sulfides that tend to melt and extract first, are known to typically have light Fe isotopic composition (e.g., Poitrasson et al., 2005Poitrasson, F., Levasseur, S., Teutsch, N. (2005) Significance of iron isotope mineral fractionation in pallasites and iron meteorites for the core–mantle differentiation of terrestrial planets. Earth and Planetary Science Letters 234, 151–164. https://doi.org/10.1016/j.epsl.2005.02.010
; Weyer et al., 2005Weyer, S., Anbar, A.D., Brey, G.P., Münker, C., Mezger, K., Woodland, A.B. (2005) Iron isotope fractionation during planetary differentiation. Earth and Planetary Science Letters 240, 251–264. https://doi.org/10.1016/j.epsl.2005.09.023
; Chernonozhkin et al., 2017Chernonozhkin, S.M., Weyrauch, M., Goderis, S., Oeser, M., McKibbin, S.J., Horn, I., Hecht, L., Weyer, S., Claeys, P., Vanhaecke, F. (2017) Thermal equilibration of iron meteorite and pallasite parent bodies recorded at the mineral scale by Fe and Ni isotope systematics. Geochimica et Cosmochimica Acta 217, 95–111. https://doi.org/10.1016/j.gca.2017.08.022
). However, several observations favour Fe and Zn isotope fractionation during metal melt extraction on the UPB: (i) the correlation between Ir/Pd, a tracer of metal evolution, and δ66/64Zn, δ56/54Fe within the ureilite metal, and the absence of such correlation within the silicates, which could be expected if evaporation and differentiation occurred simultaneously, is difficult to explain otherwise (Figs. 1b and 2); (ii) the fractionated HSE patterns (Rankenburg et al., 2008Rankenburg, K., Humayun, M., Brandon, A.D., Herrin, J.S. (2008) Highly siderophile elements in ureilites. Geochimica et Cosmochimica Acta 72, 4642–4659. https://doi.org/10.1016/j.gca.2008.07.003
; Goodrich et al., 2013Goodrich, C.A., Ash, R.D., Van Orman, J.A., Domanik, K., McDonough, W.F. (2013) Metallic phases and siderophile elements in main group ureilites: Implications for ureilite petrogenesis. Geochimica et Cosmochimica Acta 112, 340–373. https://doi.org/10.1016/j.gca.2012.06.022
) and overall depletion of ureilites in sulfide (Goodrich, 1992Goodrich, C.A. (1992) Ureilites: A critical review. Meteoritics 27, 327–352. https://doi.org/10.1111/j.1945-5100.1992.tb00215.x
) point to high degrees of Fe-S melt extraction and metal-melt fractionation; (iii) the absence of significant isotopic fractionation of Fe and Zn in the silicate parts, which would be observed in the case of PB scale evaporation.Our data show that the metal phases of the most primitive ureilites with nearly chondritic Ir/Pd ratios are characterised by δ56/54Fe values that are significantly higher than any primitive chondritic reservoir (Fig. 2a), thus requiring an additional isotopic fractionation process. This could result from both (i) mixing with injected impactor metal, as suggested in the literature (Goodrich et al., 2013
Goodrich, C.A., Ash, R.D., Van Orman, J.A., Domanik, K., McDonough, W.F. (2013) Metallic phases and siderophile elements in main group ureilites: Implications for ureilite petrogenesis. Geochimica et Cosmochimica Acta 112, 340–373. https://doi.org/10.1016/j.gca.2012.06.022
), or (ii) equilibration of metal and silicate at lower temperatures.In line with such mixing models, earlier studies on HSEs in ureilites have found hints for mixing arrays in the metal. It has been suggested that the ureilite metal formed as a result of mixing of two components: a residual metal formed by high degree batch melting of primitive Fe-FeS, and a second compound of unknown composition, depleted in HSEs but relatively enriched in less compatible elements (Pd, Au) (Rankenburg et al., 2008
Rankenburg, K., Humayun, M., Brandon, A.D., Herrin, J.S. (2008) Highly siderophile elements in ureilites. Geochimica et Cosmochimica Acta 72, 4642–4659. https://doi.org/10.1016/j.gca.2008.07.003
; Goodrich et al., 2013Goodrich, C.A., Ash, R.D., Van Orman, J.A., Domanik, K., McDonough, W.F. (2013) Metallic phases and siderophile elements in main group ureilites: Implications for ureilite petrogenesis. Geochimica et Cosmochimica Acta 112, 340–373. https://doi.org/10.1016/j.gca.2012.06.022
). Gabriel and Pack (2009)Gabriel, A.D., Pack, A. (2009) Ureilite vein metal – indigeneous or impact material? 40th Lunar and Planetary Science Conference, abstract 2462. https://www.lpi.usra.edu/meetings/lpsc2009/pdf/2462.pdf
found that Ni and Co in the vein metal are not in equilibrium with olivine, and metal fractions of ureilites were reported to have Ni nucleosynthetic anomalies different from those shown by the silicate fraction (Quitté et al., 2010Quitté, G., Markowski, A., Latkoczy, C., Gabriel, A., Pack, A. (2010) Iron-60 heterogeneity and incomplete isotope mixing in the early solar system. The Astrophysical Journal 720, 1215–1224. https://doi.org/10.1088/0004-637X/720/2/1215
). As such, it was suggested that the second metal component might represent the impactor material (Gabriel and Pack, 2009Gabriel, A.D., Pack, A. (2009) Ureilite vein metal – indigeneous or impact material? 40th Lunar and Planetary Science Conference, abstract 2462. https://www.lpi.usra.edu/meetings/lpsc2009/pdf/2462.pdf
).Isotopic fractionation by evaporation. Moynier et al. (2010)
Moynier, F., Beck, P., Yin, Q.-Z., Ferroir, T., Barrat, J.-A., Paniello, R., Telouk, P., Gillet, P. (2010) Volatilization induced by impacts recorded in Zn isotope composition of ureilites. Chemical Geology 276, 374–379. https://doi.org/10.1016/j.chemgeo.2010.07.005
and Brugier et al. (2019)Brugier, Y.-A., Barrat, J.-A., Gueguen, B., Agranier, A., Yamaguchi, A., Bischoff, A. (2019) Zinc isotopic variations in ureilites. Geochimica et Cosmochimica Acta 246, 450–460. https://doi.org/10.1016/j.gca.2018.12.009
have suggested that the heavy Zn isotopic compositions of bulk ureilites and a broad anti-correlation of Zn concentrations and isotope ratios (Fig. 1a) result from the evaporation of Zn. This is unlikely at the recorded ureilite equilibration temperatures (1120–1280 °C, Collinet and Grove, 2020Collinet, M., Grove, T.L. (2020) Incremental melting in the ureilite parent body: Initial composition, melting temperatures, and melt compositions. Meteoritics & Planetary Science 55, 832–856. https://doi.org/10.1111/maps.13471
; compared to temperatures of 453 °C for 50 % Zn condensation from the solar nebula), as the associated temperature must have remained below the evaporation point of silicate minerals, with their Zn isotopic signatures not showing covariation with the Zn contents while the temperature must have been sufficiently high to evaporate the metal. As such, the Zn signatures of ureilites were unlikely affected by the volatilisation process at the PB scale. However, it is possible that some ureilites were locally affected by impact-related evaporation. GRA 95205 is an example of that, as it has the lowest bulk Zn content among the ureilites measured (110 μg/g); its δ66/64Znsilicate value (0.99 ‰) is significantly higher than the signatures observed for the other silicate portions, and its δ56/54Femetal and δ66/64Znmetal (2.46 ‰) are anomalously heavier than what is expected based on the partial melting trend (Figs. 1 and 2). GRA 95205 is also the only ureilite among those studied in this work, which locally shows evidence of melting and recrystallisation of silicate minerals (see Supplementary Information). As such, some parts of the UPB, potentially, did reach temperatures sufficiently high to evaporate both metal and silicate, and it is likely that the isotopic signatures of at least GRA 95205 were overprinted by impact volatilisation.Fe isotopic disequilibrium in ureilites. The observed preferential extraction of heavier Fe isotopes (Fig. 2) into the early portions of Fe-FeS melts rules out equilibrium. Moreover, in five ureilites out of seven, Fe in the silicate is isotopically heavier than in metal, while overall the difference between silicate and metal is negligible (Table S-2). This finding contradicts the prediction by first-order thermodynamic constraints for metal and silicate in smaller planetesimals (Elardo et al., 2019
Elardo, S.M., Shahar, A., Mock, T.D., Sio, C.K. (2019) The effect of core composition on iron isotope fractionation between planetary cores and mantles. Earth and Planetary Science Letters 513, 124–134. https://doi.org/10.1016/j.epsl.2019.02.025
) and the observations of heavier Fe isotopic signatures in metal phases relative to silicates in e.g., iron meteorites, chondrites, pallasites, and aubrites (Zhu et al., 2001Zhu, X.K., Guo, Y., O’Nions, R.K., Young, E.D., Ash, R.D. (2001) Isotopic homogeneity of iron in the early solar nebula. Nature 412, 311–313. https://doi.org/10.1038/35085525
; Poitrasson et al., 2005Poitrasson, F., Levasseur, S., Teutsch, N. (2005) Significance of iron isotope mineral fractionation in pallasites and iron meteorites for the core–mantle differentiation of terrestrial planets. Earth and Planetary Science Letters 234, 151–164. https://doi.org/10.1016/j.epsl.2005.02.010
; Chernonozhkin et al., 2017Chernonozhkin, S.M., Weyrauch, M., Goderis, S., Oeser, M., McKibbin, S.J., Horn, I., Hecht, L., Weyer, S., Claeys, P., Vanhaecke, F. (2017) Thermal equilibration of iron meteorite and pallasite parent bodies recorded at the mineral scale by Fe and Ni isotope systematics. Geochimica et Cosmochimica Acta 217, 95–111. https://doi.org/10.1016/j.gca.2017.08.022
). The uncommon direction of Fe isotopic fractionation hints towards disequilibrium, which agrees with incongruous results of the Fe isotopic metal-olivine thermometer described by Dauphas et al. (2012)Dauphas, N., Roskosz, M., Alp, E.E., Golden, D.C., Sio, C.K., Tissot, F.L.H., Hu, M.Y., Zhao, J., Gao, L., Morris, R.V. (2012) A general moment NRIXS approach to the determination of equilibrium Fe isotopic fractionation factors: Application to goethite and jarosite. Geochimica et Cosmochimica Acta 94, 254–275. https://doi.org/10.1016/j.gca.2012.06.013
(Fig. 3). The ureilites consistently plot at unrealistically high temperatures >1800 °C, as compared to the 1170–1246 °C equilibration temperatures determined for a set of ureilites (Singletary and Grove, 2003Singletary, S.J., Grove, T.L. (2003) Early petrologic processes on the ureilite parent body. Meteoritics & Planetary Science 38, 95–108. https://doi.org/10.1111/j.1945-5100.2003.tb01048.x
). The temperatures imply Fe isotopic disequilibrium in the silicate-metal system of ureilites (Fig. 3), due to the addition of an exogenous component, or more likely due to the Fe isotope fractionation in the metal towards lighter signatures during its extraction to the core (not sampled among collected meteorites) and fractional crystallisation.Figure 3 Plot of δ57/54Femetal versus δ57/54Feolivine for ureilites and pallasites; the isotherms calculated using the NRIXS parameters for γ-iron and olivine (Dauphas et al., 2012
Dauphas, N., Roskosz, M., Alp, E.E., Golden, D.C., Sio, C.K., Tissot, F.L.H., Hu, M.Y., Zhao, J., Gao, L., Morris, R.V. (2012) A general moment NRIXS approach to the determination of equilibrium Fe isotopic fractionation factors: Application to goethite and jarosite. Geochimica et Cosmochimica Acta 94, 254–275. https://doi.org/10.1016/j.gca.2012.06.013
). The unrealistically high temperatures hint towards silicate-metal disequilibrium.An Fe isotopic profile across an olivine grain from GRA 95205 ureilite, analysed with femtosecond laser ablation MC-ICP-MS, is presented in Figure 4, and the data for two extra grains are presented in the Supplementary Information. This olivine has experienced reduction by secondary smelting in the presence of carbon, as indicated by EMPA Fa# profile, potentially related to the impact evaporation. One of the characteristic features of this olivine grain is that the average δ56/54Fe signature across the profile is lighter than that of bulk silicate or olivine Fe isotopic compositions measured via solution MC-ICP-MS. Moreover, the δ56/54Fe lateral profile in the zoned olivine grain indicates a convoluted isotope diffusion process, which can only be explained by superposition of chemical diffusion during cooling (Oeser et al., 2015
Oeser, M., Dohmen, R., Horn, I., Schuth, S., Weyer, S. (2015) Processes and time scales of magmatic evolution as revealed by Fe–Mg chemical and isotopic zoning in natural olivines. Geochimica et Cosmochimica Acta 154, 130–150. https://doi.org/10.1016/j.gca.2015.01.025
), and diffusion of light Fe isotopes into olivine during olivine-metal back reactions. The latter likely related to a late-stage episode of simultaneous melting and reduction of Fe2+ by carbon with the release of CO gas (Warren and Huber, 2006Warren, P.H., Huber, H. (2006) Ureilite petrogenesis: A limited role for smelting during anatexis and catastrophic disruption. Meteoritics & Planetary Science 41, 835–849. https://doi.org/10.1111/j.1945-5100.2006.tb00489.x
). These profiles, together with the abnormally heavy Fe and Zn isotopic signatures of metal phase of GRA 95205 hint towards a potential relation of the impact melting, evaporation and reduction of silicates. Overall, the Fe isotope systematics of ureilites demonstrate that these meteorites were driven out of isotopic equilibrium.Figure 4 Profiles of δ56/54Fe and Fa# = Fe2+/(Fe2+ + Mg2+) across an olivine grain of the GRA 95205 ureilite, measured via fsLA-MC-ICP-MS and electron microprobe. In situ pyroxene data of GRA 95205 are shown using black crosses out of distance context.
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Implications
The co-variation of the Zn and Fe isotopic compositions of ureilite metal with the HSE contents, as well as the metal-silicate isotopic disequilibrium and the olivine isotopic profiles, indicate that the Zn and Fe isotopic signatures of ureilites are controlled by metal differentiation and rapid metal melt extraction, during which the UPB mineral assemblage did not reach isotopic equilibrium. While several cases of the isotopic fractionation of Fe by metal differentiation during planetesimals core formation have been described previously, this work shows the first evidence for Zn isotopic fractionation following such process. Similar to chalcophile Cu (e.g., Williams and Archer, 2011
Williams, H.M., Archer, C. (2011) Copper stable isotopes as tracers of metal–sulphide segregation and fractional crystallisation processes on iron meteorite parent bodies. Geochimica et Cosmochimica Acta 75, 3166–3178. https://doi.org/10.1016/j.gca.2011.03.010
; Savage et al., 2015Savage, P.S., Moynier, F., Chen, H., Shofner, G., Siebert, J., Badro, J., Puchtel, I.S. (2015) Copper isotope evidence for large-scale sulphide fractionation during Earth’s differentiation. Geochemical Perspectives Letters 1, 53–64. https://doi.org/10.7185/geochemlet.1506
), isotope fractionation during Fe-FeS melt extraction into early planetesimal cores should not be uncommon for Zn, considering that Zn is known to show chalcophile properties. Although the demonstrated isotopic fractionation of Zn by metal differentiation argues against evaporative Zn fractionation at the bulk UPB scale, localised impact-induced evaporation (potentially linked with silicates reduction by carbon) remains possible, as evidenced by the case of sample GRA 95205.While evaporation from the planetesimals has often been used to explain the heavy isotope signatures of MVEs in various meteorite groups, this work demonstrates that the role of evaporation may appear exaggerated once the presence of unsampled PB reservoirs (e.g., the UPB crust and core absent in current meteorite collections) and the various potential competitive PB processes are considered. A combination of proxies of planetary differentiation should be used to this end, as a Rayleigh trend of isotopic signatures alone does not constitute a signature unique to vaporisation. The disequilibrium nature of ureilites underlines that single-mineral signatures, as well as metal and silicate separates, provide invaluable insights into the histories of meteorite PBs, which cannot be deduced based on whole rock data only.
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Acknowledgements
The authors acknowledge the support of the FWO/FNRS “Excellence of Science” project ET-HoME - ID 30442502, FRIA-FNRS, the VUB Strategic Research, BELSPO (BAMM! and DESIRED projects), ERC StG “ISoSyc”, FWO ZW15-02 – G0H6216N, and BOF-UGent. The NHM London (curated by Dr. N. Almeida), NIPR Japan, and the NASA Antarctic meteorites collection (recovered by the ANSMET, funded by the NSF and NASA, and curated by the Smithsonian Institution and NASA JSC) are thanked for the meteorite loans. W. Wegner, W. Debouge, S. Cauchies, J. de Jong, and H. Vandeput are thanked for their technical assistance.
Editor: Helen Williams
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References
Barrat, J.-A., Rouxel, O., Wang, K., Moynier, F., Yamaguchi, A., Bischoff, A., Langlade, J. (2015) Early stages of core segregation recorded by Fe isotopes in an asteroidal mantle. Earth and Planetary Science Letters 419, 93–100. https://doi.org/10.1016/j.epsl.2015.03.026
Show in context Mass-dependent variations in the isotope ratios of “non-traditional” systems in meteorites record not only the evaporation/condensation processes, but have also been applied to study planetary differentiation in the early Solar System, such as: (i) the extraction of metal phases during early core formation (Barrat et al., 2015), (ii) fractionation between solid and liquid metal during crystallisation (Ni et al., 2020), and the differentiation of silicate magmas during melting and crystallisation (Sossi and Moynier, 2017).
View in article
Brugier, Y.-A., Barrat, J.-A., Gueguen, B., Agranier, A., Yamaguchi, A., Bischoff, A. (2019) Zinc isotopic variations in ureilites. Geochimica et Cosmochimica Acta 246, 450–460. https://doi.org/10.1016/j.gca.2018.12.009
Show in context At the same time Zn, one of the most volatile elements among MVEs known for chalcophile and lithophile properties, is present in higher concentrations than that in chondrites (Goodrich, 1992), and although the exact precursors and delivery mechanisms of Zn on the ureilite parent body (UPB) are not known, isotopically heavy Zn signatures have been interpreted to result from evaporation (Moynier et al., 2010; Brugier et al., 2019).
View in article
Moynier et al. (2010) and Brugier et al. (2019) have previously suggested, based on bulk rock analyses, that such a process may be linked to evaporative loss of Zn from the UPB.
View in article
The literature data are from Warren and Huber (2006), Rankenburg et al. (2008), Moynier et al. (2010), Goderis et al. (2015), Brugier et al. (2019).
View in article
Moynier et al. (2010) and Brugier et al. (2019) have suggested that the heavy Zn isotopic compositions of bulk ureilites and a broad anti-correlation of Zn concentrations and isotope ratios (Fig. 1a) result from the evaporation of Zn.
View in article
Chabot, N.L., Jones, J.H. (2003) The parameterization of solid metal-liquid metal partitioning of siderophile elements. Meteoritics & Planetary Science 38, 1425–1436. https://doi.org/10.1111/j.1945-5100.2003.tb00248.x
Show in context The fractional melting model line shows a trajectory calculated with D Ir = 23.28 and D Pd = 0.769 after Chabot and Jones (2003), and a hypothetical α(66Zn/64Zn)solid/liquid = 1.00045.
View in article
As the ratios of highly siderophile elements (HSEs) are largely controlled by the distribution between solid metal and progressively evolving Fe-FeS melts (Chabot and Jones, 2003), HSEs are known to be fractionated in the process of metal melting during asteroid differentiation.
View in article
The model line shows fractional metal melting, calculated after Chabot and Jones (2003), using α(56Fe/54Fe)solid/liquid = 0.99994. (b) δ56/54Fe versus δ66/64Zn for ureilites. The metal fractions align with the model for fractional metal melting.
View in article
Chernonozhkin, S.M., Goderis, S., Costas-Rodríguez, M., Claeys, P., Vanhaecke, F. (2016) Effect of parent body evolution on equilibrium and kinetic isotope fractionation: a combined Ni and Fe isotope study of iron and stony-iron meteorites. Geochimica et Cosmochimica Acta 186, 168–188. https://doi.org/10.1016/j.gca.2016.04.050
Show in context Similar Fe isotopic fractionation between solid and liquid metal phases, although during fractional crystallisation, has previously been suggested to take place during formation of the metal core(s) of pallasites and iron meteorites (Chernonozhkin et al., 2016; Ni et al., 2020).
View in article
Chernonozhkin, S.M., Weyrauch, M., Goderis, S., Oeser, M., McKibbin, S.J., Horn, I., Hecht, L., Weyer, S., Claeys, P., Vanhaecke, F. (2017) Thermal equilibration of iron meteorite and pallasite parent bodies recorded at the mineral scale by Fe and Ni isotope systematics. Geochimica et Cosmochimica Acta 217, 95–111. https://doi.org/10.1016/j.gca.2017.08.022
Show in context The observed direction of the Fe isotope vs. Ir/Pd trend (Fig. 2a) is opposite to that expected from thermodynamic properties, as iron sulfides that tend to melt and extract first, are known to typically have light Fe isotopic composition (e.g., Poitrasson et al., 2005; Weyer et al., 2005; Chernonozhkin et al., 2017).
View in article
This finding contradicts the prediction by first-order thermodynamic constraints for metal and silicate in smaller planetesimals (Elardo et al., 2019) and the observations of heavier Fe isotopic signatures in metal phases relative to silicates in e.g., iron meteorites, chondrites, pallasites, and aubrites (Zhu et al., 2001; Poitrasson et al., 2005; Chernonozhkin et al., 2017).
View in article
Clayton, R.N., Mayeda, T.K. (1988) Formation of ureilites by nebular processes. Geochimica et Cosmochimica Acta 52, 1313–1318. https://doi.org/10.1016/0016-7037(88)90286-4
Show in context A diversity among the nebular components that accreted into the heterogeneous UPB is supported by non-equilibrated oxygen isotopic signatures of ureilites, plotting along the carbonaceous chondrite anhydrous minerals line (Clayton and Mayeda, 1988), and correlation of mineral and O-isotopic compositions (Rai et al., 2020).
View in article
Collinet, M., Grove, T.L. (2020) Incremental melting in the ureilite parent body: Initial composition, melting temperatures, and melt compositions. Meteoritics & Planetary Science 55, 832–856. https://doi.org/10.1111/maps.13471
Show in context Petrography and geochemistry of ureilites record high degree melting and melt extraction from an unknown lithology similar to a carbonaceous chondrite (Rankenburg et al., 2008; Collinet and Grove, 2020).
View in article
Dauphas, N., Roskosz, M., Alp, E.E., Golden, D.C., Sio, C.K., Tissot, F.L.H., Hu, M.Y., Zhao, J., Gao, L., Morris, R.V. (2012) A general moment NRIXS approach to the determination of equilibrium Fe isotopic fractionation factors: Application to goethite and jarosite. Geochimica et Cosmochimica Acta 94, 254–275. https://doi.org/10.1016/j.gca.2012.06.013
Show in context The uncommon direction of Fe isotopic fractionation hints towards disequilibrium, which agrees with incongruous results of the Fe isotopic metal-olivine thermometer described by Dauphas et al. (2012) (Fig. 3).
View in article
Plot of δ57/54Femetal versus δ57/54Feolivine for ureilites and pallasites; the isotherms calculated using the NRIXS parameters for γ-iron and olivine (Dauphas et al., 2012).
View in article
Dauphas, N., Poitrasson, F., Burkhardt, C., Kobayashi, H., Kurosawa, K. (2015) Planetary and meteoritic Mg/Si and δ30Si variations inherited from solar nebula chemistry. Earth and Planetary Science Letters 427, 236–248. https://doi.org/10.1016/j.epsl.2015.07.008
Show in context As kinetic processes under extreme heating regimes drive isotope fractionation, in the recent decade the isotope ratios of MVEs (e.g., K, Fe, Si) have been found to be a useful proxy to unravel the evaporation histories of the planetary materials (Pringle et al., 2014; Dauphas et al., 2015; Wang and Jacobsen, 2016).
View in article
Elardo, S.M., Shahar, A., Mock, T.D., Sio, C.K. (2019) The effect of core composition on iron isotope fractionation between planetary cores and mantles. Earth and Planetary Science Letters 513, 124–134. https://doi.org/10.1016/j.epsl.2019.02.025
Show in context This finding contradicts the prediction by first-order thermodynamic constraints for metal and silicate in smaller planetesimals (Elardo et al., 2019) and the observations of heavier Fe isotopic signatures in metal phases relative to silicates in e.g., iron meteorites, chondrites, pallasites, and aubrites (Zhu et al., 2001; Poitrasson et al., 2005; Chernonozhkin et al., 2017).
View in article
Gabriel, A.D., Pack, A. (2009) Ureilite vein metal – indigeneous or impact material? 40th Lunar and Planetary Science Conference, abstract 2462. https://www.lpi.usra.edu/meetings/lpsc2009/pdf/2462.pdf
Show in context It has been suggested that the ureilite metal formed as a result of mixing of two components: a residual metal formed by high degree batch melting of primitive Fe-FeS, and a second compound of unknown composition, depleted in HSEs but relatively enriched in less compatible elements (Pd, Au) (Rankenburg et al., 2008; Goodrich et al., 2013). Gabriel and Pack (2009) found that Ni and Co in the vein metal are not in equilibrium with olivine, and metal fractions of ureilites were reported to have Ni nucleosynthetic anomalies different from those shown by the silicate fraction (Quitté et al., 2010).
View in article
As such, it was suggested that the second metal component might represent the impactor material (Gabriel and Pack, 2009).
View in article
Goderis, S., Brandon, A.D., Mayer, B., Humayun, M. (2015) s-Process Os isotope enrichment in ureilites by planetary processing. Earth and Planetary Science Letters 431, 110–118. https://doi.org/10.1016/j.epsl.2015.09.021
Show in context The literature data are from Warren and Huber (2006), Rankenburg et al. (2008), Moynier et al. (2010), Goderis et al. (2015), Brugier et al. (2019).
View in article
When the Zn and Fe isotopic signatures of the ureilitic metal are compared to the Ir/Pd ratios of the bulk meteorites (Ir/Pd from Warren et al., 2006; Rankenburg et al., 2008; Goderis et al., 2015), clear correlations can be observed (Figs. 1b and 2a).
View in article
Goodrich, C.A. (1992) Ureilites: A critical review. Meteoritics 27, 327–352. https://doi.org/10.1111/j.1945-5100.1992.tb00215.x
Show in context At the same time Zn, one of the most volatile elements among MVEs known for chalcophile and lithophile properties, is present in higher concentrations than that in chondrites (Goodrich, 1992), and although the exact precursors and delivery mechanisms of Zn on the ureilite parent body (UPB) are not known, isotopically heavy Zn signatures have been interpreted to result from evaporation (Moynier et al., 2010; Brugier et al., 2019).
View in article
However, several observations favour Fe and Zn isotope fractionation during metal melt extraction on the UPB: (i) the correlation between Ir/Pd, a tracer of metal evolution, and δ66/64Zn, δ56/54Fe within the ureilite metal, and the absence of such correlation within the silicates, which could be expected if evaporation and differentiation occurred simultaneously, is difficult to explain otherwise (Figs. 1b and 2); (ii) the fractionated HSE patterns (Rankenburg et al., 2008; Goodrich et al., 2013) and overall depletion of ureilites in sulfide (Goodrich, 1992) point to high degrees of Fe-S melt extraction and metal-melt fractionation; (iii) the absence of significant isotopic fractionation of Fe and Zn in the silicate parts, which would be observed in the case of PB scale evaporation.
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Goodrich, C.A., Ash, R.D., Van Orman, J.A., Domanik, K., McDonough, W.F. (2013) Metallic phases and siderophile elements in main group ureilites: Implications for ureilite petrogenesis. Geochimica et Cosmochimica Acta 112, 340–373. https://doi.org/10.1016/j.gca.2012.06.022
Show in context However, several observations favour Fe and Zn isotope fractionation during metal melt extraction on the UPB: (i) the correlation between Ir/Pd, a tracer of metal evolution, and δ66/64Zn, δ56/54Fe within the ureilite metal, and the absence of such correlation within the silicates, which could be expected if evaporation and differentiation occurred simultaneously, is difficult to explain otherwise (Figs. 1b and 2); (ii) the fractionated HSE patterns (Rankenburg et al., 2008; Goodrich et al., 2013) and overall depletion of ureilites in sulfide (Goodrich, 1992) point to high degrees of Fe-S melt extraction and metal-melt fractionation; (iii) the absence of significant isotopic fractionation of Fe and Zn in the silicate parts, which would be observed in the case of PB scale evaporation.
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This could result from both (i) mixing with injected impactor metal, as suggested in the literature (Goodrich et al., 2013), or (ii) equilibration of metal and silicate at lower temperatures.
View in article
It has been suggested that the ureilite metal formed as a result of mixing of two components: a residual metal formed by high degree batch melting of primitive Fe-FeS, and a second compound of unknown composition, depleted in HSEs but relatively enriched in less compatible elements (Pd, Au) (Rankenburg et al., 2008; Goodrich et al., 2013). Gabriel and Pack (2009) found that Ni and Co in the vein metal are not in equilibrium with olivine, and metal fractions of ureilites were reported to have Ni nucleosynthetic anomalies different from those shown by the silicate fraction (Quitté et al., 2010).
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.) Planetary Materials. Reviews in Mineralogy, 36, Degruyter, Berlin, 523–718. https://doi.org/10.1515/9781501508806-019
Show in context The main group ureilites are coarse-grained, ultramafic olivine-pyroxene rocks with interstitial refractory components, including carbon-rich material, Fe-Ni metal and accessories (Mittlefehldt et al., 1998).
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Ureilites are thought to have formed on a heterogeneously accreted planetesimal as mantle restites after extraction of metal and silicate partial melts at 1120–1280 °C (Mittlefehldt et al., 1998).
View in article
Moynier, F., Beck, P., Yin, Q.-Z., Ferroir, T., Barrat, J.-A., Paniello, R., Telouk, P., Gillet, P. (2010) Volatilization induced by impacts recorded in Zn isotope composition of ureilites. Chemical Geology 276, 374–379. https://doi.org/10.1016/j.chemgeo.2010.07.005
Show in context At the same time Zn, one of the most volatile elements among MVEs known for chalcophile and lithophile properties, is present in higher concentrations than that in chondrites (Goodrich, 1992), and although the exact precursors and delivery mechanisms of Zn on the ureilite parent body (UPB) are not known, isotopically heavy Zn signatures have been interpreted to result from evaporation (Moynier et al., 2010; Brugier et al., 2019).
View in article
Moynier et al. (2010) and Brugier et al. (2019) have previously suggested, based on bulk rock analyses, that such a process may be linked to evaporative loss of Zn from the UPB.
View in article
Zn concentration in ureilites, earlier interpreted to result from evaporation (Moynier et al., 2010).
View in article
The literature data are from Warren and Huber (2006), Rankenburg et al. (2008), Moynier et al. (2010), Goderis et al. (2015), Brugier et al. (2019).
View in article
Moynier et al. (2010) and Brugier et al. (2019) have suggested that the heavy Zn isotopic compositions of bulk ureilites and a broad anti-correlation of Zn concentrations and isotope ratios (Fig. 1a) result from the evaporation of Zn.
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 Mass-dependent variations in the isotope ratios of “non-traditional” systems in meteorites record not only the evaporation/condensation processes, but have also been applied to study planetary differentiation in the early Solar System, such as: (i) the extraction of metal phases during early core formation (Barrat et al., 2015), (ii) fractionation between solid and liquid metal during crystallisation (Ni et al., 2020), and the differentiation of silicate magmas during melting and crystallisation (Sossi and Moynier, 2017).
View in article
Similar Fe isotopic fractionation between solid and liquid metal phases, although during fractional crystallisation, has previously been suggested to take place during formation of the metal core(s) of pallasites and iron meteorites (Chernonozhkin et al., 2016; Ni et al., 2020).
View in article
Oeser, M., Dohmen, R., Horn, I., Schuth, S., Weyer, S. (2015) Processes and time scales of magmatic evolution as revealed by Fe–Mg chemical and isotopic zoning in natural olivines. Geochimica et Cosmochimica Acta 154, 130–150. https://doi.org/10.1016/j.gca.2015.01.025
Show in context Moreover, the δ56/54Fe lateral profile in the zoned olivine grain indicates a convoluted isotope diffusion process, which can only be explained by superposition of chemical diffusion during cooling (Oeser et al., 2015), and diffusion of light Fe isotopes into olivine during olivine-metal back reactions.
View in article
Poitrasson, F., Levasseur, S., Teutsch, N. (2005) Significance of iron isotope mineral fractionation in pallasites and iron meteorites for the core–mantle differentiation of terrestrial planets. Earth and Planetary Science Letters 234, 151–164. https://doi.org/10.1016/j.epsl.2005.02.010
Show in context The observed direction of the Fe isotope vs. Ir/Pd trend (Fig. 2a) is opposite to that expected from thermodynamic properties, as iron sulfides that tend to melt and extract first, are known to typically have light Fe isotopic composition (e.g., Poitrasson et al., 2005; Weyer et al., 2005; Chernonozhkin et al., 2017).
View in article
This finding contradicts the prediction by first-order thermodynamic constraints for metal and silicate in smaller planetesimals (Elardo et al., 2019) and the observations of heavier Fe isotopic signatures in metal phases relative to silicates in e.g., iron meteorites, chondrites, pallasites, and aubrites (Zhu et al., 2001; Poitrasson et al., 2005; Chernonozhkin et al., 2017).
View in article
Pringle, E.A., Moynier, F., Savage, P.S., Badro, J., Barrat, J.-A. (2014) Silicon isotopes in angrites and volatile loss in planetesimals. Proceedings of the National Academy of Sciences 111, 17029–17032. https://doi.org/10.1073/pnas.1418889111
Show in context As kinetic processes under extreme heating regimes drive isotope fractionation, in the recent decade the isotope ratios of MVEs (e.g., K, Fe, Si) have been found to be a useful proxy to unravel the evaporation histories of the planetary materials (Pringle et al., 2014; Dauphas et al., 2015; Wang and Jacobsen, 2016).
View in article
Quitté, G., Markowski, A., Latkoczy, C., Gabriel, A., Pack, A. (2010) Iron-60 heterogeneity and incomplete isotope mixing in the early solar system. The Astrophysical Journal 720, 1215–1224. https://doi.org/10.1088/0004-637X/720/2/1215
Show in context It has been suggested that the ureilite metal formed as a result of mixing of two components: a residual metal formed by high degree batch melting of primitive Fe-FeS, and a second compound of unknown composition, depleted in HSEs but relatively enriched in less compatible elements (Pd, Au) (Rankenburg et al., 2008; Goodrich et al., 2013). Gabriel and Pack (2009) found that Ni and Co in the vein metal are not in equilibrium with olivine, and metal fractions of ureilites were reported to have Ni nucleosynthetic anomalies different from those shown by the silicate fraction (Quitté et al., 2010).
View in article
Rai, N., Downes, H., Smith, C. (2020) Ureilite meteorites provide a new model of early planetesimal formation and destruction. Geochemical Perspectives Letters 14, 20–25. https://doi.org/10.7185/geochemlet.2018
Show in context A diversity among the nebular components that accreted into the heterogeneous UPB is supported by non-equilibrated oxygen isotopic signatures of ureilites, plotting along the carbonaceous chondrite anhydrous minerals line (Clayton and Mayeda, 1988), and correlation of mineral and O-isotopic compositions (Rai et al., 2020).
View in article
Non-equilibrated oxygen isotope ratios indicate that the UPB differentiation did not reach the magma ocean stage (Rai et al., 2020).
View in article
Rankenburg, K., Humayun, M., Brandon, A.D., Herrin, J.S. (2008) Highly siderophile elements in ureilites. Geochimica et Cosmochimica Acta 72, 4642–4659. https://doi.org/10.1016/j.gca.2008.07.003
Show in context Petrography and geochemistry of ureilites record high degree melting and melt extraction from an unknown lithology similar to a carbonaceous chondrite (Rankenburg et al., 2008; Collinet and Grove, 2020).
View in article
The literature data are from Warren and Huber (2006), Rankenburg et al. (2008), Moynier et al. (2010), Goderis et al. (2015), Brugier et al. (2019).
View in article
The HSE patterns in ureilitic metal (e.g., the Ir/Pd ratios) are demonstrated to result from batch melting accompanied by the removal of a S-rich melt, followed by admixing of a metal component with near chondritic HSE pattern (Rankenburg et al., 2008).
View in article
When the Zn and Fe isotopic signatures of the ureilitic metal are compared to the Ir/Pd ratios of the bulk meteorites (Ir/Pd from Warren et al., 2006; Rankenburg et al., 2008; Goderis et al., 2015), clear correlations can be observed (Figs. 1b and 2a).
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However, several observations favour Fe and Zn isotope fractionation during metal melt extraction on the UPB: (i) the correlation between Ir/Pd, a tracer of metal evolution, and δ66/64Zn, δ56/54Fe within the ureilite metal, and the absence of such correlation within the silicates, which could be expected if evaporation and differentiation occurred simultaneously, is difficult to explain otherwise (Figs. 1b and 2); (ii) the fractionated HSE patterns (Rankenburg et al., 2008; Goodrich et al., 2013) and overall depletion of ureilites in sulfide (Goodrich, 1992) point to high degrees of Fe-S melt extraction and metal-melt fractionation; (iii) the absence of significant isotopic fractionation of Fe and Zn in the silicate parts, which would be observed in the case of PB scale evaporation.
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It has been suggested that the ureilite metal formed as a result of mixing of two components: a residual metal formed by high degree batch melting of primitive Fe-FeS, and a second compound of unknown composition, depleted in HSEs but relatively enriched in less compatible elements (Pd, Au) (Rankenburg et al., 2008; Goodrich et al., 2013). Gabriel and Pack (2009) found that Ni and Co in the vein metal are not in equilibrium with olivine, and metal fractions of ureilites were reported to have Ni nucleosynthetic anomalies different from those shown by the silicate fraction (Quitté et al., 2010).
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Savage, P.S., Moynier, F., Chen, H., Shofner, G., Siebert, J., Badro, J., Puchtel, I.S. (2015) Copper isotope evidence for large-scale sulphide fractionation during Earth’s differentiation. Geochemical Perspectives Letters 1, 53–64. https://doi.org/10.7185/geochemlet.1506
Show in context Similar to chalcophile Cu (e.g., Williams and Archer, 2011; Savage et al., 2015), isotope fractionation during Fe-FeS melt extraction into early planetesimal cores should not be uncommon for Zn, considering that Zn is known to show chalcophile properties.
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Singletary, S.J., Grove, T.L. (2003) Early petrologic processes on the ureilite parent body. Meteoritics & Planetary Science 38, 95–108. https://doi.org/10.1111/j.1945-5100.2003.tb01048.x
Show in context The ureilites consistently plot at unrealistically high temperatures >1800 °C, as compared to the 1170–1246 °C equilibration temperatures determined for a set of ureilites (Singletary and Grove, 2003).
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Sossi, P.A., Moynier, F. (2017) Chemical and isotopic kinship of iron in the Earth and Moon deduced from the lunar Mg-Suite. Earth and Planetary Science Letters 471, 125–135. https://doi.org/10.1016/j.epsl.2017.04.029
Show in context Mass-dependent variations in the isotope ratios of “non-traditional” systems in meteorites record not only the evaporation/condensation processes, but have also been applied to study planetary differentiation in the early Solar System, such as: (i) the extraction of metal phases during early core formation (Barrat et al., 2015), (ii) fractionation between solid and liquid metal during crystallisation (Ni et al., 2020), and the differentiation of silicate magmas during melting and crystallisation (Sossi and Moynier, 2017).
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Wang, K., Jacobsen, S.B. (2016) Potassium isotopic evidence for a high-energy giant impact origin of the Moon. Nature 538, 487–490. https://doi.org/10.1038/nature19341
Show in context As kinetic processes under extreme heating regimes drive isotope fractionation, in the recent decade the isotope ratios of MVEs (e.g., K, Fe, Si) have been found to be a useful proxy to unravel the evaporation histories of the planetary materials (Pringle et al., 2014; Dauphas et al., 2015; Wang and Jacobsen, 2016).
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Warren, P.H., Huber, H. (2006) Ureilite petrogenesis: A limited role for smelting during anatexis and catastrophic disruption. Meteoritics & Planetary Science 41, 835–849. https://doi.org/10.1111/j.1945-5100.2006.tb00489.x
Show in context The literature data are from Warren and Huber (2006), Rankenburg et al. (2008), Moynier et al. (2010), Goderis et al. (2015), Brugier et al. (2019).
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The latter likely related to a late-stage episode of simultaneous melting and reduction of Fe2+ by carbon with the release of CO gas (Warren and Huber, 2006).
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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 When the Zn and Fe isotopic signatures of the ureilitic metal are compared to the Ir/Pd ratios of the bulk meteorites (Ir/Pd from Warren et al., 2006; Rankenburg et al., 2008; Goderis et al., 2015), clear correlations can be observed (Figs. 1b and 2a).
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Weyer, S., Anbar, A.D., Brey, G.P., Münker, C., Mezger, K., Woodland, A.B. (2005) Iron isotope fractionation during planetary differentiation. Earth and Planetary Science Letters 240, 251–264. https://doi.org/10.1016/j.epsl.2005.09.023
Show in context The observed direction of the Fe isotope vs. Ir/Pd trend (Fig. 2a) is opposite to that expected from thermodynamic properties, as iron sulfides that tend to melt and extract first, are known to typically have light Fe isotopic composition (e.g., Poitrasson et al., 2005; Weyer et al., 2005; Chernonozhkin et al., 2017).
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Williams, H.M., Archer, C. (2011) Copper stable isotopes as tracers of metal–sulphide segregation and fractional crystallisation processes on iron meteorite parent bodies. Geochimica et Cosmochimica Acta 75, 3166–3178. https://doi.org/10.1016/j.gca.2011.03.010
Show in context Similar to chalcophile Cu (e.g., Williams and Archer, 2011; Savage et al., 2015), isotope fractionation during Fe-FeS melt extraction into early planetesimal cores should not be uncommon for Zn, considering that Zn is known to show chalcophile properties.
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Zhu, X.K., Guo, Y., O’Nions, R.K., Young, E.D., Ash, R.D. (2001) Isotopic homogeneity of iron in the early solar nebula. Nature 412, 311–313. https://doi.org/10.1038/35085525
Show in context This finding contradicts the prediction by first-order thermodynamic constraints for metal and silicate in smaller planetesimals (Elardo et al., 2019) and the observations of heavier Fe isotopic signatures in metal phases relative to silicates in e.g., iron meteorites, chondrites, pallasites, and aubrites (Zhu et al., 2001; Poitrasson et al., 2005; Chernonozhkin et al., 2017).
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Supplementary Information
The Supplementary Information includes:
- Materials and Methods
- Mineral Separation and the Significance of Individual Mineral Fractions
- Mass Balance of the Zn Isotopic Compositions of the Bulk Ureilites and Their Silicate and Metal Reservoirs
- Mixing as a Potential Mechanism for the Observed Trends of Zn Isotope Ratios in the Metal Reservoir of Ureilites
- Batch versus Fractional Melting to Model the Metal Reservoir of Ureilites
- Tables S-1 to S-4
- Figures S-1 to S-5
- Supplementary Information References
Download the Supplementary Information (PDF)
Figures
Figure 1 (a) Plot of δ66/64Zn vs. Zn concentration in ureilites, earlier interpreted to result from evaporation (Moynier et al., 2010
Moynier, F., Beck, P., Yin, Q.-Z., Ferroir, T., Barrat, J.-A., Paniello, R., Telouk, P., Gillet, P. (2010) Volatilization induced by impacts recorded in Zn isotope composition of ureilites. Chemical Geology 276, 374–379. https://doi.org/10.1016/j.chemgeo.2010.07.005
). The lines represent evaporative Rayleigh fractionation. (b) δ66/64Zn vs. bulk Ir/Pd ratio (tracer of solid-liquid metal fractionation). The fractional melting model line shows a trajectory calculated with D Ir = 23.28 and D Pd = 0.769 after Chabot and Jones (2003)Chabot, N.L., Jones, J.H. (2003) The parameterization of solid metal-liquid metal partitioning of siderophile elements. Meteoritics & Planetary Science 38, 1425–1436. https://doi.org/10.1111/j.1945-5100.2003.tb00248.x
, and a hypothetical α(66Zn/64Zn)solid/liquid = 1.00045. The starting composition is defined by the δ66/64Zn of the UPB silicate and the lowest Ir/Pd found. The literature data are from Warren and Huber (2006)Warren, P.H., Huber, H. (2006) Ureilite petrogenesis: A limited role for smelting during anatexis and catastrophic disruption. Meteoritics & Planetary Science 41, 835–849. https://doi.org/10.1111/j.1945-5100.2006.tb00489.x
, Rankenburg et al. (2008)Rankenburg, K., Humayun, M., Brandon, A.D., Herrin, J.S. (2008) Highly siderophile elements in ureilites. Geochimica et Cosmochimica Acta 72, 4642–4659. https://doi.org/10.1016/j.gca.2008.07.003
, Moynier et al. (2010)Moynier, F., Beck, P., Yin, Q.-Z., Ferroir, T., Barrat, J.-A., Paniello, R., Telouk, P., Gillet, P. (2010) Volatilization induced by impacts recorded in Zn isotope composition of ureilites. Chemical Geology 276, 374–379. https://doi.org/10.1016/j.chemgeo.2010.07.005
, Goderis et al. (2015)Goderis, S., Brandon, A.D., Mayer, B., Humayun, M. (2015) s-Process Os isotope enrichment in ureilites by planetary processing. Earth and Planetary Science Letters 431, 110–118. https://doi.org/10.1016/j.epsl.2015.09.021
, Brugier et al. (2019)Brugier, Y.-A., Barrat, J.-A., Gueguen, B., Agranier, A., Yamaguchi, A., Bischoff, A. (2019) Zinc isotopic variations in ureilites. Geochimica et Cosmochimica Acta 246, 450–460. https://doi.org/10.1016/j.gca.2018.12.009
.
Figure 2 (a) Diagram of δ56/54Fe versus bulk Ir/Pd ratio. The model line shows fractional metal melting, calculated after Chabot and Jones (2003)
Chabot, N.L., Jones, J.H. (2003) The parameterization of solid metal-liquid metal partitioning of siderophile elements. Meteoritics & Planetary Science 38, 1425–1436. https://doi.org/10.1111/j.1945-5100.2003.tb00248.x
, using α(56Fe/54Fe)solid/liquid = 0.99994. (b) δ56/54Fe versus δ66/64Zn for ureilites. The metal fractions align with the model for fractional metal melting.
Figure 3 Plot of δ57/54Femetal versus δ57/54Feolivine for ureilites and pallasites; the isotherms calculated using the NRIXS parameters for γ-iron and olivine (Dauphas et al., 2012
Dauphas, N., Roskosz, M., Alp, E.E., Golden, D.C., Sio, C.K., Tissot, F.L.H., Hu, M.Y., Zhao, J., Gao, L., Morris, R.V. (2012) A general moment NRIXS approach to the determination of equilibrium Fe isotopic fractionation factors: Application to goethite and jarosite. Geochimica et Cosmochimica Acta 94, 254–275. https://doi.org/10.1016/j.gca.2012.06.013
). The unrealistically high temperatures hint towards silicate-metal disequilibrium.
Figure 4 Profiles of δ56/54Fe and Fa# = Fe2+/(Fe2+ + Mg2+) across an olivine grain of the GRA 95205 ureilite, measured via fsLA-MC-ICP-MS and electron microprobe. In situ pyroxene data of GRA 95205 are shown using black crosses out of distance context.