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Heavy iron isotope composition of iron meteorites explained by core crystallization

Abstract

Similar to Earth, many large planetesimals in the Solar System experienced planetary-scale processes such as accretion, melting and differentiation. As their cores cooled and solidified, substantial chemical fractionation occurred due to solid metal–liquid metal fractionation. Iron meteorites—core remnants of these ancient planetesimals—record a history of this process. Recent iron isotope analyses of iron meteorites found their 57Fe/54Fe ratios to be heavier than chondritic by approximately 0.1 to 0.2 per mil for most meteorites, indicating that a common parent body process was responsible. However, the mechanism for this fractionation remains poorly understood. Here we experimentally show that the iron isotopic composition of iron meteorites can be explained solely by core crystallization. In our experiments of core crystallization at 1,300 °C, we find that solid metal becomes enriched in the heavier iron isotope by 0.13 per mil relative to liquid metal. Fractional crystallization modelling of the IIIAB iron meteorite parent body shows that observed iridium, gold and iron compositions can be simultaneously reproduced during core crystallization. The model implies the formation of complementary sulfur-rich components of the iron meteorite parental cores that remain unsampled by meteorite records and may be the missing reservoir of isotopically light iron. The lack of sulfide meteorites and previous trace element modelling predicting substantial unsampled volumes of iron meteorite parent cores support our findings.

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Fig. 1: Fe isotopic compositions for various types of terrestrial and extraterrestrial samples.
Fig. 2: Results of solid metal–liquid metal equilibrium experiments.
Fig. 3: Core crystallization fractionation modelling.
Fig. 4: Demonstration of the missing S-rich reservoir unsampled by Fe meteorites.

Data availability

The data supporting the findings of this study are available within the article and its Supplementary Information files. All new data associated with this paper will be made publicly available via figshare (https://figshare.com/).

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Acknowledgements

We thank T. D. Mock for help with the MC-ICP-MS, M. F. Horan for help in the clean lab, and E. S. Bullock for help with the electron microprobe analyses. P.N. was supported by a Carnegie Postdoctoral Fellowship while working on this project. This research is partially supported by NASA grant NNX15AJ27G to N.L.C. We thank the APL internship programme for enabling contributions by C.J.R.

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P.N., N.L.C. and A.S. designed the research project. C.J.R. and N.L.C. conducted the experiments, examined the run products and prepared them for analyses. P.N. and A.S. performed the clean lab chemistry and Fe isotope measurements and analysed the data. P.N. drafted the manuscript, and all authors contributed to writing the paper.

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Correspondence to Peng Ni.

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Supplementary discussions, Figs. 1–6 and Tables 1–4.

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Ni, P., Chabot, N.L., Ryan, C.J. et al. Heavy iron isotope composition of iron meteorites explained by core crystallization. Nat. Geosci. 13, 611–615 (2020). https://doi.org/10.1038/s41561-020-0617-y

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