Non-chondritic iron isotope ratios in planetary mantles as a result of core formation

Abstract

Information about the materials and conditions involved in planetary formation and differentiation in the early Solar System is recorded in iron isotope ratios. Samples from Earth, the Moon, Mars and the asteroid Vesta reveal significant variations in iron isotope ratios, but the sources of these variations remain uncertain. Here we present experiments that demonstrate that under the conditions of planetary core formation expected for the Moon, Mars and Vesta, iron isotopes fractionate between metal and silicate due to the presence of nickel, and enrich the bodies’ mantles in isotopically light iron. However, the effect of nickel diminishes at higher temperatures: under conditions expected for Earth’s core formation, we infer little fractionation of iron isotopes. From our experimental results and existing conceptual models of magma ocean crystallization and mantle partial melting, we find that nickel-induced fractionation can explain iron isotope variability found in planetary samples without invoking nebular or accretionary processes. We suggest that near-chondritic iron isotope ratios of basalts from Mars and Vesta, as well as the most primitive lunar basalts, were achieved by melting of isotopically light mantles, whereas the heavy iron isotope ratios of terrestrial ocean floor basalts are the result of melting of near-chondritic Earth mantle.

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Figure 1: The Fe isotopic compositions of planetary materials.
Figure 2: Results of metal–silicate equilibration experiments.
Figure 3: The range in Fe isotopic compositions of bulk planetary mantles predicted from core formation models.

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Acknowledgements

We are grateful to T. D. Mock, M. F. Horan, C. K. I. Sio and J. A. Armstrong for assistance with clean laboratory chemistry, mass spectrometry and SDD–EDS analyses. J. Labidi is thanked for providing comments on an early version of the manuscript. This work was funded by NSF grant EAR-1321858 to A.S.

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Correspondence to Stephen M. Elardo.

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Elardo, S., Shahar, A. Non-chondritic iron isotope ratios in planetary mantles as a result of core formation. Nature Geosci 10, 317–321 (2017). https://doi.org/10.1038/ngeo2896

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