Abstract
As the Earth accreted, metallic materials segregated from silicates to form the iron-rich core. The proportions of the refractory lithophile elements in the silicate part of the Earth are thought to have remained similar to that of chondrite meteorites throughout accretion1. However, although niobium (Nb) and tantalum (Ta) are both classified as refractory lithophile elements and share a similar degree of incompatibility in mineral structures2, the Nb/Ta ratio of the bulk silicate Earth is subchondritic3. To explain this paradox, it has been proposed that Nb becomes siderophile at the high pressures of core formation, and was preferentially removed from the silicate Earth4,5,6. Here we conduct metal/silicate partitioning experiments at a range of oxygen fugacities and show that Nb and Ta are both siderophile elements under reducing conditions, but become so at different oxygen fugacities, leading to fractionation. We find that pressure has a negligible influence on the Nb/Ta ratio. Applying our partitioning data to existing theoretical accretion models7,8, we reproduce the Nb/Ta ratios of the bulk silicate Earth, Mars and the differentiated asteroid 4 Vesta, and discuss the implications for Moon formation. We conclude that planetary accretion of reduced materials played an important role in the chemical evolution of Earth and, more generally, that Nb and Ta can be used to trace prevailing oxygen fugacities during the segregation of planetary cores.
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Acknowledgements
The authors thank O. Laurent for discussion. This research received funding from the European Research Council under the European Community’s Seventh Framework Program (FP7/2007-2013 Grant Agreement 209035) and from the French PNP program (INSU-CNRS). The multi-anvil apparatus of Laboratoire Magmas et Volcans is financially supported by the Centre National de la Recherche Scientifique (Instrument National de l’INSU). This is Laboratory of Excellence ClerVolc contribution no. 101.
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All authors contributed equally to this work. C.C. conducted the experiments and prepared the samples. C.C. and J-L.D. acquired and reduced the data. Modelling and manuscript preparation were carried out by C.C., T.H., M.B. and M.A.B. All authors discussed the results and implications and commented on the manuscript at all stages.
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Cartier, C., Hammouda, T., Boyet, M. et al. Redox control of the fractionation of niobium and tantalum during planetary accretion and core formation. Nature Geosci 7, 573–576 (2014). https://doi.org/10.1038/ngeo2195
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DOI: https://doi.org/10.1038/ngeo2195
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