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Silicon in the Earth’s core

Abstract

Small isotopic differences between the silicate minerals in planets may have developed as a result of processes associated with core formation, or from evaporative losses during accretion as the planets were built up. Basalts from the Earth and the Moon do indeed appear to have iron isotopic compositions that are slightly heavy relative to those from Mars, Vesta and primitive undifferentiated meteorites1,2,3,4 (chondrites). Explanations for these differences have included evaporation during the ‘giant impact’ that created the Moon (when a Mars-sized body collided with the young Earth). However, lithium5 and magnesium6, lighter elements with comparable volatility7,8,9, reveal no such differences, rendering evaporation unlikely as an explanation. Here we show that the silicon isotopic compositions of basaltic rocks from the Earth and the Moon are also distinctly heavy. A likely cause is that silicon is one of the light elements in the Earth’s core. We show that both the direction and magnitude of the silicon isotopic effect are in accord with current theory10 based on the stiffness of bonding in metal and silicate. The similar isotopic composition of the bulk silicate Earth and the Moon is consistent with the recent proposal11 that there was large-scale isotopic equilibration during the giant impact. We conclude that Si was already incorporated as a light element in the Earth’s core before the Moon formed.

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Figure 1: Silicon isotopic compositions of different Solar System objects.
Figure 2: Theoretical fractionation of silicon isotopes between silicate and metal as a function of temperature.
Figure 3: Fractionation of silicon isotopes by core formation calculated from the composition of the bulk silicate Earth (BSE).

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Acknowledgements

We are grateful to C. Smith and colleagues at the Natural History Museum, London, for aliquots of most of the meteorite samples and to CAPTEM for the lunar samples. MORB and Loihi basalt glasses were provided by M. Garcia, C. Langmuir and W. White. We also thank F. Oberli and colleagues for continuing technical assistance for the isotope facility at ETH Zurich, and SNF, PPARC (now STFC), the NSF and Oxford University for providing financial support. We thank S. Nielsen, H. Williams, D. Stevenson and K. Pahlevan for discussion. The manuscript was improved following comments from T. Elliott.

Author Contributions R.B.G. developed the isotopic methods, produced all of the analytical data and most of the tables and figures, and contributed to the modelling and interpretation. A.N.H. conceived the project, organised sample acquisition, contributed to the interpretation and modelling and wrote most of the text. E.A.S. developed the isotopic fractionation theory and associated calculations and interpretations and wrote important sections of the text. B.C.R. developed the isotopic methods with R.B.G. and made critical standard calibration measurements.

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Correspondence to Alex N. Halliday.

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Supplementary information

Supplementary Information 1

This file contains Supplementary Discussion on the calculation of the Fe isotopic fractionation expected from perovskite pumping and mantle self-oxidation and additional references. (PDF 71 kb)

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Georg, R., Halliday, A., Schauble, E. et al. Silicon in the Earth’s core. Nature 447, 1102–1106 (2007). https://doi.org/10.1038/nature05927

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