Letter | Published:

Broad bounds on Earth’s accretion and core formation constrained by geochemical models

Nature Geoscience volume 3, pages 439443 (2010) | Download Citation

Abstract

The Earth formed through the accretion of numerous planetary embryos that were already differentiated into a metallic core and silicate mantle. Prevailing models of Earth’s formation, constrained by the observed abundances of metal-loving siderophile elements in Earth’s mantle, assume full metal–silicate equilibrium, whereby all memory of the planetary embryos’ earlier differentiation is lost1,2. Using the hafnium–tungsten (Hf–W) and uranium–lead (U–Pb) isotopic dating systems, these models suggest rapid accretion of Earth’s main mass within about 10 million years3,4,5,6 (Myr) of the formation of the Solar System. Accretion terminated about 303,7 or 1004,5 Myr after formation of the Solar System, owing to a giant impact that formed the Moon. Here we present geochemical models of Earth’s accretion that preserve some memory of the embryos’ original differentiation. These disequilibrium models allow some fraction of the embryos’ metallic cores to directly enter the Earth’s core, without equilibrating with Earth’s mantle. We show that disequilibrium models are as compatible with the geochemical observations as equilibrium models, yet still provide bounds on Earth’s accretion and core formation. We find that the Hf–W data mainly constrain the degree of equilibration rather than the timing, whereas the U–Pb data confirm that the end of accretion is consistent with recent estimates of the age of the Moon8,9. Our results indicate that only 36% of the Earth’s core must have formed in equilibrium with Earth’s mantle. This low degree of equilibration is consistent with the siderophile element abundances in Earth’s mantle.

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Acknowledgements

We thank A. Corgne, J. Wade, M. Walter and B. Wood for discussions, and F. Albarède for his review.

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Affiliations

  1. Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland

    • John F. Rudge
    • , Thorsten Kleine
    •  & Bernard Bourdon
  2. Institute of Theoretical Geophysics, Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK

    • John F. Rudge
  3. Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany

    • Thorsten Kleine

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Contributions

J.F.R. derived the model equations and carried out the calculations. J.F.R. and T.K. wrote the paper. B.B. conceptually designed the study. All authors discussed the results and implications and commented on the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to John F. Rudge.

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DOI

https://doi.org/10.1038/ngeo872

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