Accretion of the Earth and segregation of its core

Abstract

The Earth took 30–40 million years to accrete from smaller ‘planetesimals’. Many of these planetesimals had metallic iron cores and during growth of the Earth this metal re-equilibrated with the Earth's silicate mantle, extracting siderophile (‘iron-loving’) elements into the Earth's iron-rich core. The current composition of the mantle indicates that much of the re-equilibration took place in a deep (> 400 km) molten silicate layer, or ‘magma ocean’, and that conditions became more oxidizing with time as the Earth grew. The high-pressure nature of the core-forming process led to the Earth's core being richer in low-atomic-number elements, notably silicon and possibly oxygen, than the cores of the smaller planetesimal building blocks.

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Figure 1: Elemental abundance in the silicate Earth versus temperature of 50% condensation.
Figure 2: The effect of pressure on Ni and Co partitioning.
Figure 3: The deep magma ocean model.
Figure 4: Conditions yielding correct core–mantle partitioning of siderophile elements during accretion.
Figure 5: Sketch of a possible mechanism by which the mantle may have self-oxidized via perovskite crystallization.
Figure 6: Two estimates of the timing of accretion and core formation on the Earth.
Figure 7: Calculated effect of perovskite crystallization on the Fe 3+ content and hence oxygen fugacity of a magma ocean of peridotite composition.

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Acknowledgements

The constructive reviews of C. Agee and K. Righter are acknowledged with thanks. B.J.W. acknowledges the award of an ARC Federation Fellowship.

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Correspondence to Bernard J. Wood.

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Wood, B., Walter, M. & Wade, J. Accretion of the Earth and segregation of its core. Nature 441, 825–833 (2006). https://doi.org/10.1038/nature04763

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