• Open Access

Compositional instability of Earth's solid inner core

Authors

  • D. Gubbins,

    1. School of Earth and Environment, University of Leeds, Leeds, UK
    2. Institute for Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, La Jolla, California, USA
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  • D. Alfè,

    1. Department of Earth Sciences and Thomas Young Centre at UCL, University College London, London, UK
    2. Department of Physics and Astronomy and London Centre for Nanotechnology, University College London, London, UK
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  • C. J. Davies

    Corresponding author
    • School of Earth and Environment, University of Leeds, Leeds, UK
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Corresponding author: C. J. Davies, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK. (c.davies@leeds.ac.uk)

Abstract

[1] All models that invoke convection to explain the observed seismic variations in Earth's inner core require unstable inner core stratification. Previous work has assumed that chemical effects are stabilizing and focused on thermal convection, but recent calculations indicate that the thermal conductivity at core temperatures and pressures is so large that the inner core must cool entirely by conduction. We examine partitioning of oxygen, sulfur, and silicon in binary iron alloys and show that inner core growth results in a variable light element concentration with time: oxygen concentration decreases, sulfur concentration decreases initially and increases later, and silicon produces a negligible effect to within the model errors. The result is a net destabilizing concentration gradient. Convective stability is measured by a Rayleigh number, which exceeds the critical value for reasonable estimates of the viscosity and diffusivity. Our results suggest that inner core convection models, including the recently proposed translational mode, can be viable candidates for explaining seismic results if the driving force is compositional.

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