Dynamic history of the inner core constrained by seismic anisotropy

Daniel A Frost,Marine Lasbleis,Brian Chandler,Barbara Romanowicz

doi:10.1038/s41561-021-00761-w

Abstract

Progressive crystallization of Earth’s inner core drives convection in the outer core and magnetic field generation. Determining the rate and pattern of inner-core growth is thus crucial to understanding the evolution of the geodynamo. The growth history of the inner core is probably recorded in the distribution and strength of its seismic anisotropy, which arises from deformation texturing constrained by conditions at the inner-core solid–fluid boundary. Here we show from analysis of seismic body wave travel times that the strength of seismic anisotropy increases with depth within the inner core, and the strongest anisotropy is offset from Earth’s rotation axis. Then, using geodynamic growth models and mineral physics calculations, we simulate the development of inner-core anisotropy in a self-consistent manner. From this we find that an inner core composed of hexagonally close-packed iron–nickel alloy, deformed by a combination of preferential equatorial growth and slow translation, can match the seismic observations without requiring hemispheres with sharp boundaries. Our model of inner-core growth history is compatible with external constraints from outer-core dynamics, and supports arguments for a relatively young inner core (~0.5–1.5 Ga) and a viscosity >1018 Pa s. The inner core underwent preferential equatorial growth and translation after nucleation ~0.5–1.5 billion years ago, according to an analysis of its seismic anisotropy and self-consistent geodynamic simulations.

Highlights

Progressive crystallisation of Earth's inner core over geological times drives convection in the outer core and the generation of the Earth’s magnetic field
Using geodynamic growth models and mineral physics calculations, we simulate the development of inner core anisotropy in a self-consistent manner
We show for the first time that an inner core model composed of hexagonally closepacked iron-nickel alloy, deformed by a combination of preferential equatorial growth and slow translation can match the seismic observations without requiring the introduction of hemispheres with sharp boundaries

Summary

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Plastic deformation of a hexagonally close packed (hcp) iron-nickel alloy (Fe93.75Ni6.2520), compatible with cosmochemical constraints[21], with slip on the pyramidal planes[22,23] produces an anisotropic inner core with up to 6.6% anisotropy (Fig. 3d) that can fit the seismic data well In this model, the fast direction of anisotropy becomes aligned with the rotation axis and the slow direction varies with depth (Extended Data Fig. 7), matching observations[24]. We conclude that the trends of velocity with depth in the two hemispheres have statistically distinct intercepts but statistically very similar gradients

South Sandwich Islands to Alaska anomaly

Mineral physics

Data availability statement

Findings

Extended Data