Fe alloy slurry and a compacting cumulate pile across Earth's inner-core boundary

TL;DR

This paper proposes a model for Earth's inner core boundary involving a slurry layer formed by fractional crystallization of Fe alloy and a compacting cumulate pile, explaining seismic observations and inner core properties.

Contribution

It introduces a new model combining a slurry layer and a cumulate pile at the ICB, explaining seismic asymmetries and velocity gradients with mineral physics data.

Findings

Fractional crystallization explains reduced seismic velocity gradient.

Lateral variations in the cumulate pile account for east-west asymmetry.

Inner core shear viscosity estimated at ~10^23 Pa s.

Abstract

Seismic observations show a reduced compressional-wave gradient at the base of the outer core relative to the preliminary reference Earth model and seismic wave asymmetry between the east-west hemispheres at the top of the inner core. Here, we propose a model for the inner core boundary (ICB), where a slurry layer forms through fractional crystallization of an Fe alloy at the base of the outer core (F layer) above a compacting cumulate pile at the top of the inner core (F' layer). Using recent mineral physics data, we show that fractional crystallization of an Fe alloy (e.g., Fe-Si-O) with light element partitioning can explain the observed reduced velocity gradient in the F layer, in cases with a solid fraction of ~15(5)% in liquid with a compositional gradient due to preferential light element partitioning into liquid. The compacting cumulate pile in the F' layer may exhibit lateral variations in thickness between the east-west hemispheres due to lateral variations of large-scale heat flow in the outer core, which may explain the east-west asymmetry observed in the seismic velocity. Our interpretations suggest that the inner core with solid Fe alloy has a high shear viscosity of ~10^23 Pa s.