# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1903.12574