Fe Alloy Slurry and a Compacting Cumulate Pile Across Earth's Inner-Core Boundary

Abstract

Seismic observations show a reduced compressional-wave velocity 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 a solid fraction of ~15 ± 5% and preferential light element partitioning into the liquid can explain the observed reduced velocity gradient in the F layer. 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 flux in the outer core, which may explain the east-west asymmetry observed in the seismic velocity. Our model suggests that the inner core solid has a high shear viscosity >1022 Pa/s.