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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 >10 22  Pa/s.

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