A Boussinesq slurry model of the F–layer at the base of Earth’s outer core

Seismic observations suggest that a stably-stratified layer, known as the F-layer, 150–300 km thick exists at the bottom of Earth’s liquid outer core. These observations contrast with the density inferred from the Preliminary Reference Earth Model (PREM), which assumes an outer core that is well-mixed and adiabatic throughout. The liquid core is composed primarily of iron alloyed with a light component. A thermal boundary layer produces the opposite effect on the density profile compared with the observations, and single phase, thermochemical models do not provide a sufficient dynamical description of how light element is transported across the F–layer into the overlying liquid outer core. We therefore propose that the layer can be explained by a slurry on the liquidus, whereby solid particles of iron crystallise from the liquid alloy throughout the layer. The slurry model provides a dynamical explanation of how light element can be transported across a stable layer. We make two key assumptions, the first of which is fast-melting where the timescale of freezing is considered short compared to other processes. The second assumption is that we consider a binary alloy where the light element is purely composed of oxygen, which is expelled entirely into the liquid during freezing. We present a steady state one-dimensional box model of a slurry formulated in a reference frame moving at the speed of inner core growth. We ascertain temperature, light element concentration and solid flux profiles by varying the layer thickness, inner core heat flux and thermal conductivity, since there is some uncertainty in these estimates. Our solutions demonstrate that the steady state slurry can satisfy the geophysical constraints on the density jump across the layer and the core-mantle boundary heat flux.