A slurry model of the F-layer in the Earth's core

Seismic observations suggest that a stably-stratified layer, known as the F-layer, 150–300 km thick exists at the base of Earth’s liquid outer core. This contrasts with the density inferred from the Preliminary Reference Earth Model, which assumes a well–mixed and adiabatic outer core. The liquid core is composed primarily of iron alloyed with a light component. We 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 dynamically consistent explanation of how light element can pass through a stable layer. We make two key assumptions, the first of which is fast-melting where the time-scale of freezing is considered short compared to the evolution of the F-layer. 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 an idealised steady state model of a slurry, where temperature, light element concentration and solid flux profiles are ascertained for various layer thicknesses, inner core heat fluxes and values of the core thermal conductivity, since there is some uncertainty in these parameters. 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. A time-dependent model is presented, in which the slurry system is coupled to the evolving global heat balance of the core. Promising results show that the origin of the F-layer and its future long-term development may be probed