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

Abstract

S U M M A R YSeismic 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 dynamic 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 crystallize from the liquid alloy throughout the layer. The slurry model provides a dynamic 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 1-D 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.

Regimes of thermo-compositional convection and related dynamos in rotating spherical shells

“…However, the thermal and compositional boundary conditions at the interface to a growing inner iron core are likely even more involved, in that, the temperature flux and the compositional flux are related to each other via a coupled set of differential equations (Braginsky andRoberts 1995, Glatzmaier andRoberts 1996). Furthermore, it is uncertain how a seismologically distinct mushy or slurry F-layer at base of Earths outer core may be (Wong et al 2018) affect boundary conditions. The outer boundary of the convective regions of gas planets and stars requires yet different boundary conditions, see discussion in (Wicht and Tilgner 2010).…”

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

“…It was further investigated by (Fearn et al, 1981), (Loper and Roberts, 1981), (Loper, 1983), , and others, from the geodynamic and thermodynamic state of the core to understand possible formation mechanisms of the slurry layer. A recent study indicates that a slurry F layer could satisfy the geophysical constraints on the density jump across ICB and the core-mantle boundary (CMB) heat flux as well (Wong et al, 2018). An approximately 100-km-thick slurry layer likely exists at the top of the present Martian core as well, according to a geodynamical model with magnetic and geodetic constraints (Davies and Pommier, 2018).…”

“…It was further investigated by a number of researchers to understand possible formation mechanisms of the slurry layer (e.g., Fearn et al, ; Loper, ; Loper & Roberts, ; Loper & Roberts, ; Loper & Roberts, ; Shimizu et al, ; Sumita et al, ). A more recent study indicates that a slurry F layer arising from particles of iron freezing out of the liquid alloy could satisfy the geophysical constraints on the density jump across ICB and the heat flux across the core‐mantle boundary (CMB; Wong et al, ). An approximately 100‐km‐thick slurry layer likely exists at the top of the present Martian core as well, according to a geodynamic model with magnetic and geodetic constraints (Davies & Pommier, ).…”

A regime diagram for the slurry F-layer at the base of Earth's outer core

“…Our study also opted to keep the layer thickness constant over time in order to ascertain solutions that are concomitant with the present-day seismic observations. Relaxing the layer thickness requires an extra constraint on the model to be developed in its place [46]. Constructing a fully time-dependent framework could provide insight into the factors controlling the growth and decline of a slurry layer, which may shed light on how the F-layer came into existence over the core's history.…”