Impact of Rotation on the Differentiation of a Terrestrial Magma Ocean

Abstract:

It is widely accepted that the Earth experienced several large impacts during its early evolution. Most likely these impacts led to the formation of one- or more magma oceans. Differentiation processes in such a magma ocean are important, since they set the stage for structure and the dynamics of the mantle. Differentiation in a magma ocean was clearly influenced by vigorous convection. Further, differentiation was possibly influenced by rotation, given the low viscosity of the molten magma and especially since the Early Earth was rotating much faster than today. In order to study the influence of rotation on the crystallization of a magma ocean, we developed a 3D model in Cartesian geometry, allowing for the simulation of silicate crystals in a vigorously convection flow, at low Prandtl number under strong rotation. A discrete element approach is employed to simulate the crystal behavior. The results show a crucial dependence of the dynamics on crystal density, rotation rate and latitude. At the pole, convection dominates at low rotation, keeping al large fraction of particles in suspension. Increasing rotation weakens convection and thus many particles settle at the bottom and form a stably stratified layer. Differently a the equator all particles settle at the bottom at low rotation, they form a layer of significant thickness with increasing rotation rate, and at highest rotation rate they are kept in suspension and form a layer at mid depth of the magma ocean. Due to Coriolis forces crystals of different densities separate from each other. At a given rotation rate lighter particles settle at the bottom while denser particles accumulate at mid depth. This results in an unstably stratified mantle in the equatorial region after magma ocean solidification. Thus rotation can lead to asymmetric crystallization of the magma ocean with an opposite layering in the polar- and equatorial region. This basically can explain the existence of local magma ocean at the core mantle boundary and the existence of mantle heterogeneities as relic structures originating from magma ocean crystallization.