Conditions for Segregation of a Crystal‐Rich Layer Within a Convective Magma Ocean

Abstract

The vigor of the thermal convection in a terrestrial magma ocean tends to prevent the sedimentation of the solid grains. Understanding of the overall dynamical behavior of this solid phase segregation is required to anticipate the solidification mechanisms in the early Earth mantle. We develop numerical models to monitor the crystal fraction evolution of a convecting magmatic reservoir. Our models show that the ability of the crystal fraction to disperse or sediment within the domain strongly depends on the crystal size, the density difference and the magma viscosity. Our models show that the critical value of the convection/buoyancy stress ratio separating sedimentation/suspension regimes can be smaller than 0.1. Hence, during the early crystallization of a magma ocean, suspension should be the dominant process. We then investigate the possibility of bridgmanite segregation by comparing the density difference between the MO and the solid crystals for different compositions and the critical density contrast above which crystal segregation is likely to occur. We define the relevant set of parameters; including the P‐V‐T equations of state of coexisting melt and bridgmanite in the mushy MO. We observe that bridgmanite grains are unlikely to segregate in a mantle of pyrolite composition. However, bridgmanite segregation is more likely to occur at the bottom of a MO enriched in SiO2, compared to pyrolite. When a solidifying layer contains 60% of bridgmanite and 40% of melt, we observe a significant SiO2 enrichment with increasing mantle depth in a primitive mantle compatible with seismic and geochemical observations.