9.04 - Magma Oceans and Primordial Mantle Differentiation

Abstract

Crystallization and chemical differentiation of the early Earth was governed by a combination of various processes. Within the uncertainties of physical parameters, the two end-member models, equilibrium crystallization and fractional crystallization, are both possible. In terms of crystal size, the boundary between these models is ∼1mm. Analysis of nucleation and crystal growth in the convecting magma ocean suggests that the crystals in the magma ocean are approximately this size. The equilibrium model is preferred because it seems to satisfy better the geochemical constraints. According to this model, at the early stages crystallization proceeds from bottom up without any substantial crystal-melt segregation. In a vigorously convecting magma ocean the thermal profile is approximately adiabatic and the crystal fraction decreases gradually with depth. The basic physical reason for equilibrium crystallization is a fast convective cooling during which the solid and the liquid phases had a very limited time window for crystal-melt segregation, about 1000 years. Chemical differentiation due to crystal-melt segregation is more significant at low pressures corresponding – with large uncertainties – to the upper mantle. It is caused by several factors. When the temperature drops below liquidus everywhere the nucleation–growth–dissolution cycle of crystals changes to continuous crystal growth. This increases the crystal size to ∼1cm indicating the beginning of crystal-melt segregation. When the crystal fraction increases to about 60% near the surface, the convective heat transport is controlled by solid-state creep which is many orders of magnitude slower. Crystal-melt segregation occurs via melt percolation, which is consistent with the geochemical constraints. Crystallization of the remaining partially molten layers takes about 107–109 years. This stage merges with the subsequent planetary evolution controlled by mantle convection and radiogenic heating. The long lifetime of the shallow magma ocean suggests that this might be the place where liquid iron equilibrates with the mantle before it sinks down to the core.