Coupled thermochemical evolution of the early Earth’s solid mantle and basal magma ocean: The role of melting and melt transport

Abstract

Differentiation of Earth’s magma ocean likely created chemical heterogeneity, leading to a gradually stratified deep mantle overlying a basal magma ocean (BMO). The Earth’s thermochemical evolution hinges on the fate of this early stratification. At this stage, the solid mantle is near-solidus, so deep convective motions are probably strongly influenced by solid-liquid phase change. However, the preservation or modification of stratification is poorly understood in the presence of partial melting. We investigate the evolution of a compositionally stratified deep mantle considering melt-related processes and interaction between the solid mantle and BMO. We determine that bottom-heating of the stratified solid mantle by a radioactively heated BMO or a cooling core powers restructuring of stratification in two regimes: (1) if produced melt can drain effectively, downward percolation of fractional melt removes the enriched, dense, stratifying component to the growing BMO; (2) otherwise, hot or melt-rich plumes stir the initially stratified material, which retains its original composition. We present a numerically-validated analytical model which predicts both the timescale and the different regimes of stratification restructuring. Applying this model to Earth, we determine that gradual stratification in the deep solid mantle would have been restructured by the early Archean by drainage of low-degree partial melts. This process would leave the early Earth with a depleted deep mantle only slightly denser than the shallow mantle, and a voluminous BMO. The deep solid reservoirs may be relatively easily entrained into the convecting shallow mantle, whereas the BMO could act as a more persistent source of heterogeneity.