Homogenization of Earth’s mantle after magma ocean solidification

As Earth’s magma ocean solidified, chemical fractionation and physical separation of silicate melt and crystal produced chemical heterogeneity, potentially resulting in the compositional stratification of Earth’s deep mantle. A stratified deep mantle would have prevented advective flux of heat or material between the deep Earth (basal magma ocean (BMO) and core) and the shallow mantle. Therefore, the thermochemical evolution of the Earth hinges on the evolution of the stratified deep mantle. How does this region evolve, especially considering that it is likely underlaid by a radioactively heated BMO and a cooling core? To what extent and in what form would heterogeneity introduced by magma ocean differentiation be preserved in Earth’s mantle over time?

We explored these questions through numerical experiments simulating the evolution of a compositionally stratified, initially solid layer underlaid by a volumetrically heated liquid layer. We model percolation as well as convection driven by density perturbations related to thermal expansion, composition (iron), and melt fraction, using pressure-dependent melting temperatures and density perturbations appropriate for Earth’s deep mantle. We explore a variety of heating rates, stratifications, and material properties.

We find that the evolution of a stratified deep mantle may proceed in two regimes, depending on the competition between the timescales of (1) melt segregation and (2) mantle stirring driven by thermochemical convection.

If stirring is efficient relative to melt segregation, bottom-heating will drive homogenization of a stratified region as heat added to deep material leads to density reduction through partial melting. In this regime, the timescale of homogenization is determined by the time it takes to deliver the energy necessary to reduce the density of the entire deep mantle to match that at the top of the stratified region. Density reduction can be achieved either by thermal expansion or melting; homogenization driven by melting-related density decrease will occur much more rapidly than homogenization driven by thermal expansion. The Earth’s solid mantle following deep mantle homogenization likely had multiple compositionally distinct layers (not including any BMO), which then would have proceeded to mix by entrainment.

If melt segregation is efficient relative to stirring, bottom-heating will still produce partial melt, which will be dense due to the incompatibility of iron and will percolate to the BMO. This process drains incompatible components from the deep mantle to the BMO, with the depleted low-density residue rising in diapirs until the deep mantle is homogenized through depletion. In this case, the Earth is left with a mantle which is more uniform and more depleted than in the stirring-dominated regime, and a thicker BMO.