Mantle Differentiation, Mixing and Interior-Exterior Exchange

Abstract: We review the formation of large-scale heterogeneity in the solid Earth from the magma ocean phase to the present day, focusing on lower-mantle structure and evolution, as well as continental formation and its impact on interior-exterior volatile exchange. The solidification of the magma ocean sets the "initial condition" for solid-state mantle structure and evolution, partitioning volatiles between the surface and interior, and differentiating major and trace elements between mantle reservoirs. A possibly long-lived basal magma ocean is a potential reservoir for iron and incompatible trace elements with important implications for the present-day structure of the lowermost mantle and the distribution of heat-producing elements. With emergent plate tectonics and mantle convection, the production of oceanic crust is the dominant differentiation mechanism, and may contribute to lower-mantle heterogeneity. In turn, convection acts to re-mix heterogeneity. Thus, the structure of the lower mantle records the early differentiation of our planet, as well as long-term evolution. The formation of continental crust and lithosphere has also an important influence on deep-mantle structure and composition, because the distribution of continents can control the scale of plate tectonics, and thus affect mantle convection patterns. While the formation of continental crust and lithosphere, scenarios can be tested by self-consistent long-term evolution models. The exchange of water between the surface and mantle also plays an important role in the dynamics of Earth\' s deep interior. By using relative sea-level change as a proxy for the interior-exterior exchange of water, it is possible to constrain the deep-mantle water cycle. Based on a comprehensive review of these topics, we conclude with several open questions that should be addressed by future studies.