The thermochemical structure and evolution of Earth's mantle: constraints and numerical models

Geochemical observations place several constraints on geophysical processes in the mantle, including a requirement to maintain several distinct reservoirs. Geophysical constraints limit plausible physical locations of these reservoirs to a thin basal layer, isolated deep 'piles' of material under large-scale mantle upwellings, high-viscosity blobs/plums or thin strips throughout the mantle, or some combination of these. A numerical model capable of simulating the thermochemical evolution of the mantle is introduced. Preliminary simulations are more differentiated than Earth but display some of the proposed thermochemical processes, including the generation of a high-mu mantle reservoir by recycling of crust, and the generation of a high-(3)He/(4)He reservoir by recycling of residuum, although the resulting high-(3)He/(4)He material tends to aggregate near the top, where mid-ocean-ridge melting should sample it. If primitive material exists as a dense basal layer, it must be much denser than subducted crust in order to retain its primitive (e.g. high-(3)He) signature. Much progress is expected in the near future.