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 di¬erentiated than Earth but display some of the proposed thermochemical processes, including the generation of a high-· 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