Earth’s mantle was extensively molten in its first 100 million years. Its solidification left a strong compositional and structural imprint on the mantle, still observable today in the geochemical and geophysical record. Isotopic variations in basalts and ancient crust reveal the existence of one or more mantle reservoirs that formed early in Earth’s history, and never fully remixed. Seismic tomography reveals large, chemically distinct, thermochemical structures in the lowermost mantle, anchored to the source of these anomalous basalts. These observations leave no doubt that the mantle still bears differentiated regions that preserve signatures of its solidification. However, their formation mechanisms and compositional evolution are still unknown. Unravelling those opens a new window in understanding the early stages of Earth and planetary evolution. The overarching question of this proposal: how did the primitive molten Earth solidify to form a tectonically and geomagnetically active planet? My goal is to address this question by investigating mantle crystallisation in the laboratory, by combining experimental petrology, thermodynamical modelling, and geodynamical simulations in a self-consistent fashion. I propose to assemble a multidisciplinary team to: (1) experimentally study the crystallisation sequence of relevant silicate melts in the laboratory under P-T conditions extending to those at of the depth of the core-mantle boundary, using a newly developed protocol in the laser-heated diamond anvil cell; (2) use these results to constrain a thermodynamical model of phase relations and compositions, and trace-element partitioning between solids and melts; (3) use DFT calculations to calculate the density and relative buoyancy of these solids and melts; (4) feed the thermodynamics and buoyancy as input to mushy two-phase 3D fluid dynamics simulations, to realistically simulate the dynamical and thermochemical evolution of Earth’s mantle as it solidified.
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