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Direct shock compression experiments on premolten forsterite and progress toward a consistent high‐pressure equation of state for CaO‐MgO‐Al2O3‐SiO2‐FeO liquids

We performed shock compression experiments on preheated forsterite liquid (Mg2SiO4) at an initial temperature of 2273 K and have revised the equation of state (EOS) that was previously determined by shock melting of initially solid Mg2SiO4 (300 K). The linear Hugoniot, US = 2.674 ± 0.188 + 1.64 ± 0.06 up km/s, constrains the bulk sound speed within a temperature and composition space as yet unexplored by 1 bar ultrasonic experiments. We have also revised the EOS for enstatite liquid (MgSiO3) to exclude experiments that may have been only partially melted upon shock compression and also the EOS for anorthite (CaAl2SiO6) liquid, which now excludes potentially unrelaxed experiments at low pressure. The revised fits and the previously determined EOS of fayalite and diopside (CaMg2SiO6) were used to produce isentropes in the multicomponent CaO‐MgO‐Al2O3‐SiO2‐FeO system at elevated temperatures and pressures. Our results are similar to those previously presented for peridotite and simplified “chondrite” liquids such that regardless of where crystallization first occurs, the liquidus solid sinks upon formation. This process is not conducive to the formation of a basal magma ocean. We also examined the chemical and physical plausibility of the partial melt hypothesis to explain the occurrence and characteristics of ultra‐low velocity zones (ULVZ). We determined that the ambient mantle cannot produce an equilibrium partial melt and residue that is sufficiently dense to be an ultra‐low velocity zone mush. The partial melt would need to be segregated from its equilibrium residue and combined with a denser solid component to achieve a sufficiently large aggregate density.