Thermodynamics of the MgO–SiO2 liquid system in Earth's lowermost mantle from first principles

Knowledge of the multi-component thermodynamics and phase equilibria of silicate melts in Earth's deep interior are key to understanding the thermal and chemical evolution of the planet, yet the melting phase diagram of the lower mantle remains poorly constrained, with large uncertainties in both eutectic composition and temperature. We use results from first-principles molecular dynamics of nine compositions along the MgO–SiO2 binary to investigate the compositional dependence of liquid state thermodynamics, applying our results to describe incongruent melting for the system at deep lower mantle pressures. Our phase diagram is bi-eutectic throughout the lower mantle, with no liquid immiscibility. Accounting for solid–liquid partitioning of Fe, we find partial melts of basaltic and peridotitic lithologies to be gravitationally stable at the core–mantle boundary, while liquidus density contrasts predict that perovskite will sink and periclase will float in a crystallizing pyrolytic magma ocean.

► Results on the MgO–SiO2 join show bi-eutectic melting throughout the lower mantle.
► High pressure eutectics correspond closely to basaltic and peridotitic bulk compositions.
► At the magma ocean liquidus perovskites tends to sink and periclase to float.
► At the mantle solidus, liquids are denser than co-existing solids.
► Partial melting a plausible explanation for ultra-low velocity zones at base of mantle.