Thermodynamics of the MgO–SiO2 liquid system in Earthʹs lowermost mantle from first principles

Knowledge of the multi-component thermodynamics and phase equilibria of silicate melts in Earthʹs 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.