The petrological evolution of Earth’s solidifying magma ocean from experimental melting phase relations

Understanding the crystallisation of silicate melts at lower mantle conditions is essential for constraining the petrological and chemical evolution of planetary magma oceans. In this study, we investigated the solidification of a pyrolitic melt through fractional crystallisation experiments in the laser-heated diamond anvil cell coupled with thermodynamic modelling. Fifteen experiments were conducted to establish the melting phase diagrams of pyrolite at four lower mantle pressures (53, 87, 107, 133 GPa). The data were then used to fit the parameters of a self-consistent thermodynamic model in the FeO-MgO-SiO2 ternary, which was then applied to calculate the crystallisation sequences of a pyrolitic melt as it solidifies in the deep mantle down to the core-mantle boundary. We confirmed that bridgmanite is the first mineral to crystallise (liquidus phase) in a pyrolitic magma ocean in this entire pressure range, followed by ferropericlase and, in the final stage of crystallisation, calcium silicate perovskite. Our results reveal a strong enrichment of the residual melt in iron oxide, which grows with pressure showing that FeO becomes increasingly incompatible (with depth) in lower mantle phases. The resulting dense, iron-rich melts provide a petrological basis for their accumulation at the base of the mantle, giving rise to a basal magma ocean in the final stages of magma ocean solidification.