Solidification of the Terrestrial Magma Ocean – Insights from Studies of Crustal Magma Chambers

In most models of terrestrial magma ocean solidification, crystals are said to accumulate above the core–mantle boundary to form a crystal mush that solidifies under equilibrium conditions. Large mafic-ultramafic crustal intrusions crystallize differently. They are dominated by adcumulates that are composed almost entirely of cumulus minerals, and the residual melt undergoes fractional crystallization. In this study, we draw on insights from studies of crustal magma chamber processes to propose a new model for the solidification of the magma ocean. Our model takes into account the phase relations of ultramafic melt under lower mantle conditions, the presence of a zone of neutral crystal buoyancy located in the lower magma ocean, and the Coriolis force, which organizes convection into separate zones aligned parallel to the planet’s axis of rotation. In this scenario, crystals initially accumulate in two annular septa in the deeper part of the magma ocean, one in each hemisphere. A combination of composition-driven convection within the crystal mush and compaction then expels melt from the septa into the magma ocean. Early on, melt circulates between different parts of the magma ocean through gaps between the septa. Once these gaps close, the two hemispheric regions evolve independently.