The potential for bridgmanite megacrysts to drive magma ocean segregation.

Abstract

Earth's early mantle probably existed as a deep, vigorously convecting magma ocean, and its solidification is considered central to the long-term chemical and dynamical evolution of the planet. Yet a notable uncertainty is the grain size of bridgmanite-the dominant lower-mantle phase-whose nucleation behaviour at extreme pressure has remained experimentally inaccessible. Here we show, using a combination of cutting-edge techniques, including large-scale molecular dynamics simulations consisting of up to 1 million atoms driven by machine learning potentials (MLPs), seeding and enhanced sampling, that crystal-melt interfacial energies of MgSiO₃ bridgmanite increase substantially with pressure, surpassing those of silicate-liquid systems at ambient pressure by a factor of up to ten (refs. ^1-3). In a deep basal magma ocean (BMO), this amplified interfacial energy, combined with the potential sluggish cooling, may permit the formation of unusually large bridgmanite crystals, up to centimetre-to-metre-scale sizes. Such potentially large crystals could drive efficient fractional crystallization and cause substantial chemical differentiation and mantle compaction. If operative, this mechanism would provide a new physical pathway linking lower-mantle material properties to early Earth stratification and it motivates future geodynamic models that explicitly incorporate supercooling, compositional convection and elemental partitioning. Our findings thus offer a plausible hypothesis connecting microscopic nucleation processes with macroscopic planetary structure, refining present views of how the Earth's interior acquired its initial compositional architecture.