Atomic structure at the surface of warm basaltic glasses

Silicate melts exist as lava flows which form when molten or partially molten magma erupts, and when they cool, magmas evolve into solid rocks. Depending on the cooling rate, they can evolve into fully crystalline rocks, partially-crystallized rocks or even glassy rocks. The composition of the glass at the cooling interface with air or water may be different than in the bulk. Here we study how some major elements could be concentrated or depleted at the surface of a cooling basaltic melt. This may have effects on how glass will interact with water at the onset of weathering. To model silicate melts, we have trained a machine-learned interatomic potential for basaltic glass, which we use to run molecular dynamics simulations of molten basalt and a basalt surface at temperatures consistent with fresh deposits of basalt during eruption. We have studied the difference between bulk molten basalt and a free surface of molten basalt by comparing the diffusion coefficient, lifetime of species and the spatial distribution of atoms between the two domains. We show that the diffusion at the surface is higher than in the bulk, indicating a higher rearrangement of the surfaces compared to the bulk, and the coordination numbers are generally lower at the surface than in the bulk. When studying the composition of a surface and bulk, our results show that most of the cations on the surface are iron, magnesium and calcium, i.e. the cations that can react with CO2 to precipitate as carbonate minerals. These simulations are relevant for the initial weathering of silicate melt, and knowledge of the composition of the surface are relevant for the potential reactions with CO2 and carbon mineralization.