Incorporation of water into olivine during nebular condensation: Insights from density functional theory and thermodynamics, and implications for phyllosilicate formation and terrestrial water inventory

Abstract

AbstractUsing density functional theory, we have examined the hydration mechanism of olivine with the objective of understanding the reaction pathways toward the formation of crystalline serpentine and brucite. It is found that further supply of water beyond saturation of the adsorption sites on olivine surfaces leads to the formation of amorphous brucite and serpentine molecules, with the latter forming in the subsurface domain. The calculated activation energy for this process is ~25 kJ mol−1, which permits formation of the amorphous materials well within the life span of the solar nebula. In addition, molecular dynamic simulations show that the adsorbed water in olivine is stable at least up to 900 K—a finding that is in accord with independent experimental studies. Thus, adsorption plus subsurface reaction of H2O in olivine could have taken place at temperatures considerably higher than the stability limit of hydrous minerals in the nebular condition. Using the DFT derived enthalpy of adsorption data, and reasonable approximation for the entropy of adsorption, we have calculated the fractional coverage of the reactive surface sites of olivine grains of spherical geometry by adsorbed water, and the corresponding ocean equivalent water (OEW) that could have been accreted into the Earth. These results suggest that adsorption and the associated subsurface hydroxylation of olivine grains might have been responsible for a significant fraction of the Earth's water budget. The adsorption of water into olivine crystals in the solar nebula might also have led to the delivery of water to other planetary bodies.