Molecular H2O as carrier for oxygen diffusion in hydrous silicate melts

Dissolved water is known to dramatically enhance oxygen diffusion in silicate melts, glasses and minerals. A quantitative theory has been developed to explain this phenomenon by transport via molecular H2O diffusion [Y. Zhang, E.M. Stolper, G.J. Wasserburg, Diffusion of a multi-species component and its role in the diffusion of water and oxygen in silicates, Earth Planet. Sci. Lett., 103 (1991) 228–240.]. Here we report experimental confirmation of the theory for rhyolitic melts by measuring both H2O and 18O diffusion profiles in a single experiment. In sorption experiments at 100 MPa and temperatures from 1041 to 1136 K isotopically enriched water diffused into doubly polished rhyolitic glass wafers. H2O profiles were analyzed by infrared spectroscopy and 18O profiles by SIMS. 18O diffusivities were found to be 1–2 orders of magnitude slower than bulk water diffusivities but 3–4 orders of magnitude faster than Eyring diffusivities calculated from viscosity. The data show that oxygen “self” diffusion under hydrothermal conditions is due to molecular H2O diffusion, not due to the self diffusion of oxygen itself. With this confirmation, experimental data on H2O diffusion in silicate melts can be used to infer 18O diffusion under hydrothermal conditions, and hydrothermal oxygen diffusion data in silicate minerals can be used to infer H2O diffusivity, as long as the concentration or solubility of H2O in the given phase is known.