MOLECULAR H2O AS CARRIER FOR OXYGEN ISOTOPE DIFFUSION IN HYDROUS SILICATE MELTS

Water dramatically enhances oxygen "self" diffusion in silicate melts, in silicate glasses as well as in many crystalline silicates. Various hypotheses have been proposed and two have often been cited: the role of fast proton transients, and hydrous species as carrier for oxygen. The former is a qualitative idea that has not been developed into a quantitative theory. The latter hypothesis suggests that hydrothermal oxygen "self" diffusion actually reflects transport via H2O diffusion for which a quantitative theory has been developed. Although the theory makes testable predictions, it has not been verified until now. Here we report the first experimental confirmation of the theory for rhyolitic melts by measuring both H2O and 18O diffusion profiles in a single substance. We analyze 18O-enriched hydrothermal experimental charges for both H2O (by infrared spectroscopy) and 18O (by SIMS) profiles. The data demonstrate that oxygen "self" diffusion under hydrothermal conditions is due to molecular H2O diffusion, and is not due to the self diffusion of oxygen in 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 either the concentration or solubility of H2O in the given phase is known.