It was recalled that dissolved water dramatically enhanced O diffusion in silicate melts, glasses and minerals. A quantitative theory was developed in order to explain this phenomenon by transport via molecular H2O diffusion (Zhang et al. 1991). Experimental confirmation of the theory was found here, for rhyolitic melts, by measuring both H2O and 18O diffusion profiles in a single experiment. In sorption experiments performed at 100MPa, and 1041 to 1136K, isotopically enriched water diffused into doubly-polished rhyolitic glass wafers. The H2O profiles were analyzed by means of infra-red spectroscopy and the 18O profiles were measured using secondary ion mass spectrometry. The 18O diffusivities were found to be 1 to 2 orders of magnitude lower than bulk-water diffusivities but 3 to 4 orders of magnitude higher than the Eyring diffusivities deduced from the viscosity. The data showed that O so-called self-diffusion under hydrothermal conditions was due to molecular H2O diffusion, and not due to the self-diffusion of O itself. With this confirmation, the experimental data on H2O diffusion in silicate melts could be used to deduce the presence of 18O diffusion under hydrothermal conditions. The hydrothermal O diffusion data for silicate minerals could be used to deduce the presence of H2O diffusivity; provided that the concentration or solubility of H2O in the given phase was known.

Molecular H2O as Carrier for Oxygen Diffusion in Hydrous Silicate Melts. H.Behrens, Y.Zhang, M.Leschik, M.Wiedenbeck, G.Heide, G.H.Frischat: Earth and Planetary Science Letters, 2007, 254[1-2], 69-76