Water Diffusion in Basaltic to Dacitic Melts and Glasses

Water diffusion in silicate melts is a controlling factor in bubble formation in magmas and volcanic eruptions. There are many published data for water diffusivity in rhyolitic melts and glasses, but only a few experimental studies have been performed to study water diffusion in other natural composition melts and glasses. We performed dehydration experiments on basaltic, andesitic and dacitic glasses with 1.1, 0.7 and 1.0 wt% total water at 400 - 675°C and 0.1 MPa. The dehydration rates were measured by in situ infrared spectroscopy and modeled by assuming total water diffusion. The diffusivities of total water in basalt and dacite were found to be dependent on total water contents, but that in andesite was independent of or weakly dependent on total water contents at water content and temperature ranges of this study. The total water diffusivities in basalt and dacite were determined by assuming the linear-dependent diffusivity on total water contents and that in andesite was obtained by assuming the constant diffusivity. The obtained diffusivities of total water in basalt, andesite dacite and previous data for water diffusivity in rhyolite show that at 0.7 wt% and 400 - 675°C, the total water diffusivity decreases from rhyolite to andesite, but the total water diffusivity in basalt is higher than that in dacite and lower than that in rhyolite. On the other hand, the results of this study predict that the diffusivities of total water in basalt to rhyolite increase with depolymerization of silicate structures at higher temperatures, which results from the crossovers of diffusivity-temperature trends. To express the dependence of total water diffusivity on anhydrous silicate composition, we introduced NBO/T as a primary measure. The pre-exponential factor and the activation energy obtained by plotting the total water diffusivity as a function of reciprocal temperature (Arrhenius plot) roughly increase with NBO/T. The preliminary model in which the total water diffusivity is expressed as a function of NBO/T reproduces the diffusivity data at low temperatures (400 - 675°C) and low water content (0.7 wt%) within the uncertainty of 0.28 log unit (2 sigma).