Diffusion of Water in Silicate Melts

Diffusion of water in silicate melts and glasses has significant impact on both high temperature igneous and low temperature alteration processes. Water diffusivity in silicates is complicated by its strong, complex dependence on composition, including water content, which has been used to infer molecular level mechanisms of water diffusion. Very little has been done on mafic melts, but such studies are important both for their petrologic significance and because their molecular structures differ considerably (e.g., they are less polymerized) from more extensively studied acidic compositions. We have measured diffusion of water in high-Al basalt, Ab50Di50, Ab75Di25 (the last two modeling haplo-basalt to andesite), and rhyolite. Hydration (water-poor melt in contact with water vapor) and diffusion couple experiments (two glasses with different water contents placed in contact) were run in an internally heated pressure vessel or a rapid-quench TZM pressure vessel (1250-1300°C, 0.5-1 kbar, up to 25 minutes for intermediate to mafic compositions; 600-950°C, 0.5-1 kbar, up to 44 hours for rhyolite). Concentration profiles, obtained by FTIR spectroscopy, were fit using water diffusion coefficients with various functional forms for the dependence on water content. The dependence of water diffusivity on water content was also determined directly using a modified Boltzmann-Matano method for calculating diffusion coefficients along profiles. Key results include: (1) Hydration and diffusion couple experiments give similar results. (2) Diffusivity of water in basaltic melts at high water contents is consistent with the single previous study. (3) The concentration dependence of water diffusivity in these melts is consistent with previous studies and with simple models that assume water diffuses as water molecules in equilibrium with immobile hydroxyl groups. (4) Diffusivity of water over a wide range of melt compositions (at 1 wt.% water) correlates well with calculated melt viscosity.