Program > Browse abstracts by speaker > Ni Huaiwei

Water diffusion in silicate melts is a fundamental process driving bubble growth and volcanic eruptions. In rhyolitic melt, water diffusivity increases with water content, a phenomenon previously explained by a model where molecular H2O is the primary diffusing species, while hydroxyl groups (OH) are considered immobile. However, studies on andesitic and basaltic melts revealed that OH contributes significantly to water diffusion at total H2O concentrations below 1 wt%, particularly due to the presence of free OH. The role of OH in more polymerized rhyolitic melt, however, remains poorly understood. Through water diffusion experiments in rhyolitic melts with H2O < 1.5 wt%, we demonstrate that (1) water diffusivity exhibits a linear correlation with water content, and (2) at H2O < 0.1 wt%, water diffusivity is comparable to that of fluorine ion, which is similar to OH in both valence and size. These findings strongly suggest that water diffusivity at low H2O concentrations (< 0.1 wt%) can be attributed to OH diffusivity (~2 μm²/s at 1473 K and 1 GPa), which is several orders of magnitude higher than the Eyring diffusivity, despite OH being predominantly bonded to Si and Al in rhyolitic melt. Additionally, experimental oxygen self-diffusivity in nominally "dry" rhyolitic melts (likely containing ~0.02 wt% H2O) supports the role of OH as an oxygen carrier. While molecular H2O dominates water diffusion at high H2O concentrations, the contribution of OH to water diffusion in rhyolitic melts becomes significant at H2O < 1 wt%. The importance of OH diffusion further increases with temperature, as higher temperatures favor the conversion of molecular H2O into OH. Moreover, mobile OH also enhances the electrical conductivity of hydrous rhyolitic melts.