Authors: みなみ 黒田
Source: Hokkaido University Collection of Scholarly and Academic Papers (Hokkaido University) (article, 2019-03-25)
Field: Materials Science — Ceramics and Composites

Water diffusion in silica glass toward understanding of atomistic diffusion mechanism in silicate melts and glasses Water diffusion in silicate melts is a fundamental process controlling physical and chemical consequences of magmatism, but mechanism of diffusion in silicate glasses and melts are not fully understood yet.In this study, with the aim of improving our understanding of the mechanisms of water diffusion in silicate melts and glasses including hydrogen isotope effects, I performed water diffusion experiments of water in silica glass and discussed the diffusion mechanism of water in silicate melt and glasses.The thesis consists of three parts as follows:(1) Effect of structural dynamical property of melt on water diffusion in rhyolite melt:Diffusion coefficients of water in rhyolite melt reported in previous studies are discussed in the context of water diffusion model in silica glass.The model well explains the water concentration dependence of water diffusivity in rhyolite melt, considering the effect of water concentration on activation energy of diffusion.The water concentration dependence of activation energy can be explained by the change of structural dynamical property of melt (i.e., viscosity), and the empirical relation between the water diffusivity and viscosity is also explained by the present model.Therefore I conclude that the water diffusion model proposed for silica glass (Kuroda et al.. 2018) can be applied to rhyolite melt and that the water diffusion in rhyolite melt is controlled by the same atomic process as in silica glass.(2) Hydrogen isotopic exchange in hydrated silica glass:Hydrogen isotopes can be a useful tracer of magmatism and eruption.I performed hydrogen isotope exchange experiments between water vapor and hydroxyls in silica glass for fundamental understanding of the hydrogen isotope fractionation between silicate glasses and water vapor.Obtained hydrogen isotope exchange profiles show that the surface isotope exchange reaction and diffusion of hydrogen isotopes, which are carried as molecular water, occur simultaneously.I also found that the surface exchange reaction between 1 H 2 O and 2 H in glass proceeds 1.7 times faster than that between 2 H 2 O and 1 H in glass, which is a dominant cause of isotope fractionation profiles inside the glasses.The large difference of surface isotope exchange rate implies that a reaction involving hydrogen atoms controls the isotope exchange at the glass surface.The large kinetic isotope fractionation of hydrogen between water vapor and hydroxyls in glass may occur in a time scale of a few hours, and could affect the hydrogen isotope fractionation between dissolved water in magma and degassed water (i.e., bubbles) in an ascending magma.(3) Fast diffusion path for water in silica glass: Diffusion experiments of 2 H 2 O at 900-750C and water vapor pressure of 50 bar found more than one-order of magnitude faster diffusion of water in SiO 2 glass than that reported previously.The fast diffusion profile of water was observed as an extended tail of the normal water diffusion profile by a line scan analysis with SIMS, and it can be fitted with a diffusion model with a constant diffusivity.The obtained fast diffusion coefficient suggests that the diffusion species responsible for the fast diffusion is not molecular hydrogen but molecular water.The diffusivity and activation energy for the fast water diffusion can be explained by the correlation between diffusivities of noble gases in silica glass and their sizes.Because noble gases diffuse through free volume in the glass structure, we conclude that molecular water can also diffuse through the free volume.The abundance of free volume in the silica glass structure estimated previously is higher than that of 2 H observed in the fast diffusion in this study, suggesting that the free volume were not fully occupied by 2 H under the present experimental condition.This implies that the contribution of the fast water diffusion to the total water transport in volcanic glass becomes larger under higher water vapor pressure conditions.These new findings improve our understanding of the mechanism of water diffusion in silicate melts/glasses, and would make a contribution to better understanding of volcanic activities. Diffusion ; Silicate ; Silicate glass ; Mechanism (biology) ; Silica glass ; Materials science ; Chemical physics ; Mineralogy ; Chemical engineering ; Chemistry ; Thermodynamics ; Composite material ; Physics ; Engineering Glass properties and applications