High-pressure, isotopic studies of the water diffusion mechanism in silicate melts and glasses

The stoichiometry and valence charge of the diffusing species responsible for the hydration of silicate glasses and melts, as well as its mechanism of transfer, have been subjects debated in the materials and geochemical literature for the past three decades. Resolving features such as these are difficult with normal methods of analysis due to the inherently transitory nature of the diffusing entity (or entities) and its method of transfer. In the case of water diffusion the complex nature of water dissolution properties already established is a further complication.;The results are used to suggest a likely diffusion mechanism which involves interstitial transport of a neutral molecular water species and a positively charged hydronium ion, with extensive dissociation-condensation reactions of the transported water with the melt lattice. The diffusion rate at a particular water content is controlled by the concentration of both the molecular water and hydroxyl species in the melt at that bulk water content.;The D;In this study a series of high-pressure experiments on water diffusion in rhyolite obsidian melts and glasses were conducted. They were intended to rule out a number of possibilities, and thus elucidate the nature of the diffusion process. The experiments conducted include; diffusion of D...