Water Diffusion in Dacitic Melt and Glass, and Explosive Volcanic Eruptions

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ZhangEAR-0228752Gas-driven volcanic eruptions are a major natural hazard. These eruptions are powered by the exsolution of H2O and other volatiles from magma. The process is complex, and understanding it requires quantification and modeling of a number of variables and processes, especially the diffusion of water. After many years of work, most of the fundamental data for high-silica rhyolite--a magma with 77 wt% silica--has been experimentally determined. However, there is a critical need for experimental studies of dacitic magmas, containing around 65 wt% silica and common to explosive eruptions such as the 1980 eruption of Mount St. Helens, the 1991 eruption of Mount Pinatubo, and the 1991 eruption of Mount Unzen. Under this grant, water diffusion in dacitic melt and glass will be investigated using dehydration and diffusion couple experiments over a wide range of temperature, pressure, and H2O contents. Experiments will be performed in one-atmosphere furnaces, rapid-quench cold-seal pressure vessels, internally-heated pressure vessels, and a piston-cylinder apparatus. Hydrous species concentrations will be analyzed by Fourier Transform Infrared Spectroscopy, and diffusion profiles modeled using a nonlinear diffusion equation. The results will be used to model bubble growth and magma fragmentation in dacite.