The Effect of H2O on the Anomalous Velocities of Rhyolitic Glasses up to 3 GPa

Rhyolite eruptions can be some of the largest and most devastating. Rhyolitic melts are prevalent in the Earth's crust and can contain up to 10 wt.% water¹. Identifying such melts and detecting where they form remain challenging for geophysical methods, as there is a need for understanding their properties at relevant P-T conditions. In this study, we employ Brillouin Spectroscopy and FTIR Spectroscopy to measure acoustic velocities and water speciation in both dry and hydrous rhyolitic glasses up to 3 GPa. All highly polymerized silicate glasses, such as rhyolitic glasses, display anomalous acoustic velocities that decrease with increasing pressure up to 1.5-2 GPa². At pressures higher than 2 GPa, acoustic velocities increase with increasing pressure. The anomalous decrease in acoustic velocities can be attributed to the polymerization of the glass and the densification of the polymerized silicate network². It has also been shown that with increasing pressure, coordination of network-forming cations, such as Si and Al, increases, which contributes to the network connectivity³. Furthermore, water in silicate glasses also affect the polymerization of the glasses. When present as hydroxyl groups, water depolymerizes the glass network, which influences acoustic velocities of the glass, as well as other physical properties, such as density and structure^4,5. We discuss the influence of water on the measured acoustic velocities and elasticity of rhyolitic glasses as a function of pressure, as well as the influence of water on the stability and buoyancy of hydrous melts in the Earth's crust.

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