Temperature dependence of the speciation of water in rhyolitic melts and glasses

Abstract

The concentrations of molecular water and hydroxyl groups in rhyolitic glasses and melts depend on temperature as well as total water content. Increasing temperature results in an increase in the proportion of water dissolved as hydroxyl groups at a constant total water content. The temperature dependence of water speciation has been determined experimentally and can be described in thermodynamic terms by using a regular-solution formulation for the interactions between water molecules, hydroxyl groups, and oxygens in the rhyolitic glass or melt. This formulation is also consistent with calorimetric data on hydrous haplogranitic glasses. The temperature dependence of water speciation in rhyolitic glasses can be used as the basis of a geothermometer for hydrous volcanic glasses. Obsidians from pyroclastic deposits from the ca. 1340 A.D. eruption of the Mono Craters, California, yield temperatures of about 600 ºC. These chips may represent samples of the cool glassy margins and roofs of feeder dikes for this eruption. Glass inclusions in quartz phenocrysts from the Plinian deposit of the Bishop Tuff yield temperatures of ~300-600 ºC; inclusions in quartz phenocrysts from the Mono Lobe ash-flow deposit of the Bishop Tuff yielded temperatures of ~200-300 ºC. These temperatures are interpreted as closure temperatures and suggest that the initially hotter ash-flow deposit cooled more slowly than the Plinian deposit.