Water, carbon dioxide, and hydrogen isotopes in glasses from the ca. 1340 A.D. eruption of the Mono Craters, California: Constraints on degassing phenomena and initial volatile content

Water and molecular carbon dioxide concentrations, the speciation of water, and hydrogen isotope ratios have been measured in a series of obsidians from the ca. 1340 A.D. eruption of the Mono Craters chain in central California. Obsidians were collected from domes and pyroclastic deposits. The maximum water contents of the obsidian clasts from the pyroclastic deposits generally declined as the eruption proceeded, but at most stratigraphic levels the obsidians display a range of water contents. The water contents of the obsidians from domes are lower than those from the pyroclastic beds. The D/H ratio varies monotonically with the total water content. The proportions of water dissolved in these glasses as hydroxyl groups and as molecules of water vary smoothly with total water content. Dissolved molecular carbon dioxide contents are low, but roughly proportional to total water content. The proportions of molecular water and hydroxyl groups in the obsidians from the pyroclastic phase of this eruptive cycle indicate temperatures of 500–600°C, much lower than magmatic temperatures. We propose that these glasses are samples of the cool glassy margins (e.g., roof, walls) of the feeder systems of the volcanoes caught up in the explosive eruption. The water contents of the glasses are interpreted in terms of the pressure (i.e., depth) at which they were frozen into the glassy rind. Modelling of the hydrogen isotopes and CO_2/H_2O ratios of the obsidians suggests that degassing of the magma initially approached that of a closed system, but changed to an open system when explosive eruptions ceased and the domes were extruded quiescently. These models also suggest that the parental rhyolitic magmas from which the erupted glasses were derived contained significant amounts of both water and carbon dioxide and imply that a vapor phase is present throughout the evolution of silicic magmas. Basaltic magma underplating rhyolitic magma in a crustal magma chamber could be the source of the CO_2