Depth to and concentrations of water in large bodies of silicic magma. Progress report, July 1, 1982-June 30, 1983

Abstract

Large bodies of silicic magma are potential sources of geothermal energy and ore. They also pose threats of catastrophic eruptions. The depths of such bodies are related to their economic potential and probably to their eruption mechanisms. The concentrations of water in the magmas are important for their eruptive and dynamical behavior and for the development of ores. Estimates of viscosity and density of melt require knowledge of concentration of water. The concentration of water in melt before ascent and eruption can be measured in inclusions of glass which became trapped in crystals before extrusion. The depth of a magma body can be estimated or delimited if we can find out the concentrations of both carbon dioxide and water in the inclusions of glass. Initial results on the Bishop Tuff of Long Valley Caldera, California yield 4.9 +- 0.5 percent H/sub 2/O for glass included in quartz from the Plinian air fall pumice. This result is comparable to the estimates of Hildreth (1977) of about 3.5 to 4.9 percent H/sub 2/O in the lowermost part of the Bishop ash flow. From January 1982 through December 1982, analyses of inclusions of glass in two additional quartz phenocrysts from the Plinian air fall unit of the Bishop Tuff revealed variable H/sub 2/O and CO/sub 2/. The corresponding partial pressures range between about 2000 and 5000 atmospheres, assuming gas saturation. The variation may be natural or caused by an analytical artifact. A computerized data file has been constructed to facilitate the storage and retrieval of published and unpublished chemical analyses of glasses and minerals. Some data on the Bishop Tuff are presently stored.