HR: 1340h
AN: V43D-1445    [Abstracts]
TI: A Record of Magmatic Water Content Preserved in Hydroxyl Concentrations of Plagioclase Phenocrysts From the 1980-1981 Eruption Sequence of Mount St. Helens
AU: * Johnson, E A
EM: johnsoel@ucla.edu
AF: University of California, Los Angeles, Department of Earth and Space Sciences 595 Charles Young Dr. East, Los Angeles, CA 90095 United States
AB: Volatiles, and particularly water, influence many of the properties of volcanic systems including melt viscosity and crystallinity, and the explosive or effusive nature of an eruption. Magmatic water content could potentially be determined by measurement of OH concentrations in phenocrysts, assuming an equilibrium partitioning of water between the phenocrysts and melt. The concentration of OH in volcanic feldspars may also reflect many factors other than magmatic water content, including melt composition, oxygen fugacity, and thermal history. In this study, the OH concentrations of plagioclase phenocryts from four eruptions of Mount St. Helens between May 18, 1980 and April 1981 were measured using infrared spectroscopy in order to evaluate this method of determining magmatic water content. The eruption temperature, oxygen fugacity, and bulk chemical composition were all fairly constant through the eruption sequence from 1980-1981 at Mount St. Helens. The water content of melts from successive eruptions decreased from 4.6 wt% H$_{2}$O for the Plinian eruption on May 18, 1980 (Rutherford et al. 1985, JGR 90, 2929-2947), to less than 1 wt% H$_{2}$O for the latest dome-forming dacites. Plagioclase from the pumice erupted during the May 18, 1980 event contains 200 ppm H$_{2}$O by weight as structural hydroxyl groups, whereas feldspars from subsequent explosive eruptions with melt water concentrations about half that of the May 18 eruption (Melson 1983, Science 221, 1387-1391) contain about half the structural OH content (about 110 ppm for the October 16, 1980 and August 7, 1980 eruptions). The effusive dome-building eruption of April 1981 contains plagioclase with very low (about 20 ppm) water content, implying possible diffusive loss of hydrogen during the prolonged period of eruption. Homogeneous distribution of OH in feldspar grains $>$ 100 micrometers is observed even for those grains with pronounced major element zoning. These data show that, in the absence of changes in oxygen fugacity or eruption temperature, and even with variations in major element composition, the water content of silicic melts is preserved in the OH concentration of volcanic plagioclase.
DE: 8439 Physics and chemistry of magma bodies
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting