Volcanic Processes in Subduction Zone Magmas: Composition, Abundance, and Role of C-O-H-S Volatiles

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RutherfordEAR-0073771This proposal involves three projects, all directed at developing a more complete understanding of how the magmatic processes such as magma mixing, crystallization, crystal-melt separation, and degassing occur in gas-rich, subduction zone volcanic systems. The tasks involve a range of analytical work (i.e., electron microprobe, ion-probe, F.T.I.R spectroscopy) on samples collected during field study, and on samples prepared experimentally under controlled P, T, and fO2, conditions. The first project is an investigation of the 1912 eruption of Mt. Katmai where three compositionally distinct magmas were erupted in a single event. The study is simultaneously an investigation of magma mixing processes, and a test of the zoned magma storage region hypothesis proposed by Hildreth. The second task is one in which fundamental data will be obtained on the composition of volcanic gases as a function of P, T, fO2, and the composition of coexisting crystals and melt. This study will consider CO2-H2O-S composition gas phases. The third project is a study of the volcanic eruptions which occurred almost simultaneously at Black Butte and adjacent Mount Shasta in California. The objective of this work is to determine the pre-eruption magma condition for the two eruptions, and to develop an expression relating magmatic conditions and the compositions of coexisting plagioclase and hornblende which are prominent phases in these eruptions.