Variations in Eruptive Style of High-Temperature Rhyolites of the Juniper Mtn. Volcanic Center, Owyhee Plateau, SW Idaho

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This project is to investigate the temperatures, pre-eruptive volatile contents, compositions, and ages of three high- temperature rhyolites that had contrasting eruptive styles. The units are a moderately-welded ash flow sequence overlain by a spatter-fed rhyolitic flow complex, which in turn is overlain by a voluminous (15 km3) effusive rhyolite lava flow. Because these units apparently erupted over a short period of time and show very different styles of eruptive activity, they provide and excellent opportunity to learn not only about the pre-eruptive volatile content of "anhydrous" high-temperature rhyolites, but also to explore the control of volatile content, temperature, and crystallinity on style of eruption. The ion microprobe will be used to analyze melt inclusions for volatile and lithophile trace elements and to investigate trace element zoning in host pheoncrysts. Bulk analyses of selected pumice and scoria lumps and of vitrophyric lava will provide information on the geochemistry of the magmatic system. Detailed field work will supply information on volumes of the units and key samples for radiometric age determinations. Among other things the study should place constraints on the petrogenesis of these rhyolites, determine whether the eruptive style relates to temperature as recorded by phenocrysts, and increase understanding of volatile composition prior to eruption.