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

该项目旨在研究三种具有不同喷发风格的高温流纹岩的温度、喷发前挥发分含量、成分和年龄。这些单元是一个中等焊接的火山灰流序列，被飞溅补给的流纹岩复合体覆盖，而流纹岩复合体又被大量(15千米)喷出的流纹岩熔岩流覆盖。由于这些单元显然是在短时间内喷发，并显示出截然不同的喷发活动风格，它们不仅提供了极好的机会，不仅可以了解“无水”高温流纹岩的喷发前挥发分含量，而且还可以探索挥发分含量、温度和结晶度对喷发风格的控制。离子探针将用于分析熔融包裹体中挥发性和亲石性的微量元素，并研究寄主晶体中的微量元素分带。对选定的浮石和渣块以及玻璃斑状熔岩的整体分析将提供有关岩浆系统的地球化学信息。详细的实地工作将提供有关这些单元的体积和用于确定辐射年龄的关键样本的信息。除其他事项外，这项研究应该限制这些流纹岩的岩石成因，确定喷发风格是否与斑晶记录的温度有关，并增加对喷发前挥发成分的了解。

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.