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

研究了三种具有不同喷发风格的高温流纹岩的温度、喷发前挥发物含量、组成和年龄。这些单元是一个适度焊接的灰流层序，上面覆盖着一个飞溅流纹岩流复合体，而流纹岩流复合体又被一个体积庞大（15 km3）的流纹岩熔岩流覆盖。由于这些单元的喷发时间明显较短，且喷发活动风格迥异，为研究“无水”高温流纹岩喷发前挥发物含量，探索挥发物含量、温度和结晶度对喷发方式的控制提供了很好的机会。离子探针将用于分析熔体包裹体中挥发性和亲石性微量元素，并研究寄主现象晶体中微量元素的区带。对选定的浮石和矿渣块以及玻璃质熔岩的大量分析将提供岩浆系统的地球化学信息。详细的实地工作将为放射性年龄测定提供有关单元体积和关键样品的信息。除此之外，这项研究应该对这些流纹岩的岩石成因进行限制，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性成分的了解。

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.