238U and 232Th Series Studies of Arc-related Magmas

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This is study of magmatic processes occurring at crust and mantle levels in volcanic arcs using 238U and 232Th decay series systematics. Part of the funding will be used to complete ongoing projects that focus on the processes and lengths of time required to generate and accumulate rhyolite in Cascade arc volcanoes. The volcanoes that are being studied (medicine Lake and Crater Lake) have erupted silicic magmas in the Holocene, and are well-known geologically and geochemically through the work of U.S.G.S. collaborators. Approximately 6-9 months will be necessary to complete the analyses of whole rocks and contained mineral phases for nuclide activities, to determine magma residence times with open-system modeling techniques, and to write up the results. The remainder of the 2-year study period will be used to investigate the origin of mafic magmas that are the geochemical and thermal parents of Central American arc magma systems. We will analyze 20-25 rocks from the Central American arc and sediments from the Cocos Plate for 238U-230Th-226Ra-232Th activities. These same rocks are being analyzed by other scientists for 10Be/Be ratios, Sr, Nd, and Pb isotopic ratios, and B, trace element, and major element concentrations. The goal is to delineate the nature of the subducting slab component in Central American arc magmas, the way it becomes involved in arc magma genesis, and the time scales required for magma generation and transport.