Residence Times of Silicic Magmas

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9706519 Reid Large, highly explosive eruptions reflect the accumulation of magma at shallow crustal levels, which in turn, requires a heat input significantly in excess of output. The duration of magma accumulation and compositional modification is therefore a record of the thermal history of a volcano which, for voluminous rhyolitic magmas, may operate over timescales of tens of thousands to millions of years. To delimit the dynamics of magma accumulation, three new methods for dating the crystallization of minerals from rhyolitic magmas will be refined and employed. Two of these methods will utilize a high resolution ion microprobe which will enable individual zircon and allanite crystals to be dated and a detailed record of crystal growth to be obtained. The third method will determine the timing of quartz crystallization from age of the glass inclusions they contain. Major centers of rhyolitic volcanism, including Long Valley (California), Yellowstone (Wyoming), and Toba (Sumatra) calderas, will be investigated. The insights gathered from these studies will be used in conjunction with the temperature-dependence of crystallization to develop models for the thermal evolution of rhyolitic magmas associated with large-volume silicic systems.