We will utilize the unique chronological framework provided by the crystallization history of zircon and allanite to investigate the relative magnitudes of thermal and mass fluxes through evolving rhyolite systems. Four major centers of silicic volcanism will be studied: 1) the Bishop Tuff and Glass Mountain rhyolites of Long Valley, California; 2) the Youngest Toba Tuff of Sumatra, Indonesia; 3) the Central Plateau Member rhyolites of the Yellowstone caldera complex, Wyoming; and 4) Coso volcanic field, California. Ages of individual crystal domains will be obtained by in situ ion microprobe 238U-206Pb and 238U-230Th dating. Guided by the age distributions, the ion microprobe will be used to determine the trace element zoning profiles of the crystals and of the compositions of their melt inclusions. Complementary isotopic analyses will be performed by microdrilling of well-characterized grains. Our goals are: 1) to determine the timing of differentiation and the differentiation "memory" of crystals in caldera-forming systems (Long Valley, Toba, and Yellowstone calderas); and 2) to investigate the chemical dynamics of evolving rhyolite systems which are relatively phenocryst-rich and poor (Toba and Coso volcanic fields). Collectively, these studies will provide a deeper understanding of the dynamic interplay between secular cooling, magmatic recharge, and crustal assimilation in rhyolite evolution.
