HR: 10:35h
AN: V42C-02    [Abstracts]
TI: Combining major and accessory phase geothermometry and geochronology to delimit the thermochemical evolution of high-silica rhyolite at Yellowstone caldera
AU: * Vazquez, J A
EM: jvazquez@csun.edu
AF: California State University, Northridge, Dept. of Geological Sciences, Los Angeles, CA 91330`, United States
AU: Reid, M R
EM: mary.reid@nau.edu
AF: Northern Arizona University, Dept. of Geology, Flagstaff, AZ 86011, United States
AU: Kyriazis, S F
EM: stephanie.kyriazis.83@csun.com
AF: California State University, Northridge, Dept. of Geological Sciences, Los Angeles, CA 91330`, United States
AU: Sehler, R C
EM: robin.sehler.993@csun.edu
AF: California State University, Northridge, Dept. of Geological Sciences, Los Angeles, CA 91330`, United States
AB: Recent advances in zircon geothermometry and microbeam analysis provide an unprecedented opportunity for employing single crystals to establish absolute age limits on the differentiation history of silicic magma reservoirs. When coupled with compositional variations and geothermometry from coexisting major and accessory phases, the results can provide a unique perspective on the thermochemical evolution of silicic magmas. To quantify the thermal and chemical history of high-silica rhyolites that make up the Central Plateau Member of the Plateau Rhyolite at Yellowstone caldera, we have combined zircon and major phase geothermometry with ²³⁸U- ²³⁰Th and U-Pb of single crystals, and have analyzed associated mineral and glass compositions. The CPM rhyolites erupted in pulses between ca. 165 ka and 70 ka, and thus serve as sequential "snapshots" into the evolution of Yellowstone's postcaldera reservoir. With decreasing eruption age, CPM glasses contain higher Rb, Y, Nb, U, LREE and Th, and lower Eu, Sr, and Ba. Clinopyroxene and sanidine phenocrysts are more evolved with decreasing eruption age. However, some individual phenocrysts contain abrupt normal or reverse zoning. CPM zircons yield average ages that range from ca. 0 to 60 thousand years before their respective eruption ages. In a single rhyolite, the composition of chevkinites (LREE-Th-silicate) correlates with apparent ²³⁸U- ²³⁰Th ages that fall within the age range of coexisting zircons. In general, zircon trace element concentrations parallel the trace element co-variations of groundmass glasses. Coupled Ti-in-zircon thermometry (with aTiO₂ calculated from groundmass and inclusion glasses) and geochronology reveals a general decrease in average crystallization temperature within the range of ca. 850° to 780° C. Zircon rims typically yield temperatures that are ~10-20° lower than their cores. Temperatures calculated from QUILF thermometry using major minerals correlate with the results from zircon thermometry. Ti-in-quartz thermometry from ion probe measurements yields temperatures ranging from ca. 800°-900° C. Although some individual quartz contain irregular Ti zoning, rims generally yield lower temperatures (by ~10-20°) than cores. Quartz crystals from a single ~100 ka rhyolite contain rims that yield higher (up to 40°) temperatures than cores. The combined results from geochronology and geothermometry, as well as the secular variation of mineral and glass compositions, suggest that batches of rhyolite in the CPM reservoir evolved over tens of thousands of years and over a relatively narrow temperature range to generally cooler and more-evolved compositions. Nevertheless, this differentiation was punctuated by periodic episodes of reheating and/or magma mixing, which locally heated melts by up to at least 40° and produced abrupt compositional zoning within some crystals, particularly in major phases.
DE: 1036 Magma chamber processes (3618)
DE: 1115 Radioisotope geochronology
DE: 1120 Isotopic disequilibrium dating
DE: 8440 Calderas
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting