HR: 0800h
AN: V41F-1531    [Abstracts]
TI: Heterogeneous Crystal Populations: Signatures, Genesis and Chronologies
AU: * Reid, M R
EM: mary.reid@nau.edu
AF: Northern Arizona University, Department of Geology, Flagstaff, AZ 86011-4099 United States
AU: Vazquez, J A
EM: jvazquez@csun.edu
AF: California State University, Department of Geological Sciences, Northridge, CA 91330-8266 United States
AB: The ongoing development of accessory phase-based techniques helps to illuminate possible scenarios for the physical state of magmatic systems between eruptions. These, in turn, may aid in the detection of magmas in the crust. At Long Valley caldera, eastern California, ion microprobe analyses of ²³⁸U-²³⁰Th in zircon from the youthful Inyo Dome rhyolites show that even though their ages are mainly younger than the 760 k.y. old caldera-forming Bishop Tuff, they are significantly older than eruption (up to 200 k.y. and more). Do these ages date the timing of magma emplacement and show that liquid was present in the system for at least 200 k.y.? Or do the ages represent sampling by the ion beam of bimodal age domains, meaning that they could have been rapidly mobilized from voluminous crystal mushes residual after the Bishop Tuff (e.g., Hildreth, 2004)? Additionally or alternatively, could the "old" zircons be derived via assimilation of young, hydrothermally altered intrusive rocks (Schmitt and Simon, 2004)? The majority of ²³⁸U-²⁰⁶Pb analyses of zircons from Deer Mountain (13 of 20) are consistent with a single crystallization age that is somewhat older than previously obtained by ²³⁸U-²³⁰Th disequilibrium dating; the remaining zircon ages are younger and range to within error of the eruption age. The absence of Bishop Tuff-aged zircons indicates that if crystal mush contributes to the Inyo Dome rhyolites, its remobilization must have involved reheating to temperatures in excess of zircon saturation conditions (T>~800°C). Ion microprobe U-Th ages of allanites from South Deadman Dome range from 150 to <10 ka, in contrast to virtually all of the zircon ages which are >100 ka. The ca. 2-fold ranges in MnO/MgO and La/Nd in the same allanites overlap and extend the range of compositions we obtained for rhyolites from Toba caldera (Vazquez and Reid, 2005). The South Deadman Dome allanites could, by analogy to experimental results for a Toba rhyolite, have crystallized between the temperature interval of ~765°C to ~745°C. Taken at face value, the temperature-age distributions of allanite and zircon are therefore suggestive of a broad cooling trend. In detail, however, the T-t evolution defined by the allanites is one of a protracted interval of INCREASING temperature before eruption. We infer that intrusions of hotter, less evolved magma progressively hybridized the magma reservoir. Given the rarity with which zircons contemporary with the allanites have (as yet) been detected, the thermochemical conditions of that reservoir were sufficiently well buffered so that extensive (re)crystallization of zircon did not occur. Repose of a crystal-rich magma is indicated. This abstract is dedicated to the pioneering work of Stan Hart and Nobu Shimizu who showed how the Cameca 3f, Serial No. 1, could be used to investigate the isotopic and chemical evolution of crystals.
DE: 1036 Magma chamber processes (3618)
DE: 1115 Radioisotope geochronology
DE: 1120 Isotopic disequilibrium dating
DE: 3618 Magma chamber processes (1036)
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005