HR: 1340h
AN: V53A-0622    [Abstracts]
TI: Magma Chamber of the 26.5 ka Oruanui Eruption, Taupo Volcano, New Zealand
AU: * Liu, Y
EM: yangl@geosci.uchicago.edu
AF: University of Chicago, Dept of Geophysical Sciences 5734 S Ellis Ave, Chicago, IL 60637 United States
AU: Anderson, A T
EM: canderso@midway.uchicago.edu
AF: University of Chicago, Dept of Geophysical Sciences 5734 S Ellis Ave, Chicago, IL 60637 United States
AU: Wilson, C J
EM: c.wilson@gns.cri.nz
AF: Institute of Geological and Nuclear Sciences, PO Box 30368 , Lower Hutt, 6315 New Zealand
AU: Davis, A M
EM: a-davis@uchicago.edu
AF: University of Chicago, Dept of Geophysical Sciences 5734 S Ellis Ave, Chicago, IL 60637 United States
AU: Davis, A M
EM: a-davis@uchicago.edu
AF: Enrico Fermi Institute, University of Chicago 5640 S Ellis Ave, Chicago, IL 60637 United States
AB: We have investigated melt inclusions and their host quartz crystals from the Bishop-Tuff-sized 26.5 ka Oruanui eruption at Taupo volcano, New Zealand. Compositions (major and trace elements, H$_{2}$O and CO$_{2}$) of melt inclusions and cathodoluminescence (CL) images of quartz were obtained for eight individual pumices from early, middle and late depositional units. All melt inclusions are high-silica weakly peraluminous rhyolites. Melt inclusions for different eruptive phases have similar ranges of H$_{2}$O contents (3.8-5.2 wt %), but late-erupted samples have higher CO$_{2}$ contents (mostly $>$ 140 ppm). A positive correlation between CO$_{2}$ and compatible trace elements such as Sr suggests that crystallization and melt entrapment occurred under gas-saturated conditions. Trace elements variations in melt inclusions are consistent with fractionation of 30-40 wt % crystals (plagioclase+quartz+pyroxene+amphibole). Crystal contents in pumices, trace-element contents in melt inclusions, and CL zoning patterns of quartz show no correlation with eruptive phases, suggesting that the Oruanui magma was well mixed before eruption. Some Oruanui quartz crystals contain distinctive CL zonings with a jagged ('restitic') core mantled by a black CL zone. Trace element variations in melt inclusions in the 'restitic' cores are consistent with fractionation of Ba-bearing minerals such as sanidine and/or biotite, both of which are rare or absent in rocks erupted from Taupo volcanic center. The above evidence suggests that Oruanui rhyolite is generated by assimilation of previous intruded rocks or country rocks, differentiated by crystal fractionation, and then mixed prior to eruption. Despite the differences in trace element and volatile contents, and crystal assemblages, both Bishop Tuff and Oruanui magmas involve crystal fractionation as one of the main differentiation mechanisms during their evolution. However, there are pronounced differences in the pre-eruptive stratification of the two chambers, which may reflect the tectonic settings, eruption rates, and ages of the systems.
DE: 8400 VOLCANOLOGY
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting