HR: 16:00h
AN: V44B-01    [Abstracts]
TI: Multiple Origins of Pyroclastic Obsidian and Implications for Changes in the Dynamics of the 1300 BP eruption of Newberry Volcano, USA
AU: * Rust, A C
EM: arust@eos.ubc.ca
AF: University of British Columbia, 6339 Stores Road, Vancouver, BC V6T1Z4 Canada
AU: Cashman, K V
EM: cashman@uoregon.edu
AF: University of Oregon, 1272 University or Oregon, Eugene, 97405 United States
AB: Like many rhyolite tephras, the pyroclastic deposits of the 1300 B.P. eruption of Newberry Volcano, USA, contain minor amounts of obsidian. The H2O and CO2 contents and textures of these clasts vary considerably and provide information on eruption history and dynamics. Early in the eruption, obsidian probably derived from veins of vanguard magma or tuffisite that, together with wall rock fragments, were eroded and incorporated into the eruption column as the vent widened. Later, following a temporary cessation of activity, the proportion of obsidian to lithic fragments increased and new types of obsidian dominated, types that represent remnants of a shallow conduit plug and welded fallback material. Analysis of bubble geometries provide flow parameters and time scales operative for deformation within the shallow conduit. Furthermore, spatial variations in CO2 help constrain welding and wall rock assimilation time scales. Comparison of obsidian characteristics from the Newberry eruption with those of the well-studied Mono Craters eruption shows intriguing differences in obsidian formation that may relate to the nature of the conduit feeding the two events. From this comparison we conclude that obsidian is less likely to provide information on magmatic fragmentation than on time scales and mechanisms of pre-fragmentation magma ascent.
DE: 8428 Explosive volcanism
DE: 8434 Magma migration and fragmentation
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005