HR: 0800h
AN: V41A-1365    [Abstracts]
TI: Petrologic and Dynamic Importance of Flow Banding in Obsidian Lavas
AU: * Castro, J M
EM: jon.castro@oberlin.edu
AF: Oberlin College, Department of Geology 52 W. Lorain St., Oberlin, oh 44074 United States
AU: Dingwell, D B
EM: dingwell@lmu.de
AF: University of Munich, Department of Earth and Environmental Science Theresienstr. 41/III, Munich, D-80333 Germany
AU: Nichols, A
EM: nichols@min.uni-muenchen.de
AF: University of Munich, Department of Earth and Environmental Science Theresienstr. 41/III, Munich, D-80333 Germany
AU: Hess, K
EM: hess@min.uni-muenchen.de
AF: University of Munich, Department of Earth and Environmental Science Theresienstr. 41/III, Munich, D-80333 Germany
AB: One of the intriguing characteristics of effusive obsidians is the abundance of flow banding, or micrometer to centimeter-scale variations in microlite concentration. As these features arise from degassing, crystallization, and deformation processes, flow bands must contain important information regarding the chemical and physical evolution of obsidian magmas. Relatively little is known about the origin of this feature, and information on the relative rheologic properties of microlite-rich and poor bands is currently unavailable. In this paper, we present: 1) textural measurements on microlitic flow bands, 2) H2O concentrations, and 3) calorimetric measurements on flow bands of variable microlite content from several late Holocene obsidian flows. The goals are to better understand the mechanism of flow band formation and how these bands affect flow rheology and emplacement dynamics. Flow banded obsidians from Obsidian Dome (OD), Big Glass Mountain (BGM), and Big Obsidian Flow (BOF), are the focus of this study. Petrographic analysis shows that all obsidians contain microlites of pyroxene, feldspar, and oxide. However, the relative abundances of these phases vary dramatically within particular samples and between analyzed suites. Flow bands are therefore classified as 1) modal, wherein adjacent bands have the same mineral assemblage but contain different volume fractions, size distributions, and/or number densities of constituent phases, or 2) mineralogic, wherein adjacent bands differ by virtue of their constituent mineral assemblages. Banding in obsidians from both OD and BOF is dominantly modal, although rare bands display mineralogic differences defined by the presence or absence of plagioclase microlites. BGM obsidians tend to be modal in character, containing pyroxene microlites whose size and number densities vary across bands. Crystal size distributions measured on BGM obsidians reveal significant differences in the size and shape of microlite populations between adjacent flow bands. In general, the microlite size is largest in microlite-poor bands. Calorimetric and H2O content measurements of microlite-rich and poor bands are currently underway. Preliminary results indicate that texturally-distinct bands underwent similar cooling and degassing histories just prior to quenching during dome emplacement. Textural differences, may therefore originate in the conduit prior to extrusion.
DE: 8414 Eruption mechanisms
DE: 8429 Lava rheology and morphology
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting