HR: 10:35h
AN: V42A-02    [Abstracts]
TI: Vesiculation Rates of Obsidian Domes Inferred From H₂O Concentration Profiles
AU: * Castro, J M
EM: castroj@si.edu
AF: smithsonian institution, Department of Mineral Sciences P.O. Box 37012 NHB-119 , Washington, DC 20013-7012 United States
AU: Manga, M
EM: manga@seismo.berkeley.edu
AF: University of California Berkeley, Department of Earth and Planetary Science 307 McCone Hall, Berkeley, CA 94720 United States
AU: Martin, M C
EM: MCmartin@lbl.gov
AF: Lawrence Berkeley National Laboratory, Advanced Light Source Division 1 Cyclotron Rd. Bldg. 6R2100 , Berkeley, CA 94720 United States
AB: We determine the timescale of vesiculation of obsidian in rhyolite flows. Millimeter-scale variations in H₂O were measured by synchrotron Fourier transform infrared (FTIR) spectromicroscopy on interlayered obsidian-pumice samples. The H₂O contents of all samples are above the 1-atm solubility value and decrease systematically towards vesicular zones, indicating that gas bubbles were growing and that degassing of melt to atmospheric pressure was incomplete. H₂O profiles are compared with models for water diffusion in order to constrain the temporal scale of vesiculation. Diffusion timescales range from 0.4 to 15 days, and represent the time between bubble nucleation and quenching. We use these results to estimate the duration of volatile exsolution and evaluate the timing of the formation of explosion craters on the surfaces of obsidian domes. Our results imply that surface explosions on obsidian domes occur days to weeks after flow emplacement.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8425 Effusive volcanism
DE: 8439 Physics and chemistry of magma bodies
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005