HR: 08:45h
AN: V41I-04    [Abstracts]
TI: Shear Brecciation of Magma and Obsidian Formation During the ca. 1340 A.D. Sub-Plinian Eruption of Mono Craters, California
AU: * Gonnermann, H M
EM: gonnermann@eps.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AB: It is thought that many silicic magma systems contain a pre-eruptive CO₂-rich vapor phase. This has implications for eruption triggering, as well as magma chamber and eruption dynamics. One magmatic system that has been extensively studied in terms of volatile content, is the 760-ka Bishop Tuff. Interpretation of fluid inclusion data from the Bishop Tuff suggests approximately 25% by volume of pre-eruptive, exsolved CO₂ and H₂O (Wallace et al., JGR, 1999). More recent volcanic activity associated with this magmatic system has been during the ca. 1340 A.D. subplinian eruption of Mono Craters, California, where CO₂/H₂O ratios in pyroclastic obsidian exhibit increased concentrations of dissolved CO₂ (Newman et al., JVGR, 1988). Any model for the Mono Craters eruption has to account for the observed CO₂/H₂O ratios, as well as the formation of nonvesicular obsidian. Here we investigate the possiblility that shear fracture in the glass transition interval, in the presence of a buffering CO₂-rich exsolved gas phase, resulted in the formation of obsidian during magma ascent (e.g., Goto, GRL, 1998; Gonnermann & Manga, Nature, 2003; Tuffen et al., Geology, 2003; Rust et al., Geology, 2004; Gonnermann & Manga, EPSL, 2005a). Using a numerical model of magma degassing and nonNewtonian flow in the volcanic conduit, we find that volatile contents of Mono Craters obsidian samples can be used to constrain conditions of magma flow in the conduit. Our conduit model includes viscous dissipation and indicates that shear-strain rates may have exceed the viscous relaxation rate of magma in the upper reaches of the conduit (≤ 50 MPa) during the sublinian eruptive phases at Mono Craters. This makes autobrecciation, by shear along the conduit walls, a plausible mechanism for obsidian formation, but only at shallow depths and magma temperatures of approximately 600 °C, consistent with the findings of Newman et al. (1988). Unless magma degassing occurred via open-system gas loss through the porous magma and/or in nonequilibrium (Gonnermann & Manga, EPSL, 2005b), our results imply a pre-eruptive volatile content, similar to the Bishop Tuff, of approximately 20% for the Mono Craters magma. In conclusion, we provide a generalized scaling for the shear-strain rate in the glass transition interval of ascending silicic magma as a function of magma pressure, temperature, and viscous dissipation.
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8430 Volcanic gases
DE: 8434 Magma migration and fragmentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005