HR: 14:40h
AN: V13G-05 INVITED     [Abstracts]
TI: Nonequilibrium Magma Degassing: Results From Modeling of the ca. 1340 A.D. Eruption of Mono Craters, California
AU: * Gonnermann, H M
EM: gonnermann@eps.harvard.edu
AF: Deptartment of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AU: Manga, M
EM: manga@seismo.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720 United States
AB: We present results from a numerical conduit model of nonequilibrium, open-system magma degassing for the ca. 1340 A.D. subplinian eruption of Mono Craters (Gonnermann & Manga, EPSL, 2005). CO₂/H₂O concentration ratios in Mono Craters pyroclastic obsidian are high, relative to equilibrium values, and have been interpreted in terms of magma degassing in the presence of a buffering CO₂-rich vapor phase (Newman et al., JVGR, 1988; Rust et al., Geology, 2004). Using estimated magma eruption rates (Bursik JVGR, 1993), we show that Mono Craters volatile concentrations may instead record nonequilibrium degassing during ascent. Our results indicate that permeability-controlled, open-system gas loss is consistent with obsidian formation and promotes nonequilibrium degassing at shallow depths. Because of the low diffusivity of CO₂ relative to H₂O, we find that CO₂ concentrations can remain above equilibrium during magma ascent. To reproduce Mono Craters volatile concentrations, our model does not require volatile supersaturation, or an exsolved gas phase, prior to magma eruption. We estimate magmatic volatile contents of approximately 400 ppm CO₂ and 4.6 wt.% H₂O. In addition, we provide a scaling law for decompression rate vs. bubble number density that describes the transition from equilibrium to nonequilibrium degassing of magmatic volatiles in general.
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