HR: 0800h
AN: V31B-0493    [Abstracts]
TI: Pre-eruptive Storage Conditions and Volatile Contents of Basaltic Plinian Eruptions: Are They Unusual?
AU: * Goepfert, K J
EM: kgoepfert@mail.utexas.edu
AF: Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin, TX 78712, United States
AU: Gardner, J E
EM: gardner@mail.utexas.edu
AF: Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin, TX 78712, United States
AB: Explosive (Plinian/sub-Plinian) basaltic eruptions are rare in the geologic record and very little is known about what triggers them and what causes basaltic magma to fragment. Some of the known deposits are from the 122 B.C. eruption of Mt. Etna, the Fontana Lapilli and San Judas Formation from the Masaya Caldera Complex, and most recently is the April 1999 sub-Plinian eruption of Shishaldin. This study looks at pre-eruptive conditions of several explosive basaltic eruptions, and in particular focuses on the concentration of volatiles dissolved in the melt. Are unusually high concentrations of volatiles required for this type of eruptive behavior? Preliminary melt inclusion analysis of the 122 B.C. Plinian eruption of Mt. Etna reveal total water concentrations of 1.52-2.77 wt.%. These concentrations give minimum pressures of approximately 25-80 MPa. We have also begun hydrothermal experiments, based on those pressure approximations, to constrain temperature and total pressure. Preliminary results indicate the magma equilibrated at a temperature of 1050 ± 25 °C just before erupting. Although this temperature is low, the high concentration of water maintains a low viscosity (< 100 Pa s). This value is significantly lower than rhyolite, which suggests that viscosity may not be a factor in controlling whether magma can explosively fragment to drive Plinian eruptions. Our measured volatile concentrations overlap those published in the literature for non-Plinian eruptions of Etna, suggesting that basaltic Plinian eruptions may in fact not be triggered by excessive volatile contents. These hypotheses will be further tested by examining other basaltic Plinian eruptions.
DE: 1043 Fluid and melt inclusion geochemistry
DE: 3618 Magma chamber processes (1036)
DE: 3640 Igneous petrology
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting