HR: 11:20h
AN: V22A-05    [Abstracts]
TI: Constraining Pre-eruptive Pressure/Temperature Variations, Transition From Chamber to Conduit, and Crystal Growth Rates: a SIMS Examination of Plagioclase Phenocrysts
AU: * Genareau, K D
EM: genareau@asu.edu
AF: Arizona State University, School of Earth and Space Exploration P.O Box 871404, Tempe, AZ 85287, United States
AU: Clarke, A B
EM: amanda.clarke@asu.edu
AF: Arizona State University, School of Earth and Space Exploration P.O Box 871404, Tempe, AZ 85287, United States
AU: Hervig, R L
EM: hervig@asu.edu
AF: Arizona State University, School of Earth and Space Exploration P.O Box 871404, Tempe, AZ 85287, United States
AB: Explosive volcanic eruptions may be triggered by physical changes in the volcanic conduit that include pressure (P) and temperature (T) variations, volatile loss, and crystal growth. These changes cannot be directly witnessed, and so must be inferred through the examination of eruptive products. Experiments show: 1) plagioclase An content varies as a function of P and T and 2) lithium is a rapidly diffusing trace element within this phase. Clasts were examined from two explosions, ExpA and ExpB, which occurred in a series of 88 vulcanian events at Soufrière Hills volcano, Montserrat in the fall of 1997. Here, we show that SIMS depth-profiling of plagioclase crystals from the clasts reveals An and Li variations that indicate late-stage, decompression-induced growth, and reveal variable P and T in the conduit. These data are consistent within clasts from the same vulcanian event but differ between the two explosions. An contents increase by 23 and 13 mol% within the final 25 μm of crystal growth for plagioclase derived from ExpA and ExpB, respectively. Overpressures that would generate the measured An increases exceed geophysical estimates for the 1997 vulcanian explosions by several tens of MPa, suggesting there was a combined effect of increasing P and T in the conduit (i.e., conditions were not isothermal). When the contribution of latent heat is included, a 100°C increase for ExpA and a 50°- 60°°C increase for ExpB, calculated overpressures are 1-33 MPa, covering the estimated range. Inflection of the near-edge Li signal allows us to identify when the crystals moved from the chamber to the conduit, where decompression-induced exsolution of volatiles began. Crystal growth rates within the conduit were calculated from the depth of Li inflection and the average time between eruptions, and they are consistent, within an order of magnitude, with values obtained from independent decompression experiments. Chemical variations in the rims of plagioclase phenocrysts preserve a geochemical record of the physical conditions in the conduit between vulcanian explosions at Soufrière Hills, allowing us to directly quantify the amount of crystal growth and the magnitude of P/T variations preceding an eruption.
DE: 8428 Explosive volcanism
DE: 8434 Magma migration and fragmentation
DE: 8439 Physics and chemistry of magma bodies
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting