HR: 1340h
AN: V13B-0534    [Abstracts]
TI: Soufrière Hills Plagioclase: Postcards From the Edge.
AU: * Genareau, K
EM: genareau@asu.edu
AF: Arizona State University Department of Geological Sciences, P.O. Box 871404, Tempe, AZ 85287 United States
AU: Clarke, A
V13B-0534 AF: Arizona State University Department of Geological Sciences, P.O. Box 871404, Tempe, AZ 85287 United States
AU: Hervig, R
V13B-0534 AF: Arizona State University Department of Geological Sciences, P.O. Box 871404, Tempe, AZ 85287 United States
AB: Secondary Ion Mass Spectrometry (SIMS) can provide sub-micron depth resolution for analyzing products of volcanic eruptions. SIMS was used to examine the outer rims of plagioclase phenocrysts derived from both explosive and effusive eruptions of the Soufrière Hills Volcano (SHV), Montserrat. Phenocrysts were separated from the host igneous rock by crushing with a mortar and pestle and then cleaned with a Branson Sonifier. A 12.5 kV O₂⁺ primary ion beam was used to examine the variation in ten elements (Ca, Na, Si, Al, Ti, Zr, K, Fe, Sr, Li) through a crystal depth of 5-9 microns. Plagioclase crystals separated from explosively produced pumice clasts show increasing anorthite (An) content with depth into the crystal surface, starting at ~10% An at the surface and reaching a constant composition of ~45% An at 2-4 microns depth. According to experimentally determined estimates of plagioclase growth rates for the SHV magma (Couch et al. 2003; J. Petrology 44, 1477-1502), the 2-4 microns depth over which An changes corresponds to 1-7 hours of growth. Sr also shows a general increase with depth into the crystal. K shows a rapid decrease in abundance with depth. Fe shows more complex patterns that may indicate late-stage crystallization of magnetite. Plagioclase derived from exogenous dome samples also have surface compositions of ~10% An increasing with depth to ~30% An, but rather than plateau, the values begin to decrease again at 2-5 microns depth. This fluctuating abundance of An may reveal the presence of micron-scale decompression-induced growth zones that have not been previously documented due to limitations in the spatial resolution of conventional analytical techniques. Explosive and effusive samples exhibit conflicting Li trends. The explosively derived plagioclase have elevated surface Li concentrations while the dome derived plagioclase have low surface Li concentrations. These differing trends may provide evidence of closed system vs. open system degassing as a function of eruptive style. Geochemical analyses of igneous phenocrysts using the SIMS depth-profiling technique can be used to constrain the style of magma decompression and eruption. Additional analyses are currently being performed on an expanded suite of samples in order to confirm these results and to relate crystal-edge chemistry to other parameters such as quench pressure and degree of magma degassing.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 8400 VOLCANOLOGY
DE: 8425 Effusive volcanism
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005