HR: 0800h
AN: V41B-1451    [Abstracts]
TI: Cooling Induced Variations in Crystallization of a Basaltic Pumice From Shishaldin Volcano, Alaska
AU: * Szramek, L A
EM: szramek@mail.utexas.edu
AF: Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712 United States
AU: Gardner, J E
EM: gardner@mail.utexas.edu
AF: Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712 United States
AU: Hort, M
EM: hort@dkrz.de
AF: Institute for Geophysics , University of Hamburg, Hamburg, 20146 Germany
AB: Nucleation and growth rates of crystals are needed to model how fast magma can fractionate and solidify. Here, we use textural changes of crystals within basaltic pumice to infer such rates in basaltic magma. Pumice ejected during volcanic eruptions cools quickly by conducting heat to cold air that is incorporated into the eruption plume. Our modeling shows that cooling is highly non-linear through a pumice, with the rim cooling within seconds and the core remaining hot for an order of magnitude longer or more. We focus on pumice erupted in April 1999 from Shishladin volcano, Alaska, in which 1.4x10⁷ m³ (DRE) of tholeiitic basalt erupted in sub-Plinian style at a mass flux of 2.5x10⁶ kg s^-1. Pre-eruptive water contents, as determined from melt inclusions via FTIR, was 1.5 wt.%. Pumices contain a groundmass assemblage of plagioclase, olivine, augite, and Fe-Ti oxides; the phenocryst assemblage also contains hypersthene. Pumice cores have >25 vol.% plagioclase and >15 vol.% Fe-Ti oxides in the groundmass, whereas rims have <20 vol. % plagioclase and no Fe-Ti oxides. Plagioclase microlites cluster in composition at An_50-55, with no observable trend from rim to core. Textures also vary across the pumice, most notability with a doubling of the number density of swallowtail-shaped plagioclase from core to rim; when present, Fe-Ti oxides are dendritic. We see localized areas within the pumice that appear to be small shards as well as areas with drastically different textures without sharp boundaries. In one sample, bulk vesicularity is 57 vol.%, varies locally along the rim from 40-60 vol.%. Those variations do not correlate with groundmass texture. We speculate that some of the textural variations arise from air infiltrating the pumice during cooling. In addition, some smaller fragments have been incorporated into the larger one while still hot. To determine areas of conductive cooling we will examine further variations in vesicularity and texture. After determining areas of the pumice that resulted from conductive cooling, we will use our textural data with modeled cooling to determine the growth and nucleation rates of plagioclase and Fe-Ti oxides.
DE: 8411 Thermodynamics (0766, 1011, 3611)
DE: 8428 Explosive volcanism
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005