HR: 17:45h
AN: V14A-08    [Abstracts]
TI: The Evolution of Water Concentration in Rhyolitic Lava Flows During Emplacement and Solidification and Effects on Development of Flow Textures
AU: * Seaman, S J
EM: sjs@geo.umass.edu
AF: University of Massachusetts, Department of Geosciences, Amherst, MA 01002, United States
AU: Bruce, L
EM: lbruce@student.umass.edu
AF: University of Massachusetts, Department of Geosciences, Amherst, MA 01002, United States
AB: Rhyolitic lava flows typically host spherulites, consist of radiating skeletal crystals of feldspar +/- quartz that nucleated on a crystal or a vapor bubble and/or flow bands. We have examined the association of mineral and rock microtextures with variations in water concentration in one flow banded, spherulite-bearing rhyolitic lava flow and two spherulite-bearing non-flow banded rhyolitic lava flows. All of the flows are approximately 24 Ma and are part of the Atascosa volcanic complex of southern Arizona. Fourier transform infrared microspectroscopy was used to analyze water concentrations and to map variations in water concentration across zones of interest in the samples. The Bartolo Mountain lava flow is flow banded, with gray thicker flow bands hosting larger, water-richer spherulites and glass, and orange thinner flow bands hosting smaller, water-poorer spherulites and glass. Skeletal crystals vary in their water concentrations, but water preferentially was partitioned into the surrounding glass during spherulite formation, which occurred during flow of the lava. Textures and water concentration variations suggest that flow banding reflects primary variations in water concentration in the melt, possibly associated with stretching of vesicles as the magma flowed. Spherulites from the Hell's Gate lava flow consist of two or more generations of skeletal radiating crystals, with each successive generation nucleating on the end of crystals of the previous generation. Single skeletal crystals are up to 300 microns in length, and are wider nearer the core of the spherulites. Water concentrations generally increases along the length of each generation of sanidine needles, although oscillation of water concentration has been observed. Water concentration also generally increases from the innermost sanidine generation to those that successively overgrow the spherulite. Overall, water concentration increases from approximately 600 ppm in the cores of spherlites to approximately 5000 ppm in the rims of spherulites. Coexisting quartz phenocrysts are deeply embayed, possibly reflecting resorption due to increased concentration of water in the melt when sanidine crystallization in spherulites caused partitioning of water into the surrounding melt. Melt inclusions in quartz phenocrysts inside of spherulites generally have higher water concentrations (6000 ppm+) than melt inclusions in quartz phenocrysts outside of spherulites. Perlitic glass that surrounds the spheulites contains 2.4 to 4.1 wt.% water, but water is heterogeneously distributed in the glass. Spherulites in the Atascosa Peak lava flow consist of up to six generations of skeletal sanidine growth, and record complex variation in water concentration. Fourier transform infrared analysis, with microscopy and mapping of water concentration, provides a local monitor of evolving water concentration during the quenching and crystallization sequence of the lava flows.
DE: 8412 Reactions and phase equilibria (1012, 3612)
DE: 8425 Effusive volcanism
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting