HR: 09:00h
AN: V41I-05    [Abstracts]
TI: Flow Banding in Rhyolites: A Manifestation of Water Concentration Heterogeneity in the Melt?
AU: * Seaman, S J
EM: sjs@geo.umass.edu
AF: University of Massachusetts, Department of Geosciences 611 North Pleasant Street, Amherst, MA 01003 United States
AU: Dyar, D
EM: mddyar@myholyoke.edu
AF: Mount Holyoke College, Department of Earth and Environment, South Hadley, MA 01075 United States
AU: Marinkovic, N
EM: marinkov@bnl.gov
AF: National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY 11973 United States
AB: Synchrotron-generated infrared radiation was used to obtain Fourier transform infrared (FTIR) spectroscopic analyses of glass and spherulites from the Bartolo lava flow, a finely flow banded Oligocene (~24 Ma) rhyodacite from the Atascosa Mountains of southern Arizona. The bright synchrotron-generated infrared beam permits analysis of spots as small as 10 �m² and rapid collection of maps of water concentration across areas of interest. The goal of this study was to explore the origin of flow-banded rhyolites that consist of darker and lighter flow bands that are not compositionally distinct magmas, but contrast in texture and color. In the Bartolo lava flow, lighter-colored, thicker flow bands consist of gray glass and large (2.5 to 5 mm) spherulites. Darker-colored, thinner orange flow bands consist of orange glass and smaller (0.1 to 0.3 mm) spherulites. Zones of brown glass separate lighter and darker flow bands. Overall, the lighter-colored, thicker flow bands, that host the larger spherulites, have higher average water concentrations (to 5000 ppm average) than the darker-colored, thinner flow bands that host the smaller spherulites (to 2000 ppm average). This difference, although not large, may indicate that flow bands result from stretching of zones in the melt that had contrasting water concentration prior to flow of the melt. Spherulites in the bands preserve a record of fluctuating water concentration at the boundary between the growing spherulite and the surrounding melt. In large spherulites in the lighter bands, in some cases, two zones of feldspar radiate from the center of the spherule. The two zones are separated by a concentric zone of glass. The innermost radiating feldspar zone typically contains less (~2500 ppm) water, and the outer radiating feldspar zone contains ~3800 ppm water. The transitional glass zones that separate the inner and outer zones contain to 7500 ppm water. These characteristics suggest that when the spherulites began to grow, sanidine incorporated as much molecular water as possible, but the surrounding melt became increasingly water-rich, eventually causing sanidine to leave the liquidus. At that point, the concentric zone of water-rich glass quenched, locally removing water from the melt. At lower water concentration, sanidine once again was capable of crystallizing, producing the second radiating sanidine zone. Sanidine continued to crystallize until it was again forced off of the liquidus by local enrichment of water in the surrounding melt. At this point, large-scale quenching of melt to water-rich glass occurred. Overall the coarser-spherule, lighter-colored bands are much more water-rich than the finer-spherule, darker-colored bands, consistent with abundant water facilitating the growth of large spherules, and generally supporting the work of Hausback (1987), who suggested that flow banding may consist of alternating layers of stretched lithophysae, within which vapor phase crystallization occurred, and glass representing originally less water-rich magma.
DE: 8425 Effusive volcanism
DE: 8429 Lava rheology and morphology
DE: 8434 Magma migration and fragmentation
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005