HR: 1330h
AN: V12A-0551    [PDF]
TI: Amphibole Reaction Rims in Response to Decompression compared to Heating: An Experimental Approach
AU: * Browne, B L
EM: ftblb@uaf.edu
AF: Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775 United States
AU: Gardner, J E
EM: gardner@mail.utexas.edu
AF: Department of Geological Sciences, University of Texas at Austin, 1 University Station C1100, Austin, TX 78712-0254 United States
AU: Larsen, J
EM: faust@gi.alaska.edu
AF: Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775 United States
AB: Amphiboles are an important magmatic mineral phase common to a variety of volcanic settings, and are especially sensitive to variations in the water content and temperature of the surrounding induced during magmatic ascent or heating accompanying magma mixing events. As magma rises toward the surface hydrous amphiboles, stable at high water pressures, break down in response to degassing of the surrounding melt. Reaction rims also form around amphiboles that are destabilized when mixing with hotter, more primitive magmas occurs. In this study, we compare the thickness, composition, and rate of formation of amphibole reaction rims during a series of isothermal decompression and isobaric heating experiments using dacite and andesite magmas erupted from Redoubt volcano, Alaska in 1989-1990. The 1989-90 Redoubt dacite pumice consists of magnesio-hornblendes, plagioclase, othropyroxene, and Fe-Ti oxides in a high-silica rhyolite glass. In contrast, 1989-90 Redoubt andesite contains pargasitic amphiboles, orthopyroxene, clinopyroxene, plagioclase, and Fe-Ti oxides in a rhyodacite glass. Our results indicate that the reaction rims formed in response to decompression are distinguishable in thickness, grain size, and type of mineral formed in the rims from those formed in response to heating, regardless of the type of amphibole studied. First, decompression induced rims were thinner (5- 50 um) compared to those developing in response to heating (25- 200 um). Decompression induced rim growth rate ranges from 0.3 to 0.6 um/day compared to heating induced rim growth rate, which ranges from 0.4 to 1.6 um/day. Second, decompression rims were finer-grained (5-15 um diameter crystals) compared to heating reaction rims (5- 35 um diameter crystals). And third, decompression rims are predominantly composed of orthopyroxene and plagioclase, with lesser amounts of Fe-Ti oxides, whereas reaction rims that grew in response to heating experiments were composed almost entirely of clinopyroxene and Fe-Ti oxides, with lesser amounts of orthopyroxene, and plagioclase. Because the results are independent of the type of amphibole studied, the physical characteristics of reaction rims around amphiboles from a given eruption may yield important information about the rates and types of pre-eruptive disequilibrium event that was the cause.
DE: 8414 Eruption mechanisms
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting