HR: 1340h
AN: V13B-0553    [Abstracts]
TI: Experimental results on decompression crystallization in an Aleutian basaltic-andesite
AU: Larsen, J F
EM: faust@gi.alaska.edu
AF: Geophysical Institute, Univ. of AK Fairbanks, Fairbanks, AK 99775 United States
AU: * Gardner, J E
EM: gardner@mail.utexas.edu
AF: Dept. of Geological Sciences, 1 University Station C1100 The University of Austin Texas, Austin, TX 78712 United States
AB: In 1991-92, Westdahl volcano in the Aleutians erupted basaltic andesitic magma, producing lava fountaining and ash clouds to 2 km asl, resulting in a lava flow that extended about 7 km from vent. In order to understand the magmatic ascent rates during that relatively non-explosive eruption, this study compares plagioclase and orthopyroxene microlite textures in the basaltic andesite to crystallization kinetics of plagioclase and pyroxene in laboratory experiments. The experiments used melt-rich basaltic andesite that was hydrated at 150 MPa, 1000° C, and an oxygen fugacity of NNO. Before decompression, the starting material was mainly water-saturated melt that contained a few percent of Fe-Ti oxides and pargasite. Pieces of the starting material were then decompressed rapidly to 25 MPa, and held for various times. Preliminary results show that when held at low pressure for less than 1 hour no crystallization occurred, except for some growth around existing pargasite and oxides. In runs held for about an hour both enstatite and plagioclase nucleate and grow, and are skeletal. In runs held from 4 to 24 hours, plagioclase growth dominates, as enstatite remains a minor phase. The single-step decompressions will be compared with runs using multiple steps to examine how the kinetics change. We can then compare the results with observed crystallization textures in the Westdahl lava and tephra to infer the path and rate of magma ascent during that eruption.
DE: 3630 Experimental mineralogy and petrology
DE: 8411 Thermodynamics (0766, 1011, 3611)
DE: 8412 Reactions and phase equilibria (1012, 3612)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005