HR: 1340h
AN: V13B-0536    [Abstracts]
TI: Plagioclase Crystal Populations in Lavas from Calbuco Volcano, Chile
AU: * McKelvey, T
EM: tiffany.mckelvey@fiu.edu
AF: Florida International University, Earth Sciences Department 11200 SW 8th Street UP Campus, PC 344, Miami, FL 33199 United States
AU: Hickey-Vargas, R
V13B-0536 AF: Florida International University, Earth Sciences Department 11200 SW 8th Street UP Campus, PC 344, Miami, FL 33199 United States
AB: Calbuco Volcano in Southern Chile is regionally unique in that it is predominantly andesitic, whereas surrounding volcanic centers are basaltic in composition. Lava from Calbuco is also hornblende-bearing, phenocryst-rich, and it contains crystal clots that are interpreted as cumulate minerals entrained in the magma. The phenocryst assemblage of the lava, and the mineral assemblage of the crystal clots, is: plagioclase, hornblende, orthopyroxene, clinopyroxene, olivine and oxide minerals. Three different species of plagioclase crystals are found in Calbuco andesite: zoned phenocrysts in the lava, unzoned grains in the crystal clots, and high-An megacrysts. The phenocrysts are predominantly anhedral and normally zoned with An mol% in the core ranging from An_68-91, and at the rim from An_58-82. Most of these crystals range in size from 50 to 700 microns, with rare grains larger, and nearly all contain melt inclusions. Plagioclase grains within the crystal clots are not zoned and have An mol% of An_70-93, which overlaps the An mol% of the zoned phenocryst cores. These grains range in size from 100 to 500 microns. Megacrysts are 2000 microns and larger in size, and are unzoned, except for the rim of the grains. They have very high An mol% of An_88-92 in the core. The 10-60 micron rims are normally zoned. Based on the overlapping An mol% contents of zoned plagioclase cores and the plagioclase from crystal clots, it is probable these two populations of crystals have formed as phenocrysts, crystallizing directly from the basaltic andesite to andesite magma. The origin of the megacrysts is more puzzling. Periodic replenishment by a more mafic magma and magma mixing could introduce these large An-rich plagioclase crystals, or they could be cumulate crystals that formed from a more mafic magma and were later entrained in andesitic magma. In either case, the narrow rim of normal zoning suggests a relatively short time span between entrainment and eruption.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3618 Magma chamber processes (1036)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3625 Petrography, microstructures, and textures
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005