HR: 0800h
AN: V41D-0813    [Abstracts]
TI: Recharge as an Eruption Trigger Revealed at El Misti, Southern Peru
AU: * Tepley, F J
EM: ftepley@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331-5506, United States
AU: Salas, G
EM: salasg@geo.oregonstate.edu
AF: Department of Geosciences, Oregon State University, Corvallis, OR 97331-5506, United States
AU: de Silva, S
EM: desilvas@geo.oregonstate.edu
AF: Department of Geosciences, Oregon State University, Corvallis, OR 97331-5506, United States
AB: The most recent major explosive eruption of El Misti in southern Peru occurred ~2000 yr BP, producing significant pyroclastic deposits and extensive lahars. As such this eruption has considerable significance for assessing the hazard posed for the city of Arequipa (pop. ~800,000) built on the lahars of this eruption. Throughout its ~100 kyr history, eruptions from Misti have recorded recharge events into an open magmatic system. The 2000 BP eruption is no exception and contains a spectacular record that suggests that recharge may have triggered the eruption. Juvenile blocks from the 2000 BP eruption reveal the intimate mingling of two magmas; amphibole-plagioclase rhyolite and amphibole-plagioclase andesite. Glass compositions range from ~67 wt.% SiO₂ in the andesite to ~75 wt.% SiO₂ in the rhyolite. Phase equilibria yield temperatures of 780±30°C in the rhyolite and 960±40°C for the andesite. Viscosity estimates yield 10⁸ and 10⁶ Pa s, respectively. Mingling at the cm scale is manifested in distinct bands and selvages that are linear or folded, while at the mm scale variably convoluted ribbons, wisps, and threads attest to strong viscosity contrasts between the magmas. The record in plagioclase phenocrysts in both phases is extremely complicated with eight different textural types in the rhyolite and four different types in the andesite. Plagioclase phenocrysts from the andesite have core compositions of ~An₈₀, are normally zoned with rims to ~An₆₀, but are texturally complex. Plagioclase phenocryst compositions in the rhyolite are much more complex; some are similar to those in the andesite, others are more typical of those that would be expected in a rhyolitic melt with cores of ~An₅₅ to rims as low as An₄₀. Microlites in the andesite are non-zoned and have compositions similar to rims of the phenocrysts. Microlites in the rhyolite are normally zoned, and define two populations; one with An₆₀ cores and An₅₀ rims, and the other with An₄₀ cores and An₃₀ rims, similar to the phenocrysts. The record in amphibole phenocrysts is simpler. Those in the rhyolite have dehydration reaction rims whereas the andesite-hosted amphiboles do not. Compositions of amphiboles in the two phases are broadly similar although the andesite does have a population with higher Mg# and lower Al^IV. We interpret the plagioclase record to represent an integrated history extending back to pre-2000 BP, whereas the amphibole and microlite data to be revealing of the 2000 BP event. We propose that a dike of hotter, less viscous andesite intruded into a stagnant or semi-solid mass of rhyolitic composition material and its associated phenocryst suite. By virtue of its lower viscosity and momentum, the andesite forced its way through the rhyolite with little interaction except for local exchange through viscous coupling at the margins of the dike. Dehydration of amphiboles in the rhyolites was induced and may have contributed volatiles to the mixture. The microlites record decompression-induced growth during ascent of the magmas. The distinct composition of residual glass in the two phases demonstrates a lack of wholesale thermal and compositional equilibration suggesting that the mingling occurred a very short time before the subsequent eruption. We interpret the record in the 2000 BP eruption of El Misti as one where mafic recharge triggered the eruption and this may serve as a model for the recurrent explosive eruptions at this volcano.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8414 Eruption mechanisms and flow emplacement
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting