HR: 0800h
AN: V41B-1444    [Abstracts]
TI: Melt Diversity and Magma Mixing During the 1991 Eruptions of Hudson Volcano, Chile.
AU: * Kratzmann, D
EM: davidk@gso.uri.edu
AF: GSO/Univ. of Rhode Island, South Ferry Road, Narragansett, RI 02882 United States
AU: Carey, S
EM: scarey@gso.uri.edu
AF: GSO/Univ. of Rhode Island, South Ferry Road, Narragansett, RI 02882 United States
AU: Scasso, R
EM: rscasso@gl.fcen.uba.ar
AF: Univ. de Buenos Aires, Ciudad Univ., Argentina, Buenos Aires, 1428 Argentina
AU: Naranjo, J A
EM: jnaranjo@sernageomin.cl
AF: Servicio Nacional de Geol. y Mineria, Casilla 10465, Santiago, 8360379 Chile
AU: Bande, A
EM: alejandrobande@advancedsl.com.ar
AF: Univ. de Buenos Aires, Ciudad Univ., Argentina, Buenos Aires, 1428 Argentina
AB: Tephra fall samples from the 1991 eruptions of Hudson Volcano, Chile, range from basalt to trachyandesite in bulk composition. Scoriaceous glass from the first eruption sequence (August 8, 1991), collected at Puerto Aisén, ~60km north-northeast of the volcano, exhibit a narrow range in composition from basalt to basaltic trachyandesite. They occur both as medium brown transparent shards and microlite-charged tachylitic clasts. The associated mineral phases are plagioclase, pyroxene, olivine and Fe-Ti oxides. Light gray to dark brown glassy inclusions are common in both plagioclase and pyroxene crystals. Glasses from the paroxsymal phase of the eruption (August 12-15), collected from a well-preserved pumice fall sequence along the main fallout axis 30 km from the volcano, are more compositionally diverse ranging from trachyandesitic to trachydacitic. Within this complex sequence the melt compositions show some systematic progression in composition from trachydacitic in the earliest phase to trachyandesitic in the latter phases. This is attributed to continuous tapping of a compositionally stratified magma chamber. Melt inclusions from different phases of the August 12-15 activity are commonly more evolved than their coexisting matrix glass, but also track the progression in compositional change from the beginning to end of the second sequence. The difference between melt inclusion and coexisting matrix glass compositions may reflect injection of more mafic magma into the chamber prior to the eruption. Distinctive banded pumices from the second eruption phase of the 1991 eruption often exhibit light and dark bands with glasses of identical major element composition. Color differences are attributed to variations in the extent of vesiculation. Examples of true magma mingling have been found in dark scoria fragments from the latter stages of the second eruption fall deposits. These clasts have domains of more mafic microlite-charged glass in sharp contact with less evolved microlite-poor, brown transparent glass. Microlite-rich domains may represent magma that had suffered degassing prior to the eruption and was mixed syneruptively with relatively undegassed magma.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3640 Igneous petrology
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005