HR: 0800h
AN: V41D-0809    [Abstracts]
TI: Petrological Characteristic of Recent Eruption Events at Galeras Volcano, Colombia
AU: * Nakada, S
EM: nakada@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Yayoi, Bunkyo, Tokyo, 113-0032, Japan
AU: Noguchi, S
EM: snoguchi@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Yayoi, Bunkyo, Tokyo, 113-0032, Japan
AU: Cortes, G P
EM: gpcortes@ingeominas.gov.co
AF: INGEOMINAS, Avenida 12 de Octubre, Manizales, 15-47, Colombia
AU: Calvache, M L
EM: mcalvache@ingeominas.gov.co
AF: INGEOMINAS, Diagonal 53, Bogota, 34-53, Colombia
AB: On-going volcanic activity at Galeras began in 1988, and major explosive eruption events occurred in 1993. Long- period seismic events had occurred before these events. In late 2004, explosive eruptive events resumed and intermittently continued by the present. Long-period events similar to those before the 1993 explosive eruptive events have been observed since early 2006. Evaluating potential of more explosive future eruptions becomes very important to minimize volcanic disasters in cities and towns around this volcano, including the city of Pasto. Investigation of temporal changes in petrological characteristics of eruption products makes us possible to understand the magma system undergone at Galeras. Whether has it changed (or developed) from the 1993 explosive events or not? Ballistics and scoria of vulcanian explosions during 2004-2006 and of the 1991 eruption were investigated in this paper. Rocks are two pyroxene andesite with various crystallinity in groundmass. Small amount of hornblende and olivine microphenocrsyts are involved. The whole rock chemistry hardly changed with time. Lines of petrological evidence suggest that magma mixing occurred throughout the eruption products during 1991-2006; 1) bimodal populations in core compositions of plagioclase phenocrysts, 2) plagioclase microlites with the composition between the two polulations, 3) plagioclase phenocrysts rims more enriched in Fe, and 4) reverse zoning of pyroxene phenocrysts that rather show single chemical population. Melt inclusions in pyroxene phenocrysts are slightly less evolved than the groundmass glass, suggesting that most pyroxenes were derived from felsic magma. These suggest mixing of low-temperature hydrous felsic magma with high-temperature anhydrous (pyroxene-free) mafic magma. Similarity in the petrographical characteristics and temperatures with the pyroxene geothermometry among all the samples shows that nearly constant mixing processes has been operated throughout the recent eruption events, including the 1991 eruption.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 8400 VOLCANOLOGY
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting