HR: 10:50h
AN: V31G-03    [PDF]
TI: Magma evolution model for the post-caldera volcanism of Aso volcano: constraints from geochemistry and mineralogy
AU: * Miyoshi, M
EM: miyoshim@es.sci.kumamoto-u.ac.jp
AF: Graduate school of Science and Technology, Kumamoto University, Kurokami 2-39-1, Kumamoto, 860-8555 Japan, Kumamoto, 860-8555 Japan
AU: Hasenaka, T
EM: hasenaka@sci.kumamoto-u.ac.jp
AF: Faculty of Science, Kumamoto University, Kurokami 2-39-1,Kumamoto, 860-8555 Japan, Kumamoto, 860-8555 Japan
AU: Sano, T
EM: sano@fuji-tokoha-u.ac.jp
AF: College of Environment and Disaster Research, Fuji Tokoha University, Ohbuchi 325, Fuji 417-0801, Fuji, 417-0801 Japan
AB: After the 90-thousand-year old caldera forming plinian eruptions, Aso volcano has produced a wide range of magma compositions from basaltic to rhyolitic (Ono and Watanabe, 1985), indicating a change in the structure of the magma plumbing system. Including this Aso-4 pyroclastic flow, Aso volcano produced four gigantic caldera-forming pyroc,Oastic flows, all of which probably represent an existence of a single zoned magma chamber, as suggested by the geochemical study of Hunter (1998). It is difficult to create a wide range of post-caldera magma compositions from a single magma chamber model, when the following observations are considered. (1) Basaltic group and silicic group of lavas show distinct trends in FeO*/MgO-SiO2 plot. (2) Two lavas with wide compositional variation commonly show mingling texture. (3) Basaltic lavas have plagioclase with dusty zones, whereas silicic lavas do not. Corroded form of plagioclase is observed in all types of lavas. (4) Orthopyroxene phenocrysts (mg\#: 64-69) in basalt always have reaction-rims of clinopyroxene (mg\#:68-74) having reverse zoning rim. (5) Estimated temperature from two-pyroxene parallel growth in glomeroporphyritic texture in most lavas show 950-1050 deg. irrespective of their composition. (6) Trace element ratios indicate that the source material for caldera-forming products and post-caldera products are identical. For example, Ba/Zr ratio ranges 2.2-3.0, K2O/Rb, 0.028-0.035. Combining all these observations, we established a dual-magma chamber model: one is fractionated, the other is supplying basaltic magma. A wide range of lava composition probably reflects different degree of magma mixing between the two end member compositions.
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting