HR: 0800h
AN: V11B-1426    [Abstracts]
TI: Evolution of Magma Plumbing System Between Large Pyroclastic Eruption Cycles in Aso Volcano, Japan
AU: * Furukawa, K
EM: furukawa@gaia.h.kyoto-u.ac.jp
AF: Kyoto University, HES, Kyoto, 606-8501 Japan
AU: Kaneko, K
AF: Kyoto University, HES, Kyoto, 606-8501 Japan
AU: Koyaguchi, T
AF: Tokyo University, ERI, Tokyo, 113-0032 Japan
AU: Kamata, H
AF: Kyoto University, HES, Kyoto, 606-8501 Japan
AB: Aso volcano is located in central Kyushu, SW Japan. Volcanic activity at Aso is characterized by four large pyroclastic eruptions, which are called as Aso-1 (occurred in 300 ka), 2 (150 ka), 3 (120 ka) and 4 (90 ka), and many small eruptions between them. Previous works have reported that the four large eruptions were basically eruptions from chemically zoned chamber of rhyolitic and basaltic magmas and their petrological features of these magmas systematically change with time; incompatible elements (e.g. $K_2$O) for a given Si$O_2$ content decrease and oxidation state increase from Aso-2 to Aso-4. By contrast, the small eruptions were mostly Plinian eruptions of silicic magmas, and their petrological features are not fully investigated. In this study, we investigate petrological features of the ejecta of the small eruptions between Aso-2 and 3 (referred to as SE-3/2) and between Aso-3 and 4 (SE-4/3) in detail and compare them with those of Aso-2, 3 and 4. The variations of SE-3/2 and SE-4/3 magmas have two common petrological features. First, $K_2$O for a given Si$O_2$ content irregularly change with time; $K_2$O for a given Si$O_2$ in SE-3/2 magmas varies in the range between that of Aso-2 and Aso-3, whereas that in SE-4/3 magmas varies in the range between that of Aso-2 and Aso-4. This means that the range of the compositional variation increases with time so that the amount of incompatible elements in the most depleted magma decreases with time. Second, oxidation state of SE-3/2 magmas is similar to that of Aso-3, and likewise, that of SE-4/3 is similar to Aso-4. The compositional variation of the magmas cannot be explained by a single fractional crystallization process, suggesting that the ejecta of the small eruptions do not come from a single magma chamber. The temporal variation of the incompatible elements may indicate that the silicic magmas are generated by partial melting of the crust due to repeated injections of basaltic magmas; the crust becomes more depleted in incompatible elements with time as a whole, whereas the degree of depletion varies spatially in the crust. The oxidation state of the whole magma system changes immediately after the large eruptions.
DE: 8404 Ash deposits
DE: 8439 Physics and chemistry of magma bodies
DE: 9320 Asia
DE: 9604 Cenozoic
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting