HR: 0800h
AN: V21A-0378    [Abstracts]
TI: Structural Development and Oxidation of the Takanoobane Rhyolite Lava in Aso Caldera, Japan
AU: * Furukawa, K
EM: kfuru@aichi-u.ac.jp
AF: Aichi University, 370 Kurozasa, Miyoshi-cho, 470-0296, Japan
AU: Uno, K
AF: Todo Senior High School, 4-1 Gounpo, Gokasyo, Uji, 611-0011, Japan
AU: Miyagi, I
AF: Geological Survey Japan, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, 305-8567, Japan
AB: The Takanoobane rhyolite lava (hereafter described as the TR lava) is distributed in the western part of Aso caldera, middle Kyushu Island, SW Japan. The TR lava is one of the central cones. The volume, SiO₂ contents and K-Ar age are 0.14km³ (Miyabuchi et al., 2004), 71-72% (Furukawa, 2006) and 51+-5ka (Matsumoto et al., 1991), respectively. The TR lava was effused in a subaerial environment. In this study, we show vertical structural variation and the development of the TR lava from the four drilling cores obtained by Aso Volcanological Laboratory in 2001-2002. The TR lava is about 90m thick in the proximal part, and the internal structures are divided into three parts: Alternation of the pumiceous layers and the obsidian layers (the upper part), the crystalline rhyolite layer (the central part), and the obsidian layer (the lower part). This structural variation apparently resembles to that of the Obsidian Dome near long valley caldera in eastern California (Manley and Fink, 1987). The central crystalline rhyolite layer of the TR lava is characterized by the development of the flow structure, which is composed of interconnected minute cavities. The shapes and sizes of the structure are varied from stubby or lens to flattened and from a few mm to above 5 cm in length, respectively. The morphology of the flow structure tends to be flattened with distance from the source region. It is probably due to shear stress caused by the lava movement We described the vertical variation of the mineral assemblage of Fe-Ti oxides. It shows that the highly oxidized Fe-Ti oxides tend to be distributed around the flow structure. Thus, the part is selectively oxidized. It is supported also by the rock magnetic experiments. Above studies and cooling history calculated by a numerical modeling show that the oxidation was caused by the increasing of fO₂ at the part. We interpret that the increasing of fO₂ was caused by the release of hydrogen from the degassing lava. Hydrogen should be passing through the flow structure, which is composed of interconnected minute cavities, and the part was selectively oxidized.
DE: 1519 Magnetic mineralogy and petrology
DE: 8425 Effusive volcanism
DE: 8429 Lava rheology and morphology
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting