HR: 1340h
AN: V33D-1492    [Abstracts]
TI: Dispersion of magmatic volatiles through groundwater system at Unzen volcano, SW Japan: a summary
AU: * Kazahaya, K
EM: kazahaya-k@aist.go.jp
AF: Geol. Surv. Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Morikawa, N
AF: Geol. Surv. Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Takahashi, H A
AF: Geol. Surv. Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Yasuhara, M
AF: Geol. Surv. Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Inamura, A
AF: Geol. Surv. Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Fourre, E
AF: LSCE-CEA CNRS, Avenue de la Terrasse Bat 9, 91191, Gif sur Yvette, 91191 France
AU: LeGuern, F
AF: LSCE-CEA CNRS, Avenue de la Terrasse Bat 9, 91191, Gif sur Yvette, 91191 France
AU: Jean-Baptiste, P
AF: LSCE-CEA CNRS, Avenue de la Terrasse Bat 9, 91191, Gif sur Yvette, 91191 France
AU: Kono, T
AF: RCEST Nihon-Bunri Univ., 1727 Ichiki, Ooita, 870-0397 Japan
AU: Ohsawa, S
AF: IGS Kyoto Univ., 3088-176 Noguchihara, Beppu, 874-0903 Japan
AU: Yusa, Y
AF: IGS Kyoto Univ., 3088-176 Noguchihara, Beppu, 874-0903 Japan
AU: Kitaoka, K
AF: DAS Okayama Univ. Sci., 1-1 Ridaimachi, Okayama, 700 Japan
AB: Geochemical studies on groundwater flow system is performed using chemical and isotopic technique at Unzen volcano placed on the Beppu-Shimabara graben in Kyushu Island, SW Japan. More than 400 samples of surface and subsurface waters are collected and analyzed for major chemistry and isotopic compositions, i. e., tritium, D/H, $^{18}$O/$^{16}$O, $^{13}$C/$^{12}$C, $^{3}$He/$^{4}$He etc. The tritium concentration is used for estimate of groundwater retention time, and stable isotopic ratios of water are used for determining recharge area. The carbon and helium isotopic ratios as well as their concentrations are measured in order to know the contribution and flux of magmatic volatiles. The main groundwater flow system around Unzen volcano is developed from the summit area to the east Shimabara city area with a large flow rate of 1.7 m$^{3}$/sec. This flow rate almost corresponds to the recharge rate from an area of newer volcanic body. The hydrogen isotopic ratio of the surface and subsurface waters collected at Shimabara city are in a narrow range from -48 to -45 $\permil$, indicating that groundwaters recharged at the slope are well-mixed in the course of flow. On the other hand, their HCO$_{3}$ concentrations significantly varied suggesting that input of magmatic carbon-bearing species into the groundwater has occurred close to the collection sites. Helium isotopic ratio of the waters also varied which is consistent with the variation of bicarbonate concentration. These magmatic volatiles unlikely come from the summit conduit through aquifers, but ascend vertically through faults. We summarize and map the flux of magmatic volatiles dissolved in groundwater and evaluate two styles of magma degassing 1) degassing from deep reservoirs through fractures or faults and 2) degassing from magma conduits through aquifers, and discuss efficiency of escape of magmatic volatiles through conduit wall during magma ascent.
DE: 8400 VOLCANOLOGY
DE: 8434 Magma migration
DE: 1040 Isotopic composition/chemistry
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting