HR: 17:30h
AN: V24B-07    [Abstracts]
TI: Transition Of Degassing Processes From Syn-Eruptive To Post-Eruptive Activities At Unzen Volcano
AU: * Shinohara, H
EM: shinohara-h@aist.go.jp
AF: Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba, 305-8567 Japan
AU: Ohba, T
EM: ohba@ksvo.titech.ac.jp
AF: Volcanic Fluid Research Center, TIT, O-okayama 2-12-1, Meguro-ku, Tokyo, 152-8551
AU: Kazahaya, K
EM: kazahaya-k@aist.go.jp
AF: Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba, 305-8567 Japan
AB: Budget of volcanic gas components during the dome-forming eruption of Unzen volcano, 1991-1995, has been evaluated with the SO2 emission rates, magma effusion rates and volatile contents in melt inclusions (Hirabayashi et al., 1995; Satoh et al., 2003). The estimate concluded a closed-system degassing implying that most of the gas component in the ascending magma was discharged through the summit degassing; there is no need of the gas-loss to the volcanic edifice from the conduit wall nor the excess gas supply from non-erupting magma. As a consequence, the intense gas emission quickly decreased after the end of magma effusion. However, the relatively minor but high-temperature (ca. 400?_qC in 2001) gas emission still continues even after five years. The post-eruptive gas emission is characterized by Cl-rich and S-poor composition, indicating that the acid gases were derived from already degassed magmas. The low delta D of water in the high-T fumarolic gases might also indicate the water discharge from the already degassed magma, either from the hot lava dome or the shallow conduit. The results of the conduit drilling revealed that the shallow conduit was already cooled; therefore the Cl-rich components need to be derived from the lava dome. In contrast, the fumarolic gases also contain high amount of CO2 with the isotopic ratio (delta 13C = -6 per mil) of the typical arc magmatic gas. Emission rate of the magmatic CO2 was estimated to be in the order of 1 t/d, based on the steam flux estimates and composition of the fumarolic gases. If the CO2 was derived from a magma containing 1000 ppm of CO2, which is estimated based of the gas budget during the eruption, the CO2 emission rate of 1 t/d requires magma degassing rate of 1000 t/d, corresponding to 0.5 m3/d, or 1000 m3 for the six-years emission. Although the volume of the source magma is much less than volume of the conduit, the CO2 needs to be derived from much deeper sources, because of the low CO2 solubility in magmas. Therefore the already cooled magma conduit serves also as a gas passage from the deep sources.
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting