HR: 0800h
AN: V21C-0624    [Abstracts]
TI: Continuous Gas Monitoring at the Unzen-Conduit-Drilling USDP-4
AU: * Tretner, A
EM: atretner@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Zimmer, M
EM: weihei@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Erzinger, J
EM: erz@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Saito, M
EM: msaito@sevo.kyushu-u.ac.jp
AF: Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shimabara, 855-0843 Japan
AU: Nakada, S
EM: nakada@eri.u-tokyo.ac.jp
AF: Volcano Research Center, University of Tokyo, Tokyo, 113-0032 Japan
AB: Unzen Volcano, located in the Shimabara Peninsula, Kyushu, southwest Japan, was selected as one of the Decade Volcanoes of the United Nation's Decade of Natural Disaster Reduction, since it represents an important type of dangerous volcano. The last eruption (1990-95) was monitored and investigated in detail, yielding several models on the eruption mechanism and magmatic processes. The focus of USDP4 - drilling was to further clarify the regional crustal structure and magma evolution processes that control the manner of growth and failure of the volcano. In addition, processes of conduit formation, magma degassing, and magma's interaction with groundwater was investigated. Petrological investigations show, that Unzen magma in the conduit contained 4-5 wt.% water, whereas water content of the lava at the surface is very small. It was modelled that the water was released at less that 1-2 km depth. In view of these constraints different physico-chemical models show a strong increase in magma viscosity and a pressure drop directly below the growing dome. The gas phase dissolved in the drill mud was analyzed continuously with an online gas monitoring system. The aim was to receive a gas profile along the whole borehole length in order to gain information on the volcano fluid circulation system, to detect possible pathways for gases produced during magma degassing, and to determine hydrothermal and/or volcanic fluid inflow horizons. N₂, O₂, CO₂, CH₄, H₂S, H₂, He and Ar analyses were achieved with a quadrupol mass spectrometer; ²²²Rn was measured with a Lucas cell alpha detecotor. This data and subsequent isotopic studies contribute in particular to the discussed models about the eruptive behavior of Mt. Unzen. However, limitations were given due to the extremely difficult drilling conditions. The highly fractured rock formation led to loss of drill mud circulation mainly in the shallow parts. First results show that significant fluid inflow horizons didn't occur above 500 m (drill string length). From that depth on, influences of invading fluids could be detected by real-time mud gas monitoring, but major changes in the mud gas composition occured only deeper than 1000 m. Cracks and fissures as well as rock formation changes were often correlated with increasing amounts of methane, radon and helium. Of major importance are fluid inflow zones with high ³He/⁴He ratios at 1555 m (7.3 R_A), 1755.5 m (7.48 R_A) and 1977.4 m (6.21 R_A) depth, indicating a significant contribution of fluids with mantle signature. With respect to major questions related to the Unzens eruption mechanism, the detected major fluid and gas inflow horizons may be capable of supporting magmatic degassing processes. Also, a correlation between lithology and gas composition was observed. Increased H₂S concentrations were detected while drilling in pyrite-rich rocks. Old lava dikes and their surroundings seem to comprise of a higher gas and fluid permeability than the other rock formations. A trend with depth, from a minor magmatic to a more magmatic influenced fluid regime was observed together with a change in type of hydrothermal alteration of the surrounding rock (chlorite-rich to pyrite-rich).
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
DE: 8430 Volcanic gases
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005