2004年浅間山噴火における地球化学的観測研究一噴出物の水溶性成分の変動と火山活動一

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(3) MM A B C D @E F G H , @I J K L ,**0 : ++ ; ,+ <=> ,**2 : , ; ,2 <=?. Geochemical Observation on the ,**. Eruption of Asama Volcano through Analysis of Water-soluble Components on the Ash ῌ and Jun-ichi HIRABAYASHIῌ Kenji NOGAMIῌ, Shin’ya ONIZAWAῌ,ῌ Monitoring of volcanic gases will provide us with important information on volcanic activity and contributes towards reduction of volcanic disasters. However, approach to an erupting volcano considerably involves danger. Analysis of water-leachates of pristine ash is also available for estimating composition of volcanic gases. It is a safe and e#ective method for monitoring of eruptive activity without having to use a particular equipment. Asama volcano lies about +/* km northwest of Tokyo and it is one of the most violent volcanoes in Japan. After about ,* years dormancy, the three-months-long eruptive activity commenced on September +, ,**. with vulcanian explosion. Non-explosive strombolian eruptions with ash emission followed the explosion, and new lava e#used at the bottom of the summit crater. After that, vulcanian explosions produced ash fall and bombs at intervals. We examined change in water-soluble F, Cl and SO. contents of volcanic ash in the sequence of the ,**. eruption of Asama volcano. The ,**. eruptive activity was divided into three phases according to the mode of the volcanic activity. The ash in Phase I issued by vulcanian explosions before the e#usion of new lava was an aggregate of rock fragments altered under acidic conditions. The contents of water-soluble Na, K, Ca, Mg, Fe, Al, F, Cl, and SO. were noticeably high. This is mainly attributed to fluoride, chloride and sulfate alteration products formed by reaction of rock fragments within the summit crater with HF, HCl and SO, in volcanic gases before the ,**. eruption. Although molar ratio of Cl to SO. in the ash leachates was not equivalent to HCl/SO, of eruptive gases in this phase, increase in the Cl/SO. in the ash leachates in the late of this phase suggests that volcanic gases significantly became hot. In Phase II, new lava e#used at the crater bottom and the ash was extremely fresh. The contents of the water-soluble components of the ash in this phase were significantly less than those of the ash in Phase I. The Cl/ SO. in the ash leachates was equivalent to the HCl/SO, ratio in the plume observed by FT-IR. This result indicats that water vapor in plume did not condense into droplets of water. Owing to reaction of the ash particles with volcanic gases at high temperature without condensation of water vapor, the contents of the water-soluble components of the ash were very low. While, the F/Cl values of the ash leachates were significantly higher than that of plume, which was due to selective fixation of HF on the ash particles. In Phase III, the contents of the water-soluble components of the ash were clearly more than those of the ash in Phase II, which suggests that the lava e#used in Phase II was gradually altered by volcanic gases. Although molar ratio of Cl to SO. in the ash leachates is probably not equivalent to that of the eruptive gases in this phase, the Cl/SO. in the ash leachates mirrored temperature of the hot region at the crater bottom. Key words : volcanic ash, ash leachate, volatile components -11ῌ+1++

(4) 0.+ῌ-0 #$%&'()*+,-./01()2 3 Volcanic Fluid Research Center, Tokyo Institute of Technology, Kusatsu 0.+ῌ-0, Agatsuma, Gunma, -11ῌ +1++, Japan. 45 : -*/ῌ2/01 6789# +ῌ+ῌ+. !" Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology, Higashi +ῌ+ῌ+, Tsukuba, Ibaraki, -*/ῌ2/01, Japan. Corresponding author : Kenji Nogami e-mail : [email protected].

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(82) (+32,) #57/0;<A=/ 0G' opqr& ,. B-+ ++ῌ,*. Hirabayashi, J., Ossaka, J. and Ozawa, T. (+32,) Relationship between volcanic activity and chemical composition of volcanic gases A case study on the Sakurajima volcano. Geochem. J., +0, ++ῌ,+.

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(84) ` | (,**.) %F0B SO, 01 (}/0 C ,**. &p +3/. Ikeda, Y., Katsui, Y., Nakagawa, M., Kawachi, S., Watanabe, T., Fujibayashi, N., Shibata, T. and Kagami, H. (+33*) Petrology of the +322ῌ23 Essential ejecta and associated glassy rocks of Tokachi-dake Volcano in Central Hokkaido, Japan. Bull. Volcanol. Soc. Japan, -/, +.1ῌ +0,. Iwasaki, I., Ozawa, T., Yoshida, M., Katsura, T., Iwasaki, B. and Kamada, M. (+300) Di#erentiation of magmatic emanation of gases. Bull. Tokyo Inst. Tech, 1., +ῌ/1. ¬a (+3/1) /0!1*S®T¯#}6. -, &±A²C +*+ῌ+*/. ¬a (+32*) /0G'³ A-B

(85) ,3ῌ.*. Katusi, Y., Kawachi, S., Kondo Y., Ikeda, Y., Nakagawa, M., Gotoh, Y., Yamagishi, T., Yamazaki, T. and Sumita, M. (+33*) The +322ῌ+323 explosive eruption of Tokachidake, central Hokkaido, its sequence and mode. Bull. Volcanol. Soc. Japan, -/, +++ῌ+,3. % (,**- a) (}¥>/0G'

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