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,**0 : ++ ; ,+ <=> ,**2 : , ; ,2 <=?Geochemical Observation on the ,**. Eruption of Asama Volcano
through Analysis of Water-soluble Components on the Ashῌ 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 asafe 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 +, ,**. withvulcanian explosion. Non-explosive strombolian eruptions with ash emission followed the explosion, and newlava e#used at the bottom of the summit crater. After that, vulcanian explosions produced ash fall and bombs atintervals.
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 anaggregate 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 thewater-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 withvolcanic gases at high temperature without condensation of water vapor, the contents of the water-solublecomponents of the ash were very low. While, the F/Cl values of the ash leachates were signiﬁcantly higher thanthat 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. Althoughmolar 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++ 0.+ῌ-0#$%&'()*+,-./01()23
Volcanic Fluid Research Center, Tokyo Institute ofTechnology, 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 : knogami@ksvo.titech.ac.jp