ABSTRACT

The chemical and isotopic compositions were determined for fumarolic gas samples collected between May 1988 and January 1991 from the summit (“A” terrace) and from the south caldera rim of Izu-Oshima volcano, Japan. The contribution of meteoric water and/or seawater to the “A” terrace fumarolic discharges increased as the fumarole temperature decreased. A large contribution of seawater to the fumarolic gas was inferred during the period from December 1988 to May 1989. The fumaroles which first appeared in June 1989 on the south caldera rim discharge the water vapor derived from local meteoric water. The isotopic compositions of water vapor followed a Rayleigh fractionation process; a part of vapor condensed during ascent in a gas conduit. A scenario for the changes in the discharge system of fumarolic gases at Izu-Oshima volcano is proposed as follows: The proportion of groundwater to the “A” terrace fumarolic gas increased with time as the magma head subsided. The fracture network which formed beneath the caldera during 1986 eruption may have allowed seawater incursion from the coast to the central part of the island. Loss of a large amount of groundwater in the form of vapor from the summit crater may have accelerated infiltration of seawater. The discharge system developed over a wider area as the deeper magma head subsided.