ABSTRACT

δD and δ¹⁸O values of fumarolic condensates collected from the Showashinzan volcano, Hokkaido, Japan, during the period from 1954 to 1974 were determined. The results indicate that the fumarolic condensates are mixtures of magmatic water and local surface waters. The mixing of these two waters is suggested to have proceeded at very shallow parts of the volcano. The local surface water infiltrating the volcano shifted its δ¹⁸O value by the oxygen isotopic exchange with hot wall rocks. The degree of oxygen isotope shift of the water varied year after year as the fumarolic activity diminished. This variation caused remarkable variations in δ¹⁸O value of fumarolic condensates with time. On the basis of variations in δD and δ¹⁸O values of fumarolic condensates with time the isotopic composition of magmatic water is estimated to be -32‰ (SMOW) in δD and +7.4‰ (SMOW) in δ¹⁸O.