Variation in fumarolic H2 gas and volcanic activity at Nasudake in Japan

The chemical and isotopic compositions of fumarolic gases collected from Nasudake in northeastern Japan were determined in order to investigate the correlations between the behaviour of H2 and volcanic activity. On the basis of the hydrogen isotope ratio for volcanic H2, the temperature of the thermal fluid reservoir producing H2 was estimated at ~ 160 †C. Large variations in the δD and δ18O values of fumarolic condensates around the summit were observed. Hydrogen content in the fumarolic gases has been gradually increasing since 1980, resulting from rock-water interaction under redox conditions governed by a mineral buffer such as pyrite and magnetite in the underlying shallow hydrothermal systems. Hydrogen concentration and isotopic composition of volcanic steam are strongly influenced by the proportion of local meteoric water circulating underground. The measured content and DH ratios of volcanic H2 suggest that the contribution of magmatic gases to the fumarolic discharges has decreased with time.