Variation in fumarolic H 2 gas and volcanic activity at Nasudake in Japan

Abstract

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 H 2 and volcanic activity. On the basis of the hydrogen isotope ratio for volcanic H 2, the temperature of the thermal fluid reservoir producing H 2 was estimated at ~ 160 †C. Large variations in the δD and δ 18O 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 D H ratios of volcanic H 2 suggest that the contribution of magmatic gases to the fumarolic discharges has decreased with time.