VARIATION IN FUMAROLIC H-2 GAS AND VOLCANIC ACTIVITY AT NASUDAKE IN JAPAN

The chemical and isotopic compositions of fumarolic gases collected fr om Nasudake in northeastern Japan were determined in order to investig ate the correlations between the behaviour of H-2, and volcanic activi ty. On the basis of the hydrogen isotope ratio for volcanic H-2,, the temperature of the thermal fluid reservoir producing H-2, was estimate d at similar to 160 degrees C. Large variations in the SD and delta(18 )O values of fumarolic condensates around the summit were observed. Hy drogen content in the fumarolic gases has been gradually increasing si nce 1980, resulting from rock-water interaction under redox conditions governed by a mineral buffer such as pyrite and magnetite in the unde rlying shallow hydrothermal systems. Hydrogen concentration and isotop ic composition of volcanic steam are strongly influenced by the propor tion of local meteoric water circulating underground. The measured con tent and D/H ratios of volcanic H-2 suggest that the contribution of m agmatic gases to the fumarolic discharges has decreased with time. (C) 1998 Elsevier Science B.V.