ABSTRACT

Fumarolic gas samples collected from the hottest fumarole of the Showashinzan volcano on 15 occasions during the period from 1954 to 1978 have been examined for chemical and isotopic compositions. The fumarolic gases considerably changed their composition and temperature with time, owing probably to exhaustion of primary volcanic gases and mixing with water vapor originated from local surface waters soaking into the volcano. It has been also shown that the atmospheric contamination, lowering of temperature and reactions with the surrounding materials were important additional factors involved in the alteration of fumarolic gases. The change in gas composition with time may be considered to be indicative of the history of a volcanic hydrothermal system formed by intrusion of dacite magma.