本邦火山ガス凝縮水の水素および酸素同位体比

抄録

The D/H and ¹⁸O/¹⁶O ratios of volcanic steam from 18 active volcanoes located over the Japanese islands have been measured. From the δD versus δ¹⁸O diagram three distinct isotopic features can be recognized; (A) low temperature steam with the isotope ratios similar to those of local meteoric water, (B) high temperature steam with only oxygen isotope shift, and (C) volcanic steam with both oxygen and hydrogen isotope shift. The samples categorized as (C) are the most in number and those as (B) are rather limited. The group (A) can simply be interpreted as an evaporation product of hot water reservoir at the boiling point. Oxygen isotopic exchange between water and rocks at high temperatures is responsible for the group (B). Oxygen and hydrogen isotope exchange between water and hydrous-silicate-bearing-rocks such as altered volcanic rocks underlying the Quaternary volcanoes could give the last isotopic feature (C). A positive correlation has been observed between chlorine content and oxygen isotope shift of volcanic steam studied here.