Helium and carbon isotope systematics at Ontake volcano, Japan

Abstract

We have measured chemical compositions, He/Ne ratios, and He and C isotopic compositions of 47 gas samples collected from crater fumaroles and hot springs associated with Ontake volcano, Japan. Geographical and temporal variations of chemical compositions, 3He/4He ratios, and δ13C values of CO2 are reported here. Between 1981 and 1993, 3He/4He ratios decreased with increasing distance from the central cone of the volcano, while δ13C values increased with the distance. Magmatic He and CO2 are probably carried by a fluid flowing through the volcanic edifice and are diluted by crustal He and CO2. A negative correlation of 3He/4He ratios and δ13C values suggests two‐component mixing between magmatic and crustal end‐members. At the sampling sites close to the fault formed by a M6.8 earthquake that occurred in September 1984, 3He/4He ratios have increased significantly with time from 1984 to 1993, while, except for three sites close to the fault, no systematic change of δ13C values is found. On the other hand, He and C isotopes have been constant at the sites distant from the fault. The temporal variations of 3He/4He ratios may be related to the recent reactivation of Ontake volcano.