HELIUM AND CARBON-ISOTOPE SYSTEMATICS AT ONTAKE VOLCANO, JAPAN

We have measured chemical compositions, He/Ne ratios, and He and C iso topic compositions of 47 gas samples collected from crater fumaroles a nd hot springs associated with Ontake volcano,Japan. Geographical and temporal variations of chemical compositions, He-3/He-4 ratios, and de lta(13)C values of CO2 are reported here. Between 1981 and 1993, He-3/ He-4 ratios decreased with increasing distance from the central cone o f the volcano, while delta(13)C values increased with the distance. Ma gmatic He and CO2 are probably carried by a fluid flowing through the volcanic edifice and are diluted by crustal He and CO2. A negative cor relation of He-3/He-4 ratios and delta(13)C values suggests two-compon ent mixing between magmatic and crustal end-members. At the sampling s ites close to the fault formed by a M6.8 earthquake that occurred in S eptember 1984,He-3/He-4 ratios have increased significantly with time from 1984 to 1993, while, except for three sites close to the fault, n o systematic change of delta(13)C values is found. On the other hand, He and C isotopes have been constant at the sites distant from the fau lt. The temporal variations of He-3/He-4 ratios may be related to the recent reactivation of Ontake volcano.