Volatile element isotope systematics at Ontake volcano, Japan

抄録

Twenty three gas samples were collected from hot and mineral springs associated with Ontake volcano in central Honshu, Japan from June 1996 to June 2000. The chemical compositions, He, Ar, C and N isotopic ratios were measured using a gas chromatography, noble gas and stable isotope mass spectrometers, respectively. The ³He/⁴He ratio decreased with increasing distance from the central cone of the volcano to the sampling site, while δ¹³C value of CO₂ increased with the distance. Such trends are consistent with those observed from November 1981 to June 1993 in the literature, suggesting that source of magmatic helium and carbon is located beneath the volcanic cone and they are continuously emitted into surroundings. The δ¹⁵N value of N₂ increased with the distance while most ⁴⁰Ar/³⁶Ar ratios were similar to the atmospheric value. Magmatic nitrogen may also be carried by a fluid flowing through the volcanic edifice and diluted by crustal nitrogen. Significant increase of ³He/⁴He ratio from 1996 to 2000 was observed at the site close to the fault formed by a M6.8 earthquake that occurred in September 1984, which agrees well with the ³He/⁴He change from November 1981 to June 1993. Anomalous increase of δ¹³C value was observed at Shirakawa site from June 1993 to June 2000. The change cannot be explained by a simple two-component mixing between magmatic and crustal end-members and may require another model with three end-members, mantle, limestone and sediment.