Spatial and temporal variations of gas geochemistry at Mt. Ontake, Japan

This report describes spatial and temporal variations of helium, carbon, and nitrogen isotopes measured in CO2-rich bubbling gases from natural springs associated with Mt. Ontake (central Honshu, Japan) from November 1981 through September 2015. During the entire period, the 3He/4He ratio decreased concomitantly with increasing distance of the sampling site from the central cone. In contrast, the CO2/3He ratios and the δ13C values of CO2 increased with distance. These spatial trends became more pronounced after the September 27, 2014 Ontake eruption, suggesting reactivation of the volcano plumbing system with enhanced emission of magmatic volatiles, although the spatial trend of nitrogen isotopes disappeared. The 3He/4He ratios of the most proximal site to the central cone remained constant until 2000, apparently increasing from June 2003 through October 2014. They became constant soon after the eruption until September 2015. The ten-year 3He enhancement might have been a precursor of the 2014 Ontake eruption. In contrast, δ13C values of CO2 at the same site remained constant through this period. The lack of δ13C anomaly might be attributable to 1) negligibly small amounts of magmatic CO2 introduced into the source of hot springs compared to ambient CO2 in the air-saturated aquifer, or 2) close resemblance of the carbon signature of magmatic CO2 to that of ambient CO2. Therefore its addition cannot change the overall δ13C value of the sampled hot springs. At distal sampling sites, no measurable change of helium, carbon, or nitrogen isotopes was observed in relation to the 2014 Ontake eruption, suggesting that the effect of this eruption on the Ontake hydrothermal system was geographically localized.