ABSTRACT

We have collected 21 gas samples of various kinds such as volcanic gas, hot spring gas and CH₄-rich natural gas in the Hokkaido Island. Helium and neon in the samples were purified in a vacuum line and the ³He/⁴He and ⁴He/²⁰Ne ratios were measured by a mass spectrometer. Observed ³He/⁴He and ⁴He/²⁰Ne ratios vary significantly from 0.242 × 10^-6 to 9.58 × 10^-6 and from 0.53 to 96, respectively. There is no correlation between the ³He/⁴He and ⁴He/²⁰Ne ratios. Taking into account present data as well as published ones, geographical distribution of ³He/⁴He ratios indicates a clear contrast, high in the volcanic arc and low in the forearc region. The distribution pattern agrees well with the data of Northeast Japan (Sano and Wakita, 1985) and those of the North Island of New Zealand (Sano et al., 1987). Higher ³He/⁴He ratios than air in the volcanic arc region can be well explained by the presence of an uprising magma. Lower ³He/⁴He ratio in the forearc region is attributed to the radiogenic helium accumulated in crustal rocks. A positive correlation is found between the common logarithms of ³He/⁴He ratio and heat discharge rate of hot springs in the area. The tendency can be explained by a combination of magma aging and the dilution of magmatic fluids by local groundwater.