Stable and noble gas isotopic study of thermal and groundwaters in northwestern Hokkaido, Japan and the occurrence of geopressured fluids

Abstract

Thermal and groundwaters are distributed along the west coast and in the inland region of Northwestern Hokkaido, Japan. The waters in the latter group have relatively high temperatures. Stable (D, O) and noble gas isotopic compositions of 23 thermal and groundwaters were analyzed to investigate their origins. The Cl concentrations vary from 45 to 19,300 mg/L and the δD values are in the range of -90 to -8‰. The δD-δ¹⁸O plots show a linear relationship which can be explained by a simple mixing of local meteoric waters and altered sea water, with oxygen isotopic shift of 5‰. However, a δD-Cl plot shows that there is also a contribution of an additional water component of different origin. This 3rd component has a δD value of -20‰ and Cl concentration of 6,000 mg/L. Multivariate statistical analysis of the observed chemical and isotopic compositions using the code M3 (M; multivariate mixing and mass balance) also supports the view that the Toyotomi thermal water has the composition closest to that of the 3rd groundwater component. This component is regarded as a geopressured fluid. The ³He/⁴He ratios in these waters are in the range of (0.1-5) × 10^-6, which indicate contributions of both mantle and crustal He to the thermal and groundwaters in the studied area. The Toyotomi thermal water has a ³He/⁴He ratio of 5.5 × 10^-7, and a high ³He/²⁰Ne ratio (∼700). These results indicate a crustal He input with a small amount of mantle-derived He and imply that the 3rd groundwater component is of deep origin. On the basis of isotopic and chemical compositions, 5 of 23 samples studied groundwater and thermal waters are contributed by a geopressured fluid component. The results show that geopressured fluids with a δD value of -20‰ and Cl concentration of 6,000 mg/L are widely distributed in northwestern Hokkaido.