Formation processes of thermal waters in Green Tuff: A geochemical study in the Hokuriku district, central Japan

Thermal waters from Green Tuff region in the Hokuriku district, central Japan were investigated to reveal formation process of water, based on the chemical and isotopic compositions of water and dissolved gases in water. The thermal waters of local meteoric origin are typical Green Tuff-type water, Na–Ca–Cl–SO4-type or Na–Cl–SO4-type, which well reacted by water–rock interaction and is found to be saturated with calcite and undersaturated with gypsum. The Green Tuff-type waters have relatively high 3He/4He ratio indicating high contribution of mantle-derived helium, and have extremely low total carbon/3He (Ct/3He) ratio compared with the ratio of MORB due to removal of carbon species from waters caused by calcite deposition. In contrast, a mineral spring water close to an active fault is found to be an immature water with less water–rock interaction containing a large amount of gaseous CO2 and is undersaturated with both calcite and gypsum. This water has high 3He/4He ratio and Ct/3He ratio indicating the high contribution of mantle-derived helium and deep-seated CO2, and likely provided through the fault. posed a chemical evolution model considering existence of both chemically immature CO2 gas-rich water and mature carbon-poor water which well reacted with rocks. Deep-seated CO2 and mantle-derived helium are upwelling via faults and are supplied to the groundwater of local meteoric origin, and an immature water containing high content of CO2 and mantle-derived helium is formed. During the circulation of this immature water in the Green Tuff layers, water–rock interaction such as ion exchanges with minerals, dissolution of gypsum and deposition of calcite occurred. At the interaction with rocks, deposition of calcite occurs and results in a loss of Ca2+, and decrease of Ca2+ brings dissolution of gypsum and more calcite deposition. Consequently, almost all the supplied CO2 is eliminated from the water. Furthermore, we made a quantitative evaluation of the chemical evolution of Green Tuff-type water using geochemical modeling code PHREEQC taking account of addition of gas components, and found the good agreement with observational results. This study reveals that the supply of large amounts of CO2 and subsequent water–rock interaction are necessary for the formation of “Green Tuff-type” water with high 3He/4He ratio and extremely low Ct/3He ratio.