Geochemical characteristics of carbonate hot-springs in Japan

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Chemical and isotopic compositions of water were measured at 46 carbonate hot-springs in Japan, by applying the methods avoiding the post-sampling alternation. Measured stable isotopic values indicate that CO2 of an endogenic origin was dissolved in water of a meteoric origin, in many cases. The carbonate springs are generally in intermediate temperature for sustaining a high solubility of CO2, and near-neutral pH as a result from the reactions between weak carbonate acid and rock-forming minerals. Although carbonate acid effectively dissolved carbonate minerals, this reaction is often unimportant among the water-rock reactions because the many water samples appear alkalinity-excess, relatively low Ca concentration, and high dissolved silica. However, a significant dissolution of 13C-enriched carbonate minerals was recognized in some samples showing a high carbon isotopic value and high equilibrate CO2 partial pressure. Dissolved oxygen content implies a contact with the atmosphere or mixing with oxidative underground water, both of which decrease the initial CO2 partial pressure and the concentration of dissolved chemical components.