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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.
日本国内の46ヶ所の炭酸泉について、採水後の成分変質に留意した方法で、化学組成・同位体組成の潤定を行った。安定同位体の測定値は、深部起源の二酸化炭素が天水起源の水に溶解したことを示す。炭酸泉は一般に、高い二酸化炭素の溶解度を保持するために中温であり、弱酸である炭酸と造岩鉱物の中和反応により中性のpHを示す。炭酸は効果的に炭酸塩鉱物を溶解する性質を持つが、多くの水試料が過剰なアルカリ度、比較的低いカルシウム濃度、高い溶存シリカ濃度を示すことから考えると、炭酸塩鉱物の溶解反応は重要でないことが多い。ただし、高い溶存炭酸の炭素同位体比と高い平衡二酸化炭素分圧を示す試料では、13Cに富む炭酸塩鉱物の溶解が重大に起こっていた。溶存酸素濃度は大気との接触もしくは酸化的な地下水との混合を示し、このような場合、初生的な二酸化炭素分圧と溶存化学成分の濃度は低下する。show more
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