Mineralogy and hydrogen isotope geochemistry of clay minerals in the Ohnuma geothermal area,Northeastern Japan |
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Authors: | Katsumi Marumo Keinosuke Nagasawa Yoshimasu Kuroda |
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Institution: | 1. Department of Earth Sciences, Nagoya University, Nagoya 464 Japan;2. Institute of Geosciences, Shizuoka University, Shizuoka 422 Japan;3. Department of Geology, Shinshu University, Matsumoto 390 Japan |
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Abstract: | Mineralogical and hydrogen isotopic studies have been made on clay minerals occurring in the Ohnuma geothermal area, northeastern Japan. Here, clay minerals such as smectite, kaolinite, dickite, sericite, and chlorite were formed by hydrothermal alteration of Miocene rocks. A chemical equilibrium can be assumed to be attained from the fact that the amount of expandable layer in the interstratified chlorite/smectite decreases and the polytype of sericite changes from 1M to 2M1 with increasing depth and temperature. The hydrogen isotopic composition (D/H) of the clay minerals is lighter than that of the geothermal and local meteoric waters by about 20–40‰. The hydrogen isotopic fractionation factors αmineral-water are as follows: 0.972–0.985 for kaolinite and dickite, 0.973–0.977 for sericite, and 0.954–0.987 for chlorite. In the temperature range from 100 to 250°C, the hydrogen isotopic fractionation factors between these minerals and water are not sensitive to the temperature. αchlorite-water depends on the kind of octahedrally coordinated cations which lie close to the hydroxyl groups; it becomes large with an increase of Mg content of chlorite. |
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