温泉水の水素と酸素の安定同位体比から推定される深部花崗岩中の水の由来

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  • The origin of thermal water from deep zones in granite rocks inferred from stable isotopic ratios of hydrogen and oxygen
  • オンセンスイ ノ スイソ ト サンソ ノ アンテイ ドウイタイヒ カラ スイテイ サレル シンブ カコウガン チュウ ノ ミズ ノ ユライ

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<p>In order to clear the origin of thermal water in fractures in deep granite bedrocks, stable isotopic ratios of hydrogen and oxygen were analyzed for thermal waters from deep boreholes through bedrocks as well as hot spring waters, cold spring waters and shallow groundwaters in Okayama and Tottori Prefectures, western Japan.</p><p>Although most thermal waters from deep boreholes can be regarded as meteoric water origin from the viewpoint of the isotopic ratio, part of the thermal waters has clearly δD and δ18O values lower than those of the local meteoric waters. On the other hand, the isotopic ratios higher than those of the meteoric water are found in the thermal waters with high chlorine in the coastal area. If such the high chlorine water is to be formed by mixing of fresh water with seawater, the isotopic ratio of the fresh water must be lower than that of the local meteoric water. Water with isotopic ratios lower than the present meteoric water can be considered to exist definitely in fractures in the granite bedrocks in the district.</p><p>The isotopic ratio of meteoric water in the Frigid Zone is lower than that in the Torrid Zone, because the isotopic ratio of meteoric water is influenced by the temperature of surface water of sea in the source area. The thermal waters with very low isotopic ratio in this area may be considered to originate from the meteoric water in a climate condition colder than the present. According to the isotopic records of deuterium and oxygen-18 in the ice cores bored through ice sheets in Greenland and Antarctica, the low isotopic ratios of ice are corresponded to be in ages more than 10,000 years old at least, suggesting that the isotopic ratios of old meteoric water more than 10,000 years before the present are lower than those of the present meteoric water over the earth. The low isotopic ratios in deep thermal water from the district might be thus considered to be very old water of the glacial epoch 10,000 years at least before the present.</p><p>On the other hand, the transit time of water through dense granite rocks may be estimated to be roughly tens of thousands years under the condition of hydraulic gradient 0.01 for a horizontal distance of 50 km, by using the permeability of 10-6 cm/s or so and the porosity of 0.001 to 0.01 on the basis of literature.</p><p>Thus a deep water circulation with a time scale of tens of thousands years can be inferred to be formed through the granite bedrocks in the district.</p>

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