報告書番号

N06034

タイトル（和文）

地下水年代測定評価技術の開発（その5）－岩石間隙水における同位体比評価のための同位体交換法の開発－

タイトル（英文）

Research and development on groundwater dating (part5)-The development of isotopic exchange method for estimation of isotopic (hydrogen and oxygen) composition of pore water-

概要 （図表や脚注は「報告書全文」に掲載しております）

同位体交換法を用いた、岩石間隙水の水素・酸素同位体比評価法の実用化を目標として研究を行った。本研究ではまず、同位体交換法による評価が可能/不可能を判別するための方法を提案した。提案した手法を様々な岩石試料に対して適用し、どのようなパラメータが同位体交換法の適用可否に影響を与えるかを評価した。その結果、岩石に含まれる間隙水の量が同位体交換法の適用可否に最も顕著に影響を与えることを示した。この結果を受け、同位体交換法が適切に行われるための試験条件（間隙水量）の設定を試みた。その結果、試験容器の内容積1.3Lに対して、試験水量2gの場合、5g程度の間隙水量を含むように岩石試料の量を調整すれば同位体交換法による評価を適切に行うことができることを示した。
上記の試験条件を幌延地区で行われたボーリングコアに適用し、水素同位体比の深さ分布を決定した。同位体交換法で評価された分布は、岩石の圧縮やポンプでの吸引で得られた間隙水を評価した分布と近く、相補的に交換法・圧縮抽水法の妥当性を検証できた。この結果から、上述した条件下で試験を行うことで、多くの岩石試料に対して同位体交換法が適用できる可能性が高いことを示すことができた。

概要 （英文）

Estimation of isotopic composition of pore water without destruction of rock samples can be achieved by "isotopic exchange method" proposed by Abu et al.Even though this method has many advantages comparing with the other methods, previous reports clarified applicableness only for Opulinus-clay and halite. To apply this method to many kind of samples, obtaining following information is required:
(1) What kind of parameter affects the estimated value at this method ?
(2) What is the appropriate condition for this method so that the isotopic (hydrogen and oxygen) composition of pore water is estimated properly ?
It this study, the methodology to tell if this method can be applicable to a sample or not, is proposed. At first step in this methodology, standard sample is obtained by exchanging the pore water with prepared water according to the procedure of isotopic exchange method. Then isotopic composition of standard samples is estimated by isotopic exchange method. Applicableness of isotopic exchange method can be known comparing the estimated isotopic value by isotopic exchange method and standard sample.
The proposed methodology is applied to many kind of rocks to obtain an information on (1) described above. It is found that the amount of pore water in rock sample affects the applicableness of isotopic exchange method, critically. Even for same kind of rock, the applicableness of isotopic exchange method is controlled by the amount of pore water in rock sample in experimental system. This result has indicated the possibility that we can apply the isotopic exchange method to all rock samples if we can estimated the appropriate amount of pore water in the system. Changing the amount of rock samples, we estimated the appropriate amount of pore water in the system for isotopic exchange method. For 1.3L vessel and 2.0 g prepared water for exchanging, about 5.0g pore water is required to estimate the isotopic composition by isotopic exchange method.
Appling this method to a drilling core, depth profile of hydrogen isotopic composition is obtained according to the experimental condition obtained above. The depth profile obtained by isotopic exchange and squeezing method are very similar indicating applicableness of these 2 methods for estimation of isotopic composition of pore water in rock.

報告者

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