地下深部における地球化学的環境の長期的変遷 : 炭酸塩鉱物中の微量元素に基づく解析例

概要

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Fracture-filling and fracture-lining calcites may preserve a signature of the chemical evolution process of groundwaters. To develop a methodology for assessing the paleo-hydrogeological evolution of groundwater systems, especially with respect to understanding variations in redox conditions, calcite fracture mineralization was studied using two deep boreholes (ca. 500 m and 1,000 m) drilled in the Tono area of central Japan. Calculated Eh values based on the variations of Uand Fe concentration in calcite were plotted in the range from -180 mV to -275 mV, from -104 mV to -268 mV, respectively in the case of pH=8. By combining the content of U and Fe of calcite, based on the cathodluminescence characteristics, it is suggested that the redox conditions have varied with a range of 90 mV as the calcite grew in the evolving groundwater in each studied grains. Thus, the present study suggests that redox conditions in this area may have changed little. Therefore, redox-condition of groundwater will be kept in reduction condition during calcite precipitation. It is shown that the methodology carried out in this study is useful for assessing the long-term hydrochemical evolution process.
日本地球化学会の論文
2006-05-25