水-岩反应的铀系不平衡判别

JUDGEMENT ON WATER_ROCK INTERACTION IN CRYSTALLINE ROCKS USING URANIUM_SERIES DISEQUILIBRIUM

天然体系中水－岩反应的铀系不平衡判别，是基于水－岩反应使岩石中天然铀衰变系子体核素２３４Ｕ，２２６Ｒａ和六价铀的选择性溶失，造成原始放射性平衡遭到不同程度破坏，因而可利用现今测得的岩石中３组核素放射性活度比值Ａ（２３４Ｕ）／Ａ（２３８Ｕ），Ａ（２３０Ｔｈ）／Ａ（２３４Ｕ）和Ａ（２２６Ｒａ）／Ａ（２３０Ｔｈ），反演判别该岩石曾经历近代水－岩反应的大致强度和出现、持续时间．这是近十余年间从国际核废物安全处置地球化学类比研究中拓展出的研究水－岩反应的新方法．以研究实例介绍了该新方法的判别原理和应用．

This paper briefly reviews a new effective method of judgement on water_rock interaction in crystalline rocks using uranium_series disequilibrium. The nuclides 238 U- 234 U- 230 Th attain radioactive secular equilibrium after 1.7Ma, and the respective activity ratios of A ( 234 U)/ A ( 238 U), A ( 230 Th)/ A ( 234 U) and A ( 230 Th)/ A ( 238 U) are all in equal unity in closed geological systems. If the systems are exposed to weathering and groundwater circulation, the preferential mobility of isotopes such as 234 U, 226 Ra and U 6+ allows them to be mobilized selectively, and chemical fraction results in isotopic disequilibria, yielding activity ratio unequal to unity. The resulting disequilibria indicate the action over last 1.5Ma, 350ka, and 8ka respectively. The degrees and time_scales of last water_rock interactions can be evaluated using the three determined parent_daughter activity ratios. The uranium_series disequlibrium method was used in a selective uranium deposit to evaluate the time and degree of water_rock interaction. The deposit has been an access to assessment of the validation of nature analog concept in radio_waste disposal.