碳酸盐岩风化剖面U和Th的富集特征及淋溶实验的指示

The enrichment characteristics of uranium and thorium in weathering profiles of carbonate rocks and the implications of their leaching experiments

通过对贵州岩溶区12条风化剖面U、Th的分布情况、岩-土界面岩粉层动态淋溶残余物的元素变化特征以及元素质量平衡的研究,初步揭示了碳酸盐岩风化剖面U、Th的富集特征。①在碳酸盐岩风化体系中,U呈现出明显的活性,而Th既有惰性的一面,又表现出活性的一面。②碳酸盐岩风化过程中,岩-土界面是U、Th产生突变性富集的重要地球化学场所,而剖面进一步发育演化中U、Th的变化是一个长期而缓慢的过程。岩-土界面反应中,由于风化残余物体积的巨大缩小,U呈现出低背景、强亏损、高富集的地球化学特征。当Th呈现活性态时,与U表现出相似的地球化学特征;当Th表现为惰性态时,其在剖面中的富集主要源于易溶组分溶蚀后的绝对残余。③碳酸盐岩风化剖面U、Th的富集系数与基岩酸不溶物含量存在明显的负相关,说明碳酸盐岩酸不溶物含量愈低,碳酸盐溶蚀的体量愈大,风化溶液带出的活性U或Th也愈多,风化残余物的体积缩小变化程度也愈强,因此残余U或Th的相对富集程度也愈高。

Based on researches on the distribution of U and Th in twelve weathering profiles of carbonate rocks, elemental variation features of dynamic leaching residues of rock-powder-layer samples at the rock-soil interface, and elemental mass balance in weatherhag profiles, the authors revealed the enrichment characteristics of U and Th in weathering profiles of carbonate rocks. Some conclu- sions have been reached： （1） In the weathering system of carbonate rocks, U is significantly active, and Th appears both inert in some special cases and a little active. （2） During the weathering of carbonate rocks, the rock-soil interface is a main geochemical place which can cause drastic enrichment of U and Th in weathering residues, but the change of U and Th is a long and slow process during the further development and evolution of weathering profiles. In rock-soil interface reaction, U is massively moved out from the weather- ing system by weathering fluid with the rapid dissolution of carbonate rocks, and shows significant losses; at the same time, the inten- sive reduction of the volume of weathering residuals leads to strong relative enrichment of U in residues, so U presents unique geo-chemical behaviors characterized by low background value, strong losses, and high enrichment during weathering of carbonate rocks. As for Th, when it is in the active state, it is geochemically similar to U; when it is in the inert state, its enrichment in profiles is main- ly due to absolute remaining after the dissolution of soluble components. （~） The enrichment factors of U and Th in weathering pro- files show significant negative correlation with acid-insoluble content of carbonate bedrocks, suggesting the following relationships： the lower the acid-insoluble content of carbonate rock, the greater the volume of dissolved carbonate, the more the active U or Th that moves out with the weathering fluid, the stronger the reduction of the volume of weathering residuals, and therefore the higher the relative enrichment degree of U or Th in residues.