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

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 weathering profiles, the authors revealed the enrichment characteristics of U and Th in weathering profiles of carbonate rocks. Some conclusions 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 weathering system by weathering fluid with the rapid dissolution of carbonate rocks, and shows significant losses; at the same time, the intensive reduction of the volume of weathering residuals leads to strong relative enrichment of U in residues, so U presents unique geochemical 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 mainly due to absolute remaining after the dissolution of soluble components. ③ The enrichment factors of U and Th in weathering profiles 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.