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 weather ing 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 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 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.