Microscale U/Th Disequilibrium and Pb Isotopic Signatures in Uranium Phosphate Minerals as Indicators of Their Origin and Fate in a Contaminated Wetland

Uranium (U) phosphate minerals are commonly found in U-contaminated environments, and understanding their origin and fate is essential for predicting U mobility at these sites. In this study, we used ²³⁸U, ²³⁵U, and ²³²Th decay series to investigate U and radiogenic lead (Pb*) behavior within individual U-phosphate minerals in a wetland affected by past U-mining activities. Three groups of U-phosphates were identified using SEM-EDXS based on their U, Ca, and P contents: ningyoite-like minerals (CaU^IV(PO₄)₂·2H₂O), near-ideal Ca-autunites (Ca(U^VIO₂)₂(PO₄)₂·11H₂O), and Ca-depleted autunite/chernikovite-like minerals ((H₃O)(U^VIO₂)(PO₄)·3H₂O). NanoSIMS analysis of (²³⁰Th/²³⁸U) activity ratios revealed substantial U losses from the ningyoite-like minerals, particularly those located in the upstream vadose zone of the wetland, likely due to their oxidative dissolution under hydrological fluctuations. In contrast, the Ca-autunites and Ca-depleted autunite/chernikovite-like minerals exhibited (²³⁰Th/²³⁸U) signatures indicative of U accumulation. This behavior, more pronounced in the vadose zone, was interpreted as various stages of dissolution-precipitation that may have occurred upon early weathering of the U-ore and/or in the wetland. Finally, ²⁰⁶Pb*/²³⁸U ratios and Pb zoning within the U-grains revealed Pb losses, likely due to Pb diffusion in the mineral structure. Overall, this study demonstrates the relevance of NanoSIMS to investigate U and Pb mobility within microsized U-bearing minerals in contaminated environments.