Mineral/Fluid interaction as a potential bias in calcite U-Pb dating

The application of U-Pb dating method performed on calcite has exponentially increased over the last years, since constraining the age of the crystallization for such syn-kinematic minerals, would provide a specific timing of faults movement. The potential gain of this approach is evident but the robustness of the U-Pb method performed on calcites has been not yet systematically tested.  Here, we firstly demonstrated that a mineral/fluid interaction indeed affects the regression of the ²³⁸U/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb data-points and therefore the age; and secondly, we propose an innovative application of U-Pb dating method and a new strategy to identify and reject analytically robust isochrons.

We explore 36 samples, combining U-Pb dating performed with different methods along with carbon and oxygen stable isotopes compositions measured on the same fibres of calcite. The extremely high precision ²⁰⁷Pb/²⁰⁶Pb measured by Thermal Ionisation Mass Spectrometry  revealed that every sample experienced a certain degree of fluid interaction. We find no correlation between ²³⁸U/²⁰⁶Pb and the spread in δ¹⁸O. The higher spread in δ¹⁸O is instead coupled with a remarkable scattered data-points that yield U-Pb ages calculated with the different methods on the same samples with a large variability. In conclusion, our study demonstrates that great care must be taken when considering radiometric ages made on calcite since LA-ICP-MS large uncertainties might obscure the isotopic reorganization.