U-isotope fractionation during chemical weathering

Complete text of publication follows: Our ability to use uranium isotopes as a tool for determining rates and timescales of weathering critically depends on the processes that produce fractionation between 234U and 238U. First, during water rock interaction, 234Th can be directly recoiled into the water phase and subsequently decays to 234U. Second, mineral dissolution can lead to an enhancement of 234U in the water. We have used both a theoretical and experimental approach to better model both processes. We used continuous leaching experiments of a ground granite reported in Andersen et al. [1] combined with major element analyses to constrain the preferential leaching rates 234U relative to 238U for each mineral. To model these experiments, we have considered that 234U consist of two pools one that is easily leachage and one that has annealed at a slow rate and becomes indistinguishable from 238U. This model provides a good fit to the experiments but can also be extended to longer timescales. In order to better describe the recoil efficiency we have used surface area measurements based on the BET method and used this data to determine the fractal dimension of the surface. This fractal dimension is then used to correct the recoil efficiency for surface roughness at the length scale of recoil (∼ 30 nm). DePaolo et al. [2] had underlined that BET measurements overestimate the surface area to calculate recoil efficiency. Our methodology now provides a refined estimate of recoil efficiency that is more consistent with observations. The combination of these approaches provides a predictive framework with which one can estimate more reliably weathering rates based on U isotopes. This will be done using examples from the literature as well our study of carbonate weathering in Eastern France. Notably, an improper estimate of preferential leaching leads to an incorrect estimate of weathering rates