Experimental evidence for ²³⁴U-²³⁸U fractionation during granite weathering with implications for ²³⁴U/²³⁸U in natural waters

The daughter to parent (²³⁴U/²³⁸U) activity ratio in natural waters is often out of secular radioactive equilibrium. The major reason for this disequilibrium is related to the energetic α-decay of ²³⁸U and differential release of ²³⁴U relative to ²³⁸U. This disequilibrium originates from (1) preferential release of more loosely bound ²³⁴U from damaged mineral lattice sites or; (2) direct recoil of ²³⁴Th into surrounding media from near mineral surface boundaries, however, it is unclear which of the two mechanisms is most important in nature. To better quantify the effects of preferential release of ²³⁴U, two continuous laboratory granite leaching experiments conducted over 1100 h were performed. The leachates were characterized by declining U concentrations with time and (²³⁴U/²³⁸U) initially greater than unity (up to 1.15), which changed to below unity during leaching (∼0.95). The early elevated (²³⁴U/²³⁸U) suggests that additional ²³⁴U is released into solution by preferential release of ²³⁴U from mineral phases. However, the excess ²³⁴U constitutes a finite pool of easy leachable ²³⁴U and the (²³⁴U/²³⁸U) values become lower than unity when this pool is used up. A model based on first-order kinetics, dissolution rates and preferential release of ²³⁴U from damaged lattice sites was developed and is able to quantitatively predict the observed pattern of (²³⁴U/²³⁸U) values and U concentrations for the two granite leaching experiments. Extending the modeling to longer time scales more comparable to natural systems shows that the production of waters with high (²³⁴U/²³⁸U) ratios can be achieved in two distinct regimes (1) slow weathering where the rate of directly recoiled ²³⁴U near mineral surfaces into waters is high; (2) fast weathering where the role of incipient chemical weathering and preferential release of loosely bound ²³⁴U are important. The model is able to explain apparent opposite correlations between physical erosion rates and (²³⁴U/²³⁸U) in waters and it provides a new framework that will be useful for examining weathering regimes, their timescales and their coupling with physical erosion.