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

The daughter to parent (U-234/U-238) activity ratio in natural waters is often out of secular radioactive equilibrium. The major reason for this disequilibrium is related to the energetic alpha-decay of U-238 and differential release of U-234 relative to U-238. This disequilibrium originates from (1) preferential release of more loosely bound U-234 from damaged mineral lattice sites or; (2) direct recoil of Th-234 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 U-234, two continuous laboratory granite leaching experiments conducted over 1100 h were performed. The leachates were characterized by declining U concentrations with time and (U-234/U-238) initially greater than unity (up to 1.15), which changed to below unity during leaching (similar to 0.95). The early elevated (U-234/U-238) suggests that additional U-234 is released into solution by preferential release of U-234 from mineral phases. However, the excess U-234 constitutes a finite pool of easy leachable U-234 and the (U-234/U-238) values become lower than unity when this pool is used up. A model based on first-order kinetics, dissolution rates and preferential release of U-234 from damaged lattice sites was developed and is able to quantitatively predict the observed pattern of (U-234/U-238) 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 (U-234/U-238) ratios can be achieved in two distinct regimes (1) slow weathering where the rate of directly recoiled U-234 near mineral surfaces into waters is high; (2) fast weathering where the role of incipient chemical weathering and preferential release of loosely bound U-234 are important. The model is able to explain apparent opposite correlations between physical erosion rates and (U-234/U-238) in waters and it provides a new framework that will be useful for examining weathering regimes, their timescales and their coupling with physical erosion. (c) 2009 Elsevier Ltd. All rights reserved.