Low-temperature isotopic fractionation of uranium

摘要

Uranium is the heaviest naturally occurring element and isotope fractionation between U-235 and U-238 is not normally considered significant given the small similar to 1% difference in mass. It is therefore usual to assume that U-238/U-235 is constant in the terrestrial environment and equal to 137.88. We have developed experimental protocols for the precise measurement of U-235/U-238 by multiple-col lector ICPMS (MC-ICPMS) and have analyzed a suite of samples formed in a range of low-temperature environments. Using a high-purity U-233-U-236 U double spike to internally monitor the large (percent-level) but essentially constant instrumental mass bias effects that are inherent to plasma source mass spectrometry, we are able to resolve variations in U-235/U-238 at the 0.4 epsilon level (2 sigma; 1 epsilon= 1 part in 10,000) on sample sizes comprising 50 ng of uranium. Here we demonstrate sizeable (13 epsilon units) natural variability in U-235/U-238, exceeding the analytical reproducibility by more than an order of magnitude. Compositions that are both isotopically heavier and lighter than our terrestrial standard, by 4 and 9 epsilon units respectively, are observed. The largest excursions are found in speleothem samples. Furthermore, U-235/U-238 appears broadly correlated with U-234/(23)8 in samples showing the most extreme isotopic compositions. The present study investigates the role of abiotic processes in fractionating U-235 from U-238. Sequential leaching experiments of U-rich minerals indicate that mineral weathering is a possible mechanism by which U-235 can be fractionated from U-238 in groundwaters and incorporated into speleothems. The observed variability in U-235/U-238 indicates that uranium isotopes may offer the potential to monitor new reaction pathways, such as those activated during the redox transition between the U(IV) and U(VI) oxidation states. Experiments involving the redox transition of U(VI) to U(IV) in the presence of zero-valent zinc did not produce a resolvable shift in U-235/U-238 towards anomalous values, although fractionation need 8not occur if the reaction is governed by a fast kinetic process. Our observations have a direct impact on the U-series and U-Th-Pb chronometers, when applied to samples formed in low-temperature environments, as these chronometers currently assume an invariant U-238/U-235 equal to 137.88. (c) 2007 Elsevier B.V. All rights reserved.