Theoretical investigations of uranium isotope fractionation caused by nuclear volume effects

Abstract:

Because the half-life times of uranium isotopes are all very long, e.g., 4.5Ga for ²³⁸U and 0.7Ga for ²³⁵U, people actually treat uranium isotope system as a stable one in many young geologic systems (e.g., Bopp et al., 2010; Basu et al., 2014). There is an increasing trend of using U isotope method to study surface geochemistry problems. For example, people start to use U isotopes as a new tracer to determine the change of redox conditions (Holmden et al., 2015; Wang et al., 2015). However, there are only a few equilibrium U isotope fractionation factors available right now. The new enterprise of U isotope method requires a much expanded data-base of equilibrium U isotope fractionation factor.

Many evidences showed that heavy isotope systems could be significantly fractionated as the consequence of the nuclear volume effect (NVE) or so-called nuclear field shift effect,which is a driving force of mass-independent fractionation induced by differences in nuclear size and nuclear shape of isotopes. Here we theoretically estimate the magnitude of equilibrium isotope fractionation factors of U-bearing gaseous and solid compounds caused by NVE via density functional theory (DFT) quantum chemistry methods with careful evaluation on quantum relativistic effects. Our calculation results show the NVE drives ²³⁸U/²³⁵U fractionations can be up to -4.43‰ between U(VI) and U(IV) species, or can be up to -1.68‰ between U(IV) and U(III) species, at room temperature. The U⁴⁺-bearing species or phases tend to enrich heavier isotopes (i.e., ²³⁸U) relative to the oxidized phases (U⁶⁺-bearing), and enrich lighter isotopes (i.e., ²³⁵U) relative to the reduced U(III) phases (U³⁺-bearing), which generally agree with the recent experimental results (Wang et al., 2015). Our results provide a base for broad applications of equilibrium U isotope fractionation in surface environments.