Low-temperature Isotopic Fractionation of Uranium

Andersen M.B.¹, Stirling C.H², Potter, E. K.³, Halliday, A.,⁴

¹Bristol Isotope Group, Department of Earth Sciences, University of Bristol, Parks Road, Bristol, United Kingdom (morten.andersen@bris.ac.uk)

²Department of Chemistry, University of Otago, PO Box 56, Union Place, Dunedin
New Zealand

³Institute for Isotope Geochemistry and Mineral Resources, ETH Zentrum, Zurich, 8092 Switzerland

⁴Department of Earth Sciences, Parks Road, Oxford, OX1 3PR, United Kingdom

Uranium is the heaviest naturally occurring element and isotope fractionation between ²³⁵U and ²³⁸U is not normally considered significant and it has been usual to assume that ²³⁸U/²³⁵U is constant in the terrestrial environment (equal to 137.88). Recent developments in measurements of precise ²³⁵U/²³⁸U ratios, using MC-ICPMS and ²³³U-²³⁶U double-spike techniques, have demonstrated sizeable (~1.3 ‰) natural variability in ²³⁵U/²³⁸U (Stirling et al., 2007, Weyer et al., 2008, Bopp et al., 2009). Compositions that are both isotopically heavier and lighter compared to assumingly unfractionated high-temperature igneous and meteoritic material are observed. The largest observed excursions are towards lighter ²³⁵U/²³⁸U for redox sensitive samples (e.g. black shales, speleothems) consistent with the theoretical predictions of isotopic fractionation of U during U(VI)-U(IV) transitions related to the nuclear field shift effect (Bigeleisen, 1996). The observed variability in ²³⁵U/²³⁸U indicates that uranium isotopes may offer the potential as a geochemical tracer for paleo-redox conditions. The observations also have a direct impact on the U-series and U-Pb chronometers, as these chronometers presently assume an invariant ²³⁸U/²³⁵U equal to 137.88.

Bigeleisen, 1996, Journal of the American Chemical Society, 118, 3676-3680

Bopp et al., 2009, Geology, 37, 611-614

Stirling et al., 2007, Earth Planetary Science Letters, 264, 208-225

Weyer et al., 2008, Geochimica et Cosmochimica Acta, 72,345-359

Last updated: September 9, 2009