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Low-temperature Isotopic Fractionation of Uranium

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Andersen M.B.1, Stirling C.H 2, Potter, E. K.3, Halliday, A.,4

1Bristol Isotope Group, Department of Earth Sciences, University of Bristol, Parks Road, Bristol, United Kingdom (

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

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