HR: 10:35h
AN: V11H-02    [PDF]
TI: Isotope Fractionation of Uranium in Low-Temperature Environments
AU: * Stirling, C
EM: stirling@erdw.ethz.ch
AF: ETH Zurich, Institute for Isotope Mineralogy and Mineral Resources ETH Zentrum, Zurich, 8092 Switzerland
AU: Potter, E
EM: potter@erdw.ethz.ch
AF: ETH Zurich, Institute for Isotope Mineralogy and Mineral Resources ETH Zentrum, Zurich, 8092 Switzerland
AU: Andersen, M
EM: andersen@erdw.ethz.ch
AF: ETH Zurich, Institute for Isotope Mineralogy and Mineral Resources ETH Zentrum, Zurich, 8092 Switzerland
AU: Halliday, A
EM: halliday@erdw.ethz.ch
AF: ETH Zurich, Institute for Isotope Mineralogy and Mineral Resources ETH Zentrum, Zurich, 8092 Switzerland
AU: Sp\"{o}tl, C
EM: christoph.spoetl@uibk.ac.at
AF: Universit\"{a}rt Innsbruck, Institute f\"{u}r Geologie und Pal\"{a}ontologie, Innsbruck, 6020 Austria
AB: Uranium is the heaviest naturally occurring element. It has three isotopes, $^{238}$U, $^{235}$U and $^{234}$U, and two redox states, U(IV) and U(VI). Large isotopic fractionations have been previously documented for $^{234}$U/$^{238}$U that are attributed to lattice damage and subsequent preferential leaching and oxidation at the $\alpha$-recoil site. However, fractionation between $^{235}$U and $^{238}$U is not expected due to the small $\sim$1% difference between the masses of these two isotopes. It is therefore usual to assume $^{238}$U/ $^{235}$U is constant in the terrestrial environment and equal to 137.88. Stable isotope fractionation is normally restricted to the light and intermediate mass elements due to relatively large mass differences of several percent. Recently, however, thallium, a heavy element with stable isotopes at masses 203 and 205, has been shown to display large, permil-level $^{205}$Tl/$^{203}$Tl variability (Rehkamper et al., 2002, Earth Planet. Sci. Lett. 197, 65). Given their similar redox chemistries, it is not unreasonable to propose that $^{235}$U/$^{238}$U may show similar variability in certain terrestrial environments. We have developed experimental protocols for the precise measurement of $^{235}$U/$^{238}$U by multiple-collector ICPMS (MC-ICPMS) and have analyzed a suite of samples formed in a range of low- and high-temperature environments. Using a Nu Instruments NuPlasma MC-ICPMS, we are able to resolve variations in $^{235}$U/$^{238}$U at the 0.5 $\epsilon$ level (2$\sigma$; 1 $\epsilon$ = 1 part in 10,000) on sample sizes comprising 30 ng of uranium. Data can be acquired on smaller 4 ng samples with 1-2 epsilon $2\sigma$ uncertainties. High quality U measurements are possible because we have used a high-purity $^{233}$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. The natural variability in $^{235}$U/$^{238}$U shown by the analyzed samples is 13 $\epsilon$ units and exceeds the analytical reproducibility by more than an order of magnitude. Reproducible compositions both heavier and lighter than our terrestrial standard are observed. Importantly, the largest excursions are observed in old samples that can not have been disturbed by anthropogenic contamination. The observed variability in $^{235}$U/$^{238}$U indicates that uranium isotopes may offer the potential to monitor redox processes during the transition between U(IV) and U(VI) oxidation states. Our observations will also impact on U-series and U-Th-Pb applications in geochemistry, paleoclimatology and cosmochemistry, which currently assume invariant $^{235}$U/$^{238}$U in all terrestrial and extraterrestrial environments.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting