HR: 08:31h
AN: V41E-03 INVITED    [Abstracts]
TI: "Stable" Isotope Fractionation of Uranium: Implications for Geochemical Cycling and Geochronology
AU: * Stirling, C H
EM: cstirling@chemistry.otago.ac.nz
AF: Dept. Chemistry, University of Otago, Dunedin, 9001, New Zealand
AU: * Stirling, C H
EM: cstirling@chemistry.otago.ac.nz
AF: Dept. Earth Sciences, ETH Zurich, Zurich, 8092, Switzerland
AU: Andersen, M B
EM: morten.andersen@bris.ac.uk
AF: Dept. Earth Sciences, ETH Zurich, Zurich, 8092, Switzerland
AU: Andersen, M B
EM: morten.andersen@bris.ac.uk
AF: Dept. Earth Sciences, University of Bristol, Bristol, BS8 1RJ, United Kingdom
AU: Warthmann, R
EM: warthmann@erdw.ethz.ch
AF: Dept. Earth Sciences, ETH Zurich, Zurich, 8092, Switzerland
AU: Halliday, A N
EM: alexh@earth.ox.ac.uk
AF: Dept. Earth Sciences, ETH Zurich, Zurich, 8092, Switzerland
AU: Halliday, A N
EM: alexh@earth.ox.ac.uk
AF: Dept. Earth Sciences, University of Oxford, Oxford, OX1 3PR, United Kingdom
AB: Uranium is the heaviest naturally occurring element. It has three natural radioactive isotopes, ²³⁸U, ²³⁵U and ²³⁴U and is thus widely utilized for geochronology, and two oxidation states, insoluble U(IV) and soluble U(VI). Mass-dependent thermodynamic isotopic fractionation between ²³⁵U and ²³⁸U (and ²³⁴U and ²³⁸U), which scales with δM/M², is not normally considered significant given the small ~1% difference in mass. It is therefore usual to assume that ²³⁸U/²³⁵U is constant in nature and presently equal to 137.88 throughout the entire solar system.
Importantly, isotopic fractionation of the very heavy elements has recently been investigated for mercury and thallium in the context of mass-independent nuclear field shift effects (Schauble 2007, GCA 71, 2170-2189), which do not scale with δM/M², and are predicted to have permil-level effects on the heavy masses, including uranium. Uranium is thus emerging as a potentially significant element for monitoring biological pathways and redox processes occurring during the transition between the U(IV) and U(VI) oxidation states.
We have developed experimental protocols for the precise measurement of ²³⁸U/²³⁵U and ²³⁸U/²³⁴U by multiple-collector ICPMS (MC-ICPMS) to investigate "stable" isotope fractionation in uranium. Using a Nu Plasma MC-ICPMS, concentrated solutions are measured at high signal intensity to enable simultaneous data collection on a stable multiple-Faraday array in place of the usual electron multiplier configuration. Using these protocols and a high-purity ²³³U-²³⁶U double-spike to internally monitor instrumental mass fractionation, we are able to resolve variations in ²³⁸U/²³⁵U and ²³⁸U/²³⁴U at the 0.4 and 0.3 epsilon level (2σ; 1 ε = 1 part in 10,000), respectively.
Terrestrial and meteoritic samples formed in high-temperature environments show no variability in ²³⁸U/²³⁵U at the 1-ε level. In contrast, measurements for samples formed in low- temperature environments reveal permil-level natural variability in ²³⁸U/²³⁵U (Stirling et al., EPSL, accepted). Laboratory experiments involving the biologically-mediated reduction of U(VI) to U(IV) generate sizeable shifts in both ²³⁸U/²³⁵U and ²³⁸U/²³⁴U towards anomalous values and can constrain the relative importance of mass-dependent versus mass-independent nuclear field shift effects during the "stable" isotope fractionation of uranium.
Natural variability in ²³⁸U/²³⁵U will also impact on the accuracy of the U-series and U-Th-Pb chronometers when applied to samples formed in low-temperature environments, particularly for high-precision methods, as these chronometers currently assume an invariant ²³⁸U/²³⁵U equal to 137.88.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1000 GEOCHEMISTRY
DE: 1094 Instruments and techniques
DE: 1120 Isotopic disequilibrium dating
DE: 1194 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting