HR: 08:46h
AN: V41E-04 INVITED    [Abstracts]
TI: Nuclear Volume-Dependent Fractionation of Uranium Isotopes
AU: * Weyer, S
EM: stefan.weyer@em.uni-frankfurt.de
AF: University of Frankfurt, Institute of Geosciences Altenhoeferallee 1, Frankfurt, 40438, Germany
AU: Schauble, E A
EM: schauble@ess.ucla.edu
AF: UCLA, Department of Earth and Space Sciences 595 Charles Young Drive East, Los Angeles, CA 90095-1567, United States
AU: Anbar, A D
EM: anbar@asu.edu
AF: ASU, School of Earth & Space Exploration, Tempe, AZ 85287, United States
AB: Chemical reactions can fractionate isotopes because the magnitudes of equilibrium and rate constants are subtly sensitive to nuclear mass. Geoscientists have exploited this fact to learn about modern environmental processes and past environmental conditions by precisely measuring variations in the isotope compositions of a wide range of elements in natural materials. Here we present evidence from natural terrestrial samples that processes related to ¡°nuclear volume¡± rather than ¡°nuclear mass¡± significantly fractionate the isotope composition of the heaviest primordial element ¨C uranium. The isotopic composition of U in nature is generally assumed to be invariant. Here, we report variations of the ²³⁸U/²³⁵U isotope ratio in natural samples (basalts, granites, seawater, corals, black shales, suboxic sediments, ferro-manganese crusts/nodules and BIFs), which span a range of δ²³⁸U values of ~ 1.3 ‰, exceeding by far the analytical precision of our method (¡Ö 0.06‰, 2SD, based on replicate measurements of individual samples). The largest isotope variations found in our survey are between oxidized and reduced depositional environments, with seawater and suboxic sediments falling in between. U isotopes were analyzed with MC-ICP-MS. A mixed ²³⁶U-²³³U isotopic tracer (double spike) was used to correct for isotope fractionation during sample purification and instrumental mass bias. Sediments formed in oxic environments, such as manganese crusts from the Atlantic and Pacific oceans, display δ²³⁸U of -0.54 to -0.62 ‰, slightly lighter than that of seawater (-0.41 ‰). However, sediments from reducing environments, such as black shales from the Black Sea (unit I and unit II) and the Cariaco basin, display heavy U isotope compositions with δ²³⁸U of up to +0.43 ‰ (0.84 ‰ heavier than seawater). Uranium enrichment in these sediments probably occurred during the reduction of soluble U(VI) (from seawater) to insoluble U(IV). Intriguingly, isotope fractionation in these sediments is opposite in direction to what would be expected from mass-dependent equilibrium or kinetic/diffusive isotope fractionation. Instead, heavy U isotope compositions of reduced U species are expected from first-principles quantum chemical modeling of fractionation driven by nuclear volume. Our modeling results predict that equilibrium nuclear volume and mass-dependent isotope fractionation operate in opposite directions for U, and that the volume effect is of greater magnitude. Combining our observations on natural samples with the results of theoretical modeling strongly indicates that nuclear volume-, rather than mass-dependent isotope fractionation is the dominant process that fractionates the isotope composition of some very heavy elements, such as U, in nature.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1030 Geochemical cycles (0330)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting