HR: 14:40h
AN: PP33D-05    [Abstracts]
TI: Equilibrium stable-isotope fractionation of thallium and mercury
AU: * Schauble, E A
EM: schauble@ucla.edu
AF: Dept. of Earth and Space Sciences, UCLA, Box 951567, Los Angeles, CA 90095-1567 United States
AB: In this study first-principles quantum mechanical and empirical force-field models are used to estimate equilibrium mass-dependent isotopic fractionations among a variety of thallium and mercury compounds. High-precision MC-ICP-MS measurements have recently uncovered evidence of stable isotope fractionation for many elements, including 2-4‰ variability in the isotopic compositions of thallium[1] (atomic no. 81) and mercury[2] (atomic no. 80). The observed thallium- and mercury-isotope fractionations are remarkable, given that the magnitude of isotopic fractionation typically decreases as atomic number increases[3]. Stable isotope measurements could improve our understanding of geochemical and biogeochemical cycling of both elements, but little is known about the mechanisms driving these fractionations. A better understanding of the chemical processes controlling stable isotope compositions could help maximize the utility of these new geochemical tracers. Standard equilibrium stable isotope fractionation theory holds that the energy driving fractionation comes from isotopic effects on vibrational frequencies, which have generally not been measured. In the present study both quantum-mechanical and empirical force fields are used to estimate unknown frequencies. Results suggest that thallium and mercury fractionations of ≥ 0.5‰ are likely during the relevant redox reactions Tl⁺ ↔ Tl³⁺ and Hg^O ↔ Hg²⁺. Methyl-mercury and mercury-halide compounds like CH₃HgCl will have ~ 1‰ higher ²⁰²Hg/¹⁹⁸Hg than atomic vapor at room temperature. Fractionations between coexisting Hg²⁺ species appear to be much smaller, however. ²⁰⁵Tl/²⁰³Tl in Tl(H₂O)_6³⁺ is predicted to be ~0.5‰ higher than in coexisting Tl⁺-bearing substances. This result is in qualitative agreement with data from ferromanganese crusts [1], suggesting that Tl³⁺ in manganese-oxides will have higher ²⁰⁵Tl/²⁰³Tl than aqueous Tl⁺. Equilibrium fractionations for both elements are much smaller than the observed range of isotopic fractionations, however, which could point to a major role for kinetic-fractionation or Rayleigh-like distillation processes. Refs.: [1] Rehämper et al. (2002) EPSL 197:65. [2] Xie et al. (2005) J. Anal. Atomic Spectrom. 20:515. [3] Bigeleisen and Mayer (1947) J. Chem. Phys. 15:261.
DE: 0330 Geochemical cycles (1030)
DE: 0478 Pollution: urban, regional and global (0345, 4251)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 4271 Physical and chemical properties of seawater
DE: 4805 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4912)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005