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Role of nuclear volume in driving equilibrium stable isotope fractionation of mercury, thallium, and other very heavy elements

2007; Elsevier BV; Volume: 71; Issue: 9 Linguagem: Inglês

10.1016/j.gca.2007.02.004

1872-9533

Edwin A. Schauble,

Geochemistry and Elemental Analysis

Equilibrium stable isotope fractionations of mercury and thallium are estimated for molecules, atoms and ions using first-principles vibrational frequency and electronic structure calculations. These calculations suggest that isotopic variation in nuclear volume is the dominant cause of equilibrium fractionation, driving 205Tl/203Tl and 202Hg/198Hg fractionations of up to 3‰ at room temperature. Mass-dependent fractionations are smaller, ca. 0.5–1‰ for the same isotopes. Both fractionation mechanisms tend to enrich the neutron-rich isotopes in oxidized mercury- and thallium-bearing phases (Tl3+ and Hg2+) relative to reduced phases (Tl+ and Hg0). Among Hg2+-bearing species, inorganic molecules and complexes like HgCl2, HgCl42- and Hg(H2O)62+ will have higher 202Hg/198Hg than coexisting methylmercury species, suggesting a possible application of Hg-isotope measurements to understanding mercury methylation and increasing methylmercury concentrations at the top of the food chain. Estimated 205Tl/203Tl fractionation between Tl(H2O)63+ and Tl(H2O)3+ is in reasonable agreement with the fractionations previously observed between seawater and Fe–Mn crusts, supporting an equilibrium-like reduction/oxidation fractionation mechanism. More generally, nuclear-volume isotope fractionation will concentrate larger (heavier) nuclei in species where the electron density at the nucleus is small—due to lack of s-electrons (e.g., Hg2+—[Xe]4f145d106s0 vs. Hg0—[Xe]4f145d106s2) or enhanced s-electron screening by extra p, d, or f electrons (e.g., Tl0—[Xe]4f145d106s26p1 vs. Tl+—[Xe]4f145d106s26p0). Nuclear-volume fractionations become much smaller for lighter elements, declining from ∼1‰/amu for thallium and mercury to ∼0.2‰/amu for ruthenium and ∼0.02‰/amu for sulfur.