Nuclear volume effects in equilibrium stable isotope fractionations of mercury, thallium and lead

The nuclear volume effects (NVEs) of Hg, Tl and Pb isotope systems are investigated with careful evaluation on quantum relativistic effects via the Dirac's formalism of full-electron wave function. Equilibrium Hg-202/Hg-198, Tl-205/Tl-203, Pb-207/Pb-206 and Pb-208/Pb-206 isotope fractionations are found can be up to 3.61 parts per thousand, 2.54 parts per thousand, 1.48 parts per thousand and 3.72 parts per thousand at room temperature, respectively, larger than fractionations predicted by classical mass-dependent isotope fractionations theory. Moreover, the NVE can cause mass-independent fractionations (MIF) for odd-mass isotopes and even-mass isotopes. The plot of Delta(199)(NV)Hgvs. Delta Hg-201(NV) for Hg-bearing species falls into a straight line with the slope of 1.66, which is close to previous experimental results. For the first time, Pb4+-bearing species are found can enrich heavier Pb isotopes than Pb2+-bearing species to a surprising extent, e.g., the enrichment can be up to 4.34% in terms of Pb-208/Pb-206 at room temperature, due to their NVEs are in opposite directions. In contrast, fractionations among Pb2+-bearing species are trivial. Therefore, the large Pb fractionation changes provide a potential new tracer for redox conditions in young and closed geologic systems. The magnitudes of NVE-driven even-mass MIFs of Pb isotopes (i.e., Delta Pb-204(NV)) and odd-mass MIFs (i.e., Delta Pb-207(NV)) are almost the same but with opposite signs.