The nuclear volume effect (i.e., NVE) is originated from differences in nuclear size and nuclear shape of isotopes. It doesn’t belong to the well-known driving forces of equilibrium isotope fractionation, which are governed by the conventional mass-dependent isotope fractionation theory. The nuclear volume effect is a peculiar and very important phenomenon for heavy metal elements.

Recently, YANG Sha and LIU Yun of the State Key Laboratory of Ore Deposit Geochemistry (SKLODG) at the Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) have performed theoretical study on the equilibrium isotope fractionations of Hg, Tl and Pb systems caused by the NVE. Those isotope fractionations are investigated with careful evaluation on quantum relativistic effects via the Dirac’s formalism of full-electron wave function.

The results show the equilibrium isotope fractionations are much larger than fractionations predicted by classical mass-dependent isotope fractionations theory. Moreover, the NVE can cause mass-independent fractionations (MIFs) for odd-mass isotopes and even-mass isotopes of Hg and Pb systems.

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 ²⁰⁸Pb/²⁰⁶Pb 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 study (Yang, S. and Liu, Y. Nuclear volume effects in equilibrium stable isotope fractionations of mercury, thallium and lead. Sci. Rep. 5, 12626; doi: 10.1038/srep12626 (2015)) can be open accessed at: www.nature.com/articles/srep12626.