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

ABSTRACT: 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 (202)Hg/(198)Hg, (205)Tl/(203)Tl, (207)Pb/(206)Pb and (208)Pb/(206)Pb isotope fractionations are found can be up to 3.61‰, 2.54‰, 1.48‰ and 3.72‰ 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 [formula in text] 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, Pb(4+)-bearing species are found can enrich heavier Pb isotopes than Pb(2+)-bearing species to a surprising extent, e.g., the enrichment can be up to 4.34‰ in terms of (208)Pb/(206)Pb at room temperature, due to their NVEs are in opposite directions. In contrast, fractionations among Pb(2+)-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., [formula in text]) and odd-mass MIFs (i.e., [formula in text) are almost the same but with opposite signs.

SUBMITTER: Yang S

PROVIDER: S-EPMC4519782 | biostudies-literature | 2015 Jul

REPOSITORIES: biostudies-literature

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

Yang Sha S Liu Yun Y

Scientific reports 20150730

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 (202)Hg/(198)Hg, (205)Tl/(203)Tl, (207)Pb/(206)Pb and (208)Pb/(206)Pb isotope fractionations are found can be up to 3.61‰, 2.54‰, 1.48‰ and 3.72‰ at room temperature, respectively, larger than fractionations predicted by classical mass-dependent isotope fractionations theory. Moreover, the ...[more]

PMID: 26224248

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Project description:Mass-independent isotope fractionations driven by differences in volumes and shapes of nuclei (the field shift effect) are known in several elements and are likely to be found in more. All-electron relativistic electronic structure calculations can predict this effect but at present are computationally intensive and limited to modeling small gas phase molecules and clusters. Density functional theory, using the projector augmented wave method (DFT-PAW), has advantages in greater speed and compatibility with a three-dimensional periodic boundary condition while preserving information about the effects of chemistry on electron densities within nuclei. These electron density variations determine the volume component of the field shift effect. In this study, DFT-PAW calculations are calibrated against all-electron, relativistic Dirac-Hartree-Fock, and coupled-cluster with single, double (triple) excitation methods for estimating nuclear volume isotope effects. DFT-PAW calculations accurately reproduce changes in electron densities within nuclei in typical molecules, when PAW datasets constructed with finite nuclei are used. Nuclear volume contributions to vapor-crystal isotope fractionation are calculated for elemental cadmium and mercury, showing good agreement with experiments. The nuclear-volume component of mercury and cadmium isotope fractionations between atomic vapor and montroydite (HgO), cinnabar (HgS), calomel (Hg2Cl2), monteponite (CdO), and the CdS polymorphs hawleyite and greenockite are calculated, indicating preferential incorporation of neutron-rich isotopes in more oxidized, ionically bonded phases. Finally, field shift energies are related to Mössbauer isomer shifts, and equilibrium mass-independent fractionations for several tin-bearing crystals are calculated from (119)Sn spectra. Isomer shift data should simplify calculations of mass-independent isotope fractionations in other elements with Mössbauer isotopes, such as platinum and uranium.

Project description:Silicon and Mg in differentiated rocky bodies exhibit heavy isotope enrichments that have been attributed to evaporation of partially or entirely molten planetesimals. We evaluate the mechanisms of planetesimal evaporation in the early solar system and the conditions that controled attendant isotope fractionations. Energy balance at the surface of a body accreted within ~1 Myr of CAI formation and heated from within by 26Al decay results in internal temperatures exceeding the silicate solidus, producing a transient magma ocean with a thin surface boundary layer of order < 1 meter that would be subject to foundering. Bodies that are massive enough to form magma oceans by radioisotope decay (? 0.1% M ?) can retain hot rock vapor even in the absence of ambient nebular gas. We find that a steady-state rock vapor forms within minutes to hours and results from a balance between rates of magma evaporation and atmospheric escape. Vapor pressure buildup adjacent to the surfaces of the evaporating magmas would have inevitably led to an approach to equilibrium isotope partitioning between the vapor phase and the silicate melt. Numerical simulations of this near-equilibrium evaporation process for a body with a radius of ~ 700 km yield a steady-state far-field vapor pressure of 10-8 bar and a vapor pressure at the surface of 10-4 bar, corresponding to 95% saturation. Approaches to equilibrium isotope fractionation between vapor and melt should have been the norm during planet formation due to the formation of steady-state rock vapor atmospheres and/or the presence of protostellar gas. We model the Si and Mg isotopic composition of bulk Earth as a consequence of accretion of planetesimals that evaporated subject to the conditions described above. The results show that the best fit to bulk Earth is for a carbonaceous chondrite-like source material with about 12% loss of Mg and 15% loss of Si resulting from near-equilibrium evaporation into the solar protostellar disk of H2 on timescales of 104 to 105 years.

Dataset Information

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

Publications

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

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