Equilibrium zinc isotope fractionation in Zn-bearing minerals from first-principles calculations

Abstract

Isotopic composition analysis contributes significantly to the investigation of the biogeochemical cycle of zinc with its economical, environmental and health implications. Interpretation of isotopic measurements is however hindered by the lack of a set of equilibrium isotopic fractionation factors between Zn-bearing minerals. In this study, equilibrium mass-dependent Zn isotope fractionation factors in Zn-bearing minerals are determined from first-principles calculations within the density functional theory (DFT) scheme. A wide range of minerals belonging to sulfide, carbonate, oxide, silicate, sulfate and arsenate mineral groups are modelled to account for the natural diversity of Zn crystal-chemical environment. Calculated reduced partition function ratios (β-factors) span a range smaller than 2 at 22 • C. All studied secondary minerals (adamite, gahnite, gunningite, hemimorphite, hydrozincite, zincite) but zinc carbonate (smithsonite) are isotopically heavier than zinc sulfide minerals (sphalerite and wurtzite) from which they could form through supergene processes. Zinc-aluminium spinel and zinc silicate are the isotopically heaviest minerals. The investigation of the crystalchemical parameters at the origin of differences in isotopic properties shows an excellent linear correlation between ln β and Zn interatomic force constants. β-factors are also observed to increase when the Zn-first neighbour bond lengths decrease and charges on atoms involved in the bonding increase and vice versa. These findings are in line with the observation of heavy isotope enrichment in systems having the largest bond strength.

Zinc isotope composition of the Earth and its behaviour during planetary accretion

“…The temperatures required to decrease the 66/64 Zn fractionation factor of eq. 5, Δ 66 ZnZnO(s)-Zn(g), to analytically unresolvable levels (<0.05‰) are >2200°C (given Δ 66 ZnZnO(s)-Zn(g) = +0.31×10 6 /2473 2 = +0.05‰; Ducher et al, 2016). Thermodynamic data for eq.…”

“…However, which end-member scenario characterises volatile loss during planetary formation, remains unconstrained, despite the fact that these processes may be distinguished by their stable isotope signatures. Namely, equilibrium isotope fractionation of Zn between condensed phase(s) and gas is smaller, especially at high temperatures (the 66 Zn/ 64 Zn fractionation factor between ZnO(s) and Zn 0 (g) is 0.31‰×10 6 /T 2 ; Ducher et al, 2016), than kinetically-driven vapour loss into a vacuum which is temperature-independent and scales with the inverse square root of the masses of the two evaporating isotopes (cf. Richter et al, 2002).…”

A global assessment of Zn isotope fractionation in secondary Zn minerals from sulfide and non-sulfide ore deposits and model for fractionation control

“…1‰ lighter than the original sample (Weiss et al, 2014). Ducher et al (2016) reported theoretical studies of Zn isotope fractionation in Zn-bearing minerals. They investigated the isotope fractionation properties of Zn in several minerals at equilibrium using density functional theory (DFT).…”

“…This means that, at equilibrium, an alteration process that releases Zn from sphalerite and reprecipitates it into secondary minerals could produce either positive or negative fractionations: hemimorphite and hydrozincite would be isotopically heavier than parental sphalerite (at 22 °C ∆ hem-sph (‰) = +1.38, ∆ hydr-sph (‰) = +0.80), whereas smithsonite would be isotopically lighter (at 22 °C ∆ sm-sph (‰) = -0.10). Crystalchemical parameters control these isotopic properties (Ducher et al, 2016): indeed, excellent linear correlations exist between ln β and Zn interatomic force constants, and β-factors increase when the Zn-first neighbor bond lengths decrease and charges on atoms involved in the bonding increase and vice versa.…”

Zinc Geochemical Cycling in a Phosphorus‐Rich Ocean During the Early Ediacaran

“…A larger Zn isotope fractionation has been reported in recent experiments, where the adsorption of Zn onto calcite can fractionate Zn isotopes, with Δ 66 Zn adsorbed-solution = 0.41 ± 0.18‰ at low ionic strength and 0.73 ± 0.08‰ at high ionic strength (Dong & Wasylenki, 2016). Nevertheless, all observed Zn isotope fractionation under natural conditions are much different from that of theoretical calculations (Δ 66 Zn hydrozincite-solution = 1.0‰, Fujii et al, 2011Fujii et al, , 2014; Δ 66 Zn smithsonite-solution = À0.73‰, Ducher et al, 2016).…”