Concentration effect on equilibrium fractionation of Mg-Ca isotopes in carbonate minerals: Insights from first-principles calculations

Abstract

Naturally occurring carbonates have a wide variation in Mg and Ca contents. Using the density-functional-theory calculations, this study examines the effect of Mg and Ca concentrations on bond lengths and equilibrium fractionation factors of Mg-Ca isotopes among calcite-type carbonate minerals (Mg x Ca 1-x CO 3). Mg content x and Ca content (1-x) of the investigated carbonate minerals range from 1/12 to 1 and from 1/36 to 1, respectively. Concentration of Ca and Mg in carbonates have significant effects on Ca-O and Mg-O bond lengths when x is close to 0, 0.5 or 1. Because equilibrium isotope fractionation factors (10 3 lnα) are mainly controlled by their relevant bond strengths, which can be measured using their average bond lengths, 10 3 lnα of 26 Mg/ 24 Mg and 44 Ca/ 40 Ca between calcite-type carbonate minerals and dolomite also vary dramatically with Mg content, especially when x is close to 0 and 1. For instance, at 300 K, 10 3 lnα of 26 Mg/ 24 Mg between Mg 1/12 Ca 11/12 CO 3 and dolomite (x=0.5) is ~-4.3 ‰, while 10 3 lnα of 44 Ca/ 40 Ca between Mg 23/24 Ca 1/24 CO 3 and dolomite is ~ 6 ‰. Dolomite is enriched in 26 Mg but depleted in 44 Ca relative to all other carbonate minerals, which is consistent with it having the shortest Mg-O bond length and the longest Ca-O bond lengths among all carbonates. At 300 K, a small change of x from 0.5 to 0.6 in dolomite could result in 1‰ variation in 10 3 lnβ of 26 Mg/ 24 Mg. Therefore, the concentration effect in carbonate minerals should be taken into account when applying the isotope fractionation factors of carbonate minerals to understand geochemical processes.

Sulfur isotope fractionation between aqueous and carbonate-associated sulfate in abiotic calcite and aragonite

“…The O isotope fractionation between synthetic magnesian calcites and water at 25°C changes by 0.06 to 0.17 ‰ for every 1 mole % of MgCO3 (Tarutani et al, 1969;Jiménez-López et al, 2004). Additionally, DFT calculations suggest a change in the Mg and Ca isotopic compositions of calcite-type minerals, which arises from a change in bond lengths with changes in the proportion of MgCO3 and CaCO3 in the solid (Wang et al, 2017). It is not unexpected, therefore, that a change in [CAS] may be associated with a change in S isotope fractionation, as well as the fractionation of isotopes of the other elements in the mineral lattice, namely O, C and Ca.…”

Carbonated mantle domains at the base of the Earth's transition zone

“…The details on the pseudopotentials for Mg, O, and C can be found in Wang et al (2017). The structure at certain pressures was optimized using the variable cell-shape damped molecular dynamic (Wentzcovitch et al 1993).…”

Site-specific equilibrium isotopic fractionation of oxygen, carbon and calcium in apatite

“…A variety of isotopes and minerals have been investigated using first-principles methods based on density functional theory (DFT), such as stable isotopes O and C in carbonates (Schauble et al, 2006), and H, O and Si in silicates . Theoretical studies of calcium isotopic fractionation have also been conducted on sulfates (Griffith et al, 2008) and carbonates (Rustad et al, 2010;Wang et al, 2017). In addition, theoretical approaches make it possible to determine the isotopic fractionation properties of specific sites in the crystal structure (e.g.…”