Determination of the equilibrium magnesium isotope fractionation factors between brucite and aqueous inorganic and organic species

Magnesium (Mg) is a major element in seawater, rock-forming minerals, and biological systems. Stable Mg isotopes fractionate during silicate weathering and carbonate mineralization, and hence are a promising tool to trace these processes. Magnesium can be present in natural aqueous solutions as a number of distinct inorganic and organic complexes including MgHCO ₃ ⁺, MgCO ₃ ⁰, MgSO ₄ ⁰, Mg(OH) ⁺, Mg(citrate) ⁻ and Mg(EDTA) ²⁻, in addition to Mg(H ₂O) ₆ ²⁺ commonly referred to as Mg ²⁺. The formation of these species can significantly alter the fractionation of Mg isotopes between minerals and natural fluids. To quantify these effects, isotope exchange experiments were performed at bulk chemical equilibrium between brucite and aqueous solutions containing different organic (citrate, EDTA) and inorganic (SO ₄ ⁻) ligands at 25 °C. The ‘three isotope’ method was used to determine the equilibrium Mg isotope fractionation factors Δ _eq ²⁶Mg between brucite and several major aqueous magnesium species. The experimentally measured equilibrium Mg isotope fractionation factor between brucite and aqueous Mg ²⁺ was found to be Δ _eq ²⁶Mg _{brucite-Mg ²⁺} = -0.35 ± 0.39‰. First-principle calculations to retvieve the brucite β-factor were performed consistently with the calculations of Pinilla et al. (2015) for Mg ²⁺(aq) β-factor. The combination of both studies yield values of Δ _eq ²⁶Mg _{brucite-Mg ²⁺} between +0.3 and +0.8 ± 1.0‰, which is the lowest theoretical estimate of this constant obtained to date. An average value Δ _eq ²⁶Mg _{brucite-MgSO 4 ⁰} = 0.48 ± 0.16‰ was retrieved from the experiments for the isotope fractionation between brucite and aqueous MgSO ₄ ⁰. Mg isotope equilibrium fractionation factors between brucite and aqueous Mg(citrate) ⁻ and between brucite and aqueous Mg(EDTA) ²⁻ retrieved from the experiments performed in the presence of these organic ligands are Δ _eq ²⁶Mg _{brucite-Mg(citrate) ⁻} = 0.35 ± 0.21, and Δ _eq ²⁶Mg _{brucite-Mg(EDTA) ²⁻} = 2.41 ± 0.20‰. The experimental values determined in this study for Δ _eq ²⁶Mg _{brucite-Mg ²⁺} agree with the experimental values reported by Li et al. (2014). There is also an excellent agreement between the experimental values of this study and Li et al. (2014) with the density functional theory (DFT) estimates from Schott et al. (2016) for Δ _eq ²⁶Mg _{Mg ²⁺-Mg(EDTA) ²⁻} . In contrast, the Mg isotope fractionation factor measured in this study between aqueous Mg ²⁺ and both aqueous Mg sulphate or citrate species is significantly smaller than predictions from the ab initio calculations reported by Schott et al. (2016). The results of the present study confirm that the mineral-fluid equilibrium fractionation of Mg isotopes is strongly dependent on the identity of the inorganic or organic ligands present in the aqueous fluid and the nature of the complexes, (e.g. inner-sphere versus outer-sphere complexes), formed by magnesium with these ligands. Therefore, Mg speciation in natural fluids and the structure of aqueous Mg complexes have to be known for an accurate interpretation of Mg isotopic signatures in natural environments.