The effect of bonding environment on iron isotope fractionation between minerals at high temperature

Central to understanding the processes that drive stable isotope fractionation in nature is their quantification under controlled experimental conditions. The polyvalent element iron, given its abundance in terrestrial rocks, exerts controls on the structural and chemical properties of minerals and melts. The iron isotope compositions of typical high temperature minerals are, however, poorly constrained and their dependence on intensive (e.g. fO₂) and extensive (e.g. compositional) variables is unknown. In this work, experiments involving a reference phase, 2 M FeCl₂·4H₂O_(l), together with an oxide mix corresponding to the bulk composition of the chosen mineral were performed in a piston cylinder in Ag capsules. The oxide mix crystallised in situ at 1073 K and 1 GPa, in equilibrium with the iron chloride, and was held for 72 h. In order to characterise the effect of co-ordination and oxidation state on the isotope composition independently, exclusively Fe²⁺ minerals were substituted in: VIII-fold almandine, VI-fold ilmenite, fayalite and IV-fold chromite and hercynite. Δ⁵⁷Fe_Min–FeCl2 increases in the order VIII < VI < IV, consistent with a decrease in the mean Fe–O bond length. Magnetite, which has mixed VI- and IV-fold co-ordination, has the heaviest Δ⁵⁷Fe by virtue of 2/3 of its iron being the smaller, ferric ion. The composition of the ^VIFe²⁺-bearing minerals is similar to that of the aqueous FeCl₂ fluid. To the degree that this represents the speciation of iron in fluids exsolving from magmas, the fractionation between them should be small, unless the iron is hosted in magnetite. By contrast, predominantly Fe²⁺-bearing mantle garnets should preserve a much lighter δ⁵⁷Fe than their lower pressure spinel counterparts, a signature that may be reflected in partial melts from these lithologies. As the Fe–O bond lengths in fayalite and ilmenite are comparable, their isotope compositions overlap, suggesting that high Ti mare basalts acquired their heavy isotopic signature from ilmenite that crystallised late during lunar magma ocean solidification.