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

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, 2M 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 1073K and 1 GPa, in equilibrium with the iron chloride, and was held for 72 hours. 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 rutile and IV-fold chromite and hercynite. ?57FeMin-Chl 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 ?57Fe by virtue of 2/3 of its iron being the smaller, ferric ion, resulting in the shortest mean bond length. 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. As the Fe–O bond lengths in fayalite and ilmenite are comparable, their isotope compositions overlap, suggesting that, in lunar mare basalts, ilmenite alone cannot explain the 57Fe-enriched composition of the high-Ti relative to low-Ti basalts. By contrast, predominantly Fe2+-bearing mantle garnets should preserve a much lighter ?57Fe than their lower pressure spinel counterparts, a signature that may be reflected in partial melts from these depths.