Mineral composition control on inter-mineral iron isotopic fractionation in granitoids

Abstract This study reports elemental and iron isotopic compositions of feldspar and its coexisting minerals from four Dabie I-type granitoids to evaluate the factors that control inter-mineral Fe isotopic fractionation in granitoids. The order of heavy iron isotope enrichment is feldspar > pyrite > magnetite > biotite ≈ hornblende. Feldspar has heavier iron isotopic compositions than its co-existing magnetite (Δ 56 Fe plagioclase–magnetite  = +0.376‰ to +1.084‰, Δ 56 Fe alkali-feldspar–magnetite  = +0.516‰ to +0.846‰), which can be attributed to its high Fe 3+ /Fe tot ratio and low coordination number (tetrahedrally-coordinated) of Fe 3+ . Δ 56 Fe magnetite–biotite of coexisting magnetite and biotite ranges from 0.090‰ to 0.246‰. Based on homogeneous major and iron isotopic compositions of mineral replicates, the inter-mineral fractionation in this study should reflect equilibrium fractionation. The large variations of inter-mineral fractionation among feldspar, magnetite and biotite cannot be simply explained by temperature variation, but strongly depend on mineral compositions. The Δ 56 Fe plagioclase–magnetite and Δ 56 Fe alkali-feldspar–magnetite are positively correlated with albite mode in plagioclase and orthoclase mode in alkali-feldspar, respectively. This could be explained by different Fe–O bond strength in feldspar due to different Fe 3+ /∑Fe or different crystal parameters. The Δ 56 Fe magnetite–biotite increases with decreasing Fe 3+ /∑Fe biotite and increasing mole (Na + K)/Mg biotite , indicating a decrease of β factor in low Fe 3+ /∑Fe and high (Na + K)/Mg biotite. High-silica leucosomes from Dabie migmatites with a feldspar accumulation petrogenesis have higher δ 56 Fe values (δ 56 Fe = 0.42–0.567‰) than leucosome that represents pristine partial melt (δ 56 Fe = 0.117 ± 0.016‰), indicating that accumulation of feldspar could account for high δ 56 Fe values of these rocks. High δ 56 Fe values are also predicted for other igneous rocks that are mainly composed of cumulate feldspar crystals, e.g., anorthosites. Feldspar accumulation, however, cannot explain high δ 56 Fe values of most high-silica granitoids reported in the literature, based on their low Sr, Ba contents and negative Eu anomalies.