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 (Δ56Feplagioclase–magnetite=+0.376‰ to +1.084‰, Δ56Fealkali-feldspar–magnetite=+0.516‰ to +0.846‰), which can be attributed to its high Fe3+/Fetot ratio and low coordination number (tetrahedrally-coordinated) of Fe3+. Δ56Femagnetite–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 Δ56Feplagioclase–magnetite and Δ56Fealkali-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 Fe3+/∑Fe or different crystal parameters. The Δ56Femagnetite–biotite increases with decreasing Fe3+/∑Febiotite and increasing mole (Na+K)/Mgbiotite, indicating a decrease of β factor in low Fe3+/∑Fe and high (Na+K)/Mg biotite.High-silica leucosomes from Dabie migmatites with a feldspar accumulation petrogenesis have higher δ56Fe values (δ56Fe=0.42–0.567‰) than leucosome that represents pristine partial melt (δ56Fe=0.117±0.016‰), indicating that accumulation of feldspar could account for high δ56Fe values of these rocks. High δ56Fe 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 δ56Fe values of most high-silica granitoids reported in the literature, based on their low Sr, Ba contents and negative Eu anomalies.