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

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.