Iron isotope fractionation during crustal anatexis: Constraints from migmatites from the Dabie orogen, Central China

Iron (Fe) isotope fractionation could occur during crustal melting, which may contribute to the isotopic variations of granites. Our current knowledge on Fe isotope fractionation during crustal melting remains rare. Thirteen leucosomes, 9 melanosomes and 1 amphibolitic schollen in 11 migmatites from the Dabie orogen, Central China were measured to investigate Fe isotope fractionation during crustal anatexis. The melanosomes and amphibolitic schollen yield delta Fe-56 values from 0.018 +/- 0.031 parts per thousand to 0.152 +/- 0.027 parts per thousand with an average of 0.106 parts per thousand. The leucosomes have delta Fe-56 values from 0.107 +/- 0.035 parts per thousand to 0.512 +/- 0.028 parts per thousand, variably higher than their coexisting melanosomes by 0.00 similar to 036 parts per thousand. The delta Fe-56 of leucosomes and melanosomes, as well as apparent isotope fractionation (Delta Fe-56(L-M)) between them, do not correlate with the abundance of crystalline carbonate, loss on ignition, and Th/U, indicating the insignificant effect of fluid components. High delta Fe-56 values of three leucosomes (>= 0.34 parts per thousand), giving large Delta Fe-56(L-M) from 0.22 to 0.36 parts per thousand, were likely produced by feldspar accumulation, evidenced by their high Eu*, low FeOt/Al2O3 and high plagioclase abundance up to 50 vol%. Capture of peritectic amphiboles in another leucosome may explain its low Delta Fe-56(L-M) similar to 0 parts per thousand. After screening out these samples, 7 leucosome - melanosome pairs yield identical Delta Fe-56(L-M) averaging 0.093 +/- 0.056 parts per thousand (2SD, N = 7) within analytical uncertainties. No correlation between Delta Fe-56(L-M) and Mg#, FeOt/Al2O3 or TiO2/FeOt rules out the possibility that the fractionation observed here is produced by fractional crystallization and/or sub -solidus isotope re equilibrium between leucosomes and melanosomes. Therefore, we suggest these consistent Delta Fe-56(L-M) should record equilibrium Fe isotope fractionation during crustal anatexis producing these migmatites. The leucosomes and melanosomes have comparable Fe3+/Sigma Fe similar to 0.40, and Delta Fe-56(L-M) approximates 0.07 parts per thousand at (Fe3+/Sigma Fe)(leucosome)/(Fe3+/Sigma Fe)(menalosome) = 1. Fe isotope fractionation during crustal anatexis revealed here thus is not dominantly controlled by distribution of Fe3+ and Fe2+, but by the difference in coordination number of iron between granitic melts and residual mafic minerals. The observation here argues that Fe isotope fractionation during crustal partial melting should also contribute to the previously revealed delta Fe-56 variation in high silica granitic rocks. (C) 2017 Elsevier B.V. All rights reserved.