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

Abstract

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 δ56Fe values from 0.018±0.031‰ to 0.152±0.027‰ with an average of 0.106‰. The leucosomes have δ56Fe values from 0.107±0.035‰ to 0.512±0.028‰, variably higher than their coexisting melanosomes by 0.00~0.36‰. The δ56Fe of leucosomes and melanosomes, as well as apparent isotope fractionation (Δ56FeL-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 δ56Fe values of three leucosomes (≥0.34‰), giving large Δ56FeL-M from 0.22 to 0.36‰, were likely produced by feldspar accumulation, evidenced by their high Eu*, low FeOt/Al2O3 and high plagioclase abundance up to 50vol%. Capture of peritectic amphiboles in another leucosome may explain its low Δ56FeL-M ~0‰. After screening out these samples, 7 leucosome – melanosome pairs yield identical Δ56FeL-M averaging 0.093±0.056‰ (2SD, N=7) within analytical uncertainties. No correlation between Δ56FeL-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 Δ56FeL-M should record equilibrium Fe isotope fractionation during crustal anatexis producing these migmatites. The leucosomes and melanosomes have comparable Fe3+/ΣFe~0.40, and Δ56FeL-M approximates 0.07‰ at Fe3+/ΣFeleucosomeFe3+/ΣFemenalosome=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 δ56Fe variation in high silica granitic rocks.