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 δ 56 Fe values from 0.018 ± 0.031‰ to 0.152 ± 0.027‰ with an average of 0.106‰. The leucosomes have δ 56 Fe values from 0.107 ± 0.035‰ to 0.512 ± 0.028‰, variably higher than their coexisting melanosomes by 0.00 ~ 0.36‰. The δ 56 Fe of leucosomes and melanosomes, as well as apparent isotope fractionation (Δ 56 Fe 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 δ 56 Fe values of three leucosomes (≥ 0.34‰), giving large Δ 56 Fe L-M from 0.22 to 0.36‰, were likely produced by feldspar accumulation, evidenced by their high Eu*, low FeOt/Al 2 O 3 and high plagioclase abundance up to 50 vol%. Capture of peritectic amphiboles in another leucosome may explain its low Δ 56 Fe L-M ~ 0‰. After screening out these samples, 7 leucosome – melanosome pairs yield identical Δ 56 Fe L-M averaging 0.093 ± 0.056‰ (2SD, N = 7) within analytical uncertainties. No correlation between Δ 56 Fe L-M and Mg#, FeOt/Al 2 O 3 or TiO 2 /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 Δ 56 Fe L-M should record equilibrium Fe isotope fractionation during crustal anatexis producing these migmatites. The leucosomes and melanosomes have comparable Fe 3 + /ΣFe ~ 0.40, and Δ 56 Fe L-M approximates 0.07‰ at Fe 3 + / ΣFe leucosome Fe 3 + / ΣFe menalosome = 1. Fe isotope fractionation during crustal anatexis revealed here thus is not dominantly controlled by distribution of Fe 3 + and Fe 2 + , 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 δ 56 Fe variation in high silica granitic rocks. [ABSTRACT FROM AUTHOR]