Isotope disequilibrium caused by the influx of fluids during crustal anatexis

Isotope disequilibrium during crustal anatexis is increasingly recognized in the magmatic systems, which is usually attributed to stoichiometry or peritectic phases of melting reactions. However, whether fluid influx can contribute to isotope disequilibrium during fluid-fluxed melting remains poorly constrained. Here we report combined Mg-O-Sr isotopic data for migmatites from Dabie orogen. Except for one sample that has experienced post-migmatization metasomatism, leucosomes show considerably lower δ²⁶Mg (-0.71‰ ~ -0.23‰) and δ¹⁸O (3.7‰ ~ 8.7‰) but higher ⁸⁷Sr/⁸⁶Sr(i) (0.7089 ~ 0.7131) than the corresponding melanosomes (-0.29‰ ~ -0.11‰, 6.9‰ ~ 10.6‰ and 0.7080 ~ 0.7119, respectively). Such isotopic difference between leucosomes and melanosomes cannot result from fractionation during either fractional crystallization, accumulation or equilibrium, incongruent to non-modal melting, but most likely records isotope disequilibrium during crustal anatexis. Co-variations of δ¹⁸O, δ²⁶Mg and ⁸⁷Sr/⁸⁶Sr(i) of leucosomes indicate influx of external fluids and their preferential partition into melts. The external fluids could have low δ¹⁸O and δ²⁶Mg but high ⁸⁷Sr/⁸⁶Sr(i) and were most likely derived from dehydration of the surrounding eclogites and gneisses with Neoproterozoic protoliths experienced meteoric-hydrothermal exchange. One leucosome is plotted off the major trends by its rather low δ²⁶Mg (~ -0.71‰), and may have been overprinted by Fe-Mg inter-diffusion. Simple diffusion calculations suggested that the timescale to reach isotopic equilibration during fluid-fluxed melting is larger by one order of magnitude than that of dehydration melting. Hence, fluid-fluxed melting may generate melts out of isotope equilibrium with restite, and cautions should be taken in interpretation of isotopic data of the resulting granitoids.