Isotope disequilibrium caused by the influx of fluids during crustal anatexis

Isotope disequilibrium is increasingly recognized during crustal anatexis, which is usually attributed to stoi-chiometry or peritectic phases of melting reactions. Instead, whether fluid influx can contribute to isotope disequilibrium during fluid-fluxed melting remains poorly constrained. Here we report combined Mg-O-Sr iso-topic data for migmatites in the Dabie orogen. Leucosomes show considerably lower & delta;26Mg (-0.71%0 - -0.23%0) and & delta;18O (3.7%0 -7.9%0) but higher 87Sr/86Sr(i) (0.7089-0.7131) than the corresponding melanosomes (-0.29%0 - -0.11%0, 6.9%0 -10.6%0 and 0.7080-0.7119, respectively). Such isotopic difference between leu-cosomes and melanosomes cannot result from either fractional crystallization, equilibrium, incongruent to non-modal melting or accumulation. Instead, co-variations of & delta;18O, & delta;26Mg and 87Sr/86Sr(i) of leucosomes indicate influx of external fluids and their preferential partition into melts. The external fluids could have low & delta;18O and & delta;26Mg 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 trend by its rather low & delta;26Mg (- -0.71%0), and may have been overprinted by inter-diffusion. Simple diffusion modeling shows that the preservation of Mg-O-Sr isotope disequilibrium in the Dabie migma-tites requires a short duration from fluid-fluxed anatexis to re-solidification (i.e., < 103-104 years). This study highlights that a combination of multiple isotopes may be conducive to understand the mechanism of disequi-librium melting.