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.