Fluid inclusion and mineral isotopic compositions (H [sbnd]C [sbnd]O) in eclogitic rocks as tracers of local fluid migration during high-pressure metamorphism

Abstract

Eclogite facies metagabbros from the Monviso ophiolitic complex (Italian Western Alps) provide a unique opportunity to trace fluid migration processes in a portion of the oceanic crust that has undergone subduction at a depth of > 40 km. We have determined the omphacites δ 18O and the abundance, δD and δ 13C of hydrous and carbonaceous compounds present in whole rocks which are believed to trace the residual phases of what was mobilized in the original rocks during subduction. Prograde dehydration reactions and eclogitization of hydrothermally altered oceanic metagabbros was accompanied by approximately 90% fluid loss. The remaining fluid was trapped as primary water-rich fluid inclusions in omphacite megacrysts that developed at the expense of the precursor magmatic pyroxene. Deformation of the eclogitic rocks resulted in continuous recycling of fluid between mylonites and omphacite veins without further fluid loss from the host ductile shear zone. Oxygen isotopes of omphacite (omp) and hydrogen isotopes of water in the fluid inclusions (FI), analyzed in low-strain rocks, mylonites and undeformed/deformed veins show marked variations, δ 18O omp and δD FI values ranging from +3.0 to +5.3‰ and from −31 to −93‰, respectively. Detailed isotopic analysis of several individual vein-wallrock pairs show that the scale of isotopic equilibration is of the order of one centimeter. Therefore, the eclogitic minerals and fluids filling the veins are concluded to be locally derived. Our results argue against recent models which suggest large-scale mass flushing of isotopically homogeneous fluids during subduction zone metamorphism. On average, the H 2O contents and δD FI value are within the upper mantle range. The carbon has been inherited from the metamorphic transformation of the original carbon present in the oceanic crust and has a mean δ 13C value of −24.2 ± 1.2‰. Two carbonaceous components can be recognized, condensed carbon which represents the major carbon species and carbonate daughter crystals present in fluid inclusions. δ 18O omp values are significantly lower than those reported for mantle minerals and are similar to pyroxene and whole-rock values from hydrothermally altered oceanic crust. It is suggested that the isotopic imprint of the Monviso eclogitic minerals and fluids represents the signature of mid-ocean ridge hydrothermal alteration and subduction zone eclogitization processes. The rocks studied could be the missing link between altered oceanic gabbros and eclogitic xenoliths. In addition, the range in δ 18O omp values recorded in the different microstructural domains covers most of the δ 18O values reported in type A, B and C eclogitic xenoliths, implying that caution must be exercised when using oxygen isotopes as an indicator of the origin of eclogitic xenoliths.