Oxygen and hydrogen isotope study of high-pressure metagabbros and metabasalts (Cyclades, Greece): implications for the subduction of oceanic crust

Oxygen and hydrogen stable isotope ratios of eclogite-facies metagabbros and metabasalts from the Cycladic archipelago (Greece) document the scale and timing of fluid-rock interaction in subducted oceanic crust. Close similarities are found between the isotopic compositions of the high-pressure rocks and their ocean-floor equivalents. High-pressure minerals in metagabbros have low delta(18)O values: garnet 2.6 to 5.9 parts per thousand, glaucophane 4.3 to 7.1 parts per thousand; omphacite 3.5 to 6.2 parts per thousand. Precursor actinolite that was formed during the hydrothermal alteration of the oceanic crust by seawater analyses at 3.7 to 6.3 parts per thousand. These compositions are in the range of the delta(18)O values of unaltered igneous oceanic crust and high-temperature hydrothermally altered oceanic crust. In contrast, high-pressure metabasalts are characterised by O-18-enriched isotopic compositions (garnet 9.2 to 11.5 parts per thousand, glaucophane 10.6 to 12.5 parts per thousand, omphacite 10.2 to 12.8 parts per thousand), which are consistent with the precursor basalts having undergone low-temperature alteration by seawater. D/H ratios of glaucophane and actinolite are also consistent with alteration by seawater. Remarkably constant oxygen isotope fractionations, compatible with isotopic equilibrium, are observed among high-pressure minerals, with Delta(glaucophane-garnet) = 1.37 +/- 0.24 parts per thousand and Delta(omphacite-garnet) = 0.72 +/- 0.24 parts per thousand. For the estimated metamorphic temperature of 500 degrees C, these fractionations yield coefficients in the equation Delta = A * 10(6)/T-2 (in Kelvin) of A(glaucophane-garnet) = 0.87 +/- 0.15 and A(omphacite-garnet) = 0.72 +/- 0.24. A fractionation of Delta(glaucophane-actinolite) = 0.94 +/- 0.21 parts per thousand is measured in metagabbros, and indicates that isotopic equilibrium was established during the metamorphic reaction in which glaucophane formed at the expense of actinolite. The preservation of the isotopic compositions of gabbroic and basaltic oceanic crust and the equilibrium fractionations among minerals shows that high-pressure metamorphism occurred at low water/rock ratios. The isotopic equilibrium is only observed at hand-specimen scale, at an outcrop scale isotopic compositional differences occur among adjacent rocks. This heterogeneity reflects metre-scale compositional variations that developed during hydrothermal alteration by seawater and were subsequently inherited by the high-pressure metamorphic rocks.