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.