Stable isotope geochemistry of Alpine ophiolites: a window to ocean-floor hydrothermal alteration and constraints on fluid-rock interaction during high-pressure metamorphism

The subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle; however, despite its potential imp ortance, fluid-rock interaction during high-pressure metamorphism is relati vely poorly understood. The stable isotope and major element geochemistry o f Pennine ophiolite rocks from Italy and Switzerland that were metamorphose d at high pressures are similar to that of unmetamorphosed ophiolites, sugg esting that they interacted with little pervasive fluid during high-pressur e metamorphism. Cover sediments also have oxygen isotope ratios within the expected range of their protoliths. In the rocks that escaped late greensch ist-facies retrogression, different styles of sub-ocean-floor alteration ma y be identified using oxygen isotopes, petrology, and major or trace elemen t geochemistry. Within the basalts, zones that have undergone high- and low -temperature sub-ocean-floor alteration as well as relatively unaltered roc ks can be distinguished. Serpentinites have delta(18)O and delta(2)H values that suggest that they were formed by hydration on or below the ocean floo r. The development of high-pressure metamorphic mineralogies in metagabbros occurred preferentially in zones that underwent sub-ocean-floor alteration and which contained hydrated, fine-grained, reactive assemblages. Given th at the transformation of blueschist-facies metabasic rocks to eclogite-faci es assemblages involves the breakdown of hydrous minerals (e.g. lawsonite, zoisite, and glaucophane), and will thus liberate considerable volumes of f luids, metamorphic fluid flow must have been strongly channelled. High-pres sure (quartz + calcite +/- omphacite +/- glaucophane +/- titanoclinohumite) veins that cut the ophiolite rocks represent one possible channel; however , stable isotope and major element data suggest that they were not formed f rom large volumes of exotic fluids. Fluids were more likely channelled alon g faults and shear zones that were active during high-pressure metamorphism . Such strong fluid channelling may cause fluids to migrate toward the accr etionary wedge, especially along the slab-mantle interface, which is probab ly a major sheer zone. This may preclude all but a small fraction of the fl uids entering the mantle wedge to flux melting. Additionally, because fluid s probably interact with relatively small volumes of rock in the channels, they cannot "scavenge'' elements from the subducting slab efficiently.