An O-isotope profile through the HP-LT Corsican ophiolite, France and its implications for fluid flow during subduction

Subduction zones are important sites for fluid generation as the downgoing slab undergoes progressive high-pressure metamorphism. These fluids are tho ught to play an important role in the generation of arc-magmas, but their a ffect on the subducted slab itself is less clear. In this study, the stable isotope geochemistry of the Corsican ophiolite, which is variably affected by subduction-related metamorphism, was investigated to assess the impact of subduction zone fluid flow on the downgoing slab. A total of 170 stable isotope analyses (oxygen and carbon) showed the following features: (1) ove rall, the ophiolite preserves the same oxygen isotope profile with depth as unmetamorphosed ophiolites; (2) the variation in oxygen isotope values for different areas does not correlate with metamorphic grade; (3) oxygen and carbon isotope values for calcite in the ophiolite are not in equilibrium w ith the ophiolite but are similar to those of the overlying calc-schists (S chistes Lustres); and (4) hydrogen isotopes indicate that serpentinitisatio n of ultramafics occurred during sub-seafloor hydrothermal alteration and p robably also during tectonic emplacement of the ophiolite. These features i ndicate that large-scale fluid flow did not affect the Corsican ophiolite d uring subduction zone metamorphism. Calcite in the ophiolite was probably p recipitated from CO2-bearing fluids derived from decompression dehydration reactions affecting the overlying Schistes Lustres during exhumation of the ophiolite-bearing Schistes Lustres nappes. Given that ocean crust must und ergo at least some dehydration during prograde subduction zone metamorphism , the evolved fluids must be either highly channelled or exit the slab rela tively quickly so as to not interact with it. The results of this study ind icate that the important fluid pathways on Corsica have yet to be identifie d, but are probably the shear zones and fault systems that bound different slices of the high-pressure nappe stack. (C) 2001 Elsevier Science B.V. All rights reserved.