The gabbro–eclogite transformation: an oxygen isotope and petrographic study of west Alpine ophiolites

Both magmatic and eclogitic parageneses are preserved in the gabbros of western Alpine ophiolites. Samples with relic magmatic mineralogies display partial transformation to eclogitic assemblages along cracks and grain boundaries. Gabbros with eclogitic mineralogies contain zoned pseudomorphs after olivine, comprising talc-rich cores with kyanite, Mg-chloritoid and omphacite in outer cores and garnet rims. The compositional zonation of these olivine pseudomorphs closely parallels that shown by olivines in hydrothermally altered ocean-floor gabbros. The eclogitic gabbros are hydrous, containing paragonite, zoisite and other water-bearing minerals, and it has been suggested that water was introduced during high-pressure metamorphism. However, the similarity of olivine alteration patterns to those of ocean-floor gabbros suggests that hydration and local metasomatism leading to the stability of aluminous minerals in olivine sites occurred during hydrothermal alteration prior to subduction. Oxygen-isotope systematics are consistent with this proposal: Alpine gabbros with magmatic relics have a mean δ¹⁸O value of 5.7±0.7, similar to that of unaltered oceanic crust, whereas eclogitic gabbros have a mean δ¹⁸O value of 4.8±0.9.This statistically significant difference is consistent with the eclogitic samples having undergone high-temperature ocean-floor alteration. The preservation of magmatic and hydrothermal δ¹⁸O values in ocean-floor gabbros that have been metamorphosed at 2–2.5 GPa (60–75 km) implies that the deeper levels of ocean crust have not experienced pervasive fluid flow during subduction or subsequent exhumation. Magmatic assemblages were preserved despite an overstep of eclogitization reactions by at least 0.6–1.1 GPa implying that equilibrium was not attained in undeformed parts of the system because of slow diffusion in water-deficient rock volumes.