Melt Impregnation of Mantle Peridotite Facilitates High‐Temperature Hydration and Mechanical Weakening: Implications for Oceanic Detachment Faults

Abstract

AbstractThe footwalls of oceanic detachment faults commonly expose shear zone rocks that appear to have compositions intermediate between those of mantle peridotite and magmatic rocks. These compositions either reflect metasomatic mass transfers or they relate to the impregnation of lithospheric mantle with basaltic or more evolved melts. We studied chlorite‐amphibole‐rich shear zone rocks from a detachment fault zone in the 15°20′N Fracture Zone area, Mid‐Atlantic Ridge, to examine their origin and role in strain localization. Geochemical compositions of these rocks imply that they formed by mixing between peridotite and gabbro. Textural observations indicate a strong contrast between the deformation intensity of these hybrid peridotite‐gabbro rocks and the host serpentinized peridotite. Geothermometry data give formation temperatures of >500 °C for synkinematic amphibole, zircon, rutile, and titanite. Chlorite appears intergrown with these phases and likely grew at similar temperatures. These results are compliant to thermodynamic computations that predict comparable mechanically weak mineralogies when hydrating hybrid rocks at 500 to 600 °C, whereas secondary assemblages after pure peridotite or gabbro are considerably stronger. Consequently, metamorphic weakening takes place to a much greater extent in rocks with a hybrid ultramafic–mafic composition than in purely ultramafic or gabbroic lithologies. Deformation may enhance fluid flow, which will in turn increase the extent of hydration and mechanical weakening. A positive feedback loop between hydration and strain localization may hence develop and facilitate the concentration of extensional tectonics into long‐lived, high‐displacement faults. We suggest that hybrid lithologies may play a key role in detachment faulting at slow spreading ridges worldwide.