To investigate the influence of hydration on brittle deformation of oceanic crustal rocks, we conducted triaxial deformation experiments on gabbroic rocks with various degrees of hydration. Additional experiments were performed on samples of serpentinite and serpentinized peridotite for comparison. Elastic wave velocities were measured during deformation to monitor the development of stress-induced cracks.
Hydrated olivine gabbros reached a maximum differential stress of 225–350 MPa, which was considerably less than that recorded for gabbros (~450 MPa), but comparable to those for serpentinized ultramafic rocks (250–300 MPa). Elastic wave velocities of hydrated olivine gabbros were almost constant during deformation and did not show a marked decrease, even immediately prior to failure. This indicated that the deformation of hydrated olivine gabbro is not associated with the opening of the stress-induced axial cracks that are responsible for dilatancy and are commonly observed during deformation of crystalline rocks. Microstructural observations of the samples recovered after deformation showed crack damage to be highly localized to shear fracture zones with no trace of stress-induced crack opening, consistent with the absence of dilatancy. These data suggest that brittle deformation of hydrated olivine gabbro can be accommodated by the development of shear cracks in hydration minerals such as serpentine and chlorite, even when they are present in only small amounts. This leads to non-dilatant brittle deformation and a weakening of fracture strength, similar to that observed during deformation of serpentinized peridotite. Our results suggest that the brittle behavior of the oceanic crust may change considerably due to hydration.