Contrasting dilatant behavior between gabbro and peridotite during the triaxial deformation experiments

Abstract:

We performed triaxial deformation experiments on cylindrical specimens of gabbro and peridotite with simultaneous measurements of ultrasonic velocity. Experiments were performed using an intra-vessel deformation and fluid flow apparatus at room temperature, a constant strain rate of ~10^-6 s^-1, and a confining pressure of 20 MPa. The onset of dilatancy of gabbro is ~40% of the ultimate strength, similar to granite, whereas dilatancy in peridotite occurred close to failure. These contrasting behaviors were also identified by change in ultrasonic velocity during deformation, where velocity is markedly decrease for gabbro in contrast to a nearly constant velocity for peridotite. Microstructural observation of recovered sample after deformation indicates that the dilatancy of gabbro were caused by opening of axial microcracks. This contrasting behaviors of dilatancy could be related to different crack modes developed during deformation, where shear microcracks along grain boundary were dominated in peridotite as evident by a nearly random crack orientation along grain boundary. As dilatancy is related to the opening of microcracks, these contrasting dilatant behaviors of mafic and ultramafic rocks can lead a marked contrast in physical and transport properties across the crust-mantle boundary.