Contrasting dilatant behaviors of mafic and ultramafic rocks based on triaxial deformation experiments

We performed triaxial deformation experiments on cylindrical specimens of mafic and ultramafic rocks to quantify their dilatant behaviors. Experiments were performed using an intra-vessel deformation and fluid flow apparatus at room temperature, a constant strain rate of similar to 10(-6 )s(-1), and a confining pressure of 20 MPa. Axial and radial strains were monitored using strain gauges glued to the specimens. The onset of dilatancy of mafic rocks ranged from 41 to 64% of the maximum differential stress, and the inelastic volumetric strain reached similar to 7 x 10(-3) at the maximum differential stress. These results are generally typical of crystalline rocks. Microstructural observation of recovered samples after deformation indicates that the dilatancy of mafic rocks were caused by opening of axial microcracks. In contrast, the onset of dilatancy of ultramafic rocks occurred at >77% of the maximum differential stress, and the inelastic volumetric strain at the maximum differential stress was less than 2.7 x 10(-3). These contrasting behaviors of dilatancy could be related to different crack modes developed during deformation, where shear microcracks along grain boundary were dominated in ultramafic rocks as is evident from 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 to different modification of the physical and transport properties during brittle deformation.