橄榄石集合体中与位错蠕变相伴随的粒间滑动

To investigate the role of grain boundary sliding during dislocation creep of dunite, a series of deformation experiments were carried out under anhydrous conditions on fine-grained samples synthesized from powdered San Carlos olivine. Triaxial compressive creep tests were conducted under a temperature of 1473 K and confining pressures of 200 and 400 MPa using a high-resolution gas-medium deformation apparatus. Each sample was deformed at several levels of differential stress between 100 and 250 MPa to yield strain rates of 10^-4 to 10^-6 S^-1. With increasing differential stress, a transition from diffusion creep to dislocation creep regime in which motion on the easy slip system is accommodated by grain boundary sliding (GBS) occurs at ~50 MPa for samples with a grain size of 15µm. This grain size sensitive creep regime is characterized by a stress exponent of 3.2 ± 0. 1, and a grain size exponent of 1. 8 ± 0. 2. Comparison of our flow law for dislocation creep accommodated by grain boundary sliding with those for diffusion creep and dislocation creep in which motion of dislocations on the hard slip system is accommodated by dislocation activity on the easy slip system reveals that the present flow law is important for flow of rocks with grain sizes of < lOOµm at differential stresses > 20 MPa. Hence, GBS-accommodated dislocation creep is likely to be the dominant deformation mechanism in deep-rooted, highly localized shear zones in the lithospheric upper mantle.