Influence of water on plastic deformation of olivine aggregates 2. Dislocation creep regime

Triaxial compressive creep experiments have been conducted over a range of hydrous conditions to investigate the effect of water fugacity on the creep behavior of olivine aggregates in the dislocation creep regime. Samples synthesized from powders of San Carlos olivine were deformed at confining pressures of 100 to 450 MPa and temperatures between 1473 and 1573 K. Water was supplied by the dehydration of talc. Water fugacities of similar to 80 to similar to 520 MPa were obtained by varying the confining pressure under water-saturated conditions with the oxygen fugacity buffered at Ni/NiO. Samples were deformed at differential stresses of similar to 20 to 230 MPa. The transition from diffusion creep to dislocation creep occurs near 100 MPa for both the hydrous case and the anhydrous case. Under hydrous conditions creep experiments yield a stress exponent of n approximate to 3 and an activation energy of Q approximate to 470 kJ/mol. The creep rate of olivine is enhanced significantly with the presence of water. At a water fugacity of similar to 300 MPa, samples crept similar to 5-6 times faster than those deformed under anhydrous conditions at similar differential stresses and temperatures. Within the range of water fugacity investigated, the strain rate is proportional to water fugacity to the 0.69 to 1.25 power, assuming values for the activation volume of 0 to 38 x 10(-6) m(3)/mol, respectively We argue that water influences creep rate primarily through its effect on the concentrations of intrinsic point defects and hence on ionic diffusion and dislocation climb. With increasing water fugacity the charge neutrality condition changes from [Fe-Me(.)] = 2[V-Me"] to [Fe-Me(.)] = [H-Me']. For the latter charge neutrality condition the concentration of silicon interstitials is proportional to f(H2O)(1) suggesting that under hydrous conditions dislocation climb is rate limited by diffusion of Si occurring by an interstitial mechanism Our experimentally determined constitutive equation permits extrapolation from laboratory to mantle conditions in order to assess the rheological behavior of regions of the upper mantle with different water contents, such as beneath a mid-ocean ridge and in the mantle wedge above a subducting slab.