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 sy nthesized from powders of San Carlos olivine were deformed at confining pre ssures of 100 to 450 MPa and temperatures between 1473 and 1573 K. Water wa s 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 w ater-saturated conditions with the oxygen fugacity buffered at Ni/NiO. Samp les were deformed at differential stresses of similar to 20 to 230 MPa. The transition from diffusion creep to dislocation creep occurs near 100 MPa f or both the hydrous case and the anhydrous case. Under hydrous conditions c reep experiments yield a stress exponent of n approximate to 3 and an activ ation energy of Q approximate to 470 kJ/mol. The creep rate of olivine is e nhanced significantly with the presence of water. At a water fugacity of si milar to 300 MPa, samples crept similar to 5-6 times faster than those defo rmed under anhydrous conditions at similar differential stresses and temper atures. 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 arg ue that water influences creep rate primarily through its effect on the con centrations of intrinsic point defects and hence on ionic diffusion and dis location climb. With increasing water fugacity the charge neutrality condit ion changes from [Fe-Me(.)] = 2[V-Me"] to [Fe-Me(.)] = [H-Me']. For the lat ter charge neutrality condition the concentration of silicon interstitials is proportional to f(H2O)(1) suggesting that under hydrous conditions dislo cation climb is rate limited by diffusion of Si occurring by an interstitia l mechanism Our experimentally determined constitutive equation permits ext rapolation from laboratory to mantle conditions in order to assess the rheo logical behavior of regions of the upper mantle with different water conten ts, such as beneath a mid-ocean ridge and in the mantle wedge above a subdu cting slab.