DISLOCATION RECOVERY IN OLIVINE UNDER DEEP UPPER-MANTLE CONDITIONS - IMPLICATIONS FOR CREEP AND DIFFUSION

The kinetics of dislocation recovery in olivine have been studied up to 10 GPa and 1500-degrees-C using a multianvil apparatus. Dislocations in the (010)[100] slip system were generated by high-temperature deformation (at room pressure), and then the specimens were statically annealed at high pressures and temperatures. The rate of dislocation recovery was measured as a function of pressure at controlled oxygen fugacity and oxide activity under anhydrous conditions. Microstructural observations indicate that dislocation recovery occurs through the climb of edge dislocations and hence is controlled by diffusion. A small activation volume (V* = 6 +/- 1 cm3/mol) was obtained, which is interpreted to be due to an interstitial mechanism of diffusion for the relevant transport-limiting species (oxygen or silicon). Combined with the previous observation of a small activation volume for Mg-Fe diffusion, we conclude that ionic diffusion in the olivine lattice in general has small activation volumes and therefore will not vary significantly with depth in the deep portions of the upper mantle. We have estimated the relative importance of dislocation and diffusion creep in the upper mantle using the present result and the published results on creep parameters (such as stress exponents, grain-size exponents and activation volume for dislocation creep). The results suggest that the dominant creep mechanism may change from dislocation creep in the shallow upper mantle to diffusion creep in the deep upper mantle.