EXPERIMENTAL CONSTRAINTS ON THE DYNAMICS OF THE PARTIALLY MOLTEN UPPER-MANTLE .2. DEFORMATION IN THE DISLOCATION CREEP REGIME

Experiments have been conducted to investigate the effect of melt on t he creep behavior of water-free olivine aggregates deformed in the dis location creep regime. The influence of the melt phase is modest at me lt fractions less than similar to 0.04. However, at melt fractions > 0 .04, the creep rate of melt-added samples is enhanced by more than an order of magnitude relative to melt-free aggregates. This unexpectedly large influence of melt on strain rate arises because deformation occ urs by grain boundary sliding (GBS) accommodated by a dislocation cree p process. Four observations support this hypothesis. (1) The strain r ate enhancement observed in the dislocation creep regime can be relate d to the Stress concentration caused by the reduction in the solid-sol id grain boundary area. (2) Both melt-free and melt-added samples exhi bit strain rates indicating that deformation is limited by slip on (01 0)[100], the easiest slip system in olivine. (3) The GBS mechanism occ urs near the transition between diffusion and dislocation creep. (4) G rains in specimens deformed in the GBS regime are not significantly fl attened, even after similar to 50% shortening. in melt-free aggregates , a transition from the GBS mechanism to dislocation creep limited by slip on (010)[001], the hardest slip system, is observed with an incre ase in grain size. A transition to (010)[001] limited creep was not ob served for partially molten aggregates because grain growth was inhibi ted by the presence of melt. The results of this study indicate that t he viscosity of the upper mantle may decrease by at least an order of magnitude if the retained melt fraction exceeds 0.04 or if the onset o f melting results in a reduction in grain size and a concomitant trans ition from (010)[001] to (010)[100] limited creep.