Rheological properties of partially molten lherzolite

Lherzolite samples synthesized from fine-grained powders prepared from a natural xenolith were deformed at P = 300 MPa and 1373 less than or equal to T less than or equal to 1573 K in a high-resolution gas-medium deformation apparatus. Below and above the solidus of T-s approximate to 1433 K, fine-grained therzolite exhibits a transition from diffusional creep at low stress to dislocation creep at high stress. The transition occurs at a differential stress sigma approximate to 100 MPa in samples with a grain size d approximate to 20 mum and a melt fraction 0.03. Extrapolation to upper-mantle conditions suggests that a similar transition will occur in the mantle for d = 2mm at sigma approximate to 0.5 MPa. Therefore, both creep mechanisms are likely to contribute to deformation of partially molten regions of the mantle. We determined an empirical relationship for the dependence of strain rate (epsilon) over dot on phi using data for melt-free olivine, analysis of the subsolidus and hypersolidus creep behavior of lherzolite and constraints on phi(T) from our experimentally determined melting curve. We find that (epsilon) over dot (phi) increases approximately exponentially with increasing phi in both the diffusional and dislocation creep regimes. The observed enhancement in (epsilon) over dot (phi) as a result Of melt is much larger than anticipated based on deformation models that consider only an isotropic distribution of melt in triple junctions and do not incorporate the presence of melt along some grain boundaries and the melt preferred orientation that develops during deformation. The results for lherzolite are similar to results reported previously for olivine plus basaltic melt samples, indicating a minor effect of pyroxene content for deformation of partially molten peridotites. Our, constitutive equation provides a basis for modeling geodynamic processes in the partially molten mantle beneath mid-ocean ridges and in the mantle wedge above subducting slabs.