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Lherzolite samples synthesized from fine-grained powders prepared from a natural xenolith were deformed at P=300 MPa and 1373 ≤ T ≤ 1573 K in a high-resolution gas-medium deformation apparatus. Below and above the solidus of Ts ≈ 1433 K, fine-grained lherzolite exhibits a transition from diffusional creep at low stress to dislocation creep at high stress. The transition occurs at a differential stress σ ≈ 100 MPa in samples with a grain size d ≈ 20 µm and a melt fraction ϕ ≈ 0·03. Extrapolation to upper-mantle conditions suggests that a similar transition will occur in the mantle for d=2 mm at σ ≈ 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 on ϕ using data for melt-free olivine, analysis of the subsolidus and hypersolidus creep behavior of lherzolite and constraints on ϕ(T) from our experimentally determined melting curve. We find that ϕ increases approximately exponentially with increasing ϕ in both the diffusional and dislocation creep regimes. The observed enhancement in ϕ 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.