Natural fine-grained gabbro was deformed in a Paterson deformation apparatus to evaluate the flow strength of lower crustal rocks containing partial melt. We performed 94 creep stepping tests on seven samples at 300 MPa confining pressure, temperatures between 950°C and 1150°C, and axial stresses of 25-510 MPa, resulting in strain rates between 2.3 × 10 ~(-4) and 6.7 × 10 ~(-8) s ~(-1). Water content of samples predried at 1000°C at 1 atm was about 0.035 wt % H _2O. The drying process induced partial melting of the starting material of ～1 vol % Si-poor and Fe-rich melt at grain boundaries, which increased further up to ～2 vol % during creep tests. Creep tests reveal strain rates increasing with duration of the tests related to increasing melt content present in the samples. Microstructural observations of deformed samples show melt in triple junctions and melt films contained in grain boundaries. The observed microstructures indicate that the samples were deformed in the dislocation creep regime. Dislocation walls are present in pyroxene and plagioclase grains. Very fine grained (about 10 m) pyroxene and olivine were produced by mineral reactions and dynamic recrystallization at temperatures >1000°C. Melt fraction φof creep test samples and annealed samples increases linearly with logarithm of time (log(t)), suggesting that strain rate enhancement by partial melting can be described by an exponential function of melt fraction with an exponent coefficient of 128. After applying a correction for the time-dependent increase of melt content the data were fitted to a power law creep equation, resulting in a stress exponent of n = 4.0±0.3, an activation energy of Q = 644±75 kJ mol ~(-1), and a preexponential factor of A = 10 ~(10.3±0.4) MPa ~n s ~(-1) for dry gabbro that contains ～1 vol % melt. The flow law for gabbro from this study is compared to published flow law parameters of basaltic composition rocks.
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