Low oxygen fugacity dependency for the deformation of partially molten lherzolite

We present the results of an experimental investigation of the influence of oxygen fugacity on the deformation of a partially molten spinel lherzolite using a 0.1MPa gas-media creep rig under temperatures of 1160-1190°C, stresses of 4-74MPa and well-controlled oxygen fugacities of 10^-7 to 10^-11MPa. The partially molten spinel lherzolite was deformed by a dislocation-mediated creep process such as dislocation creep or dislocation-accommodated grain boundary sliding (DGBS) (n=3.5±0.3) as well as in the diffusion creep regime (n=1.2±0.2). The average oxygen fugacity exponent of lherzolite is 0.04±0.02, which is significantly smaller than the values measured for olivine single crystals (0.10-0.36) and dunite rocks (0.20) deformed by dislocation creep. We attribute the low oxygen fugacity exponent of partially molten lherzolite samples either to the operation of grain boundary sliding, if DGBS dominates the deformation, or to the presence of pyroxenes whose deformation has a weak or no dependency on oxygen fugacity, if dislocation creep dominates the deformation. In the latter case, the oxygen fugacity exponent decreases rapidly with decreasing volume fraction of olivine. The low oxygen fugacity exponent for our partially molten lherzolite samples implies that dislocation creep or DGBS of mantle peridotite will most likely be insensitive to variations of oxygen fugacity in the upper mantle.