The effect of large melt fraction on the deformation behavior of peridotite

To determine the influence of a large melt fraction, 0.15 ≤ ϕ ≤ 0.30, on the rheological behavior of partially molten rocks, we performed a series of high-temperature, triaxial compressive creep experiments on dry, synthetic peridotites in both the diffusion and dislocation creep regimes. We deformed samples with an olivine grain size of either 10 or 50 μm in a gas-medium apparatus at a confining pressure of 300 MPa and temperatures from 1498 to 1556 K. Stress vs. strain rate data reveal a drop in rock viscosity of several orders of magnitude when the melt fraction is increased from ϕ = 0.25 to ϕ = 0.30, indicative of a rheologically critical melt fraction (RCMF). In all of these experiments, we observed only a small amount of grain growth. Over the range 0 ≤ ϕ ≤ 0.25, the flow behavior in both the diffusion and grain size sensitive dislocation creep regimes is well described by the published flow law for partially molten samples of olivine + MORB deformed under anhydrous conditions in which ε˙ ∝ exp(αϕ) with α = 21 for diffusion creep (Newtonian) and α = 32 for dislocation-accommodated GBS (grain boundary sliding) creep.