Effect of metallic melt on the viscosity of peridotite

To examine the dependence of viscosity of a partially molten peridotite on the wetting nature of the melt phase, we combine previously published measurements of diffusion creep on samples of olivine + MORB with new results from our creep experiments on samples of olivine + Fe–S and samples of olivine + Au. For these three partially molten systems, the melt-solid dihedral angle varies from θ = 38° for olivine + MORB to θ = 90° for olivine + Fe–S to θ = 150° for olivine + Au. In each case, the viscosity, η, decreases with increasing melt fraction, ϕ, according to the relation η ∝ exp(− αϕ). Our results reveal a substantial change in the value of the material-dependent parameter α, from α = 21 for MORB in an olivine aggregate (with θ < 60°) to α = 4 ± 1 for the two metallic melts in polycrystalline olivine (both with θ > 60°). For a melt fraction of 0.05, this difference in α corresponds to more than a factor of two higher viscosity for samples composed of olivine + a metallic melt relative to the viscosity of samples of olivine + MORB. This difference in viscosity can be attributed to two factors: (i) The enhancement of grain-scale stress due to the presence of melt is smaller for the metallic melt than for the basaltic melt because the melt-solid contact area decreases with increasing θ, and (ii) the flux of olivine through the metallic melt is small due to the low solubility of olivine in the non-silicate melts.