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 ofolivine+MORB with new results firom 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 theta=38 degrees for olivine+MORB to theta=90 degrees for olivine+Fe-S to theta=150 degrees for olivine+Au. In each case, the viscosity, eta, decreases with increasing melt fraction, phi, according to the relation eta(proportional to) exp(-alpha phi). Our results reveal a substantial change in the value of the material-dependent parameter alpha, from alpha = 21 for MORB in an olivine aggregate (with theta < 60 degrees) to alpha =4+/-1 the two metallic melts in polycrystalline olivine (both with theta>60 degrees). For a melt fraction of 0.05, this difference in alpha corresponds to more than a factor of two higer 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 theta, and (ii) the flux of olivine through the metallic melt is small due to the low solubility of olivine in the non-silicate melts. (c) 2007 Elsevier B.V. All rights reserved.