Effect of metallic melt on the viscosity of peridotite Justin Hustoft ⁎ , Ted Scott, David L. Kohlstedt ⁎ University of Minnesota, Department of Geology and Geophysics, 310 Pillsbury Dr SE, Minneapolis, MN 55455, United States Received 6 November 2006; received in revised form 3 June 2007; accepted 3 June 2007 Editor: G.D. Price Available online 12 June 2007 Abstract 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 b 60°) to α = 4 ± 1 for the two metallic melts in polycrystalline olivine (both with θ N 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. © 2007 Elsevier B.V. All rights reserved. Keywords: creep; viscosity; metallic melt; olivine 1. Introduction The dependence of strain rate on melt fraction is encompassed in the parameter α in the flow law reported for creep experiments on olivine + mid-ocean ridge basalt (MORB) (Keleman et al., 1997; Hirth and Kohlstedt, 2003; Zimmerman and Kohlstedt, 2004; Scott and Kohlstedt, 2006) Á e ¼ A r n d m expða/Þexp À Q RT   ð1Þ where ε˙ is strain rate, σ is differential stress with n the stress exponent, d is grain size with m the grain size exponent, ϕ is melt fraction, Q is activation energy, R is the ideal gas constant, and T is absolute temperature. Although melt fraction has a marked effect on strain rate, little is known about the relationship between α and material parameters such as dihedral angle, θ. In this study, we have investigated the effect of θ and ϕ on the viscosity of dry, partially molten peridotite deformed in the diffusion creep regime. Two metallic melt phases were used in order to investigate a range of dihedral angles: (1) Fe–FeS (Fe–S) with θ ≈ 90° and a dynamic viscosity μ = 0.04 Pa s (Terasaki et al., 2001) and (2) Au with θ ≈ 150° and μ = 0.003 Pa s (Tucker and Weisberg, 1986). Representative micrographs of these solid–melt Earth and Planetary Science Letters 260 (2007) 355 – 360 www.elsevier.com/locate/epsl ⁎ Corresponding authors. Tel.: +1 612 626 0572; fax: +1 612 625 3819. E-mail addresses: jhustoft@mit.edu (J. Hustoft), dlkohl@umn.edu (D.L. Kohlstedt). 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.06.011