ELSEVIER Earth and Planetary Science Letters 164 (1998) 553–568 Melt topology and seismic anisotropy in mantle peridotites of the Oman ophiolite David Jousselin Ł , David Mainprice Laboratoire de Tectonophysique, CNRS UMR 5568, Universite ´ Montpellier II, Montpellier, France Received 3 June 1998; revised version received 14 September 1998; accepted 1 October 1998 Abstract This paper presents shape measurements of plagioclase and clinopyroxene inclusions, assumed to reflect melt topology, in peridotites of the uppermost mantle section of the Oman ophiolite. The plagioclase and clinopyroxene grains are devoid of any intracrystalline deformation in all samples. In contrast, the olivine in these rocks has recorded a high temperature plastic deformation, with different strengths of the crystallographic preferred orientation (CPO) of the olivine grains. Individual ‘melt pockets’ are first described by ellipses in two dimensions. They are more elongated when they have a larger area, and they are preferentially oriented parallel to the lineation (the X structural axis) of the sample, with a better defined preferred orientation for samples that have a stronger CPO. In a second step, an average melt phase shape is defined in three dimensions for each sample, using the image autocorrelation technique. The average shape is nearly spherical for the samples with weak CPOs, and it is ellipsoidal, with a long axis parallel to X and the short axis parallel to Z (normal to the foliation) for samples with strong CPOs. The long axis of the ellipsoid is 3 times as long as the short axis for the sample with the strongest CPO. We use an anisotropic differential effective medium method to estimate the seismic properties of partially molten upper mantle peridotites. The melt pockets were modelled as basalt filled inclusions with the average shape and orientation given by the image analysis. The CPO of the olivine crystals was used to calculate the elastic properties of the anisotropic background medium. The calculated P-wave seismic anisotropies ranged from 5 to 15% with the anisotropy increasing with the CPO strength and melt fraction. The maximum P-wave velocities are found along X with velocities above 8 km=s at 0% melt and an average 0.5 km=s reduction for 10% of melt. The minimum P-wave velocities are found along Z with velocities generally below 7.5 km=s at 0% melt and an average reduction of 0.8 km=s for 10% of melt.  1998 Elsevier Science B.V. All rights reserved. Keywords: mantle; melts; anisotropy 1. Introduction The melt distribution at present day mid Ocean ridges is mainly derived from the interpretation of Ł Corresponding author. Fax: C33 46714 3603; E-mail: jousse@dstu.univ-montp2.fr. Present address: University of Ore- gon, 1272 Geol. Sciences, Eugene OR 97403, USA. seismic velocities (e.g. [1]). A key to the seismic in- terpretation is the relationship between melt fraction and seismic velocity. To date most studies addressing the problem of seismic velocity and melt distribution in the upper mantle have assumed that the solid background medium (peridotite) has isotropic prop- erties (e.g. [2–4]) and the melt filled inclusions have a random shape preferred orientation (SPO). We will 0012-821X/98/$ – see front matter  1998 Elsevier Science B.V. All rights reserved. PII:S0012-821X(98)00235-0