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Melt topology and seismic anisotropy in mantle peridotites of the oman ophiolite

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.
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David Jousselin, David Mainprice. Melt topology and seismic anisotropy in mantle peridotites of the oman ophiolite. Earth and Planetary Science Letters, Elsevier, 1998, 164 (3-4), pp.553-568. ⟨10.1016/S0012-821X(98)00235-0⟩. ⟨hal-02378474⟩

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