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

This paper presents shape measurements of plagioclase and clinopyroxene inc lusions, assumed to reflect melt topology, in peridotites of the uppermost mantle section of the Oman ophiolite. The plagioclase and clinopyroxene gra ins are devoid of any intracrystalline deformation in all samples. In contr ast, the olivine in these rocks has recorded a high temperature plastic def ormation, with different strengths of the crystallographic preferred orient ation (CPO) of the olivine grains. Individual 'melt pockets' are first desc ribed by ellipses in two dimensions. They are more elongated when they have a larger area, and they are preferentially oriented parallel to the lineat ion (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 aver age 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 par allel to X and the short axis parallel to Z (normal to the foliation) for s amples 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 anisotro pic differential effective medium method to estimate the seismic properties of partially molten upper mantle peridotites. The melt pockets were modell ed as basalt filled inclusions with the average shape and orientation given by the image analysis. The CPO of the olivine crystals was used to calcula te the elastic properties of the anisotropic background medium. The calcula ted P-wave seismic anisotropies ranged from 5 to 15% with the anisotropy in creasing with the CPO strength and melt fraction. The maximum P-wave veloci ties are found along X with velocities above 8 km/s at 0% melt and an avera ge 0.5 km/s reduction for 10% of melt. The minimum P-wave velocities are fo und along Z with velocities generally below 7.5 km/s at 0% melt and an aver age reduction of 0.8 km/s for 10% of melt. (C) 1998 Elsevier Science B.V. A ll rights reserved.