The lattice preferred orientation of olivine in Beni Bousera Ultramafic Massif, Morocco

Abstract

The roll of melt in deformation of the mantle is important for understanding mantle processes and evolution. The Beni Bousera Ultramafic Massif, northern Morocco, shows petrologic evidence that melt was an important part of its history. The massif, dominantly composed of spinel lherzolite with harzburgitic sections, has 10 to 100 meter scale zones which contain abundant pyroxenite dykes, dunite, harzburgite, and secondary lherzolite. These zones are interpreted as areas of focused melt percolation. Lattice preferred orientation (LPO) of olivine crystals from a sample transect across one of these zones was analyzed in comparison with LPO from the surrounding host lherzolite and harzburgite. The LPO from peridotites within the zone was slightly different from those outside the zone, and the LPO from the dunite analyzed was significantly different from the peridotites. Within the melt percolation zone, orientation of the olivine a-axis with respect to the mineral foliation indicates that strain increases towards the center of the zone and that shear sense changes across the zone. This suggests that focused melt flow can induce deformation in its host peridotite.