A CLOSE LOOK AT DIHEDRAL ANGLES AND MELT GEOMETRY IN OLIVINE-BASALT AGGREGATES - A TEM STUDY

Olivine-basalt aggregates sintered at high P/T have been used as a sim plest approximation of partially molten upper mantle peridotite. In th e past, geometry of partial melt in polycrystalline olivine (and other materials) has been characterised by dihedral (wetting) angles which depend upon surface free energy. However, since olivine (like most oth er crystalline materials) is distinctively anisotropic, the simple sur face energy balance defining the dihedral angles cos(Theta/2) = gamma( gb)/2 gamma(sl) is not valid and melt geometry is more complicated tha n can be expressed by a single dihedral angle value. We examine in det ail melt geometry in aggregates held at high temperature and pressure for very long times (240-612 h). We show the simple dihedral angle con cept to be invalid via transmission electron microscope images. Olivin e-basalt interfaces are frequently planar crystal faces (F-faces) whic h are controlled by the crystal structure rather than the surface area minimisation used in the simple dihedral angle concept. Nevertheless, the dihedral angles may provide useful insights in some situations. T hey may give a rough estimation of the wetting behaviour of a system, or be used to approximate the melt distribution if F-faces are not pre sent (possibly at large grain size and very low melt fraction). Our me asurements, excluding F-faces, give a range of dihedral angle values f rom 0 to 100 which is significantly lower than reported previously (20 -500). The nature of 0 degrees angles (films and layers up to 1 mu m i n thickness) is unclear but their frequency compared to dry grain boun daries depends on grain size and melt fraction (e.g. 70% for grain siz e 43 mu m and melt fraction 2%).