Grain misorientations in partially molten olivine aggregates: an electron backscatter diffraction study

In polycrystalline aggregates of olivine with mean grain sizes above 35 mu m plus a low basaltic melt fraction, both wetted and melt-free grain bounda ries are observed after equilibration times at high pressures and temperatu res of between 15 and 25 days. In order to assess a possible dependence of the wetting behaviour on the relative orientation of neighbouring grains, a SEM based technique, electron backscatter diffraction (EBSD), is used to d etermine grain orientations. From the grain orientations relative orientati ons of neighbouring grains are calculated, which are expressed as misorient ation axis/angle pairs. The distribution of misorientation angles and axes of melt-free grain boundaries differ significantly from a purely random dis tribution, whereas those of wetted grain boundaries are statistically indis tinguishable from the random distribution. The relative orientation of two neighbouring grains therefore influences the character of their common grai n boundary. However, no clustering towards special (coincident site lattice ) misorientation axes is observed, with the inference that the energy diffe rences between special and general misorientations are too small to lead to the development of preferred misorientations during grain growth.