Deformation microstructure of the chlorite dunite from the Åheim, Norway

Abstract:

The microstructure of the chlorite dunite samples from the Åheim, Norway was analyzed to understand the evolution of the microstructure through the Scandian Orogeny and the subsequent exhumation process. The lattice preferred orientation (LPO) of olivine and the slip systems of the dislocations in olivine are analyzed and compared to understand the relationship of the olivine fabrics and dislocation microstructures of olivine and their evolution during the orogenic event. To determine the LPO of olivine, an electron back-scattered diffraction (EBSD) system was used. For the olivine crystals with the subgrain boundaries, EBSD mapping along the olivine subgrain boundaries were performed to determine the dominant slip system of dislocations in subgrain walls. In addition, the thickness-fringe method was applied to weak-beam dark-field (WBDF) images of TEM to identify the Burgers vector and the slip system of dislocations in olivine.

The Åheim chlorite dunites show porphyroclastic texture with subgrain boundaries. The sample with a large grain size showed A-type olivine LPO, which corresponds to the initial stage of deformation. On the other hand, the other sample with mostly recrystallized olivine grains showed the B-type of olivine LPO and it represents the secondary stage deformation in the water-rich condition during exhumation process. The EBSD mapping and TEM observation revealed that most olivine subgrain boundaries consist of the dislocations with (001)[100] slip systems. These subgrain boundaries might be resulted from the later stage deformation with a moderate water content in olivine or low strain deformation. TEM observation with the thickness-fringe method revealed that the free dislocations with (010)[100] slip system of olivine were dominant for both samples. Our data suggest that the subgrain boundaries and free dislocations in the olivines represent the later stage deformation associated with the exhumation process.