Plastic deformation of silicate garnets. II. Deformation microstructures in natural samples

We have used Transmission Electron Microscopy to characterize the deformation microstructures in natural garnets from several localities: eclogites from Sesia Lanzo (Alps), eclogites and garnet amphibolite from Braganca (Portugal), garnet pyroxenite from Lherz (France) and eclogites from Yakutia pipe (Siberia). Two characteristic microstructures have been identified. The first, found in eclogites from the Alps, consists of microplasticity associated with microcracking that suggests brittle behavior. The remaining samples show a microstructure characteristic of dislocation creep with recovery (sub-grain boundaries). The transition between these deformation regimes occurs at ≃600°C, i.e., at the boundary between group C and group B eclogites. The deformation microstructures suggest that the dislocations experience strong lattice friction below ≃600°C whereas at high temperature, diffusion assists dislocation glide and climb (recovery). We observed the following glide systems in the whole temperature range: 1/2(111){110}, 1/2(111){112}, 1/2(111){123}, (100){010}, and (100){011}. No correlation could be established between the dislocation microstructure and the hydrous component or the chemistry of the garnets among the pyralspites. Comparison of the natural deformation microstructures and those generated in high-pressure experiments [Voegele et al., 1998 (Part I)] validates the extrapolation of experimental data to nature.