PLASTIC-DEFORMATION OF SILICATE GARNETS II - DEFORMATION MICROSTRUCTURES IN NATURAL SAMPLES

We have used Transmission Electron Microscopy to characterize the defo rmation microstructures in natural garnets from several localities: ec logites from Sesia Lanzo (Alps), eclogites and garnet amphibolite from Braganca (Portugal), garnet pyroxenite from Lherz (France) and eclogi tes from Yakutia pipe (Siberia), Two characteristic microstructures ha ve been identified. The first, found in eclogites from the Alps, consi sts of microplasticity associated with microcracking that suggests bri ttle behavior, The remaining samples show a microstructure characteris tic of dislocation creep with recovery (sub-gain boundaries), The tran sition between these deformation regimes occurs at approximate to 600 degrees C, i.e., at the boundary between group C and group B eclogites . The deformation microstructures suggest that the dislocations experi ence strong lattice friction below approximate to 600 degrees C wherea s at high temperature, diffusion assists dislocation glide and climb ( recovery), We observed the following glide systems in the whole temper ature range: 1/2[111]{1 (1) over bar 0}, 1/2[111]{11 (2) over bar}, 1/ 2[111]{12 (3) over bar}, [100]{010}, and [100]{011}. No correlation co uld be established between the dislocation microstructure and the hydr ous component or the chemistry of the garnets among the pyralspites. C omparison of the natural deformation microstructures and those generat ed in high-pleasure experiments [Voegele et al., 1998 (Part I)] valida tes the extrapolation of experimental data to nature, (C) 1998 Elsevie r Science B.V. All rights reserved.