PLASTIC-DEFORMATION OF SILICATE GARNETS I - HIGH-PRESSURE EXPERIMENTS

Deformation experiments have been carried out on garnet single crystal s (Py20 A173 Sp2 Gr5) under high confining pressure (6.5 GPa) and temp eratures in the range 700-1440 degrees C. The high-pressure sample ass embly was designed to generate high differential stress. Below 1000 de grees C, the specimens show only evidence of microplasticity associate d with microcracking. Above 1000 degrees C, garnet become ductile and the microstructure is characteristic of dislocation creep with signifi cant recovery. The observed glide systems are 1/2[111]{1 (1) over bar 0}, 1/2[111]{11 (2) over bar}, 1/2[111]{12 (3) over bar}, [100]{010} a nd [100]{011} in the whole temperature range. The most stable land hen ce less mobile) dislocation lines appear to be the 70 degrees 1/2(111) (i.e., with a large edge component) and edge [100]. Except for some r are [110] dislocations generated at low temperature, most of the dislo cations exhibit no visible dissociation. The detailed characterization of the dislocations suggests that they have an extended, non-planar c on. This core structure induces a high Peierls friction and is respons ible for the brittle behavior of garnets at low temperature. The Peier ls regime extends up to high temperature until diffusion takes place a nd enhances dislocation climb superimposed to glide. (C) 1998 Elsevier Science B.V. All rights reserved.