PLASTIC-DEFORMATION OF GARNETS - SYSTEMATICS AND IMPLICATIONS FOR THERHEOLOGY OF THE MANTLE TRANSITION ZONE

Plastic properties of materials with garnet structure have been studie d under wide temperature conditions, ranging from room temperature to similar to 95% of the melting temperatures, using uniaxial compression and hot microhardness tests. Garnets studied include single crystals of oxide garnets (Y3Al5O12, Gd3Ga5O12 and Y3Fe5O12) and silicate garne ts (various solid solutions, including grossular, almandine, andradite , pyrope, spessartine and uvarovite). Both uniaxial compression and ho t hardness tests indicate that there is a general trend in the plastic ity of garnets when the data are compared at normalized conditions (T/ T-m and sigma/mu), and that the resistance to plastic deformation in g arnets is significantly higher than most of the other minerals in the Earth's mantle. Based on both stress-dip tests and microstructural obs ervations, it is proposed that the creep strength of garnet is largely controlled by the resistance to dislocation glide rather than by reco very processes. This conclusion is consistent with the high Peierls st ress inferred from the hot hardness tests. The high Peierls stress in garnets is, presumably, due to the large unit cell (i.e., long Burgers vectors) and/or the bcc packing, which are common to all garnets. We postulate, therefore, that the present results can be applied to the s trength of high-pressure garnet (majorite) and suggest that garnet-ric h layers in the Earth, such as subducted oceanic crust in the transiti on zone or a possibly garnet-rich (bottom part of the) transition zone , will be considerably stronger than surrounding regions.