Plastic flow of omphacite in eclogites at temperatures below 500 degrees C- implications for interplate coupling in subduction zones

In subduction zones oceanic crust of the downgoing plate presumably forms a continuous interlayer between the upper and the lower plate. During subduc tion, the basaltic material is transformed to eclogite. Thus, the flow stre ngth of eclogite must pose an upper bound to shear stresses across the plat e boundary for the deeper levels of subduction zones. Up to now, experiment al flow laws for wet diopside have been applied to predict the strength of eclogites. However, based on experimental calibration the material would be essentially undeformable by dislocation creep at 650 degreesC. Even at 700 degreesC a strain rate of 10(-14) s(-1) would still imply a differential s tress of ca. 150 MPa. Omphacite microstructures in eclogites from the Piemo nte Zone, Western Alps, reveal shape-preferred orientation, subgrains and s utured high-angle grain boundaries due to migration recrystallization, and a pronounced crystallographic preferred orientation. These textures indicat e that the omphacite was deformed by dislocation creep. In contrast, garnet forms rigid inclusions within the weaker pyroxene matrix. Fe-Mg exchange t hermometry for garnet omphacite pairs indicates temperatures of 465 +/- 50 degreesC (Valle di Locana) and 475 +/- 50 degreesC (Vallone di Saint Marcel ), for a pressure of 1.5 GPa. These results are not consistent with the pre dictions based on experimental flow laws for diopside. Our data suggest tha t the flow strength of sodic pyroxene is significantly lower than that of d iopside. This finding is consistent with the homologous temperature concept , with the melting temperature of jadeite at 3 GPa being approximately 350 degreesC lower than that of diopside. In addition, dynamic migration recrys tallization of omphacite in the investigated samples can be linked to small chemical changes. This indicates that mobile grain boundaries formed an ef fective pathway for the exchange of ions and that this exchange was fast co mpared pared to the rate of boundary, migration. Thus, the combined reducti on of stored strain energy and chemical free energy provided the driving fo rce for recrystallization. The potential effects of these chemical changes on now strength remain to be explored.