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

Abstract

In subduction zones oceanic crust of the downgoing plate presumably forms a continuous interlayer between the upper and the lower plate. During subduction, the basaltic material is transformed to eclogite. Thus, the flow strength of eclogite must pose an upper bound to shear stresses across the plate boundary for the deeper levels of subduction zones. Up to now, experimental 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°C. Even at 700 °C a strain rate of 10^–14 s^–1 would still imply a differential stress of ca. 150 MPa. Omphacite microstructures in eclogites from the Piemonte Zone, Western Alps, reveal shape-preferred orientation, subgrains and sutured high-angle grain boundaries due to migration recrystallization, and a pronounced crystallographic preferred orientation. These textures indicate that the omphacite was deformed by dislocation creep. In contrast, garnet forms rigid inclusions within the weaker pyroxene matrix. Fe–Mg exchange thermometry for garnet omphacite pairs indicates temperatures of 465±50 °C (Valle di Locana) and 475±50 °C (Vallone di Saint Marcel), for a pressure of 1.5 GPa. These results are not consistent with the predictions based on experimental flow laws for diopside. Our data suggest that the flow strength of sodic pyroxene is significantly lower than that of diopside. This finding is consistent with the homologous temperature concept, with the melting temperature of jadeite at 3 GPa being approximately 350 °C lower than that of diopside. In addition, dynamic migration recrystallization of omphacite in the investigated samples can be linked to small chemical changes. This indicates that mobile grain boundaries formed an effective pathway for the exchange of ions and that this exchange was fast compared to the rate of boundary migration. Thus, the combined reduction of stored strain energy and chemical free energy provided the driving force for recrystallization. The potential effects of these chemical changes on flow strength remain to be explored.