Reaction-induced weakening of plagioclase-olivine composites

The localisation of strain into natural ductile shear zones is often associated with the occurrence of metamorphic reactions. In order to study the effects of solid-solid mineral reactions on plastic deformation of rocks, we have investigated the shear deformation of plagioclase-olivine composites during the reaction plagioclase+olivine -> orthopyroxene+clinopyroxene+spinel (+/- gamet). Microstructures of plagioclase-olivine composites were studied after shear deformation experiments in a Griggs apparatus. Experiments were performed on anorthite-forsterite (An-Fo) and labradorite-forsterite (Lab-Fo) composites at 900 T, confining pressures between 1000-1600 MPa and with constant shear strain rates of similar to 5 x 10(-5) s(-1). In absence of reaction, Lab-Fo composites are stronger than pure olivine and labradorite end-members that deform with a high temperature plasticity mechanism. Lab-Fo composites strain-harden due to the inhibition of extensive recrystallisation by interphase boundaries. In An-Fo composites, the reaction induces strain weakening by a switch from dislocation creep to grain size sensitive deformation mechanisms through the development of fine-grained (size < 0.5 mu m) polyphase reaction products. Interconnecting layers of reaction products accommodate most of the applied strain by grain size sensitive creep. Recovery processes are pronounced during syndeformational reaction: original anorthite and olivine dynamically recrystallise by subgrain rotation and bulging recrystallisation. Presumably, the dynamic recrystallisation is caused by reduced stress conditions and partitioning of strain and strain rates between the new reaction products and the relict An-Fo grains. The results of our experiments are in good agreement with natural observations of shear localisation in the lower crust and upper mantle, and imply that anhydrous mineral reactions can be important causes for localisation of deformation. (c) 2005 Elsevier B.V. All rights reserved.