High-temperature deformation of enstatite aggregates

Synthesized polycrystalline enstatite samples were deformed in a Paterson gas-medium apparatus at 1200-1300 degrees C, oxygen fugacity buffered at Ni/NiO, and confining pressures of 300MPa (protoenstatite field) or 450MPa (orthoenstatite field). At both confining pressures, the mechanical data display a progressive increase of the stress exponent from n=1 to n=3 with increasing differential stress, suggesting a transition from diffusional to dislocation creep. Nonlinear least squares fits to the high-stress data yielded dislocation creep flow laws with a stress exponent of 3 and activation energies of 600 and 720kJ/mol for orthoenstatite and protoenstatite, respectively. Deformed samples were analyzed using optical microscopy and scanning and transmission electron microscopy. Microstructures show undulatory extinction and kink bands, evidence of dislocation processes. Crystallographic preferred orientations measured by electron backscatter diffraction are axisymmetric and indicate preferential slip on (100)[001]. Most deformed grains comprise an interlayering of orthoenstatite and clinoenstatite lamellae. While many lamellae may have formed during quenching from run conditions, those in samples deformed in the orthoenstatite field are often bordered by partial [001] dislocations, suggesting transformation due to glide of partial [001] dislocations in (100) planes. Comparison of our orthoenstatite creep law with those for dislocation creep of olivine indicates that orthoenstatite deforms about a factor of 2 slower than olivine at our experimental conditions. However, as orthoenstatite has a higher activation energy and smaller stress exponent than olivine, this strength difference is likely smaller at the higher temperatures and lower stresses expected in much of the upper mantle.