Creep and strain-dependent microstructures of synthetic anorthite–diopside aggregates

We investigated plastic deformation of fine-grained synthetic diopside and two-phase anorthite–diopside aggregates in triaxial compression and torsion (up to γ ∼ 5). Temperature, confining pressure and stress ranged between 950–1180°C, 200–400 MPa and 5–500 MPa, respectively. Water content of samples ranged between ∼0.005 ± 0.002 and 0.075 ± 0.025 wt% H2O. All samples deformed in linear-viscous creep with a stress exponent of 1.0 ± 0.2. The activation energy ranged between 571 ± 53 and 290 ± 28 kJ/mol, depending on mineralogy and water content. Sample strength depended on water fugacity with an exponent of 1.55 ± 0.25. Samples deformed in torsion and coaxial compression gave similar flow laws, in spite of significant differences in the corresponding microstructures. Scanning and transmission electron microscopy of two-phase samples deformed in torsion showed phase mixing, cavitation and dislocation processes. We suggest that linear-viscous creep of fine-grained two-phase aggregates involved grain boundary sliding accommodated by grain boundary diffusion and significant dislocation accommodation at high strains. We also observed that cavity coalescence and microcracking led to sample failure. Hence, dynamic instabilities may exist in high-strain shear zones accommodating viscous deformation in the lower continental crust.