Dislocation creep near the frictional-viscous transition in blueschist: experimental constraints

Abstract. Mafic oceanic crustal rocks at blueschist facies conditions are an important rheological component of subducting slabs and the interface at subduction plate boundaries. However, the mechanical properties and deformation mechanisms of glaucophane, a rheologically-controlling sodic amphibole in blueschists, are poorly constrained. To investigate its mechanical and microstructural properties, we conducted general shear constant rate and strain rate stepping experiments on glaucophane aggregates using a Griggs apparatus at temperatures of 700–750 °C, shear strain rates of ~3x10^-6 to 9x10^-5 s^-1, varying grain sizes, and a confining pressure of ~1.0 GPa.  The constant rate experiments show an initial stage of grain-size-dependent strain hardening followed by weakening associated with brittle slip along cleavage planes, kink-band development, cataclasis resulting in a fine-grained matrix, and dislocation glide. These experiments evolved to a steady-state stress that did not depend on starting grain size, showing evidence for subgrain development and dynamic recrystallization by bulge nucleation, interpreted to reflect dislocation creep with limited recovery by climb. The mechanical behavior and microstructures of glaucophane in our experiments are consistent with experiments on other low-symmetry minerals as well as microstructural observations from natural blueschists. The strain rate stepping experiments were used to develop a dislocation creep flow law for glaucophane with values of  A = 2.23 x 10⁵ MPa^-n s^-1, n = 3, and Q = 341 ± 37  kJ/mol.  A deformation mechanism map comparing our dislocation creep flow law to an existing flow law for blueschist diffusion creep indicates dislocation creep should activate at lower temperatures, higher stresses and larger diffusion lengthscales.  Viscosities predicted by our flow law for a typical subduction strain rate of 1 x 10^-12 s^-1 lie between quartz and eclogite dislocation creep for the blueschist stability field, implying that mafic oceanic crustal rocks remain strong relative to quartz-rich metasediments all along the subduction interface.