Experimental grain size-sensitive flow of hot-pressed Brazilian quartz aggregates

Grain size-sensitive flow may be of particular importance in the flow of rocks in natural high-strain zones. Hot-pressed aggregates of synthetic, ultrafine-grained Brazilian quartz of nominal grain sizes 0.4, 1.3 and 4.5 μm were deformed at 300 MPa confining pressure and temperatures of 1273–1473 K. Fully ductile flow at differential stresses below 200 MPa followed ϵ̇=0.4σexp(−220000/RT)/d2 where ϵ̇ is strain rate (s−1), σ is stress (MPa), d is grain size (microns) and R is the gas constant. Grains remained equant and mostly dislocation-free, without significant crystallographic fabric. Water pressure (∼25–50 MPa) was estimated from water adsorbed onto grain boundaries during specimen fabrication. Flow was inferred to be controlled by silicon volume diffusion and to be sensitive to water fugacity. At differential stresses >200 MPa non-linear flow involved dislocation motion. Extrapolation to natural strain rates and temperatures that predict grain-size sensitive flow in quartz should only be important at >800 K and very small grain sizes (∼1 μm). Flow law parameters may change over the extrapolation interval, e.g. through a transition to control by silicon grain-boundary diffusion, but geological evidence suggests that flow of even very fine-grained quartzites commonly occurs by recrystallization-accommodated dislocation creep, rather than by diffusional accommodation.