Diffusion creep in feldspar aggregates: experimental evidence

Abstract

A combination of mechanical and microstructural results for experimentally deformed albite aggregates of 2–10 μm grain size indicates a regime of grain boundary diffusion creep that depends upon the presence of water, temperature and strain rate. At 900°C and 10−5 s−1, deformation occurs dominantly by recrystallization accommodated dislocation creep when the aggregates contain < ≈0.2 wt% water, but by grain boundary diffusion creep when the aggregates contain ≈0.9 wt% water. Experiments at other temperatures and strain rates indicate that fine-grained aggregates with added water may go directly from cataclastic flow to grain boundary diffusion creep with increasing temperature or decreasing strain rate, bypassing the field of dislocation creep.Microstructural evidence for grain boundary diffusion creep in the wet fine-grained aggregates includes the development of rectangular grains rather than polygonal grains; grain growth; a very low average dislocation density; open grain boundaries including pores and channels; and the presence of overgrowths of a different composition on grains in a two-plagioclase (An1 and An79) aggregate. Additional evidence for diffusion creep in the wet fine-grained aggregates is indicated by their similar microstructures and strengths when deformed at 600 and 1500 MPa; in contrast, dry feldspar aggregates deforming by dislocation creep show an increase in strength and a transition to semi-brittle behavior at lower confining pressures. Grain boundary diffusion creep of feldspars may be common in naturally deformed rocks, due to the ease of grain size reduction by both cataclasis and dynamic recrystallization.