Creep of dry clinopyroxene aggregates

We have determined diffusional and dislocation creep rheologies for clinopyroxenite Ca1.0Mg0.8Fe0.2Si2O6 under dry conditions by deforming natural and hot‐pressed samples at confining pressures of 300–430 MPa and temperatures of 1100°–1250°C with the oxygen fugacity buffered by either nickel‐nickel oxide or iron‐wüstite powders. The coarse‐grained natural Sleaford Bay clinopyroxenite yielded a stress exponent of n = 4.7 ± 0.2 and an activation energy for creep of Q = 760 ± 40 kJ mol−1, consistent with deformation in the dislocation creep regime. The strength of the natural clinopyroxenite is consistent with previous high‐temperature measurements of dislocation creep behavior of Sleaford Bay clinopyroxenite by Kirby and Kronenberg [1984] and Boland and Tullis [1986]. Fine‐grained clinopyroxenite was prepared from ground powders of the natural clinopyroxenite. Hot‐pressed samples were deformed under similar conditions to the natural samples. Mixed‐mode deformation behavior was observed, with diffusional creep (n = 1) at lower differential stresses and dislocation creep (with n and Q similar to those of the natural samples) at higher differential stresses. Within the dislocation creep field the predried hot‐pressed samples generally yielded creep rates that were about an order of magnitude faster than the natural samples. Thus, even at the highest differential stresses, a component of strain accommodation by grain boundary diffusion was present in the hot‐pressed samples. Optical and electron microscope investigations of the deformation microstructures of the natural and hot‐pressed samples show evidence for mechanical twinning and activation of dislocation slip systems. When extrapolated to geological conditions expected in the deep crust and upper mantle on Earth and other terrestrial planets, the strength of dry single‐phase clinopyroxene aggregates is very high, exceeding that of dry olivine‐rich rocks.