Grain-size sensitive rheology of orthopyroxene

The grain-size sensitive rheology of orthopyroxene is investigated using data from rheological and microstructural studies. A deformation mechanism map is constructed assuming that orthopyroxene deforms by two independent mechanisms: dislocation creep and diffusion creep. The field boundary between these mechanisms is defined using two approaches. First, experimental data from Lawlis (1998), which show a deviation from non-linear power law behavior at low stresses, are used to prescribe the location of the field boundary. Second, a new orthopyroxene grain-size piezometer is used as a microstructural constraint to the field boundary. At constant temperature, both approaches yield sub-parallel field boundaries, separated in grain size by a factor of only 2-5. Extrapolating to lithospheric conditions, the deformation mechanism transition occurs at a grain size of similar to 150-500 mu m, consistent with observations from nature. As the transition from dislocation to diffusion creep may promote shear localization, grain-size reduction of orthopyroxene may play a prominent role in plate-boundary deformation.