Low-temperature plastic rheology of granitic feldspar and quartz

Deformation of the Earth's continental crust depends largely on the mechanical properties of quartz and feldspar, including deformation by brittle, semi-brittle, and elastoplastic mechanisms. However, elastoplastic deformation of these mineral phases is difficult to achieve in laboratory experiments at temperatures relevant to the upper crust without high confining pressures to suppress microcracking and macroscopic brittle failure. In this study we use instrumented nanoindentation to determine the plastic yield strength of quartz, and orthoclase and plagioclase feldspars at temperatures of 23–500 °C and at strain rates of ∼10⁻² s⁻¹. The specimen investigated here is a medium-grained granite from southwestern Rhode Island, grains of which were characterized and oriented using scanning electron microscopy and electron backscatter diffraction (EBSD). Indentation hardness and modulus of oriented grains was measured directly using a diamond Berkovich nanoindenter; these quantities were then used to calculate yield stresses through several existing theoretical and numerical models. The calculated yield stresses are fit to constitutive flow laws for low-temperature plasticity. We find that, in contrast to most observations from nature, quartz is systematically stronger than both feldspars. The conditions of these experiments are comparable to those near the tips of cracks in the mid to upper crust and the results presented here are particularly relevant to geologic settings where brittle and semi-brittle processes dominate, including aseismic creep of faults.