High-Temperature Ductile Failure of Feldspar Rocks

We performed large-strain torsion experiments on hydrous synthetic anorthite aggregates with 3μm grain size in a Paterson-type gas deformation apparatus. Specimens with 10 mm diameter and 1-10 mm length were twisted at constant rate to a maximum shear strain of about 5 at experimental conditions of 400 MPa effective confining pressure, temperatures of 950°C to 1200°C, and maximum shear strain rates from 1×10-5s-1 to 2×10-4s-1. Resulting torques were in the range of 0.3-16 Nm, corresponding to maximum shear stresses of about 2-70 MPa at the sample periphery. We observed steady-state torques for samples that were deformed at high temperature and/or low twist rate. Strain-rate stepping tests yield a stress sensitivity of about 1, indicating linear-viscous (Newtonian) flow. The mechanical data from high-strain torsion experiments are in good agreement with data from low-strain axial compression tests at similar thermodynamic conditions that suggest grain boundary diffusion-controlled creep. Samples twisted at low strain rates showed continuous strain hardening before sudden catastrophic failure occurred at shear strains of 3-4. However, up to failure no change in stress sensitivity was observed. All samples showed cavitation and intergranular microcrack growth increasing with shear strain, resulting in sample elongation up to 15%. Cavity nucleation occurred predominantly at grain boundary edges and grain triple junctions. Cavity coalescence produced regularly spaced en echelon fissures mostly oriented about 30° to the maximum principal stress. We suggest that cavitation may occur in fine-grained mylonites associated with formation of localized shear bands and leading to catastrophic failure in high temperature shear zones.