High-strain creep of feldspar rocks: Implications for cavitation and ductile failure in the lower crust

Cavitation damage and ductile fracturing is a common phenomenon observed in high-temperature, ambient pressure deformation of superplastic metals and ceramics, but hardly described for geological materials. We performed high-pressure, high-temperature (400 MPa, 950 degrees C-1200 degrees C) torsion experiments on fine-grained (size approximate to 4 mu m, aspect ratio approximate to 2.5) synthetic feldspar aggregates containing <3 vol% residual glass. Samples deformed at constant strain rates (approximate to 2 x 10(-5) - 2 x 10(-4) s(-1)) to high strain (approximate to 2.8 - 5.6) reveal strain hardening at the lower strain rates. Microstructures show pronounced cavitation and formation of porosity bands containing redistributed glass, presumably associated with grain boundary sliding and shape-preferred orientation of high-aspect ratio feldspar grains. Sudden failure by strain-induced nucleation, growth and coalescence of the cavities occurred in one-third of the samples before deformation was terminated. In natural mylonites cavitation damage may produce increased porosity enhancing fluid flow in high-temperature shear zones.