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°C–1200°C) torsion experiments on fine‐grained (size ≈4 μ m, aspect ratio ≈2.5) synthetic feldspar aggregates containing <3 vol% residual glass. Samples deformed at constant strain rates (≈2 × 10 −5 – 2 × 10 −4 s −1 ) to high strain (≈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.