Semibrittle deformation and partial melting of perthitic K-feldspar: An experimental study

Abstract

To investigate the relationships between deformation, cracking, and partial melting in the lower continental crust, axial compression and hydrostatic experiments were performed on K-feldspar single crystals at temperatures of 700 and 900C and confining pressures between 0.75 and 1.5 GPa. Sample deformation was carried out at a constant strain rate of ~ 10-6 s-1. The samples deformed at 700C show typical brittle behavior with formation of conjugate fractures and peak stresses that increase with confining pressure. Samples deformed at 900C show formation of shear fractures, peak stresses below the Goetze criterion, and inverse confining pressure dependence of peak stress, indicating that along the fractures deformation was not dominantly friction controlled. Microstructural and chemical analyses reveal the presence of melt (<6 vol %) of inhomogeneous composition along the shear zones and chemical compositional changes of gouge fragments. In a hydrostatic experiment performed at 900C, no melt and no compositional changes were observed. These observations indicate that deformation of K-feldspars at high pressures and temperatures is controlled by the simultaneous formation of brittle fractures and melt. The formation of melt is strongly accelerated and kinetically favored by cracking, as demonstrated by the absence of melting in the hydrostatic experiments. However, the melt along fractures does not dramatically weaken the samples, as the melt domains remain isolated during deformation. The fine-grained gouge fragments formed along the fracture systems undergo chemical homogenization. The dominant deformation mechanism in the gouge is likely to be melt-enhanced diffusion creep, which may also assist the chemical homogenization process. ©2014. The Authors.