Preferred orientation of anorthite deformed experimentally in Newtonian creep

Abstract

Synthetic anorthite aggregates were deformed in a Paterson gas deformation apparatus at confining pressures up to 400 MPa in torsion and axial compression at temperatures between 950 °C and 1200 °C. Samples deformed in torsion under Newtonian creep display development of texture (or crystallographic preferred orientation) as documented with synchrotron X-ray diffraction measurements. Complex diffraction patterns were deconvoluted with the Rietveld method to obtain quantitative texture information. Torsion samples deformed up to shear strains of 4 and samples deformed in compression at higher stresses to total strains of 0.3 develop clear textures. Texture and shape preferred orientation (SPO) of torsion samples display a monoclinic pattern with an asymmetry inclined against the sense of shear, consistent with polycrystal plasticity simulations that assume the deformation is accomplished by dislocation glide. These results show that a material deforming in linear-viscous creep can develop a strong texture, in striking contrast to the paradigm that the presence of a texture precludes low-stress Newtonian behavior. Our observations show that the presence or absence of crystallographic preferred orientation is not sufficient to uniquely infer the dominant rheological/mechanical regime, as sometimes applied for interpretation of seismic anisotropy in the Earth.