X-Ray Texture-Analysis of Naturally Deformed Polymineralic Rocks

Crystallographic Preferred Orientations (CPOs) (textures) are used by numerous geologists routinely, generally for the determination of gliding systems within rock forming minerals. In the last decades a lot of methods and models have been developed in order to get information on the evolution of CPOs, especially on the mechanisms of intra- and intercrystalline deformation ("deformation mechanisms"), recrystallisation, and the influence of pressure and temperature on them. Especially the deformation mechanisms of quartz and calcite are well understood because of numerous experimental data and numerical models. The evaluation of CPOs combined with microstructural studies are the most important tools to get information on the deformational behaviour and the rheology of deformed rock materials (tectonites), and of the deformation and recrystallization mechanisms. CPOs within tectonites allow the evaluation of various factors that have been operating during deformation of the crystal lattice, and bear information on their deformation history. These factors and mechanisms are well investigated for monophase rocks. However, the influence of additional mineral phases on the evolution of CPOs is less understood. Usually, the CPOs of only one rock forming mineral does not reflect the deformation conditions and strain history of the entire rock sample; it only reflects, if at all, a distinct event along a deformation path. Furthermore, the CPOs of one rock forming mineral phase are very often highly influenced by accompaniing rock forming minerals, and different ratios of rock forming minerals highly influence the rock rheology. This results in strain partitioning and shear localization, which again influences the activity of glide and climb systems. As a consequence the CPOs are modified, too. For the determination of pole figures of distinct textures a great variety of experimental methods exists. In general, diffraction techniques (X-rays, neutrons, or electrons), optical methods (U-stage, etch pits, etc.), determination of anisotropic properties (e.g., remanent magnetism) are the methods applicated most. In this project we used X-ray Texture Goniometry which has been applied successfully for many years at our institute. This method can also be successfully applied in industry and technology, especially for metalurgy and ceramics. One goal of the project was to determine the mineral phases which can be measured within multiphase rock samples. The use of X-ray Texture Goniometry for the evaluation of CPOs is usually restricted to monomineralic rock samples. If polymineralic rocks have been analyzed the measurement of textures was only possible, if reflexion peaks of the different mineral phases did neither completely overlap nor partially interact. This typically is possible for rock samples which contain apart from one major mineral phase only minor phases of very low content in volume. Especially the reduction of the width of the diaphragms in front of the detector allowed us to seperate many distinct peaks which can be used for the determination of pole figures. However, as a consequence the measurement time had to be elevated. The most important results of this project are: Deformation partitioning between different rock forming minerals basically depends on the ductility contrast between several mineral phases, which is highly controlled by the syndeformational metamorphic grade. In quartzites containing white mica deformation partitioning occurs at low- to medium grade metamorphic conditions in terms of partitioning between pure shear and simple shear. Distinct gliding systems in monomineralic samples might be not active in polymineralic samples, but are replaced by gliding systems of other mineral phases. At high-grade conditions the ductility contrasts are reduced and, therefore, partitioning is less important. The rheological properties are generally controlled by the weakest phase.