Description: EGU2012-13016 The dynamically recrystallized grain size is the most reliable paleo-piezometer to determine the differential stress in the Earth’s crust and mantle. Knowledge on the stress magnitude is enigmatic to quantify tectonic processes in orogens and plate tectonic forces in general. Owing to this significance, a considerable number of research groups has proposed different theoretical concepts of piezometers in the last couple of years. The recrystallized grain size has been suggested to be not only a function of stress, but also of temperature, strain rate, strain and other parameters. In the meantime, data of experimental studies and from natural shear zones have been collected. Hence, empirical piezometer models and theoretical concepts can be confronted with these data sets. A recrystallized grain size compilation of quartz mylonites from shear zones worldwide indicates that specific grain sizes are less frequent corresponding to transitions in the recrystallization mechanisms. This indicates that the recrystallized grain size development is significantly controlled by the different recrystallization mechanisms in natural mylonites. This relationship should be constrained by any valid piezometer or dynamic recrystallization model which is, however, not the case. Most of the piezometer models assume a temperature dependence via an activation energy term. While the majority of these models predicts a decrease in recrystallized grain size with increasing temperature one implies an increase with increasing temperature. However, neither a decrease nor an increase has reliably been shown by deformation experiments on different minerals. In fact, experimental data on dislocation creep of quartz do not show any temperature-dependence within the error of the given stress-recrystallized grain size measurements. A strain rate dependence is – if at all – less important and also not constrained by experimental data. Also a water-dependence of the piezometer does not exist for quartz and there is contradicting experimental evidence for olivine. Experimentally deformed quartz samples display 2d- recrystallized grain size distributions close to a normal distribution with a slight tendency to a positive skewness. The dispersion of the distribution does not change over the experimental range of strain (7 – 46 %) and also not with the volume proportion of recrystallized grains (2 – 60 %). Hence, increasing strain does not change the recrystallized grain size distribution. In summary, no dependence on temperature, strain rate, strain, grain size distribution and water content can be observed for the quartz piezometer. There is only evidence for the dependence on the recrystallization mechanism. Therefore, it is recommended to refer to the original empirical piezometer relationship in which the recrystallized grain size is only a function of the differential stress when measuring stress in the Earth’s crust.

Description: EGU2012-13521 In Geoscience quantitative texture analysis here defined as the quantitative analysis of the crystallographic preferred orientation (CPO), is a common tool for the investigation of fabric development in mono- and polyphase rocks, their deformation histories and kinematics. Bulk texture measurements also allow the quantitative characterisation of the anisotropic physical properties of rock materials. A routine tool to measure bulk sample volumes is neutron texture diffraction, as neutrons have large penetration capabilities of several cm in geological sample materials. The new POWTEX (POWder and TEXture) Diffractometer at the neutron research reactor FRM II in Garching, Germany is designed as a high-intensity diffractometer by groups from the RWTH Aachen, Forschungszentrum Jülich and the University of Göttingen. Complementary to existing neutron diffractometers (SKAT at Dubna, Russia; GEM at ISIS, UK; HIPPO at Los Alamos, USA; D20 at ILL, France; and the local STRESS-SPEC and SPODI at FRM II) the layout of POWTEX is focused on fast time-resolved experiments and the measurement of larger sample series as necessary for the study of large scale geological structures. POWTEX is a dedicated beam line for geoscientific research. Effective texture measurements without sample tilting and rotation are possible firstly by utilizing a range of neutron wavelengths simultaneously (Time-of-Flight technique) and secondly by the high detector coverage (9.8 sr) and a high flux (�~1x10 7 n/cm2s) at the sample. Furthermore the instrument and the angular detector resolution is designed also for strong recrystallisation textures as well as for weak textures of polyphase rocks. These instrument characteristics allow in-situ time-resolved texture measurements during deformation experiments on rocksalt, ice and other materials as large sample environments will be implemented at POWTEX. The in-situ deformation apparatus is operated by a uniaxial spindle drive with a maximum axial load of 250 kN, which will be redesigned to minimize shadowing effects inside the cylindrical detector. The HT deformatione experiments will be carried out in uniaxial compression or extension and an upgrade to triaxial deformation conditions is envisaged. The load frame can alternatively be used for ice deformation by inserting a cryostat cell for temperatures down to 77 K with a triaxial apparatus allowing also simple shear experiments on ice. Strain rates range between 10-8 and 10-3 s-1 reaching to at least 50% axial strain. The deformation apparatus is designed for continuous long-term deformation experiments and can be exchanged between in-situ and ex-situ placements during continuous operation inside and outside the neutron detector.