Klaus Ullemeyer

Anisotropy of diamagnetic susceptibility in Thassos marble: A comparison between measured and modeled data

Abstract A study of shear zones within the calcite marble complex of the island of Thassos (Greece) shows that the low field anisotropy of magnetic susceptibility (AMS)-technique can be successfully applied to diamagnetic rocks for characterizing rock fabrics. The strain path involves both an early pure shear stage and a simple shear overprint that is documented by a transition from triaxial (neutral) to uniaxial (prolate) shapes of AMS ellipsoids. The maximum susceptibility is oriented perpendicular to the rock foliation, reflecting the preferred orientation of calcite c -axes in the protolith as well as in the mylonites. For three samples that represent different types of calcite fabrics, the AMS was recalculated from neutron and electron backscatter diffraction textural data. A comparison of the measured and modeled data shows a good coincidence for the orientation of the principal AMS axes and for the recalculated anisotropy data. Both measured and modeled data sets reflect the change from neutral to distinct prolate ellipsoids during progressive deformation.

Control of magnetic rock fabrics by mica preferred orientation: a quantitative approach

Magnetic anisotropy analysis represents a well-established and frequently used method in structural geology. Several attempts were made to relate the anisotropy of magnetic susceptibility (AMS) to rock fabrics and to discover the sources of the AMS. In gneissic rocks, paramagnetic phyllosilicates with lattice-dependent magnetic properties are assumed to control the whole rock AMS i.e. it is dominated by mica lattice preferred orientation (texture). In this study, AMS was modelled on the basis of the mica texture, the rock composition and compared to the experimentally determined AMS. The orientations of the modelled and experimental tensors agree quite well, whereas all the other characteristic parameters (shape, anisotropy, mean susceptibility) display large differences. It has been concluded that accessory high susceptible phases, as well as the diamagnetic rock constituents, may have significant influence on AMS. Consequently, mica pole figure measurements cannot be replaced by AMS measurements without control of the sources of AMS. It was also concluded, that the March model to evaluate strain from mica-preferred orientations is not valid for highly strained rocks. Beside the fact that the obtained strains are too low, the deformation is generally inhomogeneous, which leads to an overprint of the mica texture. Since the microstructural fabric elements which originate in different strain regimes (prolate or oblate) may create similar modifications of the mica orientation patterns, it is obvious that the relationship between AMS and deformation is rather complex.

Texture analysis of a muscovite-bearing quartzite: a comparison of some currently used techniques

Abstract Four well-established techniques were applied to determine the mineral textures of a muscovite-bearing quartzite: (1) directional measurements on an optical U-stage; (2) X-ray; (3) neutron; and (4) electron diffraction (EBSD). Techniques (1) and (4) are of the ‘single grain’ type and techniques (2) and (3) of the ‘statistical’ or ‘volume’ type. Experimental pole density diagrams were compared by means of the construction of pole figure differences (‘difference pole figures’), which led to the observation that EBSD- and U-stage derived pole figures agree well, even in detail. In contrast, pole figures derived from X-ray and neutron diffraction are clearly different from pole figures derived from the single grain techniques, visible as pronounced preferred orientation in the difference pole figures. Specific properties of the applied techniques may be responsible for the observed differences, such as (1) missing proportionality to the grain volume in the single grain methods, (2) the accessible sample volume, (3) erroneous data correction, or (4) statistical errors. Also the method of data treatment, which is basically different for the single grain and statistical methods, should be considered when pole figures are evaluated. Apart from purely economical constraints and availibility of equipment, the decision on the most suitable method for a texture determination should be based on the scientific goals and specific properties of particular techniques. Texture measurements of the statistical type are well suited for determination of bulk textures of rocks (e.g. as required for the calculation of anisotropic physical properties of rocks), whereas single grain measurements are advantageous for the investigation of local textures and texture forming mechanisms.

Potential of full pattern fit methods for the texture analysis of geological materials: implications from texture measurements at the recently upgraded neutron time-of-flight diffractometer SKAT

The application of Rietveld texture analysis (RTA) to time-of-flight (TOF) neutron diffraction data allows complex materials with many diffraction peaks to be investigated, for example, rocks composed of different minerals. At the recently upgraded SKAT texture diffractometer at the JINR in Dubna (Russia), which provides three alternative multidetector systems, resolution and accessible range of lattice d spacings can be adapted to sample requirements. In order to infer the optimum experimental setup and the reliability of texture estimates from complicated TOF patterns, the influence of counting statistics and various spectral resolutions on texture deconvolution was investigated. Comparing the results obtained at different resolutions and from different sections of the d patterns indicates that the textures of a four-phase sample can be determined, but using a section at small d spacings with a larger number of peak overlaps leads to smoother textures. A complex seven-phase sample shows orientation differences in addition to the smoothing effect. Weak textures and textures of the minor rock constituents are inaccurate owing to multiple peak overlaps. Consequently, good resolution is essential for RTA on such samples. Grid thinning tests confirmed that no more than 150 diffraction spectra are needed to characterize the texture of a monomineralic sample, and approximately 350 spectra are sufficient for a four-phase sample. The irregular grid point arrangement caused by the SKAT geometry has no negative consequences.

Textures and microstructures of naturally deformed amphibolites from the northern Cascades, NW USA: methodology and regional aspects

Abstract Neutron texture analyses of quartz-bearing and quartz-free amphibolite mylonites from the Windy Pass thrust, Cascades Crystalline Core (Washington/USA) reveal pronounced textures of plagioclase and clino-amphiboles (hornblende, cummingtonite) but no preferred orientation of quartz. A reliable strategy for amphibolite fabric analysis is presented by a systematic analytical approach to the experimental diffraction data processing. Clino-amphiboles show transitional textures between ideal single crystal orientations and axial symmetric great circle distributions. Plagioclase reveals a-axes distributions scattering along a great circle approximating the foliation plane as well as a-axes maxima close to the macroscopic lineation. Correlation of the textures with grain shape anisotropies of horn-blende and plagioclase and comparison with data from the literature suggest that the texture variations are due to different strain regimes rather than due to different crystallographic reorientation mechanisms. The kinematic directions deduced from the microfabric correlate well with the regional tectonic interpretations. In contrast, individual deformation paths are not yet established for the different tectonic units, as the significance of the separating Windy Pass thrust requires further structural analysis and fabric studies.

Textures and Microstructures in Peridotites from the Finero Complex (Ivrea Zone, Alps) and their Influence on the Elastic Rock Properties

In order to quantify differences in the elastic rock properties as a result of fabric differences, peridotite samples from the Finero complex were investigated with respect to their mineral textures and elastic properties. Our data indicate only weak to intermediate texture strengths and weak elastic anisotropy, which is too small to produce a significant acoustic contrast in a seismic experiment. Consequently, internal structures from peridotite bodies in the Earths crust with such fabric characteristics cannot be resolved reliably.

Bulk rock elastic moduli at high pressures, derived from the mineral textures and from extrapolated laboratory data

The elastic anisotropy of bulk rock depends on the mineral textures, the crack fabric and external parameters like, e.g., confining pressure. The texture-related contribution to elastic anisotropy can be predicted from the mineral textures, the largely sample-dependent contribution of the other parameters must be determined experimentally. Laboratory measurements of the elastic wave velocities are mostly limited to pressures of the intermediate crust. We describe a method, how the elastic wave velocity trends and, by this means, the elastic constants can be extrapolated to the pressure conditions of the lower crust. The extrapolated elastic constants are compared to the texture-derived ones. Pronounced elastic anisotropy is evident for phyllosilicate minerals, hence, the approach is demonstrated for two phyllosilicate-rich gneisses with approximately identical volume fractions of the phyllosilicates but different texture types.

Neutron time-of-flight texture measurements in Dubna: Status and developments

Introduction The multidetector texture diffractometer SKAT (from Russian: Spektrometer Kolitshestvennovo Analiza Tekstury) at the pulsed reactor IBR-2 in Dubna, Russia, started operation in March 1997 and is open for users from all countries. Application of time-of-flight (TOF) diffraction to texture measurements offers the opportunity to record complete diffraction patterns, i.e., to measure several pole figures simultaneously (Fig. 1(a)). To allow high spectral resolution for measurements on polyphase geological samples with many diffraction patterns, the instrument was placed at the end of an over 100m long flight path. In this paper, we will summarize advantages and disadvantages of the SKAT, as well as intended improvements.