Crystal distribution patterns and their anisotropy behaviour in igneous rocks: towards an automated quantification, first results

Since approximately two decades fractal geometry offers tools for the quantification of rock fabrics, and new methods are currently under development to investigate the inhomogeneity of crystal distributions, grain- and phase-boundary patterns as well as their anisotropy behaviour (Kruhl et al. 2004). These methods are now adapted for automated processing and suitable to quantify the inhomogeneity and anisotropy of rock fabrics from macro to microscale. Applications for quantifying inhomogeneity are mainly based on the box-counting and map-counting (Peternell 2002) methods, for anisotropy behaviour mainly based on modified Cantor-dust methods and provide fractal dimensions, fractal-dimension isolines and azimuthal anisotropies of fractal dimension (AAD, Volland & Kruhl 2004). For instance, the results provide information about the local variations of fabric patterns and their prefer orientation behaviour at macro and microscale.conferenc

Effect of crystallography and temperature on the development of quartz high-angle grain boundaries in metamorphic rocks

Grain boundary migration during dynamic recrystallization of quartz results in grain boundary suturing of various extent. The geometry of the sutured boundaries is affected not only by temperature, strain rate, finite strain and differential stress, but also by internal properties such as the defect distribution and crystallographic orientations. Consequently, the grain boundary geometry may provide information about these conditions and properties. In continuation of a previous study (Kuntcheva et al.) the complete crystallographic orientation of quartz grain boundaries was measured, based on a combination of electron backscatter diffraction (EBSD) and universal-stage (U-stage) measurements. For this purpose a sample of granite from the northern Aar Massif (Central Alps, Switzerland) was taken, deformed at temperatures up to 300–350°C at the end of the Lepontine event of the Alpine Orogenesis...conferenc

The equilibration of high-angle grain boundaries in dynamically recrystallized quartz : the effect of crystallography and temperature

Dynamically recrystallized and sutured quartz grains from metamorphic rocks with different strain intensities and temperature conditions ranging from ca. 350°C to ca. 700°C have been studied. Universal-stage measurements on quartz-quartz high-angle grain boundaries show that they are never curved but always consist of straight segments which preferentially occupy specific crystallographic orientations in relation to both neighboring crystals. With increasing temperature the segments preferentially concentrate in a decreasing number of orientations, mainly near the rhombohedral {101[combining macron]1} planes. The crystallographic data and the observations on grain boundary geometries suggest that: (i) grain boundary orientations are strongly crystallographically controlled, (ii) this control is the main factor on the textural equilibration of quartz-quartz grain boundaries in metamorphic rocks, and (iii) grain boundaries from dynamically recrystallized quartz should be regarded as annealed and equilibrated fabrics that are stable against subsequent annealing as long as the material is not re-deformed

Syndeformational emplacement of a tonalitic sheet - complex in a late-variscan thrust regime : fabrics and mechanism of intrusion, Monte'e Senes, northeastern Sardinia, Italy

The Monte’e Senes intrusive complex in northeastern Sardinia represents a good example of a syntectonic tonalitic sheet-complex. Its history consists of at least three intrusive events involving tonalitic magma and one subsequent intrusive event involving leucogranitic magma. The tonalites form numerous cm- to m-thick layers subparallel to the main foliation and layering of the country rocks. Emplacement occurred under greenschist-facies conditions of the country rocks. Deformation fabrics and fabrics of the contact aureole provide information on kinematics and P–T conditions during intrusion, as well as the relationship to the regional deformation. High-T deformation fabrics of quartz and plagioclase in the intrusive bodies, together with magmatic and high-T foliations and lineations parallel to foliations and lineations of the country rocks, indicate syntectonic intrusion. The deformation of cordierite, andalusite and sillimanite in the contact aureole, as well as the high-T deformation of quartz, point to a long-lasting history of partly synchronous deformation and intrusion. A relatively shallow intrusive body is indicated by the occurrence of andalusite and cordierite in the contact aureole and by granophyric microstructures. Greenschist-facies deformation fabrics in all of the igneous rocks indicate that the intrusion was outlasted by the regional deformation. The Monte’e Senes complex was emplaced during retrograde metamorphism associated with west-directed thrusting of the late Variscan continent–continent collision between Gondwana and Armorica.21 page(s

Automation of pattern recognition and fractal-geometry-based pattern quantification, exemplified by mineral-phase distribution patterns in igneous rocks

This study analyses the possibility of accurate quantification of automatically digitized mineral-phase distribution patterns in igneous rocks. Based on their colour contrast, different minerals were manually and automatically digitized on micro to macro scales. Depending on the digitized mineral phase, the accuracy of the automated digitizing procedure varies. Quantification of mineral distribution patterns was performed by box-counting. The results do not depend greatly on the pattern's pixel density and are similar to each other, even if automatic recording is performed at reduced precision in comparison to manual recording. Consequently, box-counting measurement of automatically recorded mineral distribution patterns (i) leads to fast and accurate pattern quantification, (ii) allows analysis of various phase distribution patterns from micro to macro scales, and (iii) forms an excellent basis for receiving information on pattern-forming processes, which is not available otherwise

3D modelling of the effect of thermal-elastic stress on grain-boundary opening in quartz grain aggregates

During the exhumation of rocks and the associated temperature and pressure decrease, the anisotropic thermoelastic properties of minerals lead to internal stresses on the grain scale, which in turn cause fracturing and opening of grain and phase boundaries. To gain deeper insight into the onset of grain-boundary cracking and the subsequent evolution of grain-boundary fracture networks after crack initiation, 3D grain-scale numerical modelling has been performed. In detail, the fracturing and opening of quartz grain boundaries during exhumation of quartzite with millimeter-scale grains and randomly oriented crystallographic axes, accompanied by cooling from 300 to 25 °C and decompression from 300 to 22 MPa, have been modelled with contact mechanics and the finite-element method. Model grains have an anisotropic linear-elastic rheology and an anisotropic thermal-expansion tensor. Grain boundaries are modelled as contact surfaces with a non-linear strain-softening rheology in tension and a Mohr-Coulomb rheology in shear. Grain-boundary fractures nucleate at grain vertices and triple lines, in agreement with observations in nature and experiment. Comparison with grain-boundary opening data from natural quartzite indicates that the tensile yield strength (σy) of quartz grain boundaries has an upper bound of 75 MPa, with best-fit results for σy = 25 MPa. Moreover, the best-fit model implies that quartz grain-boundary opening initiates after a temperature and pressure decrease of around 80 °C and 80 MPa.</p

Combined quantification of anisotropy and inhomogeneity of magmatic rock fabrics : an outcrop scale analysis recorded in high resolution

Magmatic mineral distribution patterns in a syntectonic syenite pluton have been recorded at high resolution over several square metres on quarried faces. The anisotropy and inhomogeneity of K-feldspar and mafic mineral distribution patterns have been quantified using two methods originally based on fractal geometry. (1) Map-counting, based on box-counting, illustrates an inhomogeneity on the decimetre to metre scale and highlights diffuse structures that can be related to mafic schlieren or felsic dykes that are not visible on the rock surface. (2) We have developed a mapping of rock fabric anisotropy (MORFA) method that leads to the detection of further magmatic structures that are not visible in the field. With MORFA a magmatic lineation and its variation over large areas is determined, as well as fabrics on the decimetre to 1 m scales, which possibly represent flow or fracture structures in the crystallising magma