MR23B-1322
Quantitative Textural Analysis of Packings of Elongate Crystals
The spatial distribution of grains in a solidifying igneous rock controls the physical properties of the crystal mush, and is in turn controlled by the rates of crystal growth and accumulation. The dominance of a non-spherical habit for minerals growing from liquid brings into question the use of spherical particles in reference packings used for the quantification of grain spatial distribution in rocks. Furthermore, details of the way crystal clustering/ordering varies on a range of lengthscales can only be discerned using statistical measures which take into account the distribution of particles beyond nearest neighbours. We advocate the use of spatial distribution functions such as Ripley's K, the pair correlation, and the mark correlation. Using random close packings of spherocylinders we demonstrate the importance of aspect ratio in controlling the aggregation index (usually known as R) and demonstrate that packings of spherical particles have more structure than those of elongate particles. Application of the spatial distribution functions to a sample of the colonnade from the Holyoke flood basalt, dominated by clusters of elongate plagioclase grains, demonstrates that the lengthscale of the clustering is 0.5 mm. Further understanding of the controls on grain spatial distribution in igneous rocks will depend on application of these techniques to rocks from well-understood environments.
MR23B-1323
The roughness of uni-axial tensile fracture and rock anisotropy in Inada granite
The roughness of uni-axial tensile fracture and rock anisotropy in Inada granite Yukiyasu FUJII, Manabu TAKAHASHI, Takato TAKEMURA, LIN Weiren Inada granite contains a lot of microcracks which are preferentially oriented along three mutually perpendicular planes (rift, grain, and hardway planes). These microcracks (fabrics) affect the physical properties of the granite. In this study, fractures parallel to each plane were produced by uni-axial tensile tests. The obtained fracture surfaces were analyzed by digital stereo-photogrammetry. From the results of fracture roughness, the fracture parallel to the rift plane is the smoothest, and the fracture parallel to the grain plane is smoother than the fracture parallel to the hardway plane. The mineral compositions are also different on the failed surfaces of different directions. The rift-parallel fractures have almost the same constituent mineral ratio as the parent rock. However, the grain-parallel fractures pass preferentially through feldspar grains, and contain less quartz grains as compared to the bulk composition of the parent rock. In addition, it is remarkable on hardway-parallel fractures. The difference of fracture roughness is proportional to the rock anisotropy, due to preferred orientations of pre- existing microcracks. The microcracks parallel to rift plane are the most pronounced, and microcracks parallel to grain plane are more pronounced than hardway plane. The difference of mineral compositions can be explained by the differece of the pre-existing intra-granular microcrack fabrics between quartz and feldspar. It is considered that the final fractures result from the development and coalescence of pre-existing microcracks along more or less a single plane.
MR23B-1324
Automated Quantification of Magmatic Rock-Fabric Anisotropy From Micro to Macro Scale
During the last decade, studies on magmatic fabrics on different scale have been made for analyzing kinematics of melt emplacement and deformation as well as cooling histories of magmatic bodies and their host rocks (Brown 2001). The problem arises for comparing data from structural analysis in thin-sections and fabrics in outcrop scale. Magmatic fabrics in the outcrop scale are often very diffusely developed or show very complex mineral distribution patterns which are mostly not quantifiable with classical field methods. On the other hand the fabrics are too large for thin section investigation. Therefore, methods have to be developed for analyzing microscale rock fabrics like crystal shape preferred orientations on larger image templates, like from outcrop photographs. For analyzing such complex fabrics on different scale, methods of fractal geometry are powerful (Mandelbrot, 1982). Especially methods like map-counting (Kruhl et al., 2004), based on the classical box-counting method, or the modified Cantor-dust method (Volland & Kruhl, 2004) may be used for analyzing meter sized magmatic mineral distribution patterns in granitic rocks and the anisotropic behavior of micro- to macro scale fracture patterns in breccias. Nevertheless, these methods are performed manually and, therefore, not applicable to larger datasets. This study shows the next step towards automated recognition and subsequent quantification of magmatic patterns from micro- to macro-scale. Microstructure shape- and crystallographic preferred orientation measurements on K-feldspar crystals from the Piquiri Syenite Body, Southern Brazil are done manually based on thin-section series parallel to the syenite magmatic foliation. In addition U-stage measurements of K-feldspar indicatrix axis and (010) have been done. Additionally, a slightly changed modified Cantor-dust method was applied on centimeter- to several meter-sized K-feldspar phase distribution patterns, gained by automated image processing of samples and field photographs of the same syenite. The results show that all three methods result in the same shape- and/or crystallographic preferred orientations for K-feldspar, indicating a magmatic lineation, which is not determinable by field observation or in the thin- sections. In addition, the modified and automated Cantor-dust method applied on K-feldspar phase images of a syenite proves the advantage of such type of modified fractal-geometry methods for quantification of magmatic fabrics on various scales. Such automated quantification is fast and precise, and it is suitable for accurate analysis of magmatic fabrics on different scales and for large datasets. References: Kruhl,J.H., Andries,F., Peternell,M. & Volland,S. (2004): Fractal geometry analyses of rock fabric anisotropies and inhomogeneities. In: D.Kolymbas (ed.), Fractals in Geotechnical Engineering. Advances in Geotechnical Engineering and Tunnelling 9. Logos, Berlin, 115-135. Mandelbrot, B.B., (1982): The Fractal Geometry of Nature. Freeman, San Francisco. Volland, S. & Kruhl, J.H. (2004): Anisotropy quantification: the application of fractal geometry methods on tectonic fracture patterns of a Hercynian fault zone in NW-Sardinia.- J. Struct. Geol. 26, 1489-1500. Brown,M. (2001): Crustal melting and granite magmatism; key issues.- Physics and Chemistry of the Earth. Part A: Solid Earth and Geodesy 26/4-5, 201-212.
MR23B-1325
The Development of Subgrains in Deformed Calcite: Influence of Stress and Temperature
The internal substructure of subgrain walls and dislocations in crystals in naturally deformed rocks is an indicator of the deformation mechanisms that were active in the material and can be used to estimate paleo-stress levels in the crust and upper mantle. In order to relate observations made in the internal parts of crystals in a meaningful way to the strength and rheology of rocks, a thorough understanding is needed of the nature of deformation induced substructures and their dependence on conditions of stress and temperature, amongst others. This requires full characterization of the microstructure rather than applying conventional averaging techniques. This is the aim of our study. We systematically investigated experimentally deformed samples of Carrara marble, uniaxially shortened to a natural strain of 0.45, at differential stresses ranging 15-85 MPa, temperatures ranging 700 to 990°C, and a confining pressure of 300 MPa. The samples were analysed using the electron backscattered diffraction technique (EBSD) to make automated 1000 x 1000 μm orientation maps (step size 1 μm). A size measurement technique was developed for separately measuring the size of recrystallised grains, deformed grains, and within the latter, core and mantle subgrains. We observed a complex microstructure in the calcite samples, consisting of deformed and recrystallised grains with heterogeneously distributed small subgrains at grain boundaries and relatively large subgrains in the core of grains. The size of the subgrains varies with stress, as expected on the basis of previous work on subgrains, but temperature, grain size and misorientation angle were also found to be of influence. Moreover, mantle and core subgrains depend differently on these parameters. Most strikingly, the subgrain sizes for mantle and core show a low sensitivity on stress, deviating from the often quoted linear inverse relationship. Mantle subgrains with high misorientation angles appear more sensitive to stress than low angle subgrains. Temperature does not have a systematic influence on the mantle subgrain size, while the core subgrain size is larger at higher temperatures. All this has the important consequence that conventional paleopiezometric relationships between stress and subgrain size cannot simply be used for any type of subgrains. We present a conceptual model involving stress gradients and variability in slip system activity between core and mantle to account for our observations.
MR23B-1326
An EBSD study of texture development and hybrid deformation mechanisms in fine grained calcite aggregates deformed in direct shear
Understanding of texture development and its relation to grain size sensitive (GSS) deformation mechanism is of great importance since the rheological behavior of rocks varies substantially depending on which deformation mechanisms are controlling. Recent studies on naturally and experimentally deformed calcite and olivine aggregates have demonstrated that even fine grained materials that are expected to deform by grain size sensitive (GSS) mechanisms, develop a weak but distinct LPO (texture) at high strain. To investigate this behavior we conducted new deformation experiments on Solnhofen limestone in direct shear. This study is part of a wider investigation on large strain deformation in different deformation modes from G. Trullenque, in collaboration with D.L. Kohlstedt (Minneapolis, USA), R. Heilbronner and H. Stuenitz (Basel, Switzerland) and the Utrecht group. The samples have been deformed to high strain both in the conditions of the transition between the GSS and GSI creep regime and in the GSS creep regime. We used Electron Backscatter Diffraction (EBSD) to determine the texture strength of the samples, the distribution of misorientation axes and the grain size distributions. The results show an oblique shape preferred orientation at 35° to 40° to the shear plane and a moderate LPO. The c-axis preferred orientation shows a girdle with one main maximum at a high angle to the shear plane, displaced towards the shortening direction of the imposed shear. The a-axes present a weak girdle perpendicular to the c-axis. Grain growth occurred during high deformation. The misorientation angle distribution has a main peak at low angle. This is due to the progressive subgrain formation and rotation which occurred mainly in the larger grains. Subgrain rotation with misorientations up to 10° occured but most boundaries are low angle (< 5°). This shows the formation of new high angle boundaries and grain size reduction. The formation of subgrains and subgrain rotation, along with the fact that the larger grain fraction shows a slightly stronger LPO than the fine grain fraction, suggest a component of dislocation creep in the coarser grains. We suggest therefore that the GSS regime in the fine grain aggregates may be a transient stage evolving into hybrid deformation (GSI and GSS mechanisms) at high strain. The next steps in this study will be to quantify the contribution of the GSI and GSS mechanisms through the EBSD study and develop a new flow law for the hybrid deformation of the Solnhofen limestone.
MR23B-1327
The geometry of random mixing: quantifying spatial distributions
There are a number of deformation mechanisms that involve mixing of particles of different phases. For example, in cataclastic flow, particles are fragmented and displaced past each other, in diffusion creep, grains of one phase nucleate and grow between grains of other phases. The resulting mixtures may form "random", "clustered" or "anticlustered" patterns. To derive the nature of the underlying process and to identify the active deformation mechanisms, it is necessary to find reliable descriptors by which random and non-random spatial distributions can be quantified and distinguished from one another. As an example of a non-random distribution of grains we studied the eclogites of the Troms Nappe (Caledonides, Norway). Previous studies indicate that the dominant deformation mechanism is diffusion creep, a process known to create anticlustered distributions of mineral grains. In the course of field observations we collected a large dataset of random and non-random 2-D geometries and compared it to results from 3-D numerical modelling. Starting with the Markov Chain analysis by Kretz (1970) we formulated two models for random spatial distributions: one based on the relative amount of grain boundary surface (surface model) and one based on the relative volume fraction of each phase (volume model). In both cases, if the aggregate consists of two phases (A and B), the relative amounts of grain contacts (AA, BB and AB) follow a binomial distribution. Because of the different grains sizes of omphacite and garnet, the eclogites were evaluated using the surface model. It can be shown that their microstructures deviate from spatial random distributions showing various degrees of anticlustering and in many cases, the degree of anticlustering depends on direction, being stronger in the direction of the stretching lineation than in direction of the foliation normal. From this we infer that diffusion creep occurred by solution-precipitation processes and heterogeneous nucleation. A more general model for random distribution considers only one phase (A) and two types of grain contacts (AA and AB, where B is the "matrix"). This model is based on fragmenting a cluster of grains into smaller clusters. It predicts the most probable distribution of cluster sizes and the probability for contact types AA and AB. A comparison of the three models for random spatial distribution of grains shows that the discrimination of random from non-random patterns and the associated statistics depend strongly on the assumptions made concerning the nature of the underlying random process.
MR23B-1328
Artificial Neural Net Assisted Edge Editing and Edge Parameter Extraction from Petrographic Images Collected With the Rotating Polarizer Stage
For any petrographic image analysis application that is aimed at the analysis of discrete objects such as mineral grains, the proper segmentation of those objects is of prime importance. However, automated edge detection algorithms are not perfect and remain problematic even in relatively clean materials such as ceramics. A practical methodology for editing edges within petrographic images is presented. The procedure uses the existing output of a standard segmentation routine as input. Because most edge detection algorithms can produce too many edges, the procedure does not find new edges; rather it examines the existing ones to identify false edges. Edges are skeletonized and converted into segments which separate two grains and join at nodal pixels. Nodal pixels connect a minimum of three separate line segments and their presence ensures that any line segment can be deleted without breaking the connectivity of any remaining boundaries. For each edge segment, an artifical neural net (ANN) evaluates differences in colour and texture parameters for the grains separated by the segment. ANN output is used to classify segments as true or false edges and can be thresholded at different levels and manually evaluated using a specialized, rapid procedure. This methodology significantly improves the speed with which edges can be edited in preparation for other studies. For each remaining individually identified edge segment, grain boundary parameters such as the length, orientation, a measure of straightness and the angle between boundary segments at nodal points can then be extracted. When combined with the size and shape parameters that can now be calculated from the correctly identified grains, this data can provide first order information about the formation or deformation of rocks.
MR23B-1329
Quantification of Flow Structures in Syntectonic Magmatic Rocks
Fabrics of syntectonic magmatic rocks provide important information on melt emplacement and crystallization conditions and, consequently, information on state and development of certain parts of the continental crust. Therefore, detailed studies on magmatic fabrics and, specifically, their quantification is a necessary prerequisite for any more detailed study. Fabric anisotropy and heterogeneity are fundamental properties of magmatic rocks. Their quantification can be performed by recently developed modified methods of fractal geometry. (i) A modified Cantor-dust method leads to a direction-related fractal dimension and, consequently, to quantification of fabric anisotropy. (ii) A modified perimeter method allows determination of fractal dimensions of complex curves in relation to their average orientations. (iii) A combination of box-counting method with kriging results in a contour map of the box-counting dimension, revealing the local fabric heterogeneity. (iv) A combination of method iii and a modified Cantor-dust method leads to mapping of fabric anisotropy (Kruhl et al. 2004, Peternell et al. subm.). Automation of these methods allows fast recording, generation of large data sets and the application of quantification methods on large areas (Gerik & Kruhl subm.). It leads to a precision of fabric analysis, not obtainable by manual execution of methods. Specifically, the direction-related Cantor-dust method has proven useful for analyzing magmatic flow structures and quantifying the intensity of flow. Application of this method to different types of syntectonic magmatic rocks will be presented and discussed. References: Gerik, A. & Kruhl, J.H.: Towards automated pattern quantification: time-efficient assessment of anisotropy of 2D pattern with AMOCADO. Computers & Geosciences (subm.). Kruhl, J.H., Andries, F., Peternell, M. & Volland, S. 2004: Fractal geometry analyses of rock fabric anisotropies and inhomogeneities. In: Kolymbas, D. (ed.), Fractals in Geotechnical Engineering, Advances in Geotechnical Engineering and Tunnelling, 9, Logos, Berlin, 115-135. Peternell, M., Bitencourt, M.F. & Kruhl, J.H.: New methods for large-scale rock fabric quantification – the Piquiri Syenite Massif, Southern Brazil. Journal of Structural Geology (subm.)
MR23B-1330
Laboratory measurement of orthogonally anisotropic and low permeability of a crystalline rock
Crystalline rocks such as granitic rocks are characteristic of anisotropic and low permeability. It is due to the distribution and connectivity of preexisting microcracks. In this study, hydraulic properties (hydraulic conductivity and specific storage) of granite was measured by transient pulse permeability test and then the relationship between the properties and microcrack distribution was analyzed based on stereology. The results showed that the hydraulic properties of the granite are strongly controlled by the geometry and distribution of microcracks. Laboratory measurements of hydraulic properties of low-permeability rocks need to be carefully conducted for precision. In general, transient pulse method suits low-permeability materials. Remarks on measuring hydraulic constants of rocks in laboratory are given and a modified analytical solution of the transient pulse method in consideration of the constituents of the test system are also introduced in the presentation. 3-D distribution of microcracks in crystalline rocks is difficult to be visualized even if micro-focus X-ray CT scanner is used. Therefore, the 3-D distribution of microcracks was reconstructed using microphotographs taken in three mutually perpendicular surfaces. Once the geometry and distribution of microcracks are estimated, the hydraulic conductivity tensor of the rock can be calculated.
MR23B-1331
Fabric and Elastic property evolution in a Granular Dynamics based sedimentation model
We study the evolution of fabric and elastic properties of a granular pack during a computationally modeled deposition and compaction process. We consider a random pack of spherical quartz grains and simulate gravity sedimentation and compaction using a granular dynamics simulation. We focus on studying the interrelationships between fabric and elastic properties near the critical porosity which is difficult to model in laboratory experiments. In the deposition model, the normal grain interactions are modeled by a visco-elastic contact law and shear interactions by a tangential elasticity model. We estimate the elastic properties of the pack using both finite element and granular dynamics calculations at intermediate stages of the simulation. For the fabric properties, we characterize pore space created by the grains using a second rank fabric tensor. The fabric tensor also gives an estimate of the fabric anisotropy in the pack. We find that the elastic properties near the critical porosity depend more on the grain rearrangements and less on the confining pressure. We also investigate the relation between fabric anisotropy and coordination number. We observe that the fabric tensor has transverse isotropy symmetry at the end of the simulation. The use of this process-based depositional simulation helps us in conducting numerical experiments to complement laboratory results in the study of loose unconsolidated sediments.