Mark Peternell

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

Abstract 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 {1011} 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.

A technique for recording polycrystalline structure and orientation during in situ deformation cycles of rock analogues using an automated fabric analyser

Two in situ plane‐strain deformation experiments on norcamphor and natural ice using synchronous recording of crystal c‐axis orientations have been performed with an automated fabric analyser and a newly developed sample press and deformation stage. Without interrupting the deformation experiment, c‐axis orientations are determined for each pixel in a 5 × 5 mm sample area at a spatial resolution of 5 μm/pixel. In the case of norcamphor, changes in microstructures and associated crystallographic information, at a strain rate of ∼2 × 10−5 s−1, were recorded for the first time during a complete in situ deformation‐cycle experiment that consisted of an annealing, deformation and post‐deformation annealing path. In the case of natural ice, slower external strain rates (∼1 × 10−6 s−1) enabled the investigation of small changes in the polycrystal aggregates crystallography and microstructure for small amounts of strain. The technical setup and first results from the experiments are presented.

Evaluating ice fabrics using fabric analyser techniques in Sørsdal Glacier, East Antarctica

Ice cores ( 4m long) obtained from areas of different surface velocities near the terminus of Sørsdal Glacier, East Antarctica, have been investigated using two versions of a fabric analyser (G50). In sections parallel to the flow plane, the microstructure is typically interlocking with elongate grains that parallel air-bubble elongation, X, reflecting their development in an earlier ductile regime. The c-axis fabric patterns vary with respect to X and vary from single–double maxima to asymmetric smallcircle girdles oblique to the planar foliation, which can be attributed to a simple shear regime. The siteto-site variations in the c-axis patterns can be related to areas of different surface velocities, the asymmetry of fabrics correlating with localized strain variations and differences in the deformation path, but not to the current strain pattern recorded by the near-surface deformation conditions. Overprinting fractures have little effect on microstructure except for local dissolution and precipitation along stylolitic surfaces. Comparison of results from the two different fabric analysers reveals that with a higher pixel resolution the incorporation of additional monochromatic light-emitting diodes and repositioning of a retarder plate produce more reliable c-axis measurements.

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 patterns 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.

Experimental deformation of deuterated ice in 3D and 2D: identification of grain-scale processes

Major polar ice sheets and ice caps experience cycles of variable flow during different glacial periods and as a response to past warming. The rate and localisation of deformation inside an ice body controls the evolution of ice microstructure and crystallographic fabric. This is critical for interpreting proxy signals for climate change, with deformation overprinting and disrupting stratigraphy deep under ice caps due to the nature of the flow. The final crystallographic fabric in polar ice sheets provides a record of deformation history, which in turn controls the flow properties of ice during further deformation and affects geophysical sensing of ice sheets. For example, identification of layering in ice sheets, using seismic or ice radar techniques, is attributed to grain size changes and fabric variations. Such information has been used to provide information on climate state and its changes over time, and as the Fourth Intergovernmental Panel on Climate Change (IPCC) Report (Solomon et al. 2007) points out there is currently still a lack of understanding of internal ice-sheet dynamics. To answer this we have recently conducted experiments at the Australian Nuclear Science and Technology Organisation (ANSTO) to collect fully quantitative microstructural data from polycrystalline heavy water (D2O) ice deformed in a dynamic regime. The ice and temperature (–7°C) chosen for this study is used as a direct analogue for deforming natural-water ice as it offers a unique opportunity to link grain size and texture evolution in natural ice at –10°C. Results show a dynamic system where steady-state rheology is not necessarily coupled to microstructural and crystallographic fabric stability. This link needs to be taken into account to improve ice-mass-deformation modelling critical for climate change predictions.

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

Introduction 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, grainand 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.