Black Hills Quartzite Research Articles

Analysis of crystallographic preferred orientations of experimentally deformed Black Hills Quartzite

Abstract. The crystallographic preferred orientations (textures) of three samples of Black Hills Quartzite (BHQ) deformed experimentally in the dislocation creep regimes 1, 2 and 3 (according to Hirth and Tullis, 1992) have been analyzed using electron backscatter diffraction (EBSD). All samples were deformed to relatively high strain at temperatures of 850 to 915 °C and are almost completely dynamically recrystallized. A texture transition from peripheral [c] axes in regime 1 to a central [c] maximum in regime 3 is observed. Separate pole figures are calculated for different grain sizes, aspect ratios and long-axis trends of grains, and high and low levels of intragranular deformation intensity as measured by the mean grain kernel average misorientation (gKAM). Misorientation relations are analyzed for grains of different texture components (named Y, B, R and σ grains, with reference to previously published prism, basal, rhomb and σ1 grains). Results show that regimes 1 and 3 correspond to clear end-member textures, with regime 2 being transitional. Texture strength and the development of a central [c]-axis maximum from a girdle distribution depend on deformation intensity at the grain scale and on the contribution of dislocation creep, which increases towards regime 3. Adding to this calculations of resolved shear stresses and misorientation analysis, it becomes clear that the peripheral [c]-axis maximum in regime 1 is not due to deformation by basal 〈a〉 slip. Instead, we interpret the texture transition as a result of different texture forming processes, one being more efficient at high stresses (nucleation or growth of grains with peripheral [c] axes), the other depending on strain (dislocation glide involving prism and rhomb 〈a〉 slip systems), and not as a result of temperature-dependent activity of different slip systems.

The grain size(s) of Black Hills Quartzite deformed in the dislocation creep regime

Abstract. General shear experiments on Black Hills Quartzite (BHQ) deformed in the dislocation creep regimes 1 to 3 have been previously analyzed using the CIP method (Heilbronner and Tullis, 2002, 2006). They are reexamined using the higher spatial and orientational resolution of EBSD. Criteria for coherent segmentations based on c-axis orientation and on full crystallographic orientations are determined. Texture domains of preferred c-axis orientation (Y and B domains) are extracted and analyzed separately. Subdomains are recognized, and their shape and size are related to the kinematic framework and the original grains in the BHQ. Grain size analysis is carried out for all samples, high- and low-strain samples, and separately for a number of texture domains. When comparing the results to the recrystallized quartz piezometer of Stipp and Tullis (2003), it is found that grain sizes are consistently larger for a given flow stress. It is therefore suggested that the recrystallized grain size also depends on texture, grain-scale deformation intensity, and the kinematic framework (of axial vs. general shear experiments).

Mapping texture domains in quartzite microstructures

The paper is concerned with the combined application of two methods for microstructure and texture analysis: (1) optical orientation and misorientation imaging and (2) autocorrelation function (ACF). Using two different samples, the usefulness of the methods and the quality of the results are discussed. One sample is a quartzite layer from the limb of a small scale fold in the Suretta Nappe (ValAvers, Switzerland) which was deformed under upper greenschist conditions, the other is a sample of Black Hills quartzite, experimentally sheared in dislocation creep. Using the method of computer-integrated polarization microscopy (CIP), c-axis orientation and misorientation images were prepared and pole figures were calculated from the azimuth and inclination images. Misorientation images were prepared and analyzed in a number of ways: (1) In as much as misorientation images constitute texture maps, they visualize the geometry and spatial distribution of texture domains; they show if the texture domains are spatially coherent, i.e., if the rock is domainal or not. (2) Thresholded misorientation images allow the calculation of the volume fraction of texture domains, which is a means of estimating the texture intensification independent of the shape of the pole figure skeleton. (3) From the ACF of thresholded misorientation images the average grain size and shape in those domains can be derived. (4) From the shape of the domains (if they exist as spatial entities), the strain or strain partitioning as well as the rheological contrast can be estimated. The paper seeks to demonstrate the importance of localized texture analysis. The aim is to discuss the quality of the results that can be obtained. The samples were chosen because one is domainal the other one is not. A full discussion of the geological implications of the results of the analysis is outside the scope of this contribution.

Dynamic recrystallization near the brittle-plastic transition in naturally and experimentally deformed quartz aggregates

The electron backscattering diffraction technique (EBSD) was used to analyze bulging recrystallization microstructures from naturally and experimentally deformed quartz aggregates, both of which are characterized by porphyroclasts with finely serrated grain boundaries and grain boundary bulges set in a matrix of very fine recrystallized grains. For the Tonale mylonites we investigated, a temperature range of 300–380 °C, 0.25 GPa confining pressure, a flow stress range of ~ 0.1–0.2 GPa, and a strain rate of ~ 10 − 13 s − 1 were estimated. Experimental samples of Black Hills quartzite were analyzed, which had been deformed in axial compression at 700 °C, 1.2–1.5 GPa confining pressure, a flow stress of ~ 0.3–0.4 GPa, a strain rate of ~ 10 − 6 s − 1 , and to 44% to 73% axial shortening. Using orientation imaging we investigated the dynamic recrystallization microstructures and discuss which processes may contribute to their development. Our results suggest that several deformation processes are important for the dismantling of the porphyroclasts and the formation of recrystallized grains. Grain boundary bulges are not only formed by local grain boundary migration, but they also display a lattice misorientation indicative of subgrain rotation. Dynamic recrystallization affects especially the rims of host porphyroclasts with a hard orientation, i.e. with an orientation unsuitable for easy basal slip. In addition, Dauphiné twins within porphyroclasts are preferred sites for recrystallization. We interpret large misorientation angles in the experimental samples, which increase with increasing strain, as formed by the activity of fluid-assisted grain boundary sliding.

Effect of water on the dislocation creep microstructure and flow stress of quartz and implications for the recrystallized grain size piezometer

Deformation experiments on Black Hills quartzite with three different initial water contents (as‐is, water‐added, and vacuum‐dried) were carried out in the dislocation creep regime in order to evaluate the effect of water on the recrystallized grain size/flow stress piezometer. Samples were deformed in axial compression at temperatures of 750°–1100°C, strain rates between 2 × 10−7 s−1 and 2 × 10−4 s−1 and strains up to 46% using a molten salt assembly in a Griggs apparatus. An increase of the initial water content at otherwise constant deformation conditions caused a decrease in flow stress, an effect known as hydrolytic weakening. The total water content of the starting material was analyzed by Karl Fischer titration (KFT) and Fourier transform infrared (IR) spectroscopy, and quenched samples were analyzed microstructurally and by IR. Changes in the dynamic recrystallization microstructure correlate with changes in flow stress, but there is no independent effect of temperature, strain rate or water content. IR absorption spectra of the deformed spectra indicate that different water contents were maintained in the three sample sets throughout the experiments. However, the amounts of water measured within the vacuum‐dried (∼260 ± 40 ppm H2O), the as‐is (∼340 ± 50 ppm H2O), and the water‐added (∼430 ± 110 ppm H2O) samples are significantly smaller than the initial content of the quartzite (∼640 ± 50 ppm H2O). Water from the inclusions in the starting material adds to the free fluid phase along the grain boundaries, which probably controls the water fugacity and the flow strength, but this water is largely lost during IR sample preparation. Vacuum‐dried as well as water‐added samples have the same recrystallized grain size/flow stress relationship as the piezometer determined for as‐is samples. No independent effect of water on the piezometric relationship has been detected.

The recrystallized grain size piezometer for quartz

In order to determine a recrystallized grain size piezometer for quartz, we deformed Black Hills quartzite in a molten salt assembly in a Griggs apparatus at 1.5 GPa, 800 to 1100°C, and strain rates between 2*10−7 and 2*10−4 s−1, conditions which include dislocation creep regimes 2 and 3 of Hirth and Tullis [1992]. Flow stresses ranged from 34 ± 16 to 268 ± 38 MPa with corresponding recrystallized grain sizes from 46 ± 15 to 3.2 ± 0.7 μm. The data are well fit by a single piezometer relation, D = 103.56±0.27 * σ−1.26 ±0.13, with no change in slope at the regime 2–3 transition and no effect of temperature or α/β stability field. Another experimental piezometer relation for regime 1 of Hirth and Tullis [1992] differs in slope, suggesting that different recrystallization mechanisms require different piezometer calibrations.

Automatic grain boundary detection and grain size analysis using polarization micrographs or orientation images

A simple procedure for creating grain boundary maps from petrographic thin sections, termed the Lazy Grain Boundary (LGB) method, has been devised. It is based on a set of macro commands programmed for NIH Image, a public domain image processing software. The grain boundary detection is based on gradient filtering. The procedure makes use of multiple input images: sets of regular polarized micrographs or—where possible—orientation/misorientation images. On each image of a given input set, only the most significant grain boundaries are detected and by combining those, a single grain boundary map is obtained. Image models and criteria for grain boundary identification are discussed briefly. The principal aim of the LGB method is to facilitate the production of large grain boundary maps (containing several thousand cross-sectional areas) to provide statistically valid samples for grain size analysis. A section of Black Hills quartzite was used to test the LGB method. Five different input sets were prepared and used for automatic LGB analysis. One set was additionally used for an interactive analysis and for a manual tracing. From all seven grain boundary maps, the 2- and 3-D grain size distributions were calculated. The results are compared and the different image sets and procedures are rated for precision, over- and undersegmentation and speed of preparation.

Effects of chemical environment on dislocation creep of quartzite

The water‐related chemical parameter that affects dislocation creep in quartzite has been determined from variations in sample strength and microstructure with chemical environment in buffered deformation and hydrostatic annealing experiments. Samples were weld‐sealed in double capsules;, , and were buffered using solid oxygen buffers, AgCl or CO2. Black Hills quartzite was deformed at 900°C and 1.5 × 10−5s−1. Two samples were deformed at ∼1700 MPa confining pressure at constant and , with and varying over 8 and 15 orders of magnitude, respectively. Both samples deformed by climb‐accommodated dislocation creep with flow stresses of 300 MPa. Two additional samples were deformed at ∼700 MPa at constant lower than for the 1700‐MPa samples, with varying over 2 orders of magnitude. Both samples faulted with a peak strength of ∼800 MPa. These four experiments suggest no dependence of dislocation creep strength on , or ; instead, a strong dependence of strength on is inferred. Previously deformed samples of Heavitree quartzite were hydrostatically annealed for 4 days at 800°C and 1200 or 500 MPa confining pressure, varying and over 2.5 and 1 order of magnitude, respectively. The microstructures of these samples show increased rates of dislocation climb and grain boundary migration with increasing but no dependence on . These buffered experiments indicate that dislocation creep is affected by alone and suggest that the exponent for the term in the power law creep flow law is >2.