Abstract Grain size distribution affects the distribution of melt in partially molten rocks, as melt is partitioned into the finer grained regions. As the creep strength of partially molten rocks depends on melt fraction, grain size distribution affects rheology in two ways: in the diffusion-creep regime the strength of the rock decreases with decreasing grain size; and by affecting the melt distribution, grain size distribution affects rock strength in the dislocation and the diffusion-creep regimes. To investigate this effect on partially molten dunites, we deformed synthetic aggregates of olivine + mid-ocean ridge basalt (MORB) with varying grain size distributions and melt fractions. We found that samples with bimodal grain size distributions did not obey the flow law derived for samples with a uniform grain size distribution. In the samples with bimodal grain size distributions the finer grained parts are enriched in melt, reducing the relative melt fraction in the coarser grained parts. The stress exponent is close to 1 for lower stresses for samples with non-uniform grain size distributions, while it is close to 3 for samples with a uniform grain size distribution. Electron backscatter diffraction (EBSD) analysis shows crystallographic preferred orientation (CPO) well developed in all samples. Microstructural investigation shows that partitioning of the liquid out of the coarser grained part of the aggregate into the finer grained part leads to a localization of the strain into the weaker, finer grained and melt-enriched part. By treating the two parts as mechanically different phases that deform at a uniform stress, we are able to approximate the rheological behaviour of the aggregates with bimodal grain size distribution.

Abstract The term “texture” encompasses a wide range of attributes of sediments/rocks and their constituent grains, including grain size and sorting, particle morphology (form and sphericity, rounding, and surface texture), grain orientation, imbrication and packing. All of those properties have significance in interpreting transport processes and depositional settings of sedimentary rocks, but they also have economic importance in a wide variety of fields ranging from engineering of construction and road materials to understanding and predicting the porosity and permeability relationships of such materials in petroleum exploration/production or hydrologic contexts. Most textural properties mentioned above are best measured in unconsolidated or easily disaggregateable materials where grains can be size-sorted by sieving, settling, laser particle analysis or other techniques or where individual grains can be viewed in three dimensions. These properties generally are far more difficult to measure accurately in thin sections of consolidated rocks. To use just one example, the size of a grain in thin section can never exceed the longest axis of the grain but it can easily be shorter because most cuts through grains are tangential or oblique to that axis. So, in most cases, the basic size-, sorting- and shape-related properties of grains are merely estimated in thin sections through the use of visual comparators. Even there, the most useful comparators are ones that have been specifically corrected for thin-section use. Detailed direct measurements on grain size and shape can, of course, be done using thin-section microscopy, especially through point-counting large numbers of grains, but there too, correction factors must be used to overcome, to the degree possible, the two-dimensional (2-D) view of three-dimensional (3-D) grains afforded in thin sections (see Harrell and Eriksson, 1979; Johnson, 1994). Advances in computerized photomicrographic image analysis can make the process of measurement and measurement correction both more accurate and far less time consuming (see, for example, Schäfer and Teyssen, 1987; Seelos and Sirocko, 2005; Syvitski, 2007). An enormous amount of effort was made in the period from the 1930s to the 1970s to perfect textural measurements and to find reliable statistical measures that could be used to identify specific environments of deposition. That work still finds application in soft and unconsolidated sediments, but it will not be discussed in detail in this book, because it is of lesser applicability to petrographic studies of hard rocks. A number of papers in the bibliography at the close of this section can be used to follow up on unconsolidated sediment studies (especially the excellent overview of statistical measures provided in Folk, 1980) and almost all textbooks on sands and sandstones include discussions of these topics.

Abstract The purpose of this paper is to investigate the potential of the use of the Hurst test as a statistical tool for quantifying the degree of clustering of high and low values of field measurements in vertical turbiditic sections within a small and confined tectonically mobile basin. The paper (1) documents the fact that significant clustering occurs in almost all sections, (2) shows that there are systematic spatial variations in the degree of clustering such that different parts of the basin have different Hurst statistics, (3) discusses the origin of those spatial trends, and (4) compares Hurst H with Chen and Hiscott's Hurst K . The case study focuses on one well-exposed reservoir-scale turbiditic unit cropping out in the eastern part of the Tertiary Piedmont Basin (NW, Italy) and representing the lower portion of an Upper Oligocene-Lower Miocene turbidite system (Castagnola Formation). The sedimentological model that has been adopted is based on the description of 41 sedimentological logs (approximately 25–35 m apart) physically correlated bed by bed. Three variables were studied: thickness of sandstones and siltstones, grain size score, and sand and silt thickness percentage (i.e. the ratio of coarse division to overlying mudstones, in percent). The Hurst test has revealed the existence of common clustering for the three studied variables (only 2 of 41 logs failed to pass the test at the 10% significance level for all the studied variables). This clustering is associated with lateral and vertical facies variations in response to changes in depositional sub-environments within the Castagnola Basin. A geostatistical analysis has been performed in order to estimate the spatial distribution of the Hurst statistics within the studied area. The analysis makes possible predictions at unsampled locations.

Abstract Quartzite samples were experimentally deformed with partial to complete dynamic recrystallization by axial compression (strain magnitude of 0.8 to 1.4) and by general shear (strain magnitude of 1.3 to 2.8) in each of the three dislocation creep regimes, and subsequently annealed with complete static recrystallization at the deformation temperature for 120 hours. The c -axis crystallographic preferred orientation (CPO), 3D grain size distribution, grain boundary surface shape, and misorientation density were measured before and after annealing. The effect of annealing on the CPO was minor, but the microstructure was greatly changed. All of the annealed samples were completely recrystallized. The recrystallized grain size increased by a factor of 2 to 5, and was greatest for samples deformed at lowest temperature. The grain boundary lobateness (PARIS factor) and misorientation density were reduced significantly. The CPOs for all the deformed samples were relatively unchanged by annealing, although the strengths are somewhat decreased; for sheared samples the asymmetry was preserved. The results suggest microstructural criteria for recognizing the occurrence of static annealing and for estimating the dynamically recrystallized grain size relevant for paleopiezometry from annealed samples.

Abstract The main petrographic variables of intrusive rocks that influence the technical test methods common used within the aggregate industry were identified and evaluated, the aim being to investigate whether petrographic description could be used as a tool for the preliminary quality evaluation of different rock groups. Evaluation of the dependence of the technical test methods on petrographic variables covered 26 intrusive rocks, including some of their altered varieties (orthogneisses), divided into two groups: 17 granitoids and 9 gabbroids. The selective designated samples were analysed to determine their petrographic characteristics and resistance to fragmentation, wear and wear by abrasion from studded tyres. In order to identify the complicated associations between the petrographic and technical properties, multivariate statistical evaluation was performed. The results of statistical evaluation highlighted that mean grain size of mica minerals, grain size distribution, content of mica minerals and mean grain size were the main variables influencing the technical properties of granitoid rocks, while for gabbroid rocks, the main influences were the frequency of microcracks, mica content, grain size distribution and mean grain size.