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 contributio

Grain size distributions of fault rocks: a comparison between experimentally and naturally deformed granitoids.

International audienceWe have investigated the grain size distribution (GSD) of granitoid fault rock by comparing experimentally produced gouge with fault rock from the Nojima Fault Zone. Triaxial experiments were carried out on wet and dry intact samples of Verzasca Gneiss at T = 300 and 500°C, Pc = 500 and 1030 MPa, ε = 0.013-1.6 x 10-4s-1. The GSD has been determined from SEM-micrographs and is characterized by the slope, D, of its log(frequency)-log(radius) histogram. The GSD is not fractal; we observe two slopes for all GSDs. The larger grains in experimentally deformed samples have a D value, D>, of 2.04 and 2.26 for feldspar and quartz gouge. Cracked grains yield values of D = 1.5-1.6.Increasing the confining pressure or temperature decreases the D-value. For grains smaller than ~2 μm another D-value, D = 2.02 for gouge and 1.64 for cracked grains; D< = 0.97. Grain size reduction in fault zones develops by a two-stage process: rupturing creates cracked grains; further displacement of fragments causes further comminution by wear and attrition. Cracked grains have been used to calculate the surface energy associated with faulting; it follows that this energy forms a small fraction in the total energy-budget of earthquakes

Dislocation creep of dry quartz

International audienceSmall-scale shear zones within the Permian Truzzo meta-granite developed during the Alpine orogeny at amphibolite facies conditions. In these shear zones magmatic quartz deformed by dislocation creep and recrystallized dynamically by grain boundary migration with minor subgrain rotation recrystallization to a grain size of around 250–750 μm, consistent with flow at low differential stresses. Fourier transform infrared (FTIR) spectroscopy reveals very low water contents in the interior of recrystallized grains (in the form of discrete OH peaks, ~20 H/10 6 Si and very little broad band absorption, <100 H/10 6 Si). The spectral characteristics are comparable to those of dry Brazil quartz. In FTIR spectra, magmatic quartz grains show a broad absorption band related with high water concentrations only in those areas where fluid inclusions are present while other areas are dry. Drainage of fluid inclusions and synkinematic growth of hydrous minerals indicates that a hydrous fluid has been available during deformation. Loss of intragranular water during grain boundary migration recrystallization did not result in a microstructure indicative of hardening. These FTIR measurements provide the first evidence that quartz with extremely low intragranular water contents can deform in nature by dislocation creep at low differential stresses. Low intragranular water contents in naturally deformed quartz may not be necessarily indicative of a high strength, and the results are contrary to implications taken from deformation experiments where very high water contents are required to allow dislocation creep in quartz. It is suggested that dislocation creep of quartz in the Truzzo meta-granite is possible to occur at low differential stresses because sufficient amounts of intergranular water ensure a high recovery rate by grain boundary migration while the absence of significant amounts of intragranular water is not crucial at natural conditions

Evolution in H2O contents during deformation of polycrystalline quartz: An experimental study

Accepted manuscript version, licensed CC BY-NC-ND 4.0. Published version available at https://doi.org/10.1016/j.jsg.2018.05.021.Shear experiments were performed in a Griggs-type apparatus at 800 °C and 1.5 GPa, at a strain rate of 2.1 × 10−5s−1 using different starting materials: (i) Powder (grain size 6–10 μm) of dry Brazil quartz with 0.15 wt% added H2O, (ii) “dry” Brazil quartz porphyroclasts (grain size ∼100–200 μm), devoid of fluid inclusions embedded in the same fine grained powder, and (iii) “wet” porphyroclasts (grain size ∼100–200 μm), containing initially a high density of μm-scale fluid inclusions embedded in the same powder. After hot pressing, samples were deformed to large shear strains (γ∼3 to 4.5), in order for the microstructures and H2O distribution to approach some state of “equilibrium”. The H2O content and speciation in quartz were analyzed by Fourier Transform Infra-Red (FTIR) spectroscopy before and after the experiments. Mechanical peak strength is generally lower in experiments with 100% hydrated matrix, intermediate in experiments incorporating wet porphyroclasts (with a proportion of 30 or 70%) and highest in those with dry porphyroclasts. All experiments with porphyroclasts show pronounced strain weakening, and the strengths of most samples converge to similar values at large strain. Wet porphyroclasts are pervasively recrystallized during deformation, while dry porphyroclasts recrystallize only at their rims and remain weakly deformed. Recrystallization of the initially fluid-inclusion-rich porphyroclasts results in a decrease in inclusion abundance and total H2O content, while H2O content of initially dry clasts increases during deformation. H2O contents of all high strain samples converge to similar values for matrix and recrystallized grains. In samples with wet porphyroclasts, shear bands with high porosity and fluid contents develop and they host the precipitation of euhedral quartz crystals surrounded by a free-fluid phase. These high porosity sites are sinks for collecting H2O in excess of the storage capacity of the grain boundary network of the recrystallized aggregate. The H2O storage capacity of the grain boundary network is determined as a H2O-boundary-film of ∼0.7 nm thickness

Dual career and job sharing - two careers or half a career?

International audienceOne option for a dual career that is often considered is job sharing. After 20 years of job sharing during the most competitive years of our careers we would like to share a few thoughts on some of the problems we came across. The typical job sharing situation is one of a young couple about to found a family. However, this need not be the case, many alternative models are thinkable, few are actually liveable. The list of problems we came across includes: 1 - The hiring age for professors keeps dropping. This adds extra stress to the competitive post doc time 2 - Postdoc positions are not designed for dual career. ... and much less for job sharing. 3 - The higher the academic position the less likely it is offered for job sharing. - because it is claimed that leadership and responsibility cannot be shared. - because two half positions do indeed cost more than one whole (what hiring institutions fail to see is that they get two instead of one fully qualified scientists in return) - because they are difficult to plan: what happens if one partner leaves the department? 4 - Age difference of dual career partners Partners of different age have different qualifications and experiences. Usually the career of the more advanced partners is promoted more, the career of the second partner falls behind

Dual career and job sharing - two careers or half a career?

International audienceOne option for a dual career that is often considered is job sharing. After 20 years of job sharing during the most competitive years of our careers we would like to share a few thoughts on some of the problems we came across. The typical job sharing situation is one of a young couple about to found a family. However, this need not be the case, many alternative models are thinkable, few are actually liveable. The list of problems we came across includes: 1 - The hiring age for professors keeps dropping. This adds extra stress to the competitive post doc time 2 - Postdoc positions are not designed for dual career. ... and much less for job sharing. 3 - The higher the academic position the less likely it is offered for job sharing. - because it is claimed that leadership and responsibility cannot be shared. - because two half positions do indeed cost more than one whole (what hiring institutions fail to see is that they get two instead of one fully qualified scientists in return) - because they are difficult to plan: what happens if one partner leaves the department? 4 - Age difference of dual career partners Partners of different age have different qualifications and experiences. Usually the career of the more advanced partners is promoted more, the career of the second partner falls behind