Khadija Alaoui, a PhD candidate at ISTO, will give her mid-thesis defense on Phyllosilicate deformation and its effect on shear zone formation and fluid circulation in the upper and middle crust: an experimental study of the deformation mechanisms of mica-quartz assemblage.
Oct 24
10:15
E018
Abstract
Phyllosilicate growth is one of the most common fluid-driven mineralogical reaction observed in upper and mid crustal shear zones and strongly influences rock mechanics. However, deformation mechanisms in phyllosilicate-rich rocks are still not well constrained, as well as if these mechanisms change according to the phyllosilicates proportion.
A first set of exploratory shear experiment (Griggs-type apparatus) using mica-quartz-aggregates were conducted to explore PT conditions, sample thickness, grain size and starting material composition. Results from the first set allowed to narrow the conditions, therefore a second set of experiments were performed in order to investigate the role of biotite-bearing systems for the development of shear zones and strain accommodation. This study used phlogopite (Phl) and quartz (Qz) assemblages deformed in different proportions (10, 20, 30, 50, 70 and 100% vol. Phl.), but at similar conditions (800°C, 10kbar, ε~10-5 s-1, 0.1% wt. H2O, 63μm < Phl < 125μm, 10μm < Qz < 20μm).
Mechanical results indicate strong samples for 10% and 20% vol. Phl, deforming at differential stresses of 918 and 1183MPa respectively. Such a stiffness is approximately equivalent to pure quartz samples (448BR in Richter et al., 2018). In all samples, an important grain size reduction is noticeable for mica flakes (less than 10mm). This grain size reduction is also present for quartz grains, especially with decreasing proportion of mica in the assembly. Most of the strain is accommodated by quartz behaving as an interconnected network in Phl-poor samples. Cathodoluminescence imaging reveals that quartz is reworked mainly by local pressure-solution and its strength controls the overall strain accommodation with deformed clasts in Phl-poor sample. A significant neo-formed micro-porosity develops in quartz (syn-kinematic). In future, in order to determine if the main process is recrystallization or reprecipitation, more localized analysis and data treatment will be conducted to define phase proportions (image analysis), how phases deform (EBSD, TEM), if it correlates with chemical composition (CL, EPMA). A few more experiments are also intended (peak stress experiments, F-Phl repeated at same conditions as in the second set, pure quartz).