Weakening mechanisms on mica-bearing rocks: an experimental approach

Experimental and field-based research has shown that the presence of mica minerals drastically reduces the mechanical resistance of rocks during both brittle and viscous deformation, which allows the localization of deformation into narrow regions such as shear zones. Furthermore, experimental work has proved that the resistance of phlogopite-quartz assemblages with a mica abundance of 30% is as weak as a sample composed entirely of mica. This phenomenon occurs as a combination of several processes, including developing interconnected networks of mica domains and grain size decrease in quartz. Nevertheless, it remains unclear which other processes enhance weakening in these rocks, to what extent each process is significant, in which stage of deformation they take place, and how the presence of mica assists the deformation of quartz. In this work, we address these questions by analyzing the microstructure of six experimental shear zones carried on samples composed of 30% muscovite and 70% quartz with different amounts of strain. By combining microstructural and mechanical information, we aim to infer how and when different weakening processes occur.

Simple shear experiments were conducted using a Griggs-type apparatus to deform 0.1 g of a powder composed of 30% muscovite with an initial grain size of 62-125 µm and 70% quartz with an initial grain size of 10-20 µm. The samples reached different amounts of gamma strain ranging from 0 to 6.  The experiments were carried out at T=800°C, P=1 GPa, added H₂O=0.1%, and ė≈1x10^-5s^-1. Afterward, in the post-mortem samples, a detailed microstructural analysis was carried out comparing SEM-BSE images, cathodoluminescence in SEM (CL), and EBSD maps. Some microstructural parameters were acquired such as the interconnectivity of each phase, grain size distribution, and grain lattice misorientation, which were compared to the CL-signal.

As strain increases, the interconnectivity of mica grains does not increase or decrease significantly, but rather, mica grains decrease in size through breaking. Quartz grains located in mica-rich domains preserve their original size and shape while micas take most of the deformation. On the other hand, in quartz-rich domains, the grain size is intensely reduced as strain increases. Additionally, a blue material in the CL maps appears along grain boundaries and microcracks. This blue material becomes more and more abundant and interconnected as deformation increases, which is the main feature appearing as a consequence of progressive deformation. The correlation of CL and EBSD maps indicates that some of the newly formed grains (blue material) present low misorientation to the parental grain, some other grains preserve exactly the orientation of the parental grain, and others present a random misorientation to the parental grain. This suggests that the coupling between continuous dissolution-precipitation and crystal-plastic deformation of quartz is the most suitable mechanism behind the formation of the new material. However, the source of luminescence of the precipitated material in the CL spectra remains unclear.