The origin of micro-porosity in quartz mylonites: insights from quartz-rich shear bands in the Ikaria granite

Micro-scale porosity is a feature commonly found in viscously deformed quartz-rich mylonites. However, the processes which may form such porosity are actively debated, and whether or not pores are formed syn-kinematically to shear zone activity remains uncertain. Yet, the production of micro-pores during rock deformation may have several critical implications, such as affecting the rock strength, possibly through the brittle-ductile transition, and/or providing fluid pathways through active shear zones.

In this study we focus on quartz-rich shear bands produced during extensional deformation of a granitic pluton below the detachment of Ikaria (Cyclades, Greece). Nearby to the detachment, quartz aggregates are often decorated by micrometric and sub-micrometric pores, of which a large proportion adopt angular, crystallographically controlled shapes. Quartz in such decorated shear bands primarily deformed by crystal plasticity and underwent dynamic recrystallisation by subgrain rotation. Using a combination of standard and High-angular Resolution (HR) Electron Back-Scatter Diffraction (EBSD) analyses alongside Scanning and Transmission Electron Microscopy (SEM/TEM), we highlight that micro-pores decorate primarily grain boundaries, as well as some intragrain substructures including subgrain boundaries. EBSD analyses show that pore-decorated substructures are characterised by high (~4°) Kernel Average Misorientation (KAM), which describes the mean lattice misorientation of one EBSD pixel with respect to its closest neighbours. (HR)EBSD maps indicate a high lattice curvature gradient across these pore-decorated substructures, which can be seen by Geometrically Necessary Dislocation (GND) densities as high as 10¹⁵ per m².

TEM analyses of Focused Ion Beam (FIB) sections across grain and intragrain boundaries reveal that quartz contains free dislocation densities around 10¹³ per m², which matches our (HR)EBSD estimates for the interior of grains and subgrains. GND estimates of some porosity-decorated subgrain boundaries are between 10¹⁴ to 10¹⁵ dislocations per m², which are not visible in TEM. Instead, nm-scale layers of amorphous SiO₂ are seen, into which porosity is often partially or fully embedded.

Our results suggest that amorphous SiO₂ and porosity are formed from the same process, since pores are embedded into amorphous SiO₂. Furthermore, in the case of pore-decorated substructures where amorphous SiO₂ is present, a factor other than dislocation climb likely accounts for their quartz lattice distortion, possibly related to a stress concentration. Although the origin of quartz amorphization remains a matter of discussion, we hypothesise a stress concentration which caused quartz to amorphize, followed by subsequent pore formation through fluid exsolution while stress was released. If this is the case, it would strongly suggest that pore nucleation occurred syn-kinematically in Ikaria.