Experimental deformation of natural monomineralic quartz to produce micro-porosity

In natural shear zones, micro-porosity is found to decorate the grain boundaries of rocks which have been deformed in viscous conditions. Whether porosity is formed during or after deformation is widely debated, and requires further investigation to test how micro-pores may be produced, particularly in monomineralic aggregates.

Using a new-generation Griggs-type apparatus, we performed two general shear experiments using a fine grained (∼ 3 μm) quartzite (white novaculite) with low to no primary porosity (< 1%). The experiments were performed at a temperature of 900 °C and pressures of 1.2 and 1.5 GPa, with bulk strain rates of ≅ 1.2×10^-4 and 2.3×10^-5 s^-1, respectively. In both, 1 wt% of water was added to the starting sample.

During deformation, both samples record a significant stress drop following a high peak of differential stress, after which a progressive strain weakening occurs over several gamma of shear strain. In the experiment at 1.2 GPa, the sample deformed above the Goetze criterion at peak stress, where σ₁-σ₃ > 1.2 GPa, which gave rise to a highly fractured sample. Within the sample, shear planes < 1 µm thick contain a material which is brighter than quartz in SEM-BSE, despite being composed of SiO₂. Microstructural observations of the same sample show the production of a penetrative secondary porosity, where most pores are < 1 µm in diameter. Pores decorate most grain boundaries, which are open and easily identifiable in the SEM.

In contrast, the experiment at 1.5 GPa did not experience any fracturing, and the maximum differential stress remained below the Goetze criterion at σ₁-σ₃ ≅ 0.8 GPa. In this experiment, a porosity of microns to tens of microns in size developed along apparent conjugate bands. Outside of these bands, there is no porosity nor open grain boundaries. Electron backscatter diffraction (EBSD) analyses reveal quartz which deformed viscously, both inside and outside of the porosity-decorated bands. However, quartz grains within the pore-decorated bands have a stronger intragrain misorientation and higher lattice curvature gradients, as well as slightly weaker lattice-preferred orientation than in the surrounding, non-decorated quartz. Finally, an interesting feature of EBSD maps is the lower indexation rate of quartz within the porosity-decorated bands, at 54% within compared to 83% outside. While the reason for this is unknown, the non-indexed area is considerably larger than the area of pores.