PhD defense of Gina McGill

Porosity is a fundamental characteristic of natural materials, particularly observed in sediments and volcanic rocks in sub-surface conditions, but in the deeper, viscous (or ductile) realm. In these zones, micro-porosity is regularly described but is still misunderstood in terms of nucleation processes. Micro-porosity is documented in both poly- and mono-mineralic systems, and mainly occurs in mylonitic rocks from ductile shear zones, which are known to be sites of fluid circulation within the middle-deep crust. Provided that deep porosity is produced syn-kinematically, its occurrence is hypothesised to be a factor contributing to deep earthquake propagation. This study focuses on quartz mylonites, and aims to understand how this porosity originates. We have first investigated mylonitic shear bands from the Ikaria granitic body (Greece), where pure quartz aggregates are decorated by geometrically-shaped micro-pores. Thanks to several high-resolution analytical techniques, we highlight pores mostly at grain boundaries, but also along intra-grain substructures, particularly in highly deformed and recrystallized areas where the crystal fabric has been partially randomized. This suggests a syn-kinematic origin of pores, likely related to crystal plasticity and grain boundary sliding. Pores being syn-kinematic is further suggested by the identification of amorphous SiO2 at grain and sub-grain boundaries, possibly related to mechanical amorphization. We then performed deformation experiments of quartz aggregates in a general shear geometry, analogous to shear bands. Using a Griggs press at 15 kbar and 900 °C, which are the required conditions to deform quartz viscously in the laboratory, a pervasive porosity was produced that decorates grain boundaries, but only in specific conditions. Necessary conditions involve the presence of H2O and a small grain size (< 4 μm), alongside a significant increase of differential stress before plastic yielding. Porosity emerges at the time of plastic yielding, and hence, as a result
of an elastic loading before the significant occurrence of plastic strain. Outside of these conditions, quartz deforms plastically without producing any porosity and experiences a ‘low’ differential stress, similar to those documented in the literature. Finally, post-mortem analyses from experiments also highlight the presence of amorphous SiO2 at grain boundaries which are decorated by pores, which demonstrates that pores can result from deformation in quartz mylonite, possibly associated to mechanical amorphization induced by stress concentration at grain boundaries.