PhD Thesis defense of Khadija Alaoui

Abstract

In this study, we aimed to investigate the role of mica in strain accommodation and the development of shear zones in the upper to middle crust, examining scales from crystal defects to polycrystalline aggregates. Previous research has explored mica’s involvement in shear zones but lacked detailed insights into its deformation processes in natural samples and experimental two-phase biotite-quartz systems. Existing studies primarily focused on single crystals, polyphased systems, or natural samples like schist, gneiss, and pegmatite, with limited attention to biotite in two-phase systems (Etheridge et al., 1973; Bell et al., 1986; Meike, 1989; Kronenberg et al., 1990; Mares and Kronenberg, 1993; Holyoke and Tullis, 2006a, b; Shea and Kronenberg, 1992, 1993; Mariani et al., 2006; Misra et al., 2012; Tullis and Wenk, 1994; Niemeijer and Spiers, 2005; Tokle et al., 2023). 

To address this gap, we employed a biotite-quartz two-phase system as an analogue for upper and mid-crustal rocks, thoroughly investigating deformation and reaction mechanisms. Our findings emphasize dissolution-precipitation as a prominent weakening process in biotite-rich systems, driven by solution-transfer diffusion creep localized at grain boundaries and facilitated by phyllosilicates. Concurrently, dynamic recrystallization was observed in quartz. Significant intracrystalline plastic deformation was not prevalent in biotite, except for localized kinking, while grain size reduction primarily occurred through dissolution-precipitation rather than basal slip or dynamic recrystallization. Increasing the proportion of mica in quartz aggregates accentuated dissolution-precipitation mechanisms, highlighting quartz’s reactivity and substantial contribution to rock weakening. This effect was evident even with low fractions of mica, where quartz maintained its role as the interconnected, load-bearing framework mineral. Furthermore, the chemical composition of mica influenced interactions at grain boundaries between mica and quartz. The dissolution of mica under stress altered the thickness and composition of the fluid-film at the mica-quartz interface, affecting solution pH, phase solubility, and consequently, dissolution-precipitation dynamics. These insights underscore the complex interplay of mineralogical reactions and deformation mechanisms in biotite-rich systems, shedding light on their role in shear zone evolution within the Earth’s crust.