Effects of water fraction on strengths and microstructures of quartz aggregates at the brittle plastic transition

Abstract:

At the plate boundary, large earthquakes as well as slow earthquakes are typically nucleated around the depth limit of the seismogenic zone. High V_p/V_s ratios indicate the presence of aqueous fluids at these regions. Thus, it is important to understand how the water content and the pore structure affect the rheology and microstructural evolution of polycrystalline materials at these conditions.

We conducted deformation experiments on quartz aggregates using a Griggs-type deformation apparatus. Silica gel was used as the starting material; quartz samples synthesized from “as-is” silica gel contain a fluid-filled porosity of ~22%; we prepared samples with lower fluid-filled porosity by pre-drying the gel at 120˚C (~15% porosity), 450˚C (~16%), and 900˚C (~7%).

The measured stress from general shear experiments on porous quartz aggregates at pressures of 1.1 and 1.5 GPa and temperatures of 800–900˚C is significantly lower than predicted by the wet quartzite flow laws (e.g., Tokle et al., 2019), and shear stress decreases with increasing porosity. The stress exponent n at 800–900˚C is 2.8–5.2, suggesting that the dislocation creep of quartz controls the overall rate-behavior in the quartz shear zone. The stress exponent at 500–700 is 4.7–19, indicating a transition to brittle fracture/friction and/or semi-brittle flow.

S-C’ mylonitic structures characterized by the CPO and water segregation are observed in recovered samples deformed at 800–900˚C. A-axes of quartz align parallel to the P direction. We also found evidence for strain localization along R₁riedel shears, structure that are characterized by high porosity. Details of these microstructures indicate that the fluid-rich bands move across the viscously deforming matrix. In contrast, deformation experiments on cores of quartzite show homogeneous dislocation creep at this pressure/temperature condition.

The low flow stress and R1 reidel shear zones indicate that a grain-scale stress enhancement process promoted by the high volume fraction of water, similar to observations on partially molten samples (e.g., Hirth and Kohlstedt, 1995). Our result suggests that a few % of porosity causes drastic weakening in the quartz shear zone from a brittle to fully plastic regime.