2D Hydro-Mechanical-Chemical Modeling of (De)hydration Reactions in Deforming Heterogeneous Rock: The Periclase-Brucite Model Reaction

Abstract

Deformation at tectonic plate boundaries involves coupling between rock deformation, fluid flow, and metamorphic reactions, but quantifying this coupling is still elusive. We present a new two-dimensional hydro-mechanical-chemical numerical model and investigate the coupling between heterogeneous rock deformation and metamorphic (de)hydration reactions. We consider linear viscous compressible and power-law viscous shear deformation. Fluid flow follows Darcy's law with a Kozeny-Carman type permeability. We consider a closed isothermal system and the reversible (de)hydration reaction: periclase and water yields brucite. Fluid pressure within a circular or elliptical inclusion is initially below the periclase-brucite reaction pressure and above in the surrounding. Inclusions exhibit a shear viscosity thousand times smaller than for the surrounding. For circular inclusions, solid deformation has a minor impact on the evolution of fluid pressure, porosity, and reaction front. For elliptical inclusions and far-field shortening, rock pressure inside the inclusion is higher compared to circular inclusions, and reaction-front propagation is faster. The reaction generates a large change in porosity (∼0.1%–55%) and in solid density (∼2,300–3,500 kg m−3), and the reaction front exhibits steep gradients of fluid pressure and porosity. Reaction-front propagating increases the weak inclusion's surface and causes an effective, reaction-induced weakening of the heterogeneous rock. Weakening evolves nonlinear with progressive strain. Distributions of fluid and rock pressure as well as magnitudes and directions of fluid and solid velocities are significantly different. The models mimic basic features of shear zones and show the importance of coupling deformation and metamorphism. The applied MATLAB algorithm is provided.