Serpentinite subduction and associated dehydration vein formation are important for subduction zone dynamics and water cycling. Field observations suggest that en échelon olivine veins in serpentinite mylonites formed by dehydration during simultaneous shearing of serpentinite. Here, we test a hypothesis of shear-driven formation of dehydration veins with a two-dimensional hydro-mechanical-chemical numerical model. We consider the reaction antigorite + brucite = forsterite + water. Shearing is viscous and the shear viscosity decreases with increasing porosity. Total and fluid pressures are initially homogeneous and in the serpentinite stability field. Initial perturbations in porosity, and hence viscosity, cause fluid pressure perturbations during simple shearing. Dehydration nucleates where fluid pressure decreases locally below the thermodynamic pressure defining the reaction boundary. During shearing, dehydration veins grow in direction parallel to the maximum principal stress and serpentinite transforms into olivine inside the veins. Simulations show that the relation between compaction length and porosity as well as the ambient pressure have a strong impact on vein formation, while the orientation of the initial porosity perturbation and a pressure-insensitive yield stress have a minor impact. Porosity production associated with dehydration is controlled by three mechanisms: solid volumetric deformation, solid density variation and reactive mass transfer. Vein formation is self-limiting and slows down due to fluid flow decreasing fluid pressure gradients. We discuss applications to natural olivine veins as well as implications for slow slip and tremor, transient weakening, anisotropy generation and the formation of shear-driven high-porosity bands in the absence of a dehydration reaction.

Serpentinite subduction and the associated formation of dehydration veins is important for subduction zone dynamics and water cycling. Field observations suggest that en-échelon olivine veins in serpentinite mylonites formed by dehydration during simultaneous shearing of ductile serpentinite. Here, we test a hypothesis of shear-driven formation of dehydration veins with a two-dimensional hydro-mechanical-chemical numerical model. We consider the reaction antigorite + brucite = forsterite + water. Shearing is viscous and the shear viscosity decreases exponentially with porosity. The total and fluid pressures are initially homogeneous and in the antigorite stability field. Initial perturbations in porosity, and hence viscosity, cause fluid pressure perturbations. Dehydration nucleates where the fluid pressure decreases locally below the thermodynamic pressure defining the reaction boundary. Dehydration veins grow during progressive simple-shearing in a direction parallel to the maximum principal stress, without involving fracturing. The porosity evolution associated with dehydration reactions is controlled to approximately equal parts by three mechanisms: volumetric deformation, solid density variation and reactive mass transfer. The temporal evolution of dehydration veins is controlled by three characteristic time scales for shearing, mineral-reaction kinetics and fluid-pressure diffusion. The modelled vein formation is self-limiting and slows down due to fluid flow decreasing fluid pressure gradients. Mineral-reaction kinetics must be significantly faster than fluid-pressure diffusion to generate forsterite during vein formation. The self-limiting feature can explain the natural observation of many, small olivine veins and the absence of few, large veins. We further discuss implications for transient weakening during metamorphism and episodic tremor and slow-slip in subduction zones.

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. Rock deformation consists of linear viscous compressible and power-law viscous shear deformation. Fluid flow follows Darcys law with a Kozeny-Carman type permeability. We consider a closed isothermal system and the reversible (de)hydration reaction: periclase and water yields brucite. In the models, 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, because we assume that periclase-water and brucite regions have different effective viscosities. In models with circular inclusions, solid deformation has a minor impact on the evolution of fluid pressure, porosity and reaction front. Models with elliptical inclusions and far-field shortening generate higher rock pressure inside the inclusion compared to circular inclusions, and show a faster reaction-front propagation. The propagating reaction-front increases the inclusion surface and causes an effective, reaction-induced weakening of the heterogeneous rock. Weakening evolves strongly 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 plate boundaries and suggest a strong impact of heterogeneous rock deformation on (de)hydration reactions and associated reaction-induced weakening. The applied MATLAB algorithm is provided.