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Hydration and dehydration reactions play pivotal roles in plate tectonics and the deep water cycle, yet many facets of (de)hydration reactions remain unclear. Here, we study (de)hydration reactions where associated solid density changes are predominantly balanced by porosity changes, with solid rock deformation playing a minor role. We propose a hypothesis for three scenarios of (de)hydration front propagation and test it using one‐dimensional hydro‐mechanical‐chemical models. Our models couple porous fluid flow, solid rock volumetric deformation, and (de)hydration reactions described by equilibrium thermodynamics. We couple our transport model with reactions through fluid pressure: the fluid pressure gradient governs porous flow and the fluid pressure magnitude controls the reaction boundary. Our model validates the hypothesized scenarios and shows that the change in solid density across the reaction boundary, from lower to higher pressure, dictates whether hydration or dehydration fronts propagate: decreasing solid density causes dehydration front propagation in the direction opposite to fluid flow while increasing solid density enables both hydration and dehydration front propagation in the same direction as fluid flow. Our models demonstrate that reactions can drive the propagation of (de)hydration fronts, characterized by sharp porosity fronts, into a viscous medium with zero porosity and permeability; such propagation is impossible without reactions, as porosity fronts become trapped. We apply our model to serpentinite dehydration reactions with positive and negative Clapeyron slopes and granulite hydration (eclogitization). We use the results of systematic numerical simulations to derive a new equation that allows estimating the transient, reaction‐induced permeability of natural (de)hydration zones.
Hydration and dehydration reactions play pivotal roles in plate tectonics and the deep water cycle, yet many facets of (de)hydration reactions remain unclear. Here, we study (de)hydration reactions where associated solid density changes are predominantly balanced by porosity changes, with solid rock deformation playing a minor role. We propose a hypothesis for three scenarios of (de)hydration front propagation and test it using one‐dimensional hydro‐mechanical‐chemical models. Our models couple porous fluid flow, solid rock volumetric deformation, and (de)hydration reactions described by equilibrium thermodynamics. We couple our transport model with reactions through fluid pressure: the fluid pressure gradient governs porous flow and the fluid pressure magnitude controls the reaction boundary. Our model validates the hypothesized scenarios and shows that the change in solid density across the reaction boundary, from lower to higher pressure, dictates whether hydration or dehydration fronts propagate: decreasing solid density causes dehydration front propagation in the direction opposite to fluid flow while increasing solid density enables both hydration and dehydration front propagation in the same direction as fluid flow. Our models demonstrate that reactions can drive the propagation of (de)hydration fronts, characterized by sharp porosity fronts, into a viscous medium with zero porosity and permeability; such propagation is impossible without reactions, as porosity fronts become trapped. We apply our model to serpentinite dehydration reactions with positive and negative Clapeyron slopes and granulite hydration (eclogitization). We use the results of systematic numerical simulations to derive a new equation that allows estimating the transient, reaction‐induced permeability of natural (de)hydration zones.
Metamorphic transformations involve important changes in material properties that can be responsible for rheological alterations of rocks. Studying the dynamics of these changes is therefore crucial to understand the weakening frequently observed in reactive rocks undergoing deformation. Here, we explore the effects of reaction dynamics on the mechanical behavior of rocks by employing a numerical model where nucleation kinetics and reaction product properties are controlled over time during deformation. Different values are tested for nucleation kinetics, density, viscosity, proportion and size of the reaction products, and pressure‐strain rate conditions relative to the brittle‐ductile transition. Our results, in good agreement with laboratory and field observations, show that rock weakening is not just a matter of the strength of the reaction products. Both density and viscosity variations caused by the transformation control local stress amplification. A significant densification can by itself generate sufficient stresses to reach the plastic yield of the matrix, even if the nuclei are stronger than their matrix. Plastic shear bands initiate in the vicinity of the newly formed inclusions in response to local stress increases. Coalescence of these shear bands are then responsible for strain weakening. We show that heterogeneous nucleation controlled by mechanical work has an even greater impact than the intrinsic properties of the reaction products. Propagation of plastic shear bands is enhanced between closely spaced nuclei that generate significant stress increases in their vicinity. This study highlights the importance of transformational weakening in strong rocks affected by fast reaction kinetics close to their brittle‐ductile transition.
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