Mechanical-chemical coupling during chemically driven dehydration in serpentinite: numerical and analytical solutions

Serpentinites are hydrous rocks with a wide pressure-temperature stability range and play a key role in subduction zone dynamics and fluid transfer within the downgoing slab. During subduction, progressive metamorphic reactions transform hydrous mineral assemblages into anhydrous phases. For example, the breakdown of brucite through reactions with antigorite, followed by the terminal breakdown of antigorite to olivine and enstatite, releases aqueous fluids that influence the deep-water cycle and induce major changes in rock density, porosity, permeability, and mechanical strength. Field observations and numerical studies suggest that chemical heterogeneities and variations in fluid composition can localize dehydration reactions and generate fluid pathways. The mechanical-chemical coupling during such fluid pathway generation can be studied with hydro-mechanical-chemical (HMC) numerical models, however, such HMC modelling remains challenging.

Here, we present a HMC model that couples poromechanical deformation and Darcy flow with a thermodynamic model for chemically driven serpentinite dehydration. We develop a two-dimensional numerical HMC model using finite-difference discretization and an accelerated pseudo-transient solution method based on an iterative, matrix-free approach. A particular focus is to make the HMC model conservative to guarantee accurate mass conservation during chemical transport. The numerical implementation integrates thermodynamically constrained look-up tables that relate chemical concentrations with solid and fluid densities to simulate the temporal and spatial evolution of reaction-induced density changes.

We use a new analytical solution for chemically driven reaction-front propagation to test the numerical model. This analytical solution has been validated with laboratory experiments. Initial applications focus on simplified configurations in which a homogeneous medium with a characteristic chemical composition is infiltrated by a fluid with a different silica content, thereby controlling the initiation and propagation of dehydration reactions. The first numerical experiments investigate how dehydration reaction fronts evolve in response to chemical variations. A primary objective of this contribution is to quantify the fundamental controls of chemical heterogeneity on dehydration dynamics and reaction-front propagation as well as the impact of mechanical deformation on such dehydration.