Hydration versus dehydration reactions: increase versus decrease of solid density with pressure rise

Hydration and dehydration reactions as well as the associated fluid flow are important features of geodynamic processes. For example, hydration of rocks can significantly decrease rock strength and generate shear localization or fluids liberated by dehydration reactions in subducting rocks can flow into the mantle wedge and cause melting and magmatism. However, several aspects of (de)hydration related fluid flow and the propagation of (de)hydration reaction fronts remain unclear.

Here, we study hydration and dehydration reactions with hydro-chemical numerical models based on continuum mechanics and local equilibrium thermodynamics. For simplicity, we mainly consider 1D isothermal models. We focus on the propagation velocity and direction of the (de)hydration reaction front. We define hydration as an increase of chemically, or lattice, bound water in the solid phase. Therefore, hydration requires fluid flow towards the hydration reaction front. Contrary, dehydration is a decrease of chemically bound water in the solid phase. Hence, dehydration requires fluid escape from the dehydration reaction front.

Our models show that hydration requires a negative sign of the solid volume change with pressure increase across the reaction boundary, whereas dehydration requires a positive sign of solid volume change with pressure increase across the reaction boundary. The reason for this difference in sign is due to the fluid flow associated with the (de)hydration reaction which is driven by the fluid pressure gradient following Darcy’s law. Thus, for hydration to happen it must occur on the lower fluid pressure side of the reaction front compared to the side with more porous fluid. Porosity is directly related to the solid density change, so it is larger on the high solid density side of the reaction front. Therefore, the hydration reaction requires that the rock that should be hydrated is on the lower fluid pressure side of the front. Opposite can be reasoned for the dehydration front. We also include in our models the case of zero porosity, and hence zero permeability, on one side of the (de)hydration reaction front. This zero-permeability limit involves a singularity at the reaction front due to the multiplication of zero permeability with an infinite pressure gradient. We resolve this singularity in our numerical algorithm by applying a fully conservative form of the governing equations. Resolving this zero-permeability limit is in agreement with the well-established theory of non-linear degenerate parabolic equations. We apply our model to two natural settings: First, eclogite shear zones in the Bergen Arcs, Norway, where hydration of dry granulite formed eclogite. Second, olivine veins in the Erro-Tobbio unit, Ligurian Alps of Italy, where dehydration of serpentinite during subduction formed olivine.