Fluid-pressure induced eclogitisation of a dry granulite: Insights from Hydro-Chemical model

Eclogitization constitutes one of the most emblematic transformations in continental subduction zones, where conversion of initially dry lower crustal rocks into eclogite facies rocks correlates with the occurrence of seismogenic events. This reaction is generally considered to occur at high pressure conditions during hydration of dry granulite. Several models using « ad hoc » diffusion equation exist to model this hydration process and the consequences of reaction-induced changes in terms of rheology and density. However, to our knowledge, there is no quantitative model allowing to physically explain how fluids propagate inside a dry rock (i.e. with no porosity at all) and how reaction-induced alteration front widens over time. In this study, we therefore propose a new fully coupled hydro-chemical model wherein a two-phase flow model is coupled with the eclogitization reaction. We use a mass conservative approach, solving total mass and solid mass equations, in a closed isothermal system. Fluid and solid densities are calculated with lookup tables from equilibrium thermodynamics. Our model shows that a fluid pressure pulse generates a pressure gradient that can be associated with the densification reaction when the pressure required for the eclogitization is reached. This reaction generates a large increase in porosity (0 to ~16%) and subsequent porous fluid flow inside the initially dry granulite. This process is then sustained as long as the fluid pulse is maintained, and ends shortly after the fluid pressure pulse stops. However, high pressure within the reacted area can persist for a long period of time. A parametric study allowing to constrain both the duration and the widening of the reaction area is proposed as well as an application to the emblematic case study of the eclogitized granulites of Holsnoy (Bergen Arcs, Norway).