Eclogitisation of dry and impermeable granulite by fluid flow with reaction-induced porosity: Insights from hydro-chemical modelling

Eclogitisation is a major metamorphic process of continental subduction zones, where transformation of dry lower crustal rocks into eclogites seems to correlate with seismogenic events. Eclogitisation can occur at high pressure during hydration of granulite, but the physical processes controlling the hydration of dry, impermeable granulite remain poorly understood. Here, we present a new fully coupled hydrochemical model of a non-deforming porous rock which undergoes metamorphic reactions in response to fluid pressure variations. Conservation equations for total and solid mass are solved, and fluid and solid densities are calculated with look-up tables computed from models relying on equilibrium thermodynamics. Our model shows that a fluid pressure pulse generates a pressure gradient that causes densification when the pressure required for eclogitisation is reached. The reaction generates porosity and subsequent porous fluid flow into the initially non-porous impermeable granulite. This process lasts as long as the pressure pulse is maintained, but high pressure within eclogite can persist for a longer time. The hydration front propagates tens of centimetres into the granulite in the order of weeks to months. We show that propagation of a hydration-reaction front is effectively a diffusive process, with diffusivity in the order of 10-9 m2 & BULL; s-1 for eclogitisation as in Holsnoy, Norway. Reactive hydration of impermeable granulite is possible because its solid density is smaller than that of eclogite. We discuss the application of our model for eclogitisation and also for other reactions for which hydration of impermeable rock is possible.& COPY; 2023 Elsevier B.V. All rights reserved.

Automated summary

This study presents a new fully coupled hydrochemical model that describes metamorphic reactions in non-deforming porous rocks undergoing fluid pressure variations. The model shows that a fluid pressure pulse can cause the densification of rocks and initiate porous fluid flow in initially non-porous impermeable rocks. The hydration front propagates in a diffusive process and lasts for weeks to months. The findings have implications for understanding eclogitisation and other reactions involving hydration of impermeable rocks.