Eclogitization front propagation in a layered lower crust: Insights from Hydro-Chemical numerical modeling

Hydration reactions play a key role in controlling fluid transport and deformation in subduction zones. These reactions are associated with changes in material properties (mainly permeability, density and strength) that can strongly influence their propagation in rocks. However, the mechanisms controlling their development remain poorly understood. The granulite-to-eclogite transformation is a striking example of a pressure- and fluid-driven metamorphic reaction and is characterized by a significant increase in density. On the island of Holsnøy (Norway), incipient eclogitization affects continental granulites and forms characteristic finger-like structures. Field observations show that these eclogite fingers are systematically aligned with the pre-existing granulite foliation, and that the orientation of this foliation appears to control the way the eclogite front propagates. However, it remains unclear whether this preferential propagation arises from mechanical anisotropy, permeability anisotropy alone, or a combination of both.

In this study, we investigate the influence of the initial granulite foliation, in terms of permeability only, on the propagation of eclogitization. We use a hydro-chemical numerical model to simulate the evolution of eclogitization in a granulitic matrix where fluid flow and pressure variations govern metamorphic reactions. For this, we solve a fully coupled system of equations (conservation of mass and Darcy's law) using Kozeny-Carman formulation for permeability associated with an equation of state. The eclogite transformation is simulated by a density increase occurring when fluid pressure is higher than the pressure of the reaction.

We propose a parametric study allowing us to assess the role of permeability layering in the initial granulite on the development and propagation of eclogite fingers. Our results are then discussed and compared with field observations from Holsnøy, providing new insights on fluid-mediated metamorphic transformations in subduction-related settings.