Weakening induced by phase nucleation: from experiments to numerical models

Metamorphic transformations are often associated with strain localization which can be observed in the field either as ductile zones, or brittle, and possibly seismogenic, structures. Deformation experiments in the laboratory not only replicate such features but also allow us to measure the associated weakening. In all these contexts, reaction overstepping and disequilibrium metamorphism appear to be the rule. Reaction rates are usually very fast once transformation initiates, in particular within highly stressed and strained volumes where the produced mechanical work is sufficient to overcome kinetic barriers. New very fine-grained and dense phases nucleate in conditions where mineral growth is impeded. Understanding how heterogeneous nucleation, along with changes in density and viscosity, affects the rock's strength during a metamorphic transformation appears therefore critical.

In that prospect, results of a 2D numerical study in which reaction products preferentially nucleate in areas of high strain energy are presented. Special attention is given to the weakening or hardening effects induced by these transformations, as well as to the deformation patterns within the model. Results of our numerical study are then discussed in the light of experimental data obtained at comparable pressure-temperature-strain rate conditions.

We show that rock weakening is not only linked to the strength of the reaction products. Indeed, (1) densification alone can generate sufficient stress to induce plastic yielding of the surrounding matrix, even when the nuclei are stronger, and (2) heterogeneous nucleation controlled by mechanical work has greater influence on the rock’s strength than the intrinsic properties of the reaction products. Weakening is primarily driven by the initiation and propagation of plastic shear bands between the closely spaced nuclei that generate significant stress concentration in their vicinity. This study highlights the importance of transformational weakening that results from fast heterogeneous nucleation in rocks close to their brittle-ductile transition.