Weakening Induced by Phase Nucleation in Metamorphic Rocks: Insights From Numerical Models

Metamorphic transformations involve important changes in material properties that can be responsible for rheological alterations of rocks. Studying the dynamics of these changes is therefore crucial to understand the weakening frequently observed in reactive rocks undergoing deformation. Here, we explore the effects of reaction dynamics on the mechanical behavior of rocks by employing a numerical model where nucleation kinetics and reaction product properties are controlled over time during deformation. Different values are tested for nucleation kinetics, density, viscosity, proportion and size of the reaction products, and pressure-strain rate conditions relative to the brittle-ductile transition. Our results, in good agreement with laboratory and field observations, show that rock weakening is not just a matter of the strength of the reaction products. Both density and viscosity variations caused by the transformation control local stress amplification. A significant densification can by itself generate sufficient stresses to reach the plastic yield of the matrix, even if the nuclei are stronger than their matrix. Plastic shear bands initiate in the vicinity of the newly formed inclusions in response to local stress increases. Coalescence of these shear bands are then responsible for strain weakening. We show that heterogeneous nucleation controlled by mechanical work has an even greater impact than the intrinsic properties of the reaction products. Propagation of plastic shear bands is enhanced between closely spaced nuclei that generate significant stress increases in their vicinity. This study highlights the importance of transformational weakening in strong rocks affected by fast reaction kinetics close to their brittle-ductile transition.