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. Plain Language Summary: When rocks are subjected to changes in pressure and temperature, for example, in areas of the Earth where tectonic plates collide, their constitutive minerals are no longer stable and react to form new phases of different physical properties. These changes can trigger significant stress reductions, a process known as weakening, which involves a concentration of the strain in specific areas sometimes associated with earthquakes. In order to better understand and quantify the effects of reaction dynamics on the way rocks deform, we use specialized computer code in which we can vary the reaction and strain rates, as well as the physical properties of the deformed material and its reaction products. Our results are in good agreement with results from deformation experiments in the laboratory and field observations on natural rocks. They show that nucleation of dense reaction products, a common case for high pressure transformations, is responsible for a local stress increase in the vicinity of the nuclei. This increase triggers fracture initiation and associated weakening. When nucleation is enhanced by the energy produced in highly strained areas, reaction products nucleate in close proximity to each other, which highly contributes to local stress increases and to the process of embrittlement. Key Points: Numerical models of work‐driven nucleation provide insights on the link between reaction dynamics and weakening in metamorphic rocksWeakening follows the nucleation and propagation of plastic shear bands in the vicinity of the reaction productsKinetics of heterogeneous nucleation plays a more important role on weakening than physical properties and proportion of reaction products [ABSTRACT FROM AUTHOR]