MECHANISMS OF RETROGRADE METAMORPHISM

Retrograde metamorphism usually includes volatilization of igneous or higher-grade metamorphic rocks. This often cause a significant reduction in rock density accompanying an increase in the volume of solids. Because such volatilization reactions are triggered by infiltration of external fluids and commonly occur relatively far from thermodynamic equilibrium, they are often fast enough to produce sufficient stress perturbations, or ‘swelling-pressure’ to drive local fracturing.

Microstructural studies show numerous examples of reaction-driven fracturing both within the reacting mineral grains and in the surrounding rock matrix. Small scale fractures often link up to form fracture networks that promotes further fluid infiltration, more reaction, more stress build up, more fracturing etc. This self-sustained coupling between reactions, fracturing, and fluid migration provides a first-order control on the rate of hydration of the Earth’s crust both during metamorphism and during lower-temperature processes such as weathering.

Examples of natural reaction-driven fracturing at a wide range of scales will be presented with focus on serpentinization. A simple mechanical model will be presented that illustrate the most pertinent features of the hierarchical fragmentation process that arise from reaction-driven fracturing. The model, as well recent experimental results, illustrate how reaction-driven fracturing acceleration the rate of retrograde metamorphism.