The material-independent effect of a differential stress on metamorphic kinetics

Deformation and metamorphism are fundamental processes that act synchronously throughout the Earth; however, their interaction remains unclear. Theoretical models predict that an applied tectonic stress has both a dramatic effect or no effect at all. While the small set of deformation experiments that document the interaction between reaction and deformation are either hard to compare or cannot provide the necessary time resolution to test the various theories. These disagreements of predictions and the gap in data invites new time-resolved experiments to be run that can probe details of model predictions and connect existing datasets from different materials deforming at a range of metamorphic conditions. To this end, we use state-of-the-art time-resolved synchrotron-based x-ray microtomography (4DSµCT) deformation experiments to map out the effect of a differential stress on the kinetics of the dehydration of polycrystalline gypsum samples. Our experiments are highly resolved in space (µm) and time (s), which allows us to track and contrast the emergence of the first small crystals (~100 µm3) and their growth through time in hydrostatic and differentially stressed conditions. We find that the kinetics of a metamorphic reaction are profoundly affected by the addition of deformational energy. Differentially stressed samples transform up to ~90% sooner than in the hydrostatic case, and reaction rates increase by a factor of ~5 with increasing differential stress. Importantly, our findings can be expanded to other published data for reactions occurring in the lower crust and the mantle to show that it is changes in the elastic strain energy that drive accelerated metamorphic kinetics. We find that, when we compare kinetic data from these different reactions and normalise the differential stress to each material’s yield strength, a trend emerges that shows stresses larger than the yield do not contribute to accelerating a reaction. Our results showcase the material-independent effect of a differential stress on metamorphic reactions and support theoretical models which place emphasis on the role of changes in stored energy. Current geodynamic models largely ignore the role of stored energy because it is assumed that it is not relevant at long time scales, our results show that its effect is important and should be accounted for when coupling deformation and metamorphism.