Contrasting fracture patterns induced by volume-increasing and -decreasing reactions: Implications for the progress of metamorphic reactions

Abstract

Hydration and dehydration reactions during metamorphism cause drastic changes in porosity and fluid pressure in a rock, and some of these reactions proceed with fracturing. For this paper we have developed a model for coupled hydraulic–chemical–mechanical processes, using the distinct element method, in order to understand the relationships among chemical reactions, fluid flow, and fracturing during metamorphism. The model considers fluid advection along fractures and the dependence of reaction rates on fluid pressure. With the help of the model, volume-decreasing dehydration reactions and volume-increasing hydration reactions are investigated in detail as analogs of prograde and retrograde reactions, respectively. Both types of reaction proceed inwards from drained boundaries or fractures, but they show contrasting fracture patterns. A volume-decreasing dehydration reaction produces tree-type fractures, with the new fractures generated as branches of a pre-existing fracture. A volume-increasing hydration reaction produces a polygonal network of fractures, where new fractures nucleate at sites far from the pre-existing fracture, and extend to form T-junctions. These contrasting fracture patterns are essentially controlled by solid volume changes during reaction rather than by the fluid pressure gradient. In the case of the tree-type fractures, the fractures are continuously generated at the reaction front, and the reaction proceeds smoothly by positive feedbacks between reaction, fracturing, and fluid flow. In contrast, in a volume-increasing hydration reaction, fracturing initially occurs in response to the irregular boundary shape, but as the reaction progresses, a compressive stress field is generated, which inhibits further fracture generation. The compressive stress field also prevents fluid flow by closing the pre-existing fractures, which slows down the reaction. These contrasting feedback systems between volume-decreasing dehydration and volume-increasing hydration reactions help to explain why prograde metamorphic reactions proceed pervasively, and why the progress of a retrograde hydration reaction tends to be localized along fractures so that relics of peak metamorphism are commonly preserved.