Feedbacks between deformation, hydrothermal reaction and permeability evolution in the crust: Experimental insights

In this paper, we present experiments that explore the interaction between crustal deformation, hydrothermal reaction and permeability evolution. We postulate that deformation should result in changes to mineral surface area, which will either favour or inhibit water-rock reactions involving the formation of secondary minerals. Mineral precipitation can greatly affect hydrologic properties such as permeability, so it is therefore crucial that the controlling parameters are well constrained. Experiments were conducted using a Paterson gas apparatus capable of independently controlling confining pressure (Pc), pore pressure (Pp) and axial load. Most experiments were conducted at Pc = 100 MPa and Pp = 50 MPa with temperatures of 200-600 °C, using a fine-grained granitic rock. In the experiments, permeability evolution was measured under different stress regimes, ranging from the isostatic to the shear failure case. We find that permeability decreases with time by a well-defined exponential function, with a rate constant, r, representing the duration of the transient in permeability. The rate constant is lowest when the specimen is in the compactional regime, where mineral surface areas are minimized. Conversely, the permeability reaction rate constant changes at the greatest rate when the specimen is stressed into the dilatant regime or undergoes shear failure. These stress states result in the generation of fresh, reactive mineral surface areas leading to enhanced reaction and a rapid associated change in permeability. Results from this study suggest that in natural systems, a negative feedback effect might be operative, in which enhanced mineral precipitation moderates permeability generated during episodes of deformation. This feedback is likely to be important in dynamic environments such as fault zones or injection sites intended as emerging hydrothermal energy sources.