An experimental study on evolution of fracture permeability and rate of water-rock reactions in ultramafic rocks at hydrothermal conditions

Abstract:

We have performed experiments on the evolution of fracture permeability during serpentinization of ultramafic rocks. The experiments were performed using a flow-through triaxial machine on samples of ultramafic rocks with a well-mated through-going tensile fracture at hydrothermal conditions at effective pressure of 30 MPa and temperature of 260°C. By determining the flow rate under a pore pressure gradient during the experiments and assuming a cubic law relating fracture aperture and permeability, the results showed that fracture permeability decreased by one to two orders of magnitude during the 200 to 340 hour experiments. Electron microprobe data and SEM images indicated the formation of needle-shaped crystals of serpentine composition along the walls of the fracture. Assuming the dissolution and precipitation reactions occur simultaneously along the fracture walls we found that the rate of transformation at the beginning of the experiments is ~10^-4-5 (molm^-2hr^-1) and decreases monotonically for about an order of magnitude towards the end of experiments. The rate of transformation is converted to rate of reaction of 10^-5-7 s^-1 at the beginning of the experiments, which is in agreement with estimates of Martin and Fyfe, [1970] of the rate of serpentinization reactions. The results suggest that the reaction rate slowed with increasing time as available reactive surface area decreased. The results also indicate that the water-rock reactions were the main mechanism contributing to the reduction in fracture aperture and cubic law is a reasonable first approximation for understanding evolution of fracture permeability. The experimental results suggest that the fracture network in long-lived hydrothermal circulation systems can be sealed rapidly as a result of mineral precipitation, and generation of new permeability resulting from a combination of tectonic and crystallization-induced stresses is required to maintain fluid circulation.