The effects of rupture and diffusion on the salinity of fault-related fluid inclusions

Abstract

The combined effects of fault rupture and diffusive transport on the salinity of fault-related fluids are addressed in the case of both an external solute source and that of rock buffered fluid compositions. The effects of injection of a dilute and a saline fluid into a fault zone, followed by diffusion, are examined in the case of both periodic and random fault rupture events. The models reproduce the large salinity variations characteristic of fault-related secondary fluid inclusions, and suggest that correlation of salinity variations may allow reconstruction of rupture events. In the case of rock buffered fluid compositions, the hydration of periclase to form brucite is used as a simple petrological model for increasing salinity in fault zones. An episodic hydration reaction, both with and without dilution effects, is capable of increasing the salinity of small volumes of fluid along microfractures and also of producing large salinity variations. These considerations suggest that large salinity variations and high fluid concentrations common in secondary fluid inclusions associated with retrograde shear zones may be due to the combined effects of diffusion and rock buffering reactions in the fault zone. The concentration spikes produced during rupture are damped by diffusion with increasing distance from the site of rupture. Concentration peaks and troughs coincide in time at different distances from the source suggesting the possibility of correlating salinity variations in the vicinity of a fault zone.