High and low density fluids in a quartz vein from the Irish Variscides

Combined microstructural and fluid inclusion studies on a multiple quartz vein from the Irish Variscides reveal a kinematic history consisting of three separate, dynamically distinct, extensional opening events. The first of these is characterized by trapped fluid at densities of 0.87-0.98 g cm^-3, indicative of pressures approximating the estimated lithostatic load, while fluid densities associated with the following two events (0.76-0.91 g cm^-3) clearly indicate sub-lithostatic fluid pressures. The combined evidence suggests that these later two opening events were extensional fractures occurring well below the critical depth for tensional σ₃ (least effective principal stress). It is argued here that the preservation of such low density fluids, in an environment that conventionally requires supra-lithostatic fluid pressures for hydraulic failure, is due to (i) the influence of a preexisting material anisotropy in the rock that would result in hybrid extensional failure, (ii) the effect of stress heterogeneities associated with the development of crude boudins in the vein.