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.