Fluid Inclusion Trails in Granite Quartz: Evidence for Aqueous Fluid Infiltration in the Oughterard Granite, Eastern Connemara

Abstract:The results of fluid inclusion petrography and microthermometry of Oughterard Granite quartz reveals the presence of a dominant and ubiquitous population of liquid rich (degree of fill ∼ 0.70-0.95) saline (<11 equivalent wt% NaCl) fluid inclusions. Temperature of homogenisation (to the liquid phase) ranges between 140°C and 380°C with the majority in the range 175°C to 250°C. These aqueous fluid inclusions invariably occur in annealed fractures that transverse the granite quartz. These represent evidence of saline hydrothermal fluid infiltration post crystallisation of granite quartz. Microthermometry also reveals that within this population rare CaCl₂-bearing fluid inclusions occur. Oxygen isotope $\Delta _{\text{quartz-feldspar}}$ values are lower than 0.70, reflecting isotope disequilibrium. The isotope data supports the fluid inclusion evidence for the influx of post crystallisation hydrothermal fluids, possibly related to granite emplacement, or to a later incursion of relatively low temperature (<200°C) meteoric waters.