Argon isotope geochemistry of inclusion fluids from granite-associated mineral veins in southwest and northeast England☆

The methodology of 40Ar-39Ar dating, extended to include the analysis of chlorine, has been applied to the analysis of 36Ar, 40Ar, K, Ca, and Cl in fluid inclusion-bearing quartz and fluorite veins from two areas of granite associated mineralisation in England. Crushing experiments have permitted the analysis of argon in the inclusion fluids. Stepped heating of the residues releases additional components from solid phases and provides approximate mineralisation ages for four samples from southwest England. These show that mineralisation extended for up to 80 Ma after the emplacement of the granite. Absolute and relative abundances of Cl, atmospheric 36Ar, and ‘parentless’ 40ArE have been determined and related to plausible sources and interactions of the mineralizing fluids. In both areas fluids are characterised by a restricted range of 40ArEClratios which reflects the source of the fluids. Cl36Ar ratios are very variable between samples due mainly to the presence of variable amounts of atmospheric argon. If present in the inclusion fluids the implied concentrations range up to an order of magnitude greater than air-saturated water. A possible source of an excess in the fluids could be dissolution of adsorbed atmospheric gases from sediments through which the fluids passed. However, we believe that a mundane explanation, in terms of atmospheric gas trapped during direct exposure to the atmosphere, is more likely.