Geuchrtmca CI Cosmochmca Am Vol. 56, pp. 221-243 Copyright 0 1992 Pergamon Press pk. Pnnted in U.S.A. 0016.7037/92/$3.00 + .OO Argon isotope geochemistry of inclusion fluids from granite-associated mineral veins in southwest and northeast England* G. TURNER’ and M. P. BANNON~ ‘Geology Department, Manchester University, Manchester M 13 9PL, UK ‘Fluid Processes Research Group, British Geological Survey, Nottingham, NG 12 5GG, UK zyxwvutsrqponmlkjihgfedcba (Received September 20, 1990; acceptedin revised form October30, 199 I ) Abstract-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 char- acterised by a restricted range of 40ArE/C1 ratios which reflects the source of the fluids. Cl/36Ar 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. INTRODUCTION BECAUSE OF THEIR chemical inertness and conservative be- haviour noble gases are now used extensively for the study of the source, movement, and interactions of modern groundwaters ( MAZOR and WASSERBURG, 1965; ANDREWS and LEE, 1979; ANDREWS et al., 1985). Air-saturated water (ASW) originating at the surface of the Earth acquires a characteristic pattern of noble gas abundances which is de- pendent mainly on the temperature and salinity of the source (SMITH and KENNEDY, 1983). The source pattern may be modified by entrainment of unfractionated air, identified through enhanced Ne/Ar ratios. Once circulating in the crust the ASW pattern can be further modified by fractionation as a result of phase separations, e.g., partitioning between oil and water, or by the formation of a separate gaseous phase (KENNEDY et al., 1985 ) . The isotopic composition may be changed by the acquisition of radiogenic isotopes, principally 4He or 40Ar ( MAZOR, 1976; ANDREW% 1985; TORGERSON and CLARKE, 1985) or in some tectonically active regions, by the addition of ‘He from the upper mantle (CRAIG and LUPTON, 1981). There is great scope for extending the observations of modem groundwater to palaeo-waters of all ages trapped in fluid inclusions. Furthermore, the use of 40Ar-39Ar meth- odology provides a means of directly linking noble gases to a range of geochemically important elements in the inclusion fluids. Elements which give rise to noble gas isotopes following neutron irradiation include K, Ca, Cl, Br, Se, I, Te, Ba, and U. Of these the halogens are particularly important constit- uents of crustal fluids. Rare gas measurements following neutron irradiation have been used extensively for I-Xe dating * Presented at PACROFI III, Third Biennial Pan-American Con- ference on Fluid Inclusions. held in Toronto, Canada, May 20-22, 1990. of meteorites and for 40Ar-39Ar dating of both terrestrial and extraterrestrial rocks. The use of neutron-produced noble gas isotopes to determine element abundances in meteorites was reported by TURNER ( 1965 ), but only recently has the scope for applying these techniques to inclusion fluids been recognised. KELLEY et al. ( 1986) attempted to use the 40Ar-39Ar method to date inclusion fluids in vein quartz from two areas of granite-related mineralisation. They found that it was pos- sible to identify several sources of argon in the quartz. 40Ar produced by the in situ decay of K was identified through a correlation with 39Ar generated by neutron interactions on K. This component was contained in cogenetic mica, in some instances trapped in the inclusions, and was released only in stepped heating. A Cl-related component, observed through correlations with neutron-produced 38Ar, was released in crushing experiments and was identified as gas dissolved in the inclusion brines. This component contained palaeo-at- mospheric 36Ar and 40Ar and parentless 40Ar. By combining the Ar/Cl ratios with freezing point measurements of salinity, KELLEY et al. ( 1986) were able to determine absolute con- centrations of argon isotopes in the inclusion fluids. They observed that the concentration of atmospheric argon was close to that expected for air-saturated water, implying that the water was mainly of meteoric origin. They also argued that the concentration of the parentless 40Ar provided a mea- sure of the interaction of this meteoric water with the country rock. BOHLKE et al. ( 1989) extended the method to include Kr. Xe, Br, and I, and in addition used laser decrepitation to release fluids from small numbers of inclusions rather than bulk samples. They studied vein quartz associated with gold mineralisation in the Sierra Nevada, California, and dem- onstrated the presence of high Xe/ Ar and I/Cl ratios. Both of these they attributed to the interaction of the fluids with 221