MULTIPLE STAGES OF FLUID TRAPPING IN THE STRIPA GRANITE INDICATED BY LASER MICROPROBE ANALYSIS OF CL, BR, I, K, U, AND NUCLEOGENIC PLUS RADIOGENIC AR, KR, AND XE IN FLUID INCLUSIONS

Abstract

Argon, Kr, and Xe were extracted from fluid inclusions (FI) in the Stripa granite by laser microprobe decrepitation of quartz and feldspar and measured in a low blank, high sensitivity, ion-counting mass spectrometer. Both unirradiated and pile-irradiated splits of Stripa samples were analyzed using identical procedures. Irradiation of samples in a nuclear pile before extraction, along with standards, enables measurement of abundances of Cl, K, Br, Se, I, Ba + Te, and U simultaneously with 36Ar, 40Ar, 84Kr, and129 Xe. Large variations in BrCl and ICl occur in FI within a single specimen of Stripa drill core. Two ''endmember'' FI compositions are represented: (1) ICl greater than 0.0012 and BrCl #0.006 (atom ratios), a distinctive halogen signature previously measured in brines associated with some oil-fields and fine-grained sediments and (2) ICl less than 1 x 10-4 and BrCl #0.0015, which resembles many fluids and FI, including high-temperature ''magmatic'' FI in some granitic rocks. Most measurements of halogen ratios are intermediate between these two extremes, with uncorrelated variations in ICl and BrCl indicating at least one additional fluid.There are large and uncorrelated variations in ratios between 40Ar, 36Ar, 84Kr, and Cl which reflect a complex history of production and trapping of radiogenic and nucleogenic isotopes in FI and possibly processes that fractionate these elements. In samples that were not pile-irradiated, nucleogenic Ar, Kr, and Xe produced by neutron capture from Cl, Br, I, and Ba +/- Te are present, plus 40Ar from radioactive decay of K and Xe from spontaneous fission of U. 40Are is defined as 40Ar in excess of the amounts attributable to atmospheric gases and that produced by decay of K in FI. Abundances of 40Are and fissiogenic Xe in FI are typically an order of magnitude greater than production from in situ decay of K and U, similar to estimates of average production per unit volume within Stripa in #2 Ga. It is inferred that a significant fraction of these isotopes were generated within Stripa and then locally transported in a fluid during episodes of recrystallization/metamorphism. Concentrations of nucleogenic 80*Kr, 82*Kr, and 128*Xe are several orders of magnitude greater than average production from Br and I within Stripa, evidence that these isotopes were produced in an I- and Br-rich source rock outside of Stripa.