Computer Science
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59..355i&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 2, pp.355-369
Computer Science
2
Scientific paper
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 36 Ar, 40 Ar, 84 Kr, and 129 Xe. Large variations in Br/Cl and I/Cl occur in FI within a single specimen of Stripa drill core. Two "endmember" FI compositions are represented: (1) I/Cl greater than 0.0012 and Br/Cl ~0.006 (atom ratios), a distinctive halogen signature previously measured in brines associated with some oil-fields and fine-grained sediments and (2) I/Cl less than 1 × 10 -4 and Br/Cl ~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 I/Cl and Br/Cl indicating at least one additional fluid. There are large and uncorrelated variations in ratios between 40 Ar, 36 Ar, 84 Kr, 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 40 Ar from radioactive decay of K and Xe from spontaneous fission of U. 40 Ar e is defined as 40 Ar in excess of the amounts attributable to atmospheric; gases and that produced by decay of K in FI. Abundances of 40 Ar e 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.
Irwin Jonathan
Reynolds James H.
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