Uranium-lead dating method at the Pará-Iso isotope geology laboratory, UFPA, Belém - Brazil

Abstract

Analytical procedures for U-Pb isotope dilution analyses at the Pará-Iso isotope geology laboratory of the Federal University of Pará (UFPA) are described in detail. The procedures are applied to zircon, titanite, rutile, apatite, columbite-tantalite and whole rock. Reagent preparation and chemical processing are done in clean-room conditions. Samples are dissolved using Teflon ™ microcapsules in steel jacket Teflon ™ Parr Instrument ™ bomb or Teflon ™ screw cap containers. U and Pb are separated using anion exchange AG 1x8 resin columns. Typical blanks for mineral sample amounts of 0.01 to 1.0 mg are less than 1 pg U and 20-30 pg Pb. Isotope analysis of the U and Pb from the same fi lament are carried out using a Finnigan MAT 262 mass-spectrometer in static and dynamic modes. The current analytical level is demonstrated on analyses of international standard zircon 91500 with three different 235 U-205 Pb and 235 U-208 Pb isotope tracers and whole rock standards. Results of analyses of two zircon samples are also presented.

The Zhireken porphyry Mo ore-magmatic system (eastern Transbaikalia): U–Pb age, sources, and geodynamic setting

“…Cathodoluminescent images were obtained on a CamScan MX2500S scanning electron microscope. The isotopic composition of Pb in minerals was determined by the technique of Krymsky et al (2007) on a Triton mass spectrometer in the static multichannel one-tape regime, using the NIST 981 standard sample.…”

Anoxygenic photosynthesis linked to Neoarchean iron formations in Carajás (Brazil)

“…The Carajás Basin, within the CMP (Figure 1b), has a Mesoarchean (c. 3.0-2.86 Ga) basement composed of mafic granulites, migmatites, and metavolcanic rocks (Machado et al, 1991). The crystalline basement is overlain unconformably by Neoarchean and Paleoproterozoic volcanosedimentary sequences from the Grão Pará Group that have undergone subgreenschist to low-greenschist facies metamorphism (Krymsky et al, 2007;Machado et al, 1991;Rosière et al, 2006).…”

“…A Late Archean to Early Paleoproterozoic sedimentary cover overlies the IFs; however, its age is still debated (Rossignol et al, 2020). The depositional age of the Carajás IFs is well constrained by U-Pb zircon ages of c. 2.755-2.740 Ga in the basalts underlying and intercalating the IFs (Gibbs et al, 1986;Krymsky et al, 2007;Machado et al, 1991) and U-Pb zircon ages from a possible tuff (2,743 ± 11 Ma) and a dolerite dyke (2,740 ± 8 Ma) cross-cutting the Carajás Formation (Trendall et al, 1998). Other age (1)…”

THE ZHIREKEN PORPHYRY Mo ORE-MAGMATIC SYSTEM (eastern Transbaikalia): U-Pb AGE, SOURCES, AND GEODYNAMIC SETTING

“…Катодолюминесцентные изображения получены на сканирующем электронном микроскопе CamScan MX2500S. Определение изотопного состава Pb в минералах проведено по методике [Krymsky et al, 2007] на масс-спектрометре Triton в одноленточном варианте в статическом многоколлекторном режиме с использованием стандарта NIST 981.…”