High spatial resolution analysis of Pb and U isotopes for geochronology by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)

Abstract

Age determinations using the 235 U and 238 U radioactive decay series to the daughter isotopes 207 Pb and 206 Pb, respectively, using the mineral zircon (ZrSiO 4 ), are widely used to decipher geological processes. A new method developed in the last couple of years, the laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), overcomes previous laborious sample preparation, and yields isotopic ratios and age data with a high spatial resolution of ten of microns. The present study describes the analytical set-up and data reduction process as presently applied at the Laboratory for Geochronology of the University of Brasília. It explores the precision and accuracy of the method by cross-analysing three international zircon standards. We arrive at a precision of 1.9 to 3.7% (2σ SD) and an accuracy of 0.6 to 3.8% (2σ SD) for and U isotopic ratios of the standards. We also apply the method to two natural zircon samples, which have previously been dated by other analytical methods. A comparison of the results shows a good conformity of the age data, being within the error limits. The data demonstrate the great analytical potential of the method for rapid, precise and accurate U-Pb isotopic analyses on the micron scale.

Rhyacian evolution of the eastern São Luís Craton: petrography, geochemistry and geochronology of the Rosário Suite

“…The analytical procedures followed the recommendations of Bühn et al (2009) and Chemale Jr. et al (2012). The zircon crystals were concentrated using conventional techniques at the Pará-Iso Laboratory of the Federal University of Pará, in Belém, Brazil, including mineral sieving (250-180 µm and 180-125 µm), magnetic separation and gravimetric separation by heavy liquid.…”

“…Modified from Gorayeb et al (1999) and Sousa et al (2012). Bühn et al (2009). Only the uncertainties of the GJ-1 were propagated to the sample values; 91500 was treated as a secondary standard and analyzed as an unknown.…”

Geocronologia e aspectos estruturais e petrológicos do Pluton Bravo, Domínio Central da Província Borborema, Nordeste do Brasil: um granito transalcalino precoce no estágio pós-colisional da Orogênese Brasiliana

“…Os grãos de zircão foram separados por catação manual, utilizando-se lupa binocular. Seguiram-se os métodos e procedimentos analíti-cos descritos por Bühn et al (2009) e Matteini et al (2009.…”

Geocronologia U-Pb em zircão de rochas intrusivas e de embasamento na região do Vale do Jacurici, Cráton do São Francisco, Bahia

“…A redução dos dados foi realizada em planilha Microsoft ® Excel (cf. Bühn et al 2009), que permite correções para branco, deriva do equipamento e chumbo comum. As incertezas associadas às razões são de 1 σ.…”