Bernhard Bühn

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)

A determinacao de idades atraves do decaimento dos isotopos radioativos 235U e 238U para os isotopos radiogenicos 207Pb e 206Pb, respectivamente, utilizando o mineral zircao (ZrSiO4), e amplamente aplicada para decifrar processos geologicos. Um novo metodo tem sido desenvolvido nos ultimos anos, a ablacao a laser multi-coletor espectrometria de massas com plasma indutivamente acoplado (LA-MC-ICP-MS), superando o laborioso trabalho anteriormente necessario em outros metodos, na preparacao de amostras, e permite obtencao de razoes isotopicas com alta resolucao espacial de micrometros. O presente estudo descreve os procedimentos analiticos e os metodos usados na reducao de dados que estao sendo aplicados no Laboratorio de Geocronologia da Universidade de Brasilia. Explora-se a precisao e exatidao do metodo atraves da analise de tres padroes internacionais de zircao. Observa-se uma precisao entre 1,9 a 3,7% (2σ desvio padrao) e uma exatidao de 0,6 a 3,8% (2σ desvio padrao) para as razoes isotopicas de Pb e U dos padroes. Tambem foram obtidas idades pelo metodo LA-ICP-MS de duas amostras de zircoes naturais, que ja foram datadas anteriormente por outros metodos analiticos. A comparacao dos resultados mostra uma boa conformidade das idades obtidas, dentro dos limites de erro.Os dados demonstram o grande potencial do metodo analitico para analises isotopicas rapidas, precisas e exatas de U-Pb, numa escala de micrometros.

Combined U-Pb and Lu-Hf isotope analyses by laser ablation MC-ICP-MS: methodology and applications

The Lutetium-Hafnium isotopic system represents one of the most innovative and powerful tools for geochronology and isotopic studies. Combined U-Pb and Lu-Hf in situ analyses on zircon by LA-MC-ICP-MS permit to characterize isotopically the host magma from which it crystallized furnishing significant information for sediment provenance and crustal evolution studies. In this paper we describe the Lu-Hf systematic by LA-MC-ICP-MS developed in the laboratory of Geochronology of the University of Brasilia and report the results obtained by repeated analyses of 176 Hf/ 177 Hf isotopic ratio of three zircon standards: GJ-1 = 0.282022 ± 11 (n=56), Temora 2 = 0.282693 ± 14 (n=25) and UQZ = 0.282127 ± 33 (n=11). The 176 Hf/ 177 Hf ratio (0.282352 ± 22, n=14) of gem quality zircon used as in-house standard have been also characterized. As a geological application, we analyzed two complex zircons selected from a migmatitic rocks from the Borborema Province, NE Brazil. On the basis of U-Pb and Lu-Hf data, two main crystallization events have been identified in both studied zircons. An older event at ca. 2.05 Ga recognized in the inherited cores represents a well-characterized paleoproterozoic magmatic event that affected the whole Borborema Province. A second crystallization event at � 575 Ma, recognized at the rims, represents a Neoproterozoic (Brazilian) high grade metamorphic-magmatic event.