EM - Escola de Minas

Resultados da Pesquisa

Agora exibindo 1 - 3 de 3

Optimization of the in-situ U-Pb age dating method via LA-Quadrupole-ICP-MS with applications to the timing of U-Zr-Mo mineralization in the Poços de Caldas Alkaline Complex, SE Brazil.
(2015) Takenaka, Lynthener Bianca; Lana, Cristiano de Carvalho; Cipriano, Ricardo Augusto Scholz; Nalini Júnior, Hermínio Arias; Abreu, Adriana Trópia de
The high spatial resolution of the LA-ICP-MS systems allows rapid extraction of vital isotopic information from individual growth zones of minerals. This paper describes in detail the optimization of a relatively inexpensive LA-ICP-MS system consisting of a UV 213 Laser Ablation and a Quadrupole ICP-MS. The results of optimization take into account laser energy, beam diameter, frequency and ICP-MS gas conditions. The optimized conditions were tested for precision and accuracy on a number of wellcharacterized zircons, commonly used as primary and secondary quality control standards. The acquisition of the U-Pb data is carried out in automated mode (pre-set points) for up to 12 h/day with only minimal operator presence. Individual U-Pb zircon analysis lasts 80 s. The 2s uncertainties of the standards ranged between 1.4 and 8.2%, and overall their relative deviations ranged from 0.02 to 0.87%. The results are comparable to techniques that use more complex and time-consuming approaches such as LA-MC-ICP-MS and ion-microprobe. We have applied this method to obtain ages of numerous granitoid rocks from the Southern São Francisco Craton and a well-known Archean granitoid of the Kaapvaal Craton, South Africa. We furthermore provide the first results of U-Pb age dating of U-Zr-Mo mineralization in the Poços de Caldas Alkaline Complex, SE Brazil, with a U-Pb age of 85 ± 3 Ma for zircon-bearing hydrothermal veins.
Evolução magmática e metamórfica de zircão do Complexo Barro Alto (GO), com base em imagens de elétrons retro-espalhados e análises químicas por microssonda eletrônica.
(1999) Takehara, Lucy; Hartmann, Leo Afraneo; Vasconcellos, Marcos Antonio Zen; Suita, Marcos Tadeu de Freitas
O zircão é muito utilizado em geocronologia U-Pb por ser considerado um sistema isotópico fechado e apresentar quantidades elevadas de U e Th. No presente trabalho, foram feitos estudos de imagens de elétrons retroespalhados (BSE) e perfis de varredura espacial por espectrômetro dispersivo em comprimento de onda dos elementos Hf, Y e U em cristais de zircão de algumas amostras do Complexo Barro Alto (GO). As imagens de BSE e os perfis evidenciam variações nas estruturas internas e na composição dos cristais de zircão, que são reflexos de processos magmáticos e metamórfïcos sofridos pela rocha hospedeira. Além das feições magmáticas presentes, observamos que os processos metamórficos encontram-se registrados de forma variável nos cristais de zircão, sendo que alguns cristais são menos afetados que outros, indicando serem mais resistentes. Em todos os cristais observados, o metamorfismo regional de médio a alto grau do Ciclo Brasiliano encontra-se registrado de uma forma mais ou menos intensa, evidenciando a abertura do sistema neste período, que afetou consequentemente o sistema isotópico.
Provenance and age delimitation of Quadrilátero Ferrífero sandstones based on zircon U–Pb isotopes.
(2006) Hartmann, Leo Afraneo; Endo, Issamu; Suita, Marcos Tadeu de Freitas; Santos, João Orestes Schneider; Frantz, José Carlos; Carneiro, Maurício Antônio; McNaughton, Neal Jesse; Barley, Mark E.
The Quadrilátero Ferrífero has some of the largest iron and gold deposits in the world and is a major geotectonic unit of the São Francisco Craton in Brazil. U–Pb zircon SHRIMP geochronology of six detrital sedimentary and metasedimentary rocks (114 zircon crystals, 118 spot analyses) has improved the understanding of the sedimentary processes and provenance ages of both rocks and the associated iron formation. The age of deposition of the iron formation is constrained between 2.58 and 2.42 Ga. The presence of an old Paleoarchean crust is dated in detrital zircon crystals, including the oldest zircon in South America (3809G3 Ma). Only high-Th/U, magmatic zircon crystals are present in the dated sedimentary rocks, and these indicate that the crust of the region was formed mostly during the Jequie´ cycle (six age peaks between 3055 and 2635 Ma). This time span ofw420 m.y. is similar to the duration of a long-lived Wilson cycle. Most of the Rio das Velhas Basin was filled during approximately 30 m.y. between 2746 and 2717 Ma, though volcanism probably started earlier. The youngest detrital zircon age from the Minas Supergroup indicates that the intracratonic basin fill, including the iron formation, was deposited after 2580 Ma. Therefore, the crust was cratonized shortly after the intrusion of minor granitic bodies at around 2.62–2.58 Ga. A large gap in orogenic activity is indicated by the absence of zircon ages of 2580–2182 Ma.

EM - Escola de Minas

Notícias

Navegar

Filtros

Configurações

Ordenar por

Resultados por página

Resultados da Pesquisa