Precambrian provenance and Silurian metamorphism of the Tsubonosawa paragneiss in the South Kitakami terrane, Northeast Japan, revealed by the chemical Th-U-total Pb isochron ages of monazite, zircon and xenotime.

Suzuki, Koji; Adachi, Mamoru

doi:10.2343/geochemj.25.357

Cited by 334 publications

(138 citation statements)

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“…However, the expense of this instrumentation hinders the wide application of this technique. Chemical dating by electron microprobe (EMPA) is another important approach for xenotime dating [21][22][23][24][25]. However, the low precision of this technique limits its applications in geological processes.…”

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confidence: 99%

In situ U-Pb dating of xenotime by laser ablation (LA)-ICP-MS

Liu

Guo

et al. 2011

Chin. Sci. Bull.

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Xenotime is an ideal mineral for U-Th-Pb isotopic dating because of its relatively high U and Th contents, but typically low concentration of common Pb. These characteristics, and the fact that it is widespread throughout various types of rocks, suggest that the U-Th-Pb dating of xenotime has broad applications. Studies of U-Pb dating on xenotime by ion microprobe (such as SHRIMP) have increased in recent years, whereas studies by laser ablation (LA)-ICP-MS are still rare. In this study, we developed a technique for U-Pb dating of xenotime using the 193 nm ArF laser-ablation system and Agilent 7500a Q-ICP-MS. To evaluate the reliability of our method, a xenotime standard, BS-1, was analyzed and calibrated against another xenotime standard, MG-1. The weighted mean 206 Pb/ 238 U ages of 510.1 ± 5.2 Ma (2 n = 21), 509.8 ± 4.3 Ma (2 n = 21) and 510.0 ± 4.6 Ma (2n = 21) were obtained using beam diameters of 16, 24 and 32 m, respectively. These ages are identical to those determined by ID-TIMS method (weighted mean 206 Pb/ 238 U age of 508.8 ± 1.4 Ma), which supports the reliability of our LA-ICP-MS method. We also analyzed xenotimes in leucogranites from South Tibet and granites from Xihuashan in southern China, and obtained accurate and precise ages. Nevertheless, we observed systematic differences in Pb/U fractionation among xenotime, monazite and zircon. The matrix-effect resulted in either under-correction or over-correction of fractionation, and thus led to inaccurate ages. Thus, a matrix-matched material is required for U-Pb dating of xenotime by LA-ICP-MS.

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confidence: 99%

In situ U-Pb dating of xenotime by laser ablation (LA)-ICP-MS

Liu

Guo

et al. 2011

Chin. Sci. Bull.

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“…As the Pb concentration is directly related to the age of the monazite and to the U and Th concentration, older is the monazite, better is the relative error. Since the first monazite dating study by EPMA [1] there has been improvements in data age processing, but only few evaluations of uncertainties in the Pb, U, Th determinations. As the error calculated on the age for each analysed point is the whole inaccuracies on Pb, U, Th determinations, this method requires high accuracies determinations of trace elements [3] as well as of major elements.…”

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Evaluation of Uncertainties in Dating of Monazite by Electron Microprobe

Merlet

2005

MAM

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Over the last decades, determination of ages in monazite using Electron Probe Microanalysis (EPMA) has progressed to the point that it is now considered as a routine procedure within several laboratories worldwide [1,2]. Nevertheless, the typical age errors of individual electron probe spots reported in the literature are an order of magnitude larger than attainable by Secondary Ion Mass Spectrometry (SIMS) and two orders of magnitude larger than attainable by Thermal Ionization Mass Spectrometry (TIMS). The age errors frequently range from 40 Ma to 120 Ma, although after statistical treatment of a set of homogeneous ages, a precision of approximately 20 Ma can be obtained. Despite the low accuracy, the numerous studies have shown that the Th-U-totalPb method using the EPMA is a reliable method, easy to use, for detecting age variations on a very small scale. The effective limitation of the EPMA monazite ages determination concerns its relatively poor accuracy regarding mainly Pb determination. Similarly the relative uncertainties for U and Th are generally larger than 2%, which can lead to large errors. The error on the age for each analysed point depends on the U, Th and Pb concentration. As the Pb concentration is directly related to the age of the monazite and to the U and Th concentration, older is the monazite, better is the relative error. Since the first monazite dating study by EPMA [1] there has been improvements in data age processing, but only few evaluations of uncertainties in the Pb, U, Th determinations. As the error calculated on the age for each analysed point is the whole inaccuracies on Pb, U, Th determinations, this method requires high accuracies determinations of trace elements [3] as well as of major elements.The aim of the present experimental and theoretical study is firstly for evaluating precisely the uncertainties of EPMA dating of monazite and secondarily to give suggestions for minimizing them. Precise evaluation is not straightforward because sources of uncertainties include not only counting statistic but also many parameters like microprobe operating conditions (e.g., accelerating voltage, beam current, beam diameter, pulse height analyser), spectral background subtraction, quantification procedure, overlap, line shape, standards, sample preparation (polish, carbon coat, contamination), irradiation damage etc.. To estimate experimentally these sources, measurements were done on a large homogeneous monazite from the Itambe pegmatite district (Brazil [4]). The crystal homogeneous at the micrometer scale is free of fluid and solid inclusions. The composition (6.5 wt. % Th, 0.12 wt. % U and 0.15 wt. % Pb) is typical for natural monazites. Multiple Isotope Dilution Thermal Ionisation Mass Spectrometer (IDTIMS) analyses yield concordant U-Pb ages of 474 ± 1 Ma [5] and document the isotopic homogeneity of the monazite. The precise knowledge of age and the low Pb and U content in this monazite allows us to estimate the main sources of uncertainties in age determination using EPMA. As th...

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“…Alguns trabalhos demonstram que a análise por microssonda eletrônica (ME) pode ser um método preciso para a datação da monazita por meio de idades químicas U-Th-Pb (por exemplo, Suzuki & Adachi, 1991, Montel et al, 1996Cocherie et al, 1998). A idade é determinada pela medição de U, Th e Pb, partindo do princípio de que não há incorporação de chumbo comum durante a cristalização da monazita (Montel et al, 1996, Cocherie et al, 1998.…”

Section: Introductionunclassified

ORIGEM E IDADE U-Th-Pb DO DISTRITO PEGMATÍTICO DE SANTA MARIA DE ITABIRA (MG) REVELADAS PELA MONAZITA

Ferreira¹,

Chaves²

2017

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Resumo: Imagens de elétrons retro-espalhados e microanálises químicas por microssonda eletrônica foram obtidas a partir de cristais de monazita pertencentes ao Distrito Pegmatítico de Santa Maria de Itabira nas lavras de Morro Escuro, Ponte da Raíz e Euxenita. Essas lavras estão nos municípios de Santa Maria de Itabira, Ferros, Guanhães e Sabinópolis, em Minas Gerais. As amostras coletadas fazem parte da Província Pegmatítica Oriental Brasileira (PPOB), situada em um contexto pós-colisional do Orógeno Araçuaí correspondente ao final do Ciclo Brasiliano. Os cristais mostram-se homogêneos, ou seja, livres de domínios/zoneamentos composicionais e seus teores de U, Th e Pb permitiram a obtenção de idade química média de 474 Ma para o distrito pegmatítico da região estudada. Seus padrões de terras raras normalizados ao condrito mostram anomalia positiva de Sm, elemento fortemente particionado em anfibólio. Esta anomalia sugere que no contexto pós-colisional do Orógeno Araçuaí houve a fusão parcial do biotita-hornblenda gnaisse (Grupo Guanhães), rocha encaixante dos pegmatitos, durante o processo de descompressão regional associado ao colapso do orógeno, gerando um magma granítico hidratado o bastante para permitir o avolumado crescimento de cristais dos pegmatitos desse Distrito Pegmatítico, incluindo a monazita. Este mineral atua, portanto, não só como um geocronômetro, mas também como importante indicador petrogenético dos pegmatitos estudados. This anomaly suggests that in the post-collisional context of the Araçuaí Orogen there was partial melting of the host rocks of the Guanhães Group (biotite-hornblende gneiss) during the regional decompression process linked with the collapse of the orogen. This contributed to formation of a granitic melt, which was hydrated enough to allow the large crystal growth of the Itambé pegmatites, including monazite. Therefore, this mineral represents not only a geochronometer, but also a petrogenetic indicator of the studied pegmatites.Keywords: Monazite; Santa Maria de Itabira (MG); U-Th-Pb chemical dating, electron microprobe; petrogenetic indicator INTRODUÇÃOOs estudos referentes a corpos pegmatíticos apresentam grande importância desde as últimas décadas, por se tratarem de rochas fonte da grande maioria dos minerais-gema, minerais industriais e de elementos raros. Os pegmatitos são interpretados como um produto de fracionamento ígneo de fusões graníticas ou se formam por processos anatéticos, a partir da fusão parcial da rocha encaixante (Černý, 1982). A maior parte dos minerais encontrados nos pegmatitos pertence a três classes predominantes: silicatos, fosfatos e óxidos. Além da importância econômica é possível extrair deles informações no que tange à caracterização químico-mineralógica e genética dos corpos pegmatíticos, bem como sua relação, quando existente, com corpos graníticos (Newman, 2009).A monazita, um fosfato de elementos terrasraras (ETR) essencialmente leves (Montel et al., 1996), é um mineral acessório muito comum em pegmatitos e difundido em diversas litologi...

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Precambrian provenance and Silurian metamorphism of the Tsubonosawa paragneiss in the South Kitakami terrane, Northeast Japan, revealed by the chemical Th-U-total Pb isochron ages of monazite, zircon and xenotime.

Cited by 334 publications

References 29 publications

In situ U-Pb dating of xenotime by laser ablation (LA)-ICP-MS

In situ U-Pb dating of xenotime by laser ablation (LA)-ICP-MS

Evaluation of Uncertainties in Dating of Monazite by Electron Microprobe

ORIGEM E IDADE U-Th-Pb DO DISTRITO PEGMATÍTICO DE SANTA MARIA DE ITABIRA (MG) REVELADAS PELA MONAZITA

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