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The Diamantina Monazite: A New Low‐Th Reference Material for Microanalysis
Authors:Guilherme O Gonçalves  Cristiano Lana  Ricardo Scholz  Ian S Buick  Axel Gerdes  Sandra L Kamo  Fernando Corfu  Daniela Rubatto  Michael Wiedenbeck  Hermínio A Nalini Jr  Luiz Carlos A Oliveira
Institution:1. Applied Isotope Research Group, Departamento de Geologia, Escola de Minas, Universidade Federal de Ouro Preto, Ouro Preto, MG, Brazil;2. Department of Earth Sciences, Stellenbosch University, Stellenbosch, South Africa;3. Institute of Geosciences, Goethe University, Frankfurt am Main, Germany;4. Jack Satterly Geochronology Laboratory, Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada;5. Department of Geosciences and CEED, University of Oslo, Oslo, Norway;6. Research School of Earth Sciences, Australian National University, Canberra, Australian Capital Territory, Australia;7. Helmholtz Zentrum Potsdam, Deutsches GeoForschungsZentrum GFZ, Potsdam, Germany;8. Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil
Abstract:Most monazite reference materials (RMs) for in situ U‐Pb geochronology are rich in Th; however, many hydrothermal ore deposits contain monazite that is low in trace element contents, including Th, U and Pb. Because of potential problems with matrix effects and the lack of appropriate matrix‐matched RMs, such variations can bias dating of hydrothermal deposits. In this paper, we describe a polycrystalline low‐U and low‐Th Diamantina monazite from the Espinhaço Range, SE Brazil. It has a U‐Pb ID‐TIMS weighted mean 207Pb*/235U ratio of 0.62913 ± 0.00079, 206Pb*/238U of 0.079861 ± 0.000088 and 207Pb*/206Pb* of 0.057130 ± 0.000031, yielding a weighted mean 206Pb*/238U date of 495.26 ± 0.54 Ma (95% c.l.). In situ dates acquired with different methods (LA‐(Q, SF, MC)‐ICP‐MS and SIMS) are within uncertainty of the ID‐TIMS data. U‐Pb LA‐(Q, MC)‐ICP‐MS runs, using Diamantina as a primary RM, reproduced the ages of other established RMs within < 1% deviation. The LA‐MC‐ICP‐MS analyses yielded homogeneous Sm‐Nd isotopic compositions (143Nd/144Nd = 0.511427 ± 23, 2s; 147Sm/144Nd = 0.1177 ± 13, 2s) and εNd(495 Ma) of ?18.7 ± 0.5 (2s). SIMS oxygen isotope determinations showed measurement reproducibility better than ± 0.3‰ (2s), confirming Diamantina's relative homogeneity at test portion masses below 1 ng.
Keywords:reference material  Diamantina monazite  U‐Pb geochronology  Sm‐Nd  oxygen isotopes  Araç  uaí  orogen
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