The Diamantina Monazite: A New Low‐Th Reference Material for Microanalysis

• Published in: Geostandards and geoanalytical research 2018-03, Vol.42 (1), p.25-47
• Main Authors: Gonçalves, Guilherme O., Lana, Cristiano, Scholz, Ricardo, Buick, Ian S., Gerdes, Axel, Kamo, Sandra L., Corfu, Fernando, Rubatto, Daniela, Wiedenbeck, Michael, Nalini, Hermínio A., Oliveira, Luiz Carlos A.
• Format: Article
• Language: English
• Subjects: Araçuaí orogen; Diamantina monazite; Geochronometry; Homogeneity; Hydrothermal deposits; Isotopes; Lead; Monazite; Oxygen isotopes; reference material; Sm‐Nd; Trace elements; U‐Pb geochronology
• ISSN: 1639-4488; 1751-908X
• DOI: 10.1111/ggr.12192

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. Key Points The hydrothermal Diamantina monazite is proposed as a primary reference material for U‐Pb geochronology. The only low‐Th monazite reference material currently available in large quantities for interlaboratory distribution. Ideal candidate for a Sm‐Nd and oxygen isotope reference material.