An assessment of monazite from the Itambe pegmatite district for use as U-Pb isotope reference material for microanalysis and implications for the origin of the "Moacyr" monazite

作者：Goncalves Guilherme O^*; Lana Cristiano; Scholz Ricardo; Buick Ian S; Gerdes Axel; Kamo Sandra L; Corfu Fernando; Marinho Moacyr M; Chaves Alexandre O; Valeriano Claudio; Nalini Herminio A Jr

来源：Chemical Geology, 2016, 424: 30-50.

DOI：10.1016/j.chemgeo.2015.12.019

• Bananeira monazite
• Moacyr monazite
• Reference material
• U-Pb geochronology
• Sm-Nd isotope composition
• LA-ICP-MS
• ID-TIMS
• Aracuai orogen
• Eastern Brazillian pegmatite belt
• Pegmatites

Large quantities of monazite from different pegmatite bodies of the Itambe pegmatite district were investigated to assess their suitability as U-Pb and Sm-Nd isotope reference materials for LA-ICP-MS and to track the origin of a piece of the Moacyr monazite (termed here Itambe), a widely used reference material for LA-ICP-MS U-Pb geochronology. Monazite from the largest pegmatite bodies in the district (the Bananeira, Coqueiro and Paraiso pegmatites) are Ce-monazite, with negligible amounts of the huttonite and brabantite components. They are homogeneous in major and trace elements, which makes them potential candidates as compositional reference materials. U-Pb LA-ICP-MS and ID-TIMS analyses yielded identical ages within error. Although the ID-TIMS ages (507.7 +/- 1.3 (Pb-207U-235) and 513.6 +/- 1.2 Ma (Pb-206U-238)) were reversely discordant, spot ages determined by LA-ICP-MS geochronology were concordant at ca 508 Ma. The Bananeiro monazite was assessed as a LA-ICP-MS U-Pb primary reference material against other known reference materials (treated as unknowns). This approach successfully reproduced the previously published ages of the reference materials. MREE/HREE fractionation (ie, (La/Gd)(N) and (Gd/Lu)(N) values), Eu/Eu* and the chondrite-normalized REE patterns suggest that the "Itambe" monazite aliquot is very similar to that from the Coqueiro pegmatite. This similarity is likewise apparent in their Sm-Nd isotope compositions. Moreover, the epsilon Ndi values of the "Itambe" monazite fragment (epsilon Ndi = -4.2) and those from all the major pegmatites in the district, are distinct from other reference materials (eg, Managountry; epsilon Ndi = -22.3) as well as gem-quality monazite from c. 490-520 Ma pegmatites from the Aracuai Orogen, further to the south. The eNdi can provide a further distinction for tracing Brazillian gem-quality monazite reference materials.

• 出版日期2016-4-10