Journal
CHEMICAL GEOLOGY
Volume 512, Issue -, Pages 69-84Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.chemgeo.2019.03.001
Keywords
Columbite-tantalite; Niobium; Tantalum; U-Pb dating; SIMS; ID-TIMS; Geochronology
Categories
Funding
- Observatoire des Sciences de L'Univers (OSU), Otelo (Nancy, France)
- CT5 CESSUR Ressources geologiques et developpement durable (Programme Tellus, CNRS)
- French National Research Agency through the Labex Resources 21 [ANR-10-LABX-21-RESOURCES21]
- Natural Science Foundation of China [41672065]
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Columbite-tantalite group minerals are the most common Nb-Ta minerals. Columbite-tantalite is particularly suitable for U-Pb dating due to its high U and low common Pb contents. In situ isotopic dating of columbite-tantalite by LA-ICP-MS or SIMS requires certified reference material to properly account for potential matrix effects linked to substitutions between Nb and Ta and between Mn and Fe. Our study has two objectives: i) establish a database of reference materials for in situ U-Pb isotopic dating of columbite-tantalite minerals and ii) test the capability of SIMS to in situ U-Pb date columbite-tantalite minerals of different chemical composition. Tests of in situ U-Pb dating demonstrate that SIMS can easily be used to date columbite-tantalite minerals with errors and precisions overlapping the reference ID-TIMS age. There are, however, significant matrix effects for non-matching Nb-Ta-Fe-Mn compositions of sample and reference material. Matrix effects are highly correlated with the Ta/(Ta+Nb) ratio of columbite-tantalite, due to the significant difference in the atomic mass of Nb and Ta. The Mn/(Mn+Fe) ratio does not significantly contribute to the observed matrix effect as the two elements have similar atomic masses. The linear correlation between Ta/(Nb+Ta) and ((Pb-206/U-238)(SIMS)/(Pb-206/U-238) (ID-TIMS)) obtained for columbite-tantalite minerals of known ID-TIMS age demonstrates that the SIMS matrix-effect can be properly accounted for by using the chemical composition as determined by EMPA. The ability to measure Pb-204 by SIMS also allows the use of reference materials with a small common lead contribution and to calculate accurate and precise ages for columbite-tantalite minerals with contributions of common lead.
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