U-Pb isotopic dating of columbite-tantalite minerals: Development of reference materials and in situ applications by ion microprobe

Abstract

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 ((206 Pb/ 238 U) SIMS /(206 Pb/ 238 U) 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 204 Pb 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.

Columbite U-Pb Geochronology of Kalu’an Lithium Pegmatites in Northern Xinjiang, China: Implications for Genesis and Emplacement History of Rare-Element Pegmatites

“…Matrix effect during in situ dating may be another reason for the variation of the age data. Significant matrix effect during dating CGMs using SIMS was observed by [42]. According to [42], the matrix effect is mainly caused by #Ta values of CGMs, whereas #Mn values have no impact.…”

“…Significant matrix effect during dating CGMs using SIMS was observed by [42]. According to [42], the matrix effect is mainly caused by #Ta values of CGMs, whereas #Mn values have no impact. However, the matrix effect during LA-ICP-MS U-Pb isotopic analyses were not encountered in most previous studies [14,21].…”

“…In Figure 13, data points K805.6 and K805.15 are affected by microfractures and show age concordance <95%, although these two data points still plot close to the Concordia. Micro-scale inclusions (e.g., feldspar) may be present in the fractures [42] and would potentially affect the dating results. It is also noted that those microfractures were not obvious on BSE images but can be revealed using high-resolution SE imaging (Figure 13).…”

“…Therefore, we excluded the dating analyses that were obviously affected by the presence of microfractures or possible metamictization for Concordia age calculation. The discordant ages are likely caused by: (1) metamictization [43], (2) presence of U-rich inclusions [43], (3) common lead [42], and (4) microfractures. According to [43], both metamictization and presence of U-rich mineral inclusions (e.g., uraninite) can result in anomalous ages.…”

“…inclusions (e.g., feldspar) may be present in the fractures [42] and would potentially affect the dating results. It is also noted that those microfractures were not obvious on BSE images but can be revealed using high-resolution SE imaging (Figure 13).…”