LCT02: a new natural reference material for U–Pb isotopic microanalysis of columbite

Abstract

Spot U–Pb isotopic dating of columbite–tantalite minerals provides a useful way to directly constrain the timing of Nb–Ta mineralization and fingerprint the provenance of columbite–tantalite ore concentrates. However, application of accurate columbite–tantalite U–Pb isotopic dating using microbeam techniques, such as laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) or secondary ion mass spectrometry, relies largely on the availability of well-characterized matrix-matched reference materials. Although some columbite–tantalite reference materials have been developed for microanalysis, columbite–tantalite minerals are typically highly variable in chemical composition, which may strongly affect the accuracy and quality of spot U–Pb dating. Here, a columbite megacryst LCT02 is investigated using a combined LA-ICPMS and isotope dilution-thermal ionization mass spectrometry (ID-TIMS) approach to assess its suitability as a reference material for spot U–Pb dating. The LA-ICPMS U–Pb measurements of LCT02 in two laboratories yield reproducible U–Pb dates, which are in good agreement with the ID-TIMS date within analytical uncertainties, indicating that LCT02 has the potential to become a new reference material for spot columbite U–Pb isotopic microanalysis. The weighted mean ID-TIMS ²⁰⁷Pb/²⁰⁶Pb date of 908.6 ± 1.4 Ma (2σ) is recommended as the best estimated crystallization age for LCT02.