Columbite SN3: A New Potential Reference Material for U-Pb Dating by LA-ICP-MS

Columbite-tantalite LA-ICP-MS U-Pb dating is a fast and useful method to determine the age of rare-metal deposits and fingerprint the provenance of columbite-tantalite ore concentrates. Accurate LA-ICP-MS U-Pb dating requires matrix-matched reference materials. We analysed three columbite-tantalite samples (SN3, HND and RL2) from China using ID-TIMS and LA-ICP-MS to assess their potential as reference materials for in situ U-Pb dating. Coltan 139 and these three columbite-tantalite samples with variable compositions yielded internally consistent LA-ICP-MS U-Pb ages when using each other for calibration and the weighted mean ²⁰⁶Pb/²³⁸U ages are comparable to respective ID-TIMS ages. Composition-dependent U-Pb fractionation seems to be insignificant under the LA-ICP-MS conditions used. Sample SN3 has a low percentage of heterogeneity for ²⁰⁶Pb/²³⁸U ages (4%) with low common Pb contributions (f₂₀₆ < 1%) and shows a good potential in calibrating unknown samples as primary reference material for LA-ICP-MS analysis. Samples RL2 and HND have altered sections characterised by high LREE contents, flat LREE patterns and old ²⁰⁶Pb/²³⁸U apparent ages, and are not suited as reference materials. The low ²⁰⁷Pb/²⁰⁶Pb intercepts for samples RL2 and HND lack geological meaning but provide strong evidence that the disturbed U-Pb systematics with anomalous apparent ²⁰⁶Pb/²³⁸U ages is a secondary feature.