Characterization of reference materials for in situ U–Pb dating of columbite group minerals by LA-ICP-MS

Abstract

Columbite group minerals generally incorporate considerable amounts of U, making them promising minerals for in situ U–Pb dating and constraining the time of Nb–Ta mineralization. Columbite group minerals may have uranium-rich inclusions and metamict domains and may show complex internal textures with recrystallization. To avoid such disturbances, isotopic measurements with high spatial resolution are needed. The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) technique, which is widely used for U–Pb dating with high spatial resolution, requires homogeneous reference materials for external calibration. Herein, we present in situ U–Pb ages, ranging from ∼2050 Ma to ∼136 Ma and chemical compositions for ten columbite group minerals considered potential reference materials. Our study indicates that CT3, Buranga, Rongi and SN3 contain very limited amounts of common Pb and can be used as primary reference materials for in situ U–Pb dating. Coltan17, ZKW, DDB, HND and RL2 show variable common Pb contents but have robust U–Pb ages that allow them to be used as primary reference materials. Among them, Coltan17 needs to be further characterized by applying the isotope dissolution thermal ionization mass spectrometry (ID-TIMS) method. A new ID-TIMS weighted average ²⁰⁶Pb/²³⁸U age of Coltan139 of 507.8 ± 1.3 Ma (2s, MSWD = 0.9; n = 6) is reported. This study demonstrates that Coltan139 contains relatively high and variable common Pb contents. Therefore, special attention should be given to the selected measurement areas when using Coltan139 as the primary reference material.