LA-ICP-MS dating of high‑uranium columbite from no. 1 pegmatite at Dakalasu, the Chinese Altay orogen: Assessing effect of metamictization on age concordance

Abstract High‑uranium (~4200 to 8500 ppm U) columbite-group minerals (CGMs) from No. 1 pegmatite at Dakalasu, the Chinese Altay orogen were used to assess the effect of metamictization on the U Pb isotopic systematics. Backscattered electron (BSE) imaging shows that the high U concentrations are due to the U in the crystal structure itself. In the rims, the part of CGM next to cracks or pores filled by titan-uranoan fluorcalciomicrolite is altered. The microlite was likely formed via a dissolution-reprecipitation process that resulted in fractionation between U and Pb and potential discordance of the U Pb isotopic systematics in the altered CGM. Raman spectroscopy indicates that the columbite samples are metamictized to various extents. However, the CGM samples dated by laser ablation – inductively coupled plasma – mass spectrometer (LA-ICP-MS) show excellent U Pb age concordance, with thirty eight out of forty analyses displaying concordant U Pb ages (229.0 ± 1.0 Ma to 228.1 ± 0.6 Ma) overlapping within analytical uncertainty (2σ). The alpha-decay doses are estimated to be 1.53 to 2.31 × 1015 α-decay events/mg, indicating that the CGMs are at a transition stage from being crystalline to completely metamict states. Although spatial separation between U and radiogenic Pb isotopes caused by α-recoil is inevitable, the displacement is on sub-micron scale, much smaller than the scale of laser ablation pits (43 μm) for LA-ICP-MS dating. Therefore, the U Pb isotopic systematics of partially metamict, U-rich CGM may still remain concordant for CGMs that have not suffered hydrothermal alteration or metasomatism and do not contain U- or Pb-rich inclusions within the laser ablation pits. Additionally, the chemical compositions of the CGM have no control over the U Pb isotopic systematics.