Application of high-resolution laser multi collector ICP-MS U–Pb dating to columbite-group minerals with compositional zonation: reassessment of matrix effects among columbite-group minerals†

Abstract

Columbite-group minerals (CGMs) have been widely used in U–Pb geochronological analysis of granite and pegmatite Nb–Ta deposits due to their high U and low common Pb characteristics. Due to the equivalent replacement of elements in the mineral structure and complex geological processes, CGMs often exhibit strong, local, and complex chemical zoning. Multiple types of composition zonation of CGMs can reflect the process of niobium–tantalum mineralization. However, the laser resolution of the currently established U–Pb methods cannot be used for in situ dating of CGMs with zonation with a width less than 20 μm. Additionally, there is no consensus on whether significant matrix effects could be observed among CGMs when nonmatrix-matched calibrations were performed. These limitations hinder the advancing understanding of niobium–tantalum differentiation and enrichment mechanisms in the formation of CGMs. In this study, we utilized the widely used geochronological reference material Coltan139 as the primary standard, established a high-resolution LA-MC-ICP-MS U–Pb analysis method using homogeneous CGM samples from the Dakalasu and Jingerquan deposits in the Altai and East Tianshan orogenic belts, Xinjiang, China. This method has a resolution of up to 10 μm. We applied this method to sample JEQ-2, which was collected from the Jingerquan Li–Be–Nb–Ta deposit and exhibited a variety of compositional zonations. Through a comparison of the ages calibrated using the ferrocolumbite Coltan139 and ferrotapiolite CT1 standards, we propose that the major matrix effect for U–Pb dating of CGMs correlates with the Ta/(Nb + Ta) ratios under small beam spot conditions. The matrix effect is likely to increase with the increase in Ta/(Nb + Ta) differences between reference materials and the samples. Furthermore, the U–Pb age results of sample JEQ-2 confirm that CGM grains with normal and oscillatory zonings are magmatic contemporaneous products. A new in-house standard material sample DKLS-27 was also reported with a reliable and stable U–Pb age for CGM U–Pb dating analysis. The ²⁰⁶Pb/²³⁸U weighted mean age obtained from long-term monitoring is 250.2 ± 0.3 Ma (2 s, MSWD = 0.52, n = 141). This study offers a new analysis method for the fine geochronology of rare metal deposits.