Precise and accurate Lu-Hf isotope analysis of columbite-group minerals by MC-ICP-MS

Due to their relatively high Hf contents (i.e., several hundreds to thousands of ppm), columbite-group minerals (CGMs) are suitable for Lu-Hf isotope analysis, which can provide new insights into the source of such minerals in combination with the U-Pb age of rare-metal deposits. However, few studies have reported Lu-Hf isotope data for CGMs. Tantalum is a major element in CGMs (12-48 wt% Ta2O5) and has a significant effect on measurements of Hf isotopes using MC-ICP-MS. This effect was first evaluated in this study by analysis of mixed Hf-Ta standard solutions with different Ta/Hf ratios and LA-MC-ICP-MS analysis of various CGMs. This analytical artifact reflects serious tailing of the large Ta-181 signal onto the Hf-180 and Hf-179 masses during analysis. Therefore, in order to obtain accurate Hf isotope data for CGMs by MC-ICP-MS, we present an improved chemical separation procedure for Hf from Ta, and also a novel analytical protocol for LA-MC-ICP-MS analysis of CGMs. Accurate in situ(176)Hf/Hf-177 ratios for CGMs can be obtained by normalization to Hf-178/Hf-177 = 1.4672, rather than Hf-179/Hf-177 = 0.7325, using the exponential law. This approach was validated by solution and laser ablation MC-ICP-MS analysis of four CGM samples from China and Africa. The Lu-Hf isotopic composition of CGMs provides a new geochemical tracer for rare-metal deposits.

Automated summary

Columbite-group minerals (CGMs) with high Hf contents are suitable for Lu-Hf isotope analysis, providing insights into their sources in rare-metal deposits. The study evaluated the impact of tantalum on Hf isotope measurements and proposed improved analytical methods for accurate results.