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 181Ta signal onto the 180Hf and 179Hf 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 situ176Hf/177Hf ratios for CGMs can be obtained by normalization to 178Hf/177Hf = 1.4672, rather than 179Hf/177Hf = 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.