Validation of digestion and element separation methods and a new data reduction program (IsotopeHf®) for Lu-Hf isotope dilution analysis by MC-ICP-MS

The Lu-Hf isotope system is not only an important geochronometer but also a powerful tool for petrogenetic studies. To determine reliable analytical results of this system from rock and mineral samples, a protocol is established that includes appropriate sample digestion, a three-stage HREE-Hf ion-exchange separation procedure, a data acquisition protocol by MC-ICP-MS, and an R-based data reduction software package (IsotopeHf (R)) that transforms raw mass spectrometry data into meaningful isotopic ratios, including all the necessary corrections for spiked samples. Our Hf-176/Hf-177 results of both mafic and felsic geochemical reference standards agree with those reported by other authors. However, results obtained using PicoTrace (R) pressure digestion system (DAS (R)) yield significantly higher Lu-176/Hf-177 for felsic rocks with high Hf content (> 15 ppm) compared to data obtained by Parr (R) bomb dissolution, indicating incomplete digestion of HFSE-rich phases like zircon under specific conditions of DAS (R) digestion. The high degree of Hf purity attained using our chemical procedure, MC-ICP-MS measurement protocol, and the IsotopeHf (R) program yielded accurate data with reliable and reproducible Lu-Hf isotope ratios within typical analytical uncertainties.