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

Abstract

The Lu-Hf isotope system is not only an important geochrono- meter 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®) that transforms raw mass spectrome- try data into meaningful isotopic ratios, including all the necessary corrections for spiked samples. Our 176Hf/177Hf results of both mafic and felsic geochemical reference standards agree with those reported by other authors. However, results obtained using PicoTrace® pressure digestion system (DAS®) yield significantly higher 176Lu/177Hf for felsic rocks with high Hf content (> 15 ppm) compared to data obtained by Parr® bomb dissolution, indicating incomplete digestion of HFSE-rich phases like zircon under specific conditions of DAS® digestion. The high degree of Hf purity attained using our chemical procedure, MC- ICP-MS measurement protocol, and the IsotopeHf® program yielded accurate data with reliable and reproducible Lu-Hf isotope ratios within typical analytical uncertainties.