A rapid and efficient ion-exchange chromatography for Lu-Hf, Sm-Nd, and Rb-Sr geochronology and the routine isotope analysis of sub-ng amounts of Hf by MC-ICP-MS

摘要

The development and improvement of MC-ICP-MS instruments have fueled the growth of Lu-Hf geochronology over the last two decades, but some limitations remain. Here, we present improvements in chemical separation and mass spectrometry that allow accurate and precise measurements of ~(176)Hf/~(177)Hf and ~(176)Lu/~(177)Hf in high-Lu/Hf samples (e.g., garnet and apatite), as well as for samples containing sub-nanogram quantities of Hf. When such samples are spiked, correcting for the isobaric interference of ~(176)Lu on ~(176)Hf is not always possible if the separation of Lu and Hf is insufficient. To improve the purification of Hf, the high field strength elements (HFSE, including Hf) are first separated from the rare earth elements (REE, including Lu) on a first-stage cation column modified after Patchett and Tatsumoto {Contrib. Mineral. Petrol., 1980, 75, 263-267). Hafnium is further purified on an Ln-Spec column adapted from the procedures of Muenker et at. (Geochem., Geophys., Geosyst, 2001, DOI:10.1029/2001gc000183) and Wimpenny er al. (Anal. Chem., 2013, 85, 11258-11264) typically resulting in Lu/Hf ＜ 0.0001, Zr/Hf ＜ 1, and Ti/Hf ＜ 0.1. In addition, Sm-Nd and Rb-Sr separations can easily be added to the described two-stage ion-exchange procedure for Lu-Hf. The isotopic compositions are measured on a Thermo Scientific Neptune Plus MC-ICP-MS equipped with three 10~(12) Ω resistors. Multiple ~(176)Hf/~(177)Hf measurements of international reference rocks yield a precision of 5-20 ppm for solutions containing 40 ppb of Hf, and 50-180 ppm for 1 ppb solutions (=0.5 ng sample Hf 0.5 in ml). The routine analysis of sub-ng amounts of Hf will facilitate Lu-Hf dating of low-concentration samples.