Separation of high field strength elements (Nb, Ta, Zr, Hf) and Lu from rock samples for MC-ICPMS measurements

Abstract

The application of multiple collector inductively coupled plasma source mass spectrometry (MC-ICPMS) to 176Lu-176Hf and 92Nb-92Zr chronometry has been hampered by complex Zr-Hf purification procedures that involve multiple ion exchange column steps. This study presents a single-column separation procedure for purification of Hf and Lu by ion exchange using Eichrom® Ln-Spec resin. The sample is loaded in pure HCl, and element yields are not dependent on the sample matrix. For 92Nb-92Zr chronometry, a one-column procedure for purification of Zr using Biorad® AG-1-× 8 resin is described. Titanium and Mo are completely removed from the Zr, thus enabling accurate 92Zr measurements. Zirconium and Nb are quantitatively separated from rock samples using Eichrom Ln-Spec resin, allowing measurements of Zr/Nb with a precision of better than ±5% (2σ). The Ln-Spec and anion resin procedures may be combined into a three-column method for separation of Zr-Nb, Hf, Ta, and Lu from rock samples. For the first time, this procedure permits combined isotope dilution measurements of Nb/Ta, Zr/Hf, and Lu/Hf using a mixed 94Zr-176Lu-180Hf-180Ta tracer. Analytical protocols for Zr and Hf isotope measurements using the Micromass Isoprobe, a second generation, single-focusing MC-ICPMS, are reported. Using the Isoprobe at Münster, 2σ external precisions of ±0.5ε units for Hf and Zr isotope measurements are achieved using as little as 5 ng (Hf) to 10 ng (Zr) of the element. The 176Hf/177Hf and Lu/Hf for rock reference materials agree well with other published MC-ICPMS and thermal ionization mass spectrometry (TIMS) data.