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 ±0.5ɛ 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.