APPLICATION OF ISOTOPE DILUTION FOR PRECISE MEASUREMENT OF ZR/HF AND 176HF/177HF RATIOS BY MASS SPECTROMETRY (ID-TIMS/ID-MC-ICP-MS)

Abstract

This paper presents a Hf and Zr chemical purification method and calibration procedure used to measure Zr/Hf ratios in terrestrial and extraterrestrial rocks by isotope dilution mass spectrometry with a mixed 96Zr-179Hf spike. Sample preparation and mass spectrometric procedures for thermal ionization mass spectrometry (TIMS) and magnetic sector multiple-collector (MC-ICP-MS) analyses are described. We used a new combination of chemical procedures to achieve a highly reliable method for Hf and Zr isotopic measurements [Minster, J.F., Allegre, C.J., 1981. The isotopic composition of zirconium in terrestrial and extraterrestrial samples: implications for extinct 92Nb. Geochim. Cosmochim. Acta, 46: 565-573; Barovich, K.M., Beard, B.L., Capple, J.B., Johnson, C.M., Kyser, T.K., Morgan, B.E., 1995. A chemical method for hafnium separation from high-Ti whole rock and zircon samples. Chem. Geol., 121: 303-308]. It involves two-stage anion exchange with minimal amounts of sulphuric acid. Undesirable elements for hafnium and zirconium isotopic analyses are removed and negligible blanks are obtained. For Hf and Zr isotopic measurements performed by thermal ionization mass spectrometry (VG 354) and magnetic sector multiple-collector ICP-MS (Plasma 54), two sample preparations schemes are proposed. A specific load of less than 5 μg of zirconium with 200 ng of hafnium was used in TIMS to avoid the inhibitor effect of zirconium on Hf thermal ionization. For ICP-MS analyses, storage of samples as oxalates stabilizes both elements in aqueous solution without interferences on mass spectra. Zr/Hf ratios are measured with similar internal precision for both ID-TIMS and ID-MC-ICP-MS and a reproducibility of 2.5%% (RSD). This is a factor of 10 better than that obtained by classical techniques used to measure the concentrations of zirconium and hafnium. 176Hf/177Hf isotope ratios were obtained with an internal precision ranging from 0.04%% to 0.1%% and a reproducibility of 0.1%% (RSD).