Application of isotope dilution for precise measurement of Zr and Hf in low-abundance samples and international reference materials by inductively coupled plasma mass spectrometry: implications for Zr (Hf)/REE fractionations in komatiites

Precise analysis of all REEʹs and HFSEʹs in mantle-derived rocks is critical to understanding the chemical compositions of different mantle reservoirs and the development of mantle lithosphere. An isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) study of Zr and Hf in komatiites and some low-abundance international reference materials (i.e. BIR-1) was undertaken, in an effort to evaluate previous data obtained by ICP-MS using external calibration with pure elemental standards. ic spikes enriched in 91Zr and 179Hf were employed. The isotope ratios used, 91Zr/90Zr and 179Hf/178Hf, are virtually free of isobaric interferences (e.g., formation rate of 162Dy16O and 163Dy16O is 0.4–0.6%, translating into uncertainties in measured 179Hf/178Hf of ∼ ±0.02%). Mass bias, showing reduced response for light masses, is insensitive to analyte solution concentration in the range 0.08–1.8 ppm for Zr, and 0.003–0.051 ppm for Hf. Mass bias for spiked samples was corrected with reference to the natural isotope ratios of Zr and Hf in unspiked counterparts. The results for Zr and Hf in unknowns and standards are in good agreement with previous results obtained by ICP-MS using external calibration, but with improved precision. Precision for Zr in BCR-1 (184 ppm, 0.005% RSD) is comparable to that obtained by thermal ionization mass spectrometry. The results confirm previously reported positive or zero normalized anomalies of Zr and Hf relative to MREEʹs in Al-undepleted komatiites, but negative anomalies in Al-undepleted komatiites, that have significance for the depths of mantle melting and Archean geodynamics.