Determination of Zr and Hf in a Flux-Free Fusion of Whole Rock Samples using Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) with Isotope Dilution Calibration

Isotope dilution calibration has been applied to the determination of Zr and Hf in whole rocks by laser ablation (LA)-ICP-MS. Enriched isotopes were added during the preparation of flux-free, synthetic whole rock glasses and homogenised through a combination of grinding and fusion. This method avoids problems, such as solution instability and the chemical resistance of minerals such as zircon, inherent in acid digestion sample preparation. The use of isotope dilution removes the need for external calibration using certified reference material glasses such as NIST SRM 612 for which certified Zr and Hf values are not available. The precision of Zr and Hf determinations were found to be < 1% and 3.5% respectively, limited by Poisson counting statistics which contributed to 50% of the final precision of analysis. Measured values correlate closely with compiled literature values. La methode de dilution isotopique a ete appliquee a la determination de Zr et Hf en roche totale par ablation laser (LA)-ICP-MS. Des isotopes enrichis ont ete ajoutes au cours de la preparation des verres synthetiques de roche totale, verre sans fondant et homogeneises par une procedure combinant broyage et fusion. Cette methode evite les problemes comme l'instabilite de la solution ou la resistance chimique de mineraux tels que le zircon, inherents a toute preparation d'echantillon par attaque acide. L'emploi de la dilution isotopique supprime la necessite d'une calibration externe par rapport a des verres de reference certifies, tel que NIST SRM 612 pour lequel les valeurs certifiees en Zr et Hf ne sont pas connues. La precision des mesures de Zr et Hf est respectivement < 1% et 3.5%. Elle est limitee par le comptage statistique de Poisson qui contribue pour plus de 50%a la precision finale de l'analyse. Les valeurs mesurees sont tres proches des donnees compilees dans la litterature.

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