Problems Associated with the Determination of Rare Earth Elements of a “Gem” Quality Zircon by Inductively Coupled Plasma‐Mass Spectrometry

A gem quality zircon crystal from the Mud Tank Carbonatite, Australia, was carefully crushed, sieved, acid leached and hand-picked to produce three different zircon fractions: 1) clear and apparently free of inclusions; 2) with inclusions; 3) with inclusions and adhering alteration/matrix material. Each separate and the bulk sample was dissolved by 1) lithium metaborate fusion and 2) Parr bomb digestion utilizing 29 mol l−1 hydrofluoric (HF) and 16 mol l−1 nitric (HNO3) acids. In situ analyses were performed using laser ablation ICP-MS and demonstrate that, on the basis of REE, two types of inclusions are present: 1) those with a positive Ce anomaly and 2) those with no Ce anomaly. The former type of inclusion appears to be smaller than the latter. Analysis of the digested samples for the rare earth elements (REE) was then performed using a quadrupole ICP-MS. Comparison of the chondrite-normalized REE patterns for the different fractions gave apparent positive anomalies at Sm and Eu in the acid digestions but not in the fused samples. Zirconium has five isotopes (90Zr, 91Zr, 92Zr, 94Zr and 96Zr) and there is a strong tendency for Zr to combine with fluorine (19 F). Based on calculations and experiments, the increased levels of Sm and Eu in the acid digested samples were due to interferences of 90Zr19 F3 on 147 Sm and of 94Zr19 F3 on 151 Eu. Decreasing the ratio of HF:HNO3 did not remove these interferences, nor did post-treatment with perchloric acid. A mathematical method was developed to correct for these polyatomic interferences. Un cristal-gemme de zircon provenant de la carbonatite de Mud Tank, Australie, a ete soigneusement broye, tamise, lessivea l'acide et triea la main pour obtenir trois fractions de zircon: 1) limpide et sans inclusions apparentes; 2) avec inclusions; 3) avec inclusions et matrice/produits d'alteration. Chaque fraction separee et la roche totale ont ete dissoutes par 1) une fusion avec metaborate de lithium et 2) une attaque acide en bombe de Parr avec HF 29 mol l−1 et HNO3 16 mol l−1. Des analyses in situ ont ete realisees par ICP-MS avec ablation laser et mettent en evidence, du point de vue des Terres Rares, deux types d'inclusions: 1) avec une anomalie positive en Ce et 2) sans anomalie en Ce. Le premier type d'inclusions semble plus petit que le second. L'analyse des Terres Rares des echantillons issus de l'attaque acide a ete realisee par ICP-MS a quadrupole. La comparaison des spectres de Terres Rares normalises aux chondrites des differentes fractions montre des anomalies positives apparentes en Sm et Eu seulement pour les attaques acides. Le zirconium a cinq isotopes (90Zr, 91Zr, 92Zr, 94Zr et 96Zr) et une forte tendance a se combiner au fluor (19 F). A partir de calculs et d'experimentations, l'enrichissement en Sm et en Eu des echantillons issus de l'attaque acide serait du aux interferences 90Zr19 F3 sur 147 Sm et 94Zr19 F3 sur 151 Eu. La diminution du rapport HF/HNO3 ne supprime pas ces interferences, pas plus qu'un traitement final a l'acide perchlorique. Une methode mathematique a ete developpee pour corriger ces interferences polyatomiques.