Hf isotope analysis of small zircon and baddeleyite grains by conventional Multi Collector-Inductively Coupled Plasma-Mass Spectrometry

• Solution MC-ICP-MS Hf isotope analysis is accurate for non-purified zircon sample.
• Matrix effects are observed between pure JMC475 and natural zircon reference materials.
• Isobars are efficiently removed by adjusting the 176Yb/173Yb ratio for each session.
• Solution Hf isotope analysis is accurate for [Hf] = 2 ppb within ± 2.66 εHf units (2SD).
• Sample-Standard bracketing is used to measure Hf isotope in zircon as small as ca. 24 ng.

Protocols for solution nebulization (SN) multi collector (MC) ICP-MS analysis of the Hf isotopic composition of accessory minerals (zircon and baddeleyite) selected for high-precision ID-TIMS UPb geochronology have improved significantly during the last two decades. Precise crystallization age and Hf isotopic composition from the exact same volume of mineral can be retrieved, provided that time-consuming sample preparation and purification is possible. The present work focuses on the analytical capabilities of SN-MC-ICP-MS for Hf isotope analysis on non-purified reference materials. Analytical issues arise for (i) igneous and metamorphic zircons, often enriched in trace elements and REEs, which increase the amount of isobaric interferences. (ii) Small minerals that challenge the limits of quantification of the apparatus. JMC475 Hf standard, Plešovice and Temora-2 zircons and Phalaborwa baddeleyite were used to characterize the sources of analytical bias and to set up an optimized protocol for Hf isotope analysis of small accessory minerals with accurate correction of the isobaric interferences and accounting for the matrix effects. Doping of JMC475 with major (Zr, Si), trace (U, Th, Y, Ti) and rare earth elements (Ce, Nd, Gd, Dy, Ho, Er) shows that oxide species produced in the ICP induce an offset of the measured 176Hf/177Hf ratios that is corrected by normalization to the reference value. The 176Yb isobaric interference is efficiently removed for samples with Yb/Hf ratios < 0.6 using in-run determined 176Yb/173Yb reference ratios from Yb doped reference material. This isotopic ratio varies (i) with time and (ii) sample matrix. Plasma loading by Zr is not the source of these variations. We propose that Yb oxidation in the plasma torch is responsible for variations in the 176Yb/173Yb ratio. Our dataset shows that matrix matched standards should be preferred for data reduction, and that Plešovice is a good zircon standard because of its availability, quantifiable REEs composition and potential for wide range of Yb doping. Consequently, low Hf concentration solutions from Temora-2 zircons were accurately measured in a small sample volume (< 120 μl), using a short acquisition protocol (50 × 1 s cycles), and sample-standard bracketing for mass bias coefficient determination in a [Hf]~ 10 ng/g Plešovice solution measured through 80 × 4 s cycles. The repeatability of the measurement (2SD) is improved by factors of up to ~ 4.4 and the limit of quantification is brought down to < 2 ng/g Hf, corresponding to zircon minerals of < 24 ng.