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

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 U-Pb 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, Plesovice 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 Hf-176/Hf-177 ratios that is corrected by normalization to the reference value. The Yb-176 isobaric interference is efficiently removed for samples with Yb/Hf ratios <0.6 using in-run determined Yb-176/Yb-173 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 Yb-176/Yb-173 ratio. Our dataset shows that matrix matched standards should be preferred for data reduction, and that Plesovice 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 mu l), using a short acquisition protocol (50 x 1 s cycles), and sample-standard bracketing for mass bias coefficient determination in a [Hf]similar to 10 ng/g Plesovice solution measured through 80 x 4 s cycles. The repeatability of the measurement (2SD) is improved by factors of up to similar to 4.4 and the limit of quantification is brought down to <2 ng/g Hf, corresponding to zircon minerals of <24 ng. (C) 2016 Elsevier B.V. All rights reserved.