In situ measurement of hafnium isotopes in rutile by LA-MC-ICPMS: Protocol and applications

This work presents a rigorous assessment of the accuracy and precision with which (176)Hf/(177)Hf can be measured in rutile by laser ablation (LA) MC-ICPMS. For rutile with >= 40 ppm Hf, we demonstrate that (176)Hf/(177)Hf can be measured accurately and reproducibly with adequate precision for application to petrological problems. We present an analytical and data reduction protocol tailored to the specific challenges of measuring Hf isotope ratios in this low-Hf mineral. Precision and accuracy are optimised by interpolating between baselines measured every similar to 10 analyses. For many rutiles, the advantages of determining the Hf mass bias coefficient, beta(Hf), during analysis are negated by the low precision with which it can be measured at low Hf contents. Hf mass bias is therefore monitored by regular analysis of a synthetic rutile doped with similar to 5000 ppm Hf (SR-2) to facilitate an external mass bias correction. Because rutile contains negligible Yb, the Yb mass bias coefficient beta(Yb) must be inferred from beta(Yb)/beta(Hf) measured on zircon in the same session. To account for a systematic instrument bias of 0.5-1 epsilon(Hf) units, rutile analyses are normalised to SR-2, for which (176)Hf/(177)Hf has been determined by solution MC-ICPMS. In situ (176)Hf/(177)Hf measurements for two natural ruble samples with 40-50 ppm Hf are in excellent agreement with solution MC-ICPMS values. Ti/Hf exerts no influence on the accuracy of (176)Hf/(177)Hf measurements on our LA-MC-ICPMS, as illustrated by indistinguishable (176)Hf/(177)Hf measurements for a series of synthetic rutiles doped with varying amounts of Hf. Rutile and zircon from the Duria mantle peridotite (Central Alps), which formed at different parts of the P-T-time path, record contrasting Hf isotope signatures and emphasise the complementary nature of Hf isotope analysis of rutile and zircon. (C) 2010 Elsevier B.V. All rights reserved.