In situ measurement of hafnium isotopes in rutile by LA-MC-ICPMS

Abstract

This work presents a rigorous assessment of the accuracy and precision with which 176Hf/177Hf can be measured in rutile by laser ablation (LA) MC-ICPMS. For rutile with ≥40ppm Hf, we demonstrate that 176Hf/177Hf 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 ~10 analyses. For many rutiles, the advantages of determining the Hf mass bias coefficient, βHf, 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 ~5000ppm Hf (SR-2) to facilitate an external mass bias correction. Because rutile contains negligible Yb, the Yb mass bias coefficient βYb must be inferred from βYb/βHf measured on zircon in the same session. To account for a systematic instrument bias of 0.5-1βHf units, rutile analyses are normalised to SR-2, for which 176Hf/177Hf has been determined by solution MC-ICPMS. In situ 176Hf/177Hf measurements for two natural rutile samples with 40-50ppm Hf are in excellent agreement with solution MC-ICPMS values. Ti/Hf exerts no influence on the accuracy of 176Hf/177Hf measurements on our LA-MC-ICPMS, as illustrated by indistinguishable 176Hf/177Hf 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. Show more