Determination of Zr isotopic ratios in zircons using laser-ablation multiple-collector inductively coupled-plasma mass-spectrometry

Abstract

This study presents the first high precision method to analyze stable Zr isotope ratios in zircons using laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). Interference problems for in situ Zr isotope analysis were investigated in detail. The polyatomic interferences of ⁸⁹Y¹H⁺ and doubly charged ion interference of ¹⁸⁰Hf⁺⁺ did not influence the Zr isotope analysis. The addition of 8 ml min⁻¹ nitrogen to the central gas flow in LA-MC-ICP-MS was found to increase the sensitivity of Zr by a factor of 1.95. The analysis mode of small laser spot sizes (16–32 μm) and low laser frequency (1 Hz) combined with a signal-smoothing device was used. Compared to that without the signal-smoothing device, the obtained analytical precision was improved by about 60.6 times by using the signal-smoothing device at the low laser frequency (1 Hz). The accuracy of the in situ method is validated by comparing the data to those of a well-established double spike solution method. Data for six international zircon standards and six additional natural zircons are given. The data are reported as standard delta notation as the permil deviation of the ⁹⁴Zr/⁹⁰Zr (δ^94/90Zr_GJ-1) and ⁹⁶Zr/⁹⁰Zr (δ^96/90Zr_GJ-1) ratios relative to the GJ-1 zircon standard. Typical intermediate precisions for δ^94/90Zr_GJ-1 and δ^96/90Zr_GJ-1 were 0.11 and 0.18‰ (2SD, standard deviation). The δ^94/90Zr_GJ-1 values for the four zircon reference materials (91500, Plešovice, Penglai and Mud Tank) determined by LA-MC-ICP-MS were in good agreement with those of double spiked solution measurements, confirming the accuracy of the in situ method. The zircon reference material FC1 showed the largest stable isotope fractionation of 4.93‰ for δ^94/90Zr_GJ-1. One natural zircon (08KQ02-1) with a complex history showed a significant δ^94/90Zr_GJ-1 isotope variation in the single grain. These results suggest that Zr isotopes are a potentially powerful geochemical tracer and that high spatial-resolution, in situ determination like by the LA-MC-ICP-MS method in this study is rather important to identify Zr isotope variation in single zircon grains.