Isotopic analysis of zirconium using enhanced sensitivity-laser ablation-multiple collector-inductively coupled plasma mass spectrometry

Abstract

Evidence for the presence of ⁹² Nb, which constrains the timescale of nucleosynthesis and formation sequence of the early solar system, can be detected by precise determination of the isotopic composition of Zr. Enhanced sensitivity-multiple collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) coupled with laser ablation was used to determine Zr isotopic ratios for five terrestrial zircon samples. A frequency-quadrupled Nd:YAG UV laser (266 nm), which produces pit sizes of 10-15 µm, was used to ablate the zircon samples. In order to deal with the extremely small amounts of material produced from the 10-15 µm ablation pit, an instrument of high sensitivity is required. This was achieved by the addition of a large capacity rotary pump for the first vacuum stage (expansion chamber) and by utilising a shielded ICP. Typical analytical precisions of the ⁹² Zr/ ⁹⁰ Zr, ⁹⁴ Zr/ ⁹⁰ Zr and ⁹⁶ Zr/ ⁹⁰ Zr isotopic ratio measurements for zircons achieved by the present laser ablation-MC-ICP-MS technique were 0.01-0.02, 0.02-0.03 and 0.03-0.04% (2σ), respectively, and these values were a factor of 2-3 worse than those achieved by solution analysis. The resultant Zr isotopic ratios for zircons show excellent agreement with those for chemical reagents obtained by conventional solution nebulisation; normalised to ⁹¹ Zr/ ⁹⁰ Zr≡0.21814, the isotopic ratios are ⁹² Zr/ ⁹⁰ Zr=0.333939±0.000020, ⁹⁴ Zr/ ⁹⁰ Zr=0.339172±0.000041 and ⁹⁶ Zr/ ⁹⁰ Zr=0.054627±0.000009 (2σ). Although neither isotopic variation in the ⁹² Zr/ ⁹⁰ Zr ratio due to radiogenic contribution from ⁹² Nb nor isotopic heterogeneity could be found for any of the Zr isotopic data obtained from terrestrial zircons and chemical reagents, the data presented here clearly demonstrate that laser ablation-MC-ICP-MS has the potential to become a strong tool for the detection of possible ⁹² Zr excess in older zircon samples.