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

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.