Precise 206Pb/238U age determination on zircons by laser ablation microprobe-inductively coupled plasma-mass spectrometry using continuous linear ablation

Abstract

Laser ablation microprobe-inductively plasma-mass spectrometry (LAM-ICP-MS) with a frequency quadrupled Nd-YAG UV laser (266 nm) has been used as a fast and relatively low cost technique to successfully determine the 207Pb/ 206Pb age of single zircon grains with ages older than 1000 Ma; however, it has a problem with serious fractionation in Pb/U during the ablation process of in-situ depth profile analysis. Alternatively, a linear-scan laser ablation technique was carried out in this study with an ablation size of 15-20 μm in width, 100-120 μm in length and 15-20 μm in depth on single zircon crystals. Constant focusing of the laser beam on the polished zircon crystal is maintained, which successfully minimized the fractionation in Pb/U during the ablation process. Using this method, typical precision of the measurement for 206Pb/ 238U ratios determined on single zircons is improved to 0.8-5%. Thus, the LAM-ICP-MS technique is capable not only of achieving precise 207Pb/ 206Pb ages at about the 1% level for > 1000 Ma zircons, but also of dating 206Pb/ 238U ages with precision of 2-5% for Mesozoic (156-126 Ma) zircons. It is likely that this simple and relatively low cost technique is able to achieve age results even comparable to the SHRIMP-type ion probe in the measurement of the relatively large (> 100 μm) and homogeneous zircons. However, it still has the disadvantage of lower spatial resolution, which limits its application to precise dating of small and/or heterogeneous zircons of Phanerozoic ages. © 2001 Elsevier Science B.V. All rights reserved.