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.