Evaluation of Pb-Pb and U-Pb laser ablation ICP-MS zircon dating using matrix-matched calibration samples with a frequency quadrupled (266 nm) Nd-YAG laser

This paper reports the successful application of laser ablation (LA) ICP-MS to the in situ analysis of (207)pb/Pb-206 and (206)pb/U-238 isotopic ratios on zircon crystals using matrix-matched calibration samples as external calibrators. For Pb-207/Pb-206 analyses, LA-ICP-MS results on reference materials (UQ-Z1 and G91500) indicated individual precisions in the range 1-10% (2s), most analyses being better than 6%. The resulting weighted means were associated with errors typically better than 1% with ages of 1148 +/- 5 Ma (2s) and 1069 +/- 9 Ma (2s) respectively. Analyses of well-dated late Archaean granitic rocks from the western margin of the Yilgarn Craton (Australia) are presented and show a close agreement with the reference values. An orthogneiss dated at 2662 +/- 4 Ma (2s) by ion microprobe (SHRIMP) gave a Pb-207/Pb-206 age of 2657 +/- 6 Ma (2s). A more complex zircon population from a syenite emplaced at 2654 +/- 5 Ma containing a less than or equal to 3.25 Ga inherited component has been investigated using a spot size of approximately 45 mum. LA-ICP-MS provides a 207Pb/206Pb age of 2653 +/- 6 Ma with older grains yielding ages of up to 3.23 Ga. Dating of younger rocks (< 1 Ga), however, was limited by poor precision in the measurement of the 207Pb/206Pb isotopic ratios and by inter-element fractionation between Pb and U during the ablation processes. Using a high power density, variations of the Pb-206/U-238 ratios during one spot analysis appeared to correlate positively with time over the first minute of ablation. A linear fit of the data acquired during this period allowed a Pb-206/U-238 ratio to be calculated, thus reducing the magnitude of the fractionation and improving precision to around 5% (2s). Results on the G91500 zircon reference sample yielded a Pb-206/U-238, age of 1057 +/- 14 Ma, in good agreement with the published reference value (1062.4 Ma). Late Hercynian zircons from a Corsican alkali granite dated at ca.285 Ma by TIMS and SHRIMP yielded a younger but consistent age of 277 +/- 11 Mo. These results show that using a somewhat simple apparatus (quadrupole ICP-MS and 266 nm Nd:YAG laser), the technique has the capability of producing precise and reliable Pb-207/Pb-206 and Pb-206/U-238 ages with a minimal sample preparation and. a high throughput. The present limitations are associated with the high density power used in this study, as analyses must be conducted on grains larger than 80 mum that are free of inclusions and fractures, which often result in "catastrophic" ablation. Shorter wavelength losers, which yield a better laser-target coupling and which produce smaller ablated particles, should help to reduce these drawbacks.