In situ Pb geochronology of zircon with laser ablation-inductively coupled plasma-sector field mass spectrometry

The in situ Pb geochronological capabilities of a laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) coupling a magnetic sector ICP-MS with a Nd:YAG laser probe working at 213 mn have been tested on three zircon populations with different age (150-294-577 Ma) and radiogenic Pb contents (0.7-10-40 ppm). The influence of scan mode and spatial resolution on age precision and accuracy has also been evaluated. All the signals necessary to independently determine the Pb-206/U-238, Pb-207/U-235, Pb-207/Ph-206 and Pb-208/Th-232 ratios have been acquired. The external standardization approach has been used to correct for laser induced U/Pb elemental fractionation, instrumental mass bias and sequential acquisition of transient signals. The efficiency of the external standardization correction has been carefully evaluated in each analytical session and residual error with a spot size of 40 mum has been estimated to be in the 0.1-1.5% range. Geologically meaningful ages can be achieved in zircons with less than I ppm of radiogenic Pb and with a spatial resolution down to 20 gm. With a spatial resolution of 40 mum, the E-scan mode is more efficient and offers, for zircon with about 40 ppm of radiogenic Pb, an internal precision (2sigma) on the apparent age to better than 1.1% for all isotope ratios. At lower radiogenic Ph contents, internal precision decreases and, for zircons with radiogenic Ph contents lower than I ppm, is better than 7% and 2% on the Pb-207/U-235 and Pb-206/U-238 ages, respectively. Accuracy is strictly related to Ph* in zircon and ranges between 1% and 5%. At a 20 mum of spot size, internal precision and accuracy are approximately 1.5-2 times lower than at 40 mum. (C) 2003 Elsevier Science B.V. All rights reserved.