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 coupling a magnetic sector ICP-MS with a NdYAG laser probe working at 213 nm were 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 206Pb/238U, 207Pb/235U, 207Pb/206Pb and 208Pb/232Th ratios have been acquired. 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 microm has been estimated to be in the 0.1-1.5% range. Geologically meaningful ages can be achieved in zircons with less than 1 ppm of radiogenic Pb and with a spatial resolution down to 20 microm. With a spatial resolution of 40 microm, E-scan mode is more efficient and offers, for zircon with about 40 ppm of radiogenic Pb, an internal precision on the apparent age to better than 1.1% for all isotope ratios. At lower radiogenic Pb contents, internal precision decreases and, for zircons with radiogenic Pb contents lower than 1 ppm, is better than 7% and 2% on the 207Pb/235U and 206Pb/238U ages, respectively.