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 nm 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 206Pb/238U, 207Pb/235U, 207Pb/206Pb and 208Pb/232Th 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 μm 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 μm. With a spatial resolution of 40 μm, the E-scan mode is more efficient and offers, for zircon with about 40 ppm of radiogenic Pb, an internal precision (2σ) 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. Accuracy is strictly related to Pb* in zircon and ranges between 1% and 5%. At a 20 μm of spot size, internal precision and accuracy are approximately 1.5–2 times lower than at 40 μm.