IN SITU PB GEOCHRONOLOGY OF ZIRCON WITH LASER ABLATION-INDUCTIVELY COUPLED PLASMA-SECTOR FIELD MASS SPECTROMETRY

Abstract

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.