Time-of-flight ICP-MS laser ablation zircon geochronology: assessment and comparison against quadrupole ICP-MS

摘要

This study presents U-Pb isotopic results from the mineral zircon measured by nanosecond laser ablation coupled to either a time-of-flight inductively coupled plasma mass spectrometer (ICP-MS), a quadrupole ICP-MS or both via a split stream arrangement. Results show both ICP-MS types produce similar levels of precision and accuracy for most U-Pb ages in the mineral zircon. The time-of-flight ICP-MS however shows slight non-linearity at high detector counts (>7000 for ~(238)U) producing a bias in zircon U-Pb ages from published values outside analytical uncertainty. Use of ~(235)U instead of ~(238)U is shown to resolve the inaccuracy in the U-Pb age at the expense of analytical precision due to lower abundance of ~(235)U. Trace element concentrations also measured along with U-Pb isotope data in zircon show the time-of-flight ICP-MS to provide similar accuracy and precision as the quadrupole ICP-MS, except for isotopes in regions around high intensity major element peaks (e.g. Zr) where accuracy and levels of detection of the time-of-flight ICP-MS are matrix dependent relative to the quadrupole ICP-MS. A comparison is made between the analytical capabilities of the time-of-flight ICP-MS and the quadrupole ICP-MS highlighting some advantages and disadvantages of the time-of-flight technology for zircon U-Pb geochronology.