Impact of air, laser pulse width and fluence on U–Pb dating of zircons by LA-ICPMS

Abstract

The accuracy of zircon U–Pb dating by LA-ICPMS is limited by matrix effects related to differences in U–Pb fractionation between an unknown and the calibration standard. Zircon radiation dose has a strong influence on the ablation rate, which in turn affects the amount of U–Pb fractionation and subsequent age calculations. Presented in this study is a previously unrecognized source of uncertainty in LA-ICPMS U–Pb ages due to variations of atmospheric air absorbed onto the surfaces of samples. Laser properties such as fluence (J cm⁻²) will change the laser ablation rate, and therefore the degree of U–Pb fractionation, and this can change the amount of zircon matrix effect. To improve the accuracy of U–Pb dating by LA-ICPMS, a systematic error component related to radiation dose needs to be included, or alternatively a correction to U–Pb dates can be applied as a function of radiation dose relative to the primary calibration standard. This study proposes a method to calculate a correction factor for the zircon matrix effect by measurement of several zircon reference materials with varying radiation dosages.