Effects of H2O- and O2-containing He carrier gases on the 206Pb/238U system bias and down-hole fractionation in LA-ICPMS of zircon

This paper deals with the effects the addition of H2O or O-2 to He aerosol carrier gas has on Pb-206/U-238 ratios during laser ablation inductively-coupled plasma mass spectrometry (LA-ICPMS) of natural zircon and synthetic silicate glass. The following parameters were studied: (i) system bias defined as constant but LA-ICPMS system-dependent shifts in Pb-206/U-238 ratios, (ii) down-hole fractionation given by the slope of the Pb-206/U-238 ratio vs. LA time, and (iii) the sum of (i) and (ii) referred to as the overall elemental fractionation. To isolate and study changes of Pb-206/U-238 ratios arising independently from either LA sampling or ICPMS analysis of the generated aerosols, a diffusion-based gas exchange device was used. The supply of He + O-2 caused relative changes in mean Pb-206/U-238 values and temporal gradients due to the LA process of less than 40%. By contrast, addition of H2O to the He carrier gas changed Pb-206/U-238 ratios by up to 150% through processes occurring during LA as well as ICPMS analysis. Here, the opposing temporal gradients of Pb-206/U-238 ratios canceled out at relative humidity levels close to the saturation vapor pressure under ambient lab conditions, i.e., 20 degrees C room temperature. As a consequence, changes in Pb-206/U-238 ratios caused by down-hole fractionation could be suppressed. However, the admixture of H2O also affected the system bias at the same time, resulting in increased offsets in Pb-206/U-238 ratios such that the accuracy of LA-ICPMS analyses worsened by up to 6% when applied to the age determination of zircon. Neither worsening nor significant improvements in the accuracy of Pb-206/U-238 ages were observed in the case of LA-ICPMS using He + O-2 carrier gas mixtures.