Improved LA-Q-ICP-MS U-Pb Zircon Age Estimates for Young (< 10 Ma) Zircons & Quantification of Bias in CL Guided Spot Selection

Charles Lewis, Adam Kent, Chris Russo, Shan de Silva

We present a new approach to reducing U-Pb data from zircons obtained by laser ablation-quadrupole-inductively coupled plasma-mass spectrometry (LA-Q-ICP-MS), with emphasis on young (< 10 Ma) zircons. In young zircons the 207Pb yield is extremely low, generating a relatively high abundance of zero values during analyses. This impacts the use of 207Pb/206Pb in application of Tera-Wasserburg Concordia, widely used to assess discordance and correct for common Pb. To improve estimates of 207Pb and 207Pb/206Pb in zircon we explore the use of two distributions that deal with zero values explicitly: the Zero-Inflated Poisson (ZIP) and the Zero-Truncated Normal (ZTN). From this we develop an approach to apply 207Pb and 207Pb/206Pb distributions that are appropriately bound at zero by application of the ZIP, which produces smaller overall uncertainties, but slightly higher discordance. This approach improves precision relative to assuming a Gaussian distribution while producing ratios and zircon ages that are within error despite the small increase in discordance (~ 0.5% under optimized analytical conditions).
Improved precision on LA-Q-ICP-MS zircons ages facilitates more rigorous cross-method comparison of ages gathered by LA-multicollector (MC)-ICP-MS in this study as well as previously collected Secondary Ion Mass Spectrometry (SIMS) ages on the same rocks. Both sets of LA ages are systematically older than SIMS ages, which are interpreted as higher incidence of older antecrystic and xenocrystic populations in the LA datasets. Qualitative analysis of CL-images suggested that this bias is due to targeted spot-selection (i.e., avoidance of specific CL-textures in cores). Following through, quantitative bias methodology is applied to quantify age bias based on user-guided spot selection via CL-texture. Based on the bias found, we quantitatively corroborate the important point that interpretations of magmatic systems using zircon U-Pb geochronology should account for any bias (conscious or unconscious) during spot selection.