Review of gchron-2023-6

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Although the loss of radiogenic Pb from zircon is known to be a major factor that can cause inaccuracy in the U-Pb geochronological system, the distribution of Pb loss in natural samples has not been well characterized. Treatment of zircon by chemical abrasion (CA) has become standard practice in isotope dilution-thermal ionization mass spectrometry (ID-TIMS), but CA is much less commonly employed prior to <em>in-situ</em> analysis via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) or secondary ionization mass spectrometry (SIMS). Differentiating the effects of low levels of Pb loss in Phanerozoic zircon with relatively low precision <em>in-situ</em> U-Pb dates, where the degree of Pb loss is insufficient to cause discernible discordance, is challenging. We show that U-Pb dates that have been perturbed by Pb loss may be modeled by convolving a Gaussian distribution, that represents the unperturbed U-Pb date distribution, with a distribution that characterizes Pb loss. We apply this mathematical framework to model the distribution of apparent Pb loss in 10 igneous samples that have both non-CA LA-ICP-MS or SIMS U-Pb dates and an estimate of the crystallization age, either through CA U-Pb or ⁴⁰Ar/³⁹Ar geochronology. All but one sample showed negative age offsets that were unlikely to have been drawn from an unperturbed U-Pb date distribution. Of the eight continuous distribution types we considered, modeling apparent Pb loss using the Weibull distribution produced, on average, the closest match with the non-CA U-Pb date distributions. We show two contrasting patterns in apparent Pb loss: samples where most zircon U-Pb dates undergo a bulk shift and samples where most zircon U-Pb dates exhibited low age offset but fewer grains had more significant offset. Our modeling framework allows comparison of relative degrees of apparent Pb loss between samples of different age, with the first and second Wasserstein distances providing useful estimates of the total magnitude of apparent Pb loss. Given that the large majority of <em>in-situ</em> U-Pb dates are acquired without the CA treatment, this study highlights a pressing need for improved characterization of apparent Pb loss distributions in natural samples to aid in interpreting non-CA <em>in-situ</em> U-Pb data and to guide future data collection strategies.