Mitigating the role of Pb loss on Maximum Depositional Age estimation using Chemical Abrasion-Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry

Abstract. It is increasingly recognized that Pb loss in zircon of even Cenozoic age is pervasive and can lead to ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U dates that are younger than the mineral’s crystallization date. Therefore, applications of U-Pb geochronology that rely on identifying a sample’s youngest age component, like the determination of a detrital zircon sample’s maximum depositional age (MDA), can be compromised if Pb loss is not mitigated. Here, we test whether the use of bulk chemical abrasion (CA) prior to laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) detrital zircon U-Pb geochronology can eliminate Pb loss, specifically to improve low-n MDA estimation methods of Cenozoic zircon. We analyzed CA-treated and untreated aliquots of two Cenozoic sandstones from southern Alaska and found all untreated fractions have zircon with younger apparent ²⁰⁶Pb/²³⁸U dates than treated zircon fractions. To assess whether this discrepancy is a result of Pb loss, the youngest zircon grains in all aliquots were extracted and dated using chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS). All chemically abraded date pairs agree within 2σ uncertainty, whereas untreated LA-ICP-MS analyses give ²⁰⁶Pb/²³⁸U dates that are distinctly younger than their corresponding CA-ID-TIMS dates by 4.1–15.9 %. We conclude the CA-LA-ICP-MS method provides an avenue to improve the accuracy of low-n MDA estimations and subsequent geological interpretations.