Past the first date: Resolving successive lead-loss episodes in zircon

Abstract. Zircon uranium–lead (U–Pb) geochronology is a cornerstone of Earth science for resolving the timing of deep-time processes, yet the U–Pb system in zircon does not always behave as a closed chronometer. Selective loss of radiogenic Pb can shift isotopic ages and generate discordant analyses that record later alteration, fluid-rock interaction, or heating. Many zircon datasets preserve evidence for more than one Pb-loss episode, yet many Pb-loss modelling approaches return a single "best" loss time. Here, we extend the concordant-discordant comparison (CDC) framework to recover multi-episode Pb-loss histories by using concordant analyses as a reference age distribution and scoring candidate Pb-loss times by how well reconstructed discordant ages reproduce that reference. The updated workflow can partition discordant analyses into internally coherent sub-arrays, retain reproducible local optima across Monte Carlo realisations rather than collapsing each realisation to a single optimum, and summarise statistically supported candidates as an ensemble catalogue with empirical 95 % uncertainty intervals and support values that quantify run-to-run stability. Synthetic benchmarks spanning single-stage and two-stage discordance geometries across three scatter tiers show that CDC achieves lower overall median absolute error and higher event-wise coverage than a discordia-likelihood discordance-dating (DD) approach. CDC performs best for single-stage benchmarks and for mixtures in which episodes remain well separated, whereas DD variants are more accurate and attain higher coverage in higher-scatter two-stage cases where likelihood surfaces are broad and competing modes occur. By reporting reproducible local optima rather than a single optimum, the CDC ensemble catalogue enables explicit recovery of multi-episode Pb-loss histories from discordant zircon U–Pb populations. Future work will focus on strongly overlapping multi-episode scenarios that remain difficult to deconvolve when candidate Pb-loss ages are tested one at a time.