Technical Note: A retroactive method for identifying subpopulations of zoned zircon in (U-Th)/He data

Abstract. Zircon (U-Th)/He thermochronometery (ZHe) is a widely used tool for investigating and dating thermal events such as uplift, exhumation, incision, and plutonic emplacement, among others. The utility of this thermochronometer relies on its ability to measure and predict the production, distribution, and ultimately diffusion of radioactive decay products, a thermoregulated process. Natural zircon crystals frequently display internal chemical zonation as a result of precipitation environment, and metamorphism. This internal chemical heterogeneity unevenly partitions radioactive actinides, alpha particle production, and resultant accumulated radiation damage within the grain. The heterogeneous distribution of radiogenic He and radiation damage accumulation change the diffusion kinetics of these zircon, hindering our ability to calculate dates and ultimately develop thermal history interpretations. Models reveal that the effect of zoned actinides and associated radiation damage is magnified for samples characterized by negative ZHe date-eU trends produced by protracted thermal histories including extended residence in the partial retention zone. Measuring the degree and distribution of zoned radiation damage accumulation requires destructive characterization processes which precludes characterized zircon from whole grain ZHe dating. It is otherwise difficult to isolate zoned zircon during standard grain picking procedures, implying that many zircon grains that could be used for (U-Th)/He dating have spatially heterogeneous accumulated radiation damage. I developed a retroactive test to identify populations of potentially zoned zircon within a dataset utilizing the HeFTy v2.3.1 (Ketcham, 2025) forward modeler and “zoned” grain function. This test allows us to reproduce the behavior of endmember zonation styles in date-eU space, with the ultimate goal of identifying and reclaiming these data to develop more robust thermal history interpretations. Considering zircon damage zonation may be helpful when interrogating samples with complex ZHe date-eU patterns that are otherwise difficult to interpret.