| 11 May 2026
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.
This work attempts to explain the dispersion present within a selection of highly dispersed samples. By generating a series of synthetic samples with different zoning characteristics and predicting ages using forward modelling. These new data are shown to overlap with measured dates as a means of showing than zoning may have played a significant role in the dispersion of grains. The workflow presented is understandable and can be seen as a simple way of screening grains to better understand dispersion and extract some thermal history information. However, in its current form, this article feels underdeveloped and highly localized to just this small collection of samples. Several inclusions could be made to make this more impactful and to ensure this approach is more broadly applicable.
Scientific significance:
I feel this work does not offer a ‘substantial’ contribution to geochronology as its limited scope within a small collection of samples from the Northern Madison Range in Montana. Testing this method on a number of other cases studies would be more convincing, which would possibly allow for more zoning combinations to be attempted. From my understanding the data need to test this workflow would be readily available in any publication of zircon (U-Th-Sm)/He dates and could warrant a wider selection of samples to be tested.
Additionally, could the newly acquired information about zoning be then reincorporated into an inverse model that shows how this work can help explore the model space better? This thermal history approach feels like it only is capable of forward modelling based on a given thermal history, and it use in inverse modelling could be very beneficial.
Scientific quality:
The work is well written, though a deeper analysis of the results is merited. In its current form the comparison between observed and modelled data appears to be visually based (overlapping in plots). A more robust statistical test that actually compares this dataset would help show how well these modelled data are. Some comparisons could help better fine tune forward modelling and find a better fit for zone/rim ratios.
Presentation quality:
Though not necessary for publication, the quality of figures and inclusion of perhaps an additional one would benefit this work. I do feel the plotting of data could be improved (perhaps coloring samples point by eU would benefit). Additionally, a conceptional figure showing a zircon with enriched cores or rims would perhaps help non-specialists.