Coupled CT/U-PB Measurements Elucidate Grain Size and Shape Fractionation Biases in Detrital Zircon Analysis

Abstract

The effects of sediment sorting and fractionation during fluvial transport have been well documented over the past century. Researchers frequently study the uranium-lead (U-Pb) age spectra of detrital zircons collected from modern or ancient rivers to document the geologic history of a basin. However, the typical single variable dissimilarity metrics that are commonly applied for sediment provenance interpretations, such as the Kuiper and Kolmogorov-Smirnov (KS) tests, do not account for the impact that grain size fractionation may have on U-Pb age distributions. Improvements in x-ray computed tomography (CT) 3D image processing software now permit for the acquisition of accurate size and shape measurements of individual zircons. Here, we present a high-resolution detrital zircon dataset coupling single grain CT scans with U-Pb dates of large-n (n > 1,000) samples from the unconsolidated sediments of northern Argentina’s Río Bermejo to evaluate the natural biases that hydraulic sorting introduces into geologic interpretations based on U-Pb age spectra. Our samples show that detrital zircon diameters vary positively with those of their host bulk sediments. Younger age modes are more variable in their zircons’ sizes and shapes than older age modes. Samples that share a common sediment provenance yield statistically significant dissimilarities in KS and Kuiper tests of their age spectra. We also discuss the impacts of zircon age-size relationships and preferential sorting on maximum deposition age estimates. Overall, our results demonstrate that sediment fractionation biases detrital zircon age spectra. Thus, future studies will need to develop new strategies to perform intersample statistical comparisons that take these effects into account and to enhance the robustness of detrital zircon provenance interpretations.