LEAVING NO ZIRCON UNTURNED: QUANTIFYING THE IMPACT OF HANDPICKING VS RANDOM ANALYSIS OF DETRITAL ZIRCON ON MAXIMUM DEPOSITIONAL AGE ANALYSIS

A primary application of detrital zircon (DZ) U-Pb geochronology is using the youngest date(s) to constrain the maximum depositional age (MDA) of a sedimentary deposit. While many MDA studies analyze zircon grains at random, we propose that handpicking zircon grains with sharp facets and euhedral morphologies will increase the likelihood of encountering first-cycle zircons that approach the true depositional age (TDA). Here we evaluate this procedure by conducting intra-sample comparisons of randomly selected and handpicked zircon aliquots analyzed via laser ablation inductively coupled mass spectrometry. To quantify the difference between these groups, we utilize the first Wasserstein distance (W₁), the area between two empirical cumulative distribution functions. To compute W₁, we estimate the TDA for each sample and then find W₁ between the handpicked and random distributions to the TDA and W₁ between the handpicked and random distributions. Using 29 samples (6,485 analyses) from Cenozoic and Mesozoic MDA studies, our results indicate that handpicked zircon produced a systematic young-shift in U-Pb age distributions relative to their randomly analyzed counterparts, with an average difference of 445 Ma between the two groups. A comparison of handpicked samples with the TDA yielded a W₁ distance of 320 Ma, while randomly analyzed samples yielded a W₁ distance of 740 Ma. Additionally, in randomly analyzed samples, only 2.2% of zircon grains analyzed were within 5% of the estimated depositional age, while handpicked samples contained 20.7%, an approximately 10-fold increase. However, handpicking can also lead to selection of older grains if these zircons have been minimally transported, as was the case of one sample in which 81% of grains analyzed were 1.1 Ga zircon and interpreted to have been derived from the Pikes Peak granite. Overall, handpicking increases the proportion of first-cycle zircon grains analyzed for a given sample, including crystals derived from contemporaneous volcanism, thus increasing the likelihood that an MDA will approximate the TDA. Future work could involve using scanning electron microscopy to identify zircon grain surfaces with less abrasion and thus indicating less transport distance.