ALTERNATE APPRAISALS OF YOUNGEST U-PB GRAIN AGES IN DETRITAL ZIRCON POPULATIONS OF MESOZOIC STRATA ON THE COLORADO PLATEAU

U-Pb ages for youngest detrital zircon grains in sedimentary strata constrain maximum depoages of stratigraphic units. We have tested alternate estimates of youngest valid grain age in detrital zircon populations against a database of 5386 U-Pb zircon ages in 61 samples of Mesozoic sandstone from the Colorado Plateau and contiguous areas for which depoages can be inferred independently within 4-14 myr by correlation of stratigraphic stages with the GTS 2004 timescale. Approximately 100 U-Pb ages were determined for individual zircon grains in each sample by LA-ICP-MS using a beam diameter of 35 microns, but ages >20% discordant or with age uncertainties >10% were rejected (average of 88 grain ages retained per sample). The very youngest grains present in each case may not have been selected for laser ablation, but we tested five measures of youngest dated grains for consistency with depoage: (1) youngest single grain age (206Pb/238U) including 1ó analytical uncertainty; (2) weighted mean average (with 1ó uncertainty) of youngest cluster of two or more grain ages overlapping at 1ó uncertainty (to provide minimal reproducibility); (3) youngest graphical age peak controlled by two or more grain ages on a probability density plot (age distribution curve); (4) youngest age peak controlled by three or more grain ages that overlap at 2ó using an in-house algorithm; (5) TuffZirc age (with uncertainties) for youngest cluster of coherent ages selected by the 2002 algorithm of Ludwig and Mundil. Youngest single grain ages overlap depoages in 48% of samples, but are older than depoages in 44% of samples and younger than depoages in 8% of samples. Half the discrepant grains (younger than depoage) may be spurious but half probably reflect correlation problems. Despite the risk of relying upon single data points, the youngest single dated zircon grain apparently provides a satisfactory measure of youngest grain age in 95% of samples provided overlap with depoage is acceptable. Using alternate measures for youngest grain age involving multiple grains reduces overlap with depoage to 13%-26% of samples, but still leaves age discrepancies (grains too young) unresolved for 5% of samples. Multi-grain measures for youngest age are thus more conservative (not necessarily an advantage depending upon the purpose of a youngest-age appraisal).