Herriott, T.M., Crowley, J.L., Wartes, M.A., LePain, D.L., and Schmitz, M.D., 2025, Accuracy and validity of maximum depositional ages in light of tandem (laser ablation and isotope dilution) U-Pb detrital zircon geochronology, including results from northern Alaska: Geochronology, v. 7, p. 513-543, https://doi.org/10.5194/gchron-7-513-2025.
Sound geologic reasoning underpins detrital zircon (DZ) maximum depositional ages (MDAs) via the principle of inclusions, although interpreting in situ U-Pb date distributions requires many geologically, analytically, and statistically driven decisions. Existing research highlights strengths and challenges of various algorithm approaches to deriving MDAs from DZ dates, yet community consensus on best practices remains elusive. Here, we first address new laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) and chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb geochronology for five DZ samples from a ~1 km thick section of mid-Cretaceous strata in Alaska's Colville foreland basin. Youthful DZ yields are extremely sparse, and the MDAs are n = 1. LA-ICPMS and CA-ID-TIMS dates from the same grains (i.e., tandem dating) adhere to a uniform pattern: laser ablation dates are younger than paired isotope dilution dates, with in situ offsets ranging from -0.3 % to -6.4 %. Existing biostratigraphic constraints suggest a ~110-94 Ma sedimentation window for the sampled section, but the CA-ID-TIMS MDAs reduce by ~8.5 Myr the maximum geologic time recorded by the stratigraphy. A simple age-depth analysis incorporating the CA-ID-TIMS MDAs and correlation of a new CA-ID-TIMS tephra zircon age yields geologically reasonable minimum stratigraphic accumulation rates, but an LA-ICPMS-based interpretation would render an improbable and inaccurate chronostratigraphy. We then explore the new tandem data and two previously published Mesozoic tandem DZ datasets for their broader MDA research implications, focusing on tandem-date pair relations and youthful-population sampling densities rather than conducting the typical MDA algorithm output assessment. Percent-offset plots document impactful (~2 %-3 % on average) and pervasive (~87 %-100 % of pairs per study) young bias for the laser ablation dates, likely reflecting a complex combination of analytical dispersion, low-temperature Pb loss, and matrix effects, which are topics we review in detail. Deconvolving offset sources without elaborate geochronologic experiments is difficult, but our tandem-date analysis provides critical context, and follow-up CA-ID-TIMS can diminish or eliminate analytical, systematic, and geologic offset sources. We also (1) redefine the reference value for MDA accuracy as the crystallization age of the youngest analyzed DZ population in a sample and (2) reframe LA-ICPMS-based DZ MDA algorithm evaluations around validity – how capable are the metrics of accurately measuring what they are intended to measure? – rather than MDA benchmarking by existing age constraints. These new perspectives follow straightforward geochronologic and stratigraphic principles, and our synthesis intends to identify and clarify opportunities to further refine DZ MDA research.
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