LA-ICPMS–CA-TIMS TANDEM DATING OF DETRITAL ZIRCON: INSIGHTS FROM N=1 MDAS OF MID-CRETACEOUS COLVILLE FORELAND BASIN STRATA, SLOPE MOUNTAIN, NORTHERN ALASKA

Recent advances in detrital zircon (DZ) maximum depositional age (MDA) research are notable, yet the persistent array of laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS)-based DZ MDA algorithms portends opportunity for further progress. Sound geologic reasoning must underpin MDAs, but interpreting youthful date distributions requires many analytically and statistically driven decisions, including whether to use single- or multi-grain MDAs. Furthermore, the reliability of MDA algorithms is usually assessed by chronostratigraphic benchmarking that does not ascertain MDA accuracy, and even careful consideration of known challenges can still lead to inaccurate MDAs. Here, we present tandem LA-ICPMS–chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) U–Pb results for five DZ samples from a ~950-m-thick section of mid-Cretaceous Torok and Nanushuk Formations at Slope Mountain. Youthful DZ yields are extremely sparse and only permit n=1 MDAs. LA-ICPMS dates are 0.3–6.4% younger than CA-TIMS ages from the same grains. The biostratigraphy suggests ~110–94 Ma sedimentation; the CA-TIMS-based MDAs reduce this window by ~8.5 Ma. These MDAs and a new CA-TIMS tephra zircon age correlated to the section’s top render reasonable minimum sedimentation rates (100s m/Ma). However, using the youngest single-grain LA-ICPMS dates as MDAs yields an improbably rapid rate (~5 km/Ma) that affirms their inaccuracy. We examine the new results and two published tandem DZ datasets to assess whether analytical dispersion and Pb-loss are formidable or discountable. These tandem dates indicate pitfalls for LA-ICPMS MDAs, with too-young offsets per study that are impactful (~2–3% avg.) and pervasive (~85–100% of pairs). Tandem date-pair plots reveal relations that DZ MDA algorithm comparisons can obscure, and too-young biases in LA-ICPMS dates likely reflect complex combinations of analytical scatter, matrix effects, and low-temperature Pb-loss. The youngest date in an LA-ICPMS distribution maximizes any too-young bias regardless of the contributing sources, but multi-grain MDAs are also prone to negative offset. We demonstrate the value of tandem LA-ICPMS–CA-TIMS geochronology for establishing DZ MDAs—and assessing the validity of MDA algorithms—in a demanding, n=1 application.