Exploring the law of detrital zircon: LA-ICP-MS and CA-TIMS geochronology of Jurassic forearc strata, Cook Inlet, Alaska, USA

Uranium-lead (U-Pb) geochronology studies commonly employ the law of detrital zircon: A sedimentary rock cannot be older than its youngest zircon. This premise permits maximum depositional ages (MDAs) to be applied in chronostratigraphy, but geochronologic dates are complicated by uncertainty. We conducted laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) of detrital zircon in forearc strata of southern Alaska (USA) to assess the accuracy of several MDA approaches. Six samples from Middle–Upper Jurassic units are generally replete with youthful zircon and underwent three rounds of analysis: (1) LA-ICP-MS of ∼115 grains, with one date per zircon; (2) LA-ICP-MS of the ∼15 youngest grains identified in round 1, acquiring two additional dates per zircon; and (3) CA-TIMS of the ∼5 youngest grains identified by LA-ICP-MS. The youngest single-grain LA-ICP-MS dates are all younger than—and rarely overlap at 2σ uncertainty with—the CA-TIMS MDAs. The youngest kernel density estimation modes are typically several million years older than the CA-TIMS MDAs. Weighted means of round 1 dates that define the youngest statistical populations yield the best coincidence with CA-TIMS MDAs. CA-TIMS dating of the youngest zircon identified by LA-ICP-MS is indispensable for critical MDA applications, eliminating laser-induced matrix effects, mitigating and evaluating Pb loss, and resolving complexities of interpreting lower-precision, normally distributed LA-ICP-MS dates. Finally, numerous CA-TIMS MDAs in this study are younger than Bathonian(?)–Callovian and Oxfordian faunal correlations suggest, highlighting the need for additional radioisotopic constraints—including CA-TIMS MDAs—for the Middle–Late Jurassic geologic time scale. INTRODUCTION Detrital zircon (DZ) U-Pb geochronology is a staple of modern stratigraphic research that proliferated with increasingly widespread use of laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) (e.g., Gehrels, 2014). Rapid data acquisition renders LA-ICP-MS well suited for the DZ analyses that are extensively used in provenance work and maximum depositional age (MDA) assessments (e.g., Schaltegger et al., 2015). This study examined MDAs, which are based on a logical premise that Gehrels (2014) referred to as the law of DZ: A sedimentary rock cannot be older than the youngest zircon crystal it contains (Houston and Murphy, 1965). The validity of a DZ MDA is always complicated by uncertainty, including analytical, systematic, and geologic sources. Laboratory-reported confidence intervals, however, principally reflect analytical precision and reproducibility of standard materials, and repeat measurements do not mitigate sample-specific systematic uncertainty (Schoene, 2014). Inter-element fractionation during laser ablation requires frequent within-session analyses of reference zircon and is a significant source of systematic uncertainty in LA-ICP-MS geochronology (e.g., Schaltegger et al., 2015). Well-characterized zircon yield LA-ICP-MS dates that typically coincide with associated chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) dates, but systematic offsets, likely reflecting matrix effects, are observed (Schoene, 2014). In fact, LA-ICP-MS dates of relatively young (i.e., Mesozoic–Cenozoic) zircon are prone to incorporating fractionation-associated matrix effects, imparting too-young biases of as much as ∼5% (Allen and Campbell, 2012). Mesozoic–Cenozoic strata are common in basin analysis, and MDAs that are younger than existing stratal age constraints may have considerable implications (e.g., Surpless et al., 2006). Lead loss is largely unconstrained by single LA-ICP-MS analyses of Mesozoic–Cenozoic zircon (Spencer et al., 2016) but is often cited to account for DZ dates that are ostensibly too young. Additionally, the impact of material properties on ablation behavior—the above-noted matrix effects—and the statistical nature of overlapping dates within youthful (i.e., near stratal age) DZ populations are rarely discussed (Coutts et al., 2019). Fortunately, total uncertainty can be reduced with complementary CA-TIMS geochronology, which mitigates and assesses Pb loss for Mesozoic–Cenozoic zircon, is not subject to laser-induced matrix effects, and yields dates commonly ∼50× more precise than LA-ICP-MS. Recent DZ studies have combined LA-ICP-MS and CA-TIMS to determine MDAs (e.g., Wainman et al., 2018), but experiments that explicitly explore the law of DZ and compare dates and MDAs from these two methods are lacking. Within this context, we conducted LA-ICP-MS and CA-TIMS geochronology of DZ in Jurassic forearc strata of southern Alaska (United States). An oceanic island arc provenance for the sampled sandstones, which have large proportions of youthful zircon, renders these strata an Published online 23 September 2019