Abstract
AbstractUranium-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.
Highlights
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)
This study examined maximum depositional ages (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)
Of ∼115 zircon grains per sample, with one date per grain; (2) two additional LA-ICP-MS dates per zircon for the ∼15 youngest grains per sample identified in round 1; and (3) chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) of the ∼5 youngest grains per sample based on sorting LA-ICP-MS multiple-analysis results by weighted mean (WM) date where n = 3 and probability of fit (PoF) is >0.05
Summary
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). 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). 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)
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