EXPLORING THE LAW OF DETRITAL ZIRCON IN ALASKA’S COOK INLET: LA-ICPMS AND CA-TIMS GEOCHRONOLOGY OF JURASSIC FOREARC STRATA

A sedimentary rock cannot be older than its youngest zircon. This premise—the law of detrital zircon—permits maximum depositional age (MDA) determinations, but geochronologic dates are complicated by uncertainty. We conducted U–Pb laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) and chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) of detrital zircon in forearc basin strata of southern Alaska to assess the accuracy of several MDA approaches. Six samples from Middle–Late Jurassic units are generally replete with youthful (i.e., near stratal age) zircon and underwent three rounds of analysis: 1) LA-ICPMS of ~115 grains, with one date per zircon; 2) LA-ICPMS 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-ICPMS.

Youngest single-grain LA-ICPMS dates are all younger than—and rarely overlap at 2σ uncertainty with—the CA-TIMS MDAs, indicating that random statistical fluctuations during analysis and subtle Pb-loss render these youngest LA-ICPMS dates poorly suited to characterizing the age of the densely sampled youthful populations. Youngest kernel density estimation modes are typically several m.y. older than the CA-TIMS MDAs, with the full probability distributions incorporating truly older dates. Weighted means of round 1 dates that define youngest statistical populations are our preferred LA-ICPMS constraints, as this high-n approach extracts a normally distributed sub-sample from the youngest tail of each full density estimation—thus tying these determinations to statistical fluctuations during analysis—and yields the best overall coincidence with the CA-TIMS MDAs.

CA-TIMS dating of the youngest detrital zircon grains identified by LA-ICPMS is indispensable for critical chronostratigraphic applications, eliminating laser-induced matrix effects, mitigating and evaluating Pb-loss, and resolving complexities of interpreting lower precision, normally distributed LA-ICPMS dates. Finally, numerous CA-TIMS MDAs in this study are younger than Bathonian(?)–Callovian and Oxfordian faunal correlations suggest and underscore the need for additional high-precision radioisotopic constraints to refine the Middle–Late Jurassic geologic time scale.