Exploring the law of detrital zircon in Alaska's Cook Inlet: LA-ICPMS and CA-TIMS geochronology of Jurassic forearc strata (presentation): Geological Society of America, Annual Meeting, 22–25 September 2019, Phoenix, Arizona
A sedimentary rock cannot be older than its youngest zircon. This premisethe 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
... . Six samples from Middle-Upper 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 Stevens Goddard et al.