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

Abstract

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.

U-Pb zircon geochronology and depositional age models for the Upper Triassic Chinle Formation (Petrified Forest National Park, Arizona, USA): Implications for Late Triassic paleoecological and paleoenvironmental change

“…The CA-TIMS maximum depositional cluster ages are also typically older than the maximum depositional cluster ages obtained by the LA-ICP-MS analyses, but there is no apparent systematic offset (Fig. 2), which is in agreement with previous studies that applied LA-ICP-MS and CA-TIMS analyses to the same crystals (e.g., Herriott et al, 2019). The difference between the CA-TIMS and LA-ICP-MS ages can be quite small (e.g., 182Q-1 and in particular 196Q-3) or relatively large, such as in samples 52Q-2, 158Q-2, 243Q-2, and 297Y-2, where the maximum depositional ages obtained by LA-ICP-MS are ca.…”

Supplemental Material: Oligocene-Neogene lithospheric-scale reactivation of Mesozoic terrane accretionary structures in the Alaska Range suture zone, southern Alaska, USA

“…Maximum depositional ages are estimated to be the mean age of the youngest statistical population of detrital dates (Herriott et al, 2019).…”

Análisis de procedencia del registro sin-orogénico Maastrichtiano de la Formación Cimarrona, flanco occidental de la Cordillera Oriental colombiana

“…Las relaciones Th/U varían de 0,09 a 1,9, sugiriendo la predominancia de circones ígneos (Rubatto, 2002), excepto por tres granos con relaciones Th/U comprendidas entre 0,01 y 0,05 con edades de 257,4±3,6 Ma, 464,6±6,8 Ma y 491,6±8 Ma, lo que podría indicar un posible origen metamórfico (Rubatto, 2002). La apropiada definición de la edad máxima de acumulación establecida con la geocronología detrítica ha sido objeto de discusión (Dickinson y Gehrels, 2009;Spencer et al, 2016;Herriott et al, 2019;Copeland, 2020). Existen diferentes propuestas, incluyendo la edad del pico de distribución de edades más joven en el histograma o la media ponderada de tres o más granos (Dickinson y Gehrels, 2009), cuyo valor descansa en el supuesto de que la repetición de la edad en diferentes granos no podría estar relacionada a posibles trayectorias de pérdidas de Pb.…”

“…Existen diferentes propuestas, incluyendo la edad del pico de distribución de edades más joven en el histograma o la media ponderada de tres o más granos (Dickinson y Gehrels, 2009), cuyo valor descansa en el supuesto de que la repetición de la edad en diferentes granos no podría estar relacionada a posibles trayectorias de pérdidas de Pb. Sin embargo, recientemente algunos autores consideran que esta aproximación puede ser inapropiada, ya que los granos no necesariamente están relacionados a la misma fuente, así que, salvo que se tenga otro amarre composicional en el circón, es más apropiado analizar la edad máxima de acumulación a partir del grano de circón más joven (Spencer et al, 2016;Herriott et al, 2019;Copeland, 2020).…”