1. Laser-ablation ICP-MS zircon U-Pb ages for key Pliocene-Pleistocene tephra beds in unglaciated Yukon and Alaska.
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Buryak, Serhiy D., Reyes, Alberto V., Jensen, Britta J.L., Davies, Joshua H.F.L., Westgate, John A., DuFrane, S. Andrew, Luo, Yan, Froese, Duane G., Pearson, D. Graham, and Benowitz, Jeff A.
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LASER ablation inductively coupled plasma mass spectrometry ,VOLCANIC ash, tuff, etc. ,ZIRCON ,FISSION track dating - Abstract
Tephrochronology is one of the most effective ways to correlate and date Quaternary deposits across large distances. However, it can be challenging to obtain direct ages on tephra beds when they are beyond the limit of radiocarbon dating, do not contain mineral phases suitable for
40 K-40 Ar (or40 Ar/39 Ar) dating, or suitable glass shards for fission-track dating are not available. Zircon U-Pb dating by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is an emerging technique for dating young (<1 Ma) tephra. Here, we demonstrate that LA-ICP-MS zircon U-Pb dating can produce reliable ages for key tephra beds found in Yukon and Alaska. We assessed five different techniques for calculating tephra maximum depositional ages from zircon U-Pb ages for eight tephra beds. Our preferred zircon U-Pb ages (reported with 2σ uncertainties), based on a Bayesian model for calculating maximum depositional ages, are broadly consistent with previously established chronology constructed from stratigraphy, paleomagnetism, and/or glass fission track and40 Ar/39 Ar ages: Biederman tephra (178 ± 17 ka), HP tephra (680 ± 47 ka), Gold Run tephra (688 ± 44 ka), Flat Creek tephra (708 ± 43 ka), PA tephra (1.92 ± 0.06 Ma), Quartz Creek tephra (2.62 ± 0.08 Ma), Lost Chicken tephra (3.14 ± 0.07 Ma), and GI tephra (542 ± 64 ka). We also present newly revised glass fission-track and40 Ar/39 Ar ages recalculated from previous determinations using updated ages for the Moldavite tektite and Fish Canyon Tuff standards, and updated K decay constants. For Pleistocene age zircon crystals, corrections for230 Th disequilibrium and common-Pb are significant and must be treated with caution. Similarly, apparent tephra ages are sensitive to the choice of method used to calculate a maximum depositional age from the assemblage of individual crystallization ages. This study demonstrates that LA-ICP-MS zircon U-Pb dating can be successfully applied to numerous Pliocene-Pleistocene Alaskan-Yukon tephra, providing confidence in applying this method to other stratigraphically important tephra in the region. • LA-ICP-MS method is well suited for rapid dating of very young zircon crystals. • Eight key tephra beds from Eastern Beringia were dated by zircon U-Pb method. • Young U-Pb ages are particularly sensitive to common-Pb and230 Th corrections. • Bayesian modeling reduces subjectivity in calculating ages. [ABSTRACT FROM AUTHOR]- Published
- 2022
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