Expanding the toolbox for dating basaltic lava sequences: <sup>40</sup>Ar–<sup>39</sup>Ar dating of silicic volcanic glass from interbeds.

Published on 2020-07-30T10:22:33Z (GMT) by
<sup>40</sup>Ar–<sup>39</sup>Ar dating of glass shards from silicic tuffs of the Ellensburg Formation (NW, USA) interbedding basaltic lavas yielded accurate, precise, reproducible plateau and isochron ages that are within error at the 2σ level. The age-spectra have flat plateaus and the inverse isochrons have atmospheric <sup>40</sup>Ar/<sup>36</sup>Ar at the 2σ level. Ages of 12.00 ± 0.24 Ma, 11.37 ± 0.15 Ma, 10.67 ± 0.21 Ma, 10.70 ± 0.18 Ma are consistent with the stratigraphy of four of the dated layers, the age of 10.77 ± 0.18 Ma for a fifth layer is at odds with the stratigraphy. This discrepancy arises due to the effect of glass alteration that induced K- and Ar-loss. There is no evidence of excess <sup>40</sup>Ar or <sup>39</sup>Ar recoil. The new ages indirectly constrain the timing of eruption of the lavas above and below the ash beds. This demonstrates that volcanic glass from interbeds can be used as an additional tool for indirectly dating basaltic lava sequences, that is independent of the lavas, and complementary to other materials. Considering the numerous studies in which volcanic glass failed to provide reliable <sup>40</sup>Ar–<sup>39</sup>Ar ages, additional and supportive constraints are still needed to assess the validity of the ages from glass shards.

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Cogliati, Simone; Sherlock, Sarah; Halton, Alison; Ebinghaus, Alena; Kelley, Simon; Jolley, David; et al. (2020): Expanding the toolbox for dating basaltic lava sequences: 40Ar–39Ar dating of silicic volcanic glass from interbeds.. Geological Society of London. Collection. https://doi.org/10.6084/m9.figshare.c.5077705.v1