New unspiked K-Ar ages of Holocene lava flows and pumices from the Ecuadorian arc

Abstract : In the northern Andes, the Ecuadorian arc presents more than 80 Quaternary volcanoes, including 25 active during the Holocene. Reconstruction of the past eruptive history of these volcanoes is essential to understand their activity frequency and eruptive dynamics evolution, and for hazard assessment. The groundmass unspiked K-Ar dating method is particularly suitable for dating young subduction lava flows, which contain low K and high Ca contents. Our youngest ages obtained for Holocene lava flows from Tungurahua and Chimborazo volcanoes and Calpi cones range between 8 ± 2 and 4 ± 2 ka, with a K content of about 1.5-2%, and a radiogenic argon content varying between 0.06 and 0.18%, for a narrow density range groundmass. These new ages are consistent with stratigraphy and morphologic observations, and are in agreement with previous published 14C age determinations. We also performed K-Ar measurement on pumice glass shards from the Chalupas ignimbrite, which originate from an ultra-plinian eruption and constitute a major stratigraphic marker in Ecuador. The pumice blocks were gently crushed by hand in an agate mortar, sieved, then grains were separated using heavy liquids. Such procedure allows to preserve massive structure of the glass shards and remove altered or vesicular shards. We obtained the age of 216 ± 5 ka, which agrees with a previous unpublished 40Ar/39Ar age (211 ± 14 ka), and is in agreement with age models of Pacific marine cores, where tephra of this eruption were founded. Our results therefore show that the K-Ar dating method on groundmass can be successfully applied for fresh Holocene lava flows, and for pumice glass shards of the most explosive eruptions, which cannot be dated by the 40Ar/39Ar technique due to 39Ar recoil occurring during irradiation.
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Contributor : Mathilde Bablon <>
Submitted on : Friday, November 15, 2019 - 3:52:19 PM
Last modification on : Thursday, December 19, 2019 - 2:24:08 PM

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  • HAL Id : hal-02365938, version 1

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Mathilde Bablon, X. Quidelleur, Pablo Samaniego, Jean-Luc Le Pennec, Silvana Hidalgo. New unspiked K-Ar ages of Holocene lava flows and pumices from the Ecuadorian arc. Cities on Volcanoes 10, 2018, Napoli, Italy. ⟨hal-02365938⟩

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