Axel K. Schmitt
Martin Danisik
Andrew T. Calvert
Napoleon Pempena
Chun-Yuan Huang
Chuan-Chou Shen
Thomas W. Sisson
2019
<div id="abstracts" class="Abstracts u-font-serif"><div id="ab0005" class="abstract author" lang="en"><div id="as0005"><p id="sp0170"><span>The <a title="Learn more about Dacite from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/dacite" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/dacite">dacite</a> of Sunset Amphitheater, Mount Rainier (USA), illustrates the difficulties in establishing accurate ages of Pleistocene <a title="Learn more about Tephra from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/tephra" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/tephra">tephra</a> eruptions. Nearly uniform whole-rock, glass, and mineral compositions, texture, and <a title="Learn more about Phenocryst from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/phenocryst" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/phenocryst">phenocryst</a> assemblage establish that certain conspicuous dissected <a title="Learn more about Pumice from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/pumice" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/pumice">pumice</a> exposures scattered from Mount Rainier to southern Puget Sound are products of the same Pleistocene <a title="Learn more about Plinian Eruption from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/plinian-eruption" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/plinian-eruption">Plinian eruption</a>. Deposit thicknesses and pumice sizes support an eruption on the order of low Volcanic Explosivity Index (VEI) 5, atypically explosive for dominantly lava-producing Mount Rainier. Statistically permissible </span><sup>40</sup>Ar/<sup>39</sup><span>Ar plateau ages of <a title="Learn more about Plagioclase from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/plagioclase" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/plagioclase">plagioclase</a> phenocryst separates are 138 ± 20 ka and 101 ± 11 ka (2σ). A previously published result of 206 ± 11 ka is herein shown to result from a sample selection error. <a title="Learn more about Zircon from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/zircon" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/zircon">Zircon</a> from the pumice yields a U-Th crystallization age of 147 ± 8 ka if the isochron is required to pass through the tephra U-Th <a title="Learn more about Isotopic Composition from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/isotopic-composition" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/isotopic-composition">isotopic composition</a>. In contrast, pooled (U-Th)/He measurements on the zircon yield an age of 85 ± 6 ka (2σ), which accords with well-behaved </span><sup>40</sup>Ar/<sup>39</sup>Ar ages of stratigraphically associated lavas high on Mount Rainier, and is the best estimate of the pumice's true eruption age. Inclusions of undegassed melt (glass) in the plagioclase separates are proposed as biasing apparent<span> </span><sup>40</sup>Ar/<sup>39</sup><span>Ar plateau ages to old values through coupling of undegassed magmatic excess Ar with radiogenic Ar that accumulated post-eruptively from relatively K-rich glass. U-Th ages record zircon growth prior to eruption, consistent with a possible complex history of advanced <a title="Learn more about Solidification from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/solidification" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/solidification">solidification</a> followed by <a title="Learn more about Remobilization from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/remobilization" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/remobilization">remobilization</a>. The ca. 85 ka eruption age confirms that bracketing glacial tills on the flanks of Mount Rainier were products of the Penultimate <a title="Learn more about Glaciation from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/glaciation" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/glaciation">Glaciation</a> (MIS 6) and <a title="Learn more about Last Glacial Maximum from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/last-glacial-maximum" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/last-glacial-maximum">Last Glacial Maximum</a> (MIS 2). This eruption age also provides an important time marker for glacial and other sedimentary deposits in southern Puget Sound lowland that, excepting the Vashon Drift (MIS 2), generally lack reliable age determinations.</span></p></div></div></div><ul id="issue-navigation" class="issue-navigation u-margin-s-bottom u-bg-grey1"></ul>
application/pdf
10.1016/j.jvolgeores.2019.03.003
en
Elsevier
Age of the dacite of Sunset Amphitheater, a voluminous Pleistocene tephra from Mount Rainier (USA), and implications for Cascade glacial stratigraphy
article