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Age of the dacite of Sunset Amphitheater, a voluminous Pleistocene tephra from Mount Rainier (USA), and implications for Cascade glacial stratigraphy

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Author(s): Thomas W. Sisson, Axel K. Schmitt, Martin Danisikc, Andrew T. Calvert, Napoleon Pempena, Chun-Yuan Huang, Chuan-Chou Shen

Document Type:
Publisher: Journal of Volcanology and Geothermal Research
Published Year: 2019
Volume: 376
Pages: 27 to 43
DOI Identifier: 10.1016/j.jvolgeores.2019.03.003
ISBN Identifier:
Keywords: Pleistocene 40Ar/39Ar Plagioclase (U-Th)/He Zircon Tephra Till Marine isotope stage Puget Sound

The dacite of Sunset Amphitheater, Mount Rainier (USA), illustrates the difficulties in establishing accurate ages of Pleistocene tephra eruptions. Nearly uniform whole-rock, glass, and mineral compositions, texture, and phenocryst assemblage establish that certain conspicuous dissected pumice exposures scattered from Mount Rainier to southern Puget Sound are products of the same Pleistocene Plinian eruption. 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 40Ar/39Ar plateau ages of plagioclase 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. Zircon 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 isotopic composition. In contrast, pooled (U-Th)/He measurements on the zircon yield an age of 85 ± 6 ka (2σ), which accords with well-behaved 40Ar/39Ar 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 40Ar/39Ar 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 solidification followed by remobilization. The ca. 85 ka eruption age confirms that bracketing glacial tills on the flanks of Mount Rainier were products of the Penultimate Glaciation (MIS 6) and Last Glacial Maximum (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.

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Suggested Citations:
In Text Citation:
Sisson and others (2019) or (Sisson et al., 2019)

References Citation:
Sisson, T.W., A.K. Schmitt, M. Danisikc, A.T. Calvert, N. Pempena, C. Huang, and C. Shen, 2019, Age of the dacite of Sunset Amphitheater, a voluminous Pleistocene tephra from Mount Rainier (USA), and implications for Cascade glacial stratigraphy: Journal of Volcanology and Geothermal Research, Vol. 376, pp. 27-43, doi: 10.1016/j.jvolgeores.2019.03.003.