HR: 11:05h
AN: V11I-04    [PDF]
TI: Pleistocene-Recent Growth and Collapse of an Island arc Volcano: Precise $^{40}$Ar/$^{39}$Ar Dating of Seguam Island, Central Aleutian arc, Alaska
AU: * Jicha, B R
EM: bjicha@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706 United States
AU: Singer, B
EM: bsinger@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706 United States
AB: Quantifying the long term growth of arc volcanoes can be done through geologic mapping supported by K-Ar or $^{40}$Ar/$^{39}$Ar age determinations, and is essential to connect rates of geochemical and petrologic processes to a volcano's eruptive history. Yet, few island arcs have benefitted from K-Ar or $^{40}$Ar/$^{39}$Ar dating. No $^{40}$Ar/$^{39}$Ar data is published from the 24 active volcanoes in the Aleutian Island arc. Seguam Island, located in the central Aleutian Island arc, is a $\sim$ 80 km$^{3}$, low-K, tholeiitic complex with multiple eruptive centers. Previous K-Ar dating of 11 whole rock samples from Seguam indicated a 1.07 myr eruptive history. Using $^{40}$Ar/$^{39}$Ar furnace incremental heating techniques on replicate samples of 200-400 mg groundmass separates, we have obtained precise ages from Seguam lavas and pyroclastics. Twenty one of the 23 new $^{40}$Ar/$^{39}$Ar age determinations constrain the duration of most of the Pleistocene-recent volcanism to 142 $\pm$ 2 ka. Experiments from two different 0.5 km$^{3}$ eroded dike swarms yielded older, less precise ages of 155 $\pm$ 78 ka and 230 $\pm$ 70 ka. We suspect that the $\sim$ 1 Ma K-Ar ages obtained from a basalt and basaltic andesite reflect low K$_{2}$O contents (0.3-0.4 wt.$%$) and thus low radiogenic Ar yields, weathering leading to K loss from matrix glass, or the incorporation of xenocrysts into the enormous ($\sim$ 25 g) whole rock samples melted for the K-Ar analyses. $^{40}$Ar/$^{39}$Ar plateau ages indicate that over 85$%$ of the complex, $\sim$ 70 km$^{3}$ of material, was erupted almost continuously between 142 ka and 32 ka with pulses of increased activity at 80 and 55 ka, which included the construction of a 5 km$^{3}$ andesitic stratocone on the eastern half of the island between 80 and 53 ka. At 7.6 $\pm$ 1.9 ka, the stratocone partially collapsed producing a 0.5 km$^{3}$ andesitic ash flow tuff, which was followed by the eruption of several rhyolitic domes in the newly created caldera floor, and continued basaltic eruptions from Pyre Peak to the west. Estimating eruptive rates is complicated by glacial erosion and exposures limited by vegetation or ash. Nonetheless, eruptive rates based on estimates of individual flow volumes and $^{40}$Ar/$^{39}$Ar ages were 0.5 km$^{3}$/k.y. for the predominantly mafic activity between 142 and 8 ka, but slowed to 0.15 km$^{3}$/k.y. for the post-collapse rhyolitic volcanism.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 3640 Igneous petrology
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting