Thermobarometry, argon dating and oxygen isotope geochemistry of the Pleistocene Pt. Tebenkof ignimbrite, Makushin Volcano, AK
Abstract
The Holocene activity of Makushin Volcano in the Aleutian volcanic arc has built a stratovolcano 1810 m high that threatens the towns of Dutch Harbor and Unalaska, however relatively little is known about Makushin’s Plio-Pleistocene history. Students of a Keck Geology Consortium REU in July 2009 performed the first detailed investigation of a 42 m thick ignimbrite to establish its stratigraphic context and petrologic history. Four distinct phases of activity created the scoriaceous Pt. Tebenkof Ignimbrite (PTI) whose compositions is andesitic (59.7-60.8 wt. % SiO2 and 2.1-2.7 wt. % MgO) and quite homogenous. A 40Ar/39Ar age for the overlying flow constrains the age of the ignimbrite to earlier than 139 ka. Although overlying flows are both more and less mafic than the PTI, trace element patterns are similar to the ignimbrite and typical of a subduction zone with a high sediment input; trace elements show that magma generation has not changed significantly from the Pleistocene to the Holocene. Cumulophyric textures seen in overlying lavas are uncommon in the relatively glassy, mingled scoria clasts that contain rare cpx, opx, plag, ±ilm. Electron microprobe analyses show consistent compositions for ignimbrite plagioclase (An40-60) that lack sieve texturing in contrast to overlying flows. Thermobarometry indicates pre-eruption magma temperatures of approximately 970-1015 ± 26°C and pyroxene-glass barometry suggests the magma crystallized at 2 kbar prior to eruption. This depth of 4.2-7.1 km below the surface may have been the site of magma-crust interaction as δ18O values for the ignimbrite (δ18Oplag= 5.29-5.46 ‰; δ18Ool= 4.85-4.9 ‰ VSMOW) show significant disequilibrium (Δ18Oplag-ol= 0.44-0.56‰). The oxygen isotope disequilibrium is seen in two of three overlying lavas and may result from melting of hydrothermally altered wallrock. The Pt. Tebenkof Ignimbrite deposit is the third low-δ18O ignimbrite discovered in the Aleutians (after ignimbrites from Okmok and Fisher calderas) and is the earliest major explosive eruption of Makushin yet documented.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V11D2334N
- Keywords:
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- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 8404 VOLCANOLOGY / Volcanoclastic deposits;
- 8413 VOLCANOLOGY / Subduction zone processes;
- 8428 VOLCANOLOGY / Explosive volcanism