ZIRCON PETROCHRONOLOGY REVEALS THE HIDDEN MAGMATIC HISTORY OF BOGOSLOF, ALEUTIAN ISLANDS, ALASKA

Bogoslof is an emergent submarine volcano found 50 km north of the main Aleutian arc front in Alaska. It is one of the more active Aleutian volcanoes since at least the late 1700s and last erupted from December 2016 to August 2017. All subaerial features have been erupted historically (since 1796), so the eruptive history inferred from the in crystal cargo represents the only evidence we have for its geologic history. New zircon ages show that that Bogoslof has been active with zircon-saturated magmas perhaps as early as 500 ka, a similar lifespan to typical arc volcanoes in Alaska.

Bogoslof erupts basalt and trachybasalt but inclusions of both trachyandesite and trachyte were found intermingled in the 2016-17 eruptive products. Clasts of crystal-rich trachyte pumice were found in 2019 showing assimilation and mingling textural features, implying that the 2016-17 basalt potentially rejuvenated an unerupted trachyte mush from the ~1796 CE Castle Rock trachyte dome (Loewen et al., 2019).

We extracted zircon (n=39) from three trachyte pumice samples and dated them by SHRIMP-RG in June 2023 using the U-Th disequilibria and U-Pb as well as trace element compositions at the same analysis locations. Zircon crystallization ages range from within error of eruption age (1796 CE) to ~500 ka. Zircon from two samples were < 25 k.y. old while the other sample contained mostly zircon from ~129 to 524 ka. A lack of zircon with ages from 40 to 129 ka may represent a period where little to no zircon-saturated magma existed under Bogoslof, either due to magmatic quiescence or thermal conditions.

These data allow for assessment of the magmatic system at Bogoslof for periods during which no known eruptive products have been sampled or observed. Zircon Eu/Eu* is between ~0.7 and 0.85 with some suggestion of higher Eu/Eu* in older crystals. Zircon from >129 ka are restricted to >7000 ppm Hf while zircon from <25 ka are ~5000-9000 ppm Hf. This age-related dispersion is reflected in other trace elements such as Ti, Th, U and Gd.

Diffusion ages in pyroxene and plagioclase growth rates suggest that Bogoslof trachyte magma heated from solidus to 1000°C in ~180 days (Moshrefzadeh et al., 2023) as new basalt intruded. Ti-in-zircon temperatures from Bogoslof zircon range from ~750-950°C. This implies that the trachyte magma heated 50-250°C, or an average of 0.3-1.4°C/day.