RESEARCH ARTICLE Petrology of the 2016–2017 eruption of Bogoslof Island, Alaska Matthew W. Loewen 1 & Pavel Izbekov 2 & Jamshid Moshrefzadeh 2 & Michelle Coombs 1 & Jessica Larsen 2 & Nathan Graham 2 & Michelle Harbin 3 & Christopher Waythomas 1 & Kristi Wallace 1 Received: 8 April 2019 /Accepted: 22 October 2019 # This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply 2019 Abstract The 2016–2017 eruption of Bogoslof primarily produced crystal-rich amphibole basalts. The dominant juvenile tephra were highly microlitic with diktytaxitic vesicles, and amphiboles had large reaction rims. Both observations support a magma history of slow ascent and/or shallow stalling prior to eruption. Plagioclase-amphibole-clinopyroxene mineralogy are also suggestive of shallow magma crystallization. Lavas were emplaced as shallow submarine lava domes and cryptodomes that produced 70 relatively short-lived and water-rich explosions over the course of the 9-month-long eruption. The explosions ejected older trachyandesite lavas that were likely uplifted by cryptodome emplacement that began in December 2016 and continued for many months. Trachyte pumice, similar in composition to a 1796 lava dome, was entrained in basalts by the end of the eruption. The pumice appears to be a largely crystalline magma that was rejuvenated, entrained in the basalt, and heated to ~ 1000 °C. The composition of trachytes require differentiation through stronger amphibole control than the apparent shallow crustal evolution implied for the basalt. This suggests that they are magmas derived from a mid-crustal zone of amphibole crystallization. Nearby arc-front volcanoes that notably lack amphibole have strikingly similar compositional trends. Trace element signatures of the Bogoslof basalts, however, suggest derivation from a mantle source with residual garnet and lower-degree melting than basalts from nearby arc-front volcanoes. The diversity of magmas erupted at Bogoslof thus provides an opportunity not only to probe rare backarc compositions from the Aleutian arc but also to examine the apparent role of amphibole in generating evolved compositions more broadly in arc environments. Keywords Backarc . Lava domes . Surtseyan eruptions . Amphibole basalt . Trachyte Introduction Bogoslof volcano, an emergent stratovolcano located about 50 km behind the central Aleutian arc front, erupted from December 2016 to August 2017 in a series of 70 explosions and at least two dome building events (Fig. 1; Coombs et al. 2019; Waythomas et al. 2019a). Historically, Bogoslof is one of the more active Aleutian volcanoes with at least nine doc- umented eruptions since 1796 (Waythomas and Cameron 2018). The most recent eruption prior to 2016 was effusion of a basaltic lava dome in 1992. Only two detailed petrologic studies exist for Bogoslof: Byers (1961) provided a petrographic and geochemical survey of the 1926 and 1796 Bogoslof lava domes in an overview study that also included the nearby Unmak Island arc front volcanoes, and Arculus et al. (1977) provided a more focused study and added additional analyses of the 1883 lava dome. These studies showed that Bogoslof lavas were alkali- rich amphibole basalts to trachyandesites. Editorial responsibility: J.J. Lyons; Special Issue Editor: N. Fournier This paper constitutes part of a topical collection: The 2016-17 shallow submarine eruption of Bogoslof volcano, Alaska Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00445-019-1333-6) contains supplementary material, which is available to authorized users. * Matthew W. Loewen mloewen@usgs.gov 1 U.S. Geological Survey, Alaska Volcano Observatory, Anchorage, AK, USA 2 University of Alaska Fairbanks, Alaska Volcano Observatory, Fairbanks, AK, USA 3 University of Alaska Fairbanks, Alaska Satellite Facility, Fairbanks, AK, USA Bulletin of Volcanology https://doi.org/10.1007/s00445-019-1333-6 (2019) 81: 72 /Published online: 28 November 2019