INITIAL CHARACTERIZATION OF SILICIC NUNATAKS FROM ÞÓRÐARHYRNA (THORDARHYRNA), ICELAND

Vatnajökull is a large ice cap in southeastern Iceland that covers several active central volcanoes and associated fissure swarms, including the Grimsvötn- Þórðarhyrna-Laki system. Subglacial volcanoes such as these harbor the potential for unique hazards, including jökulhlaups (glacial outburst floods), phreatomagmatic explosions, and decompression melting and magma destabilization triggered by climate change and ice loss. We focus on Þórðarhyrna, which last erupted in 1903 CE (mafic, phreatomagmatic, associated jökulhlaups; Global Volcanism Project, 2022), and comprises several silicic (SiO₂ ≥65 wt%) nunataks: Eystri- and Vestri-Geirvarta, Húsbóndi, Nibba, Pálsfjall, and the eponymous Þórðarhyrna. These nunataks reveal the capacity for Þórðarhyrna to generate and erupt abundant silicic magma. Since silicic magmas can erupt explosively and with widespread effects, it is important to understand their origin, evolution, longevity, and hazard potential. Initial characterization of Þórðarhyrna nunataks reveals: mineral assemblages typical of Icelandic rhyolites (plagioclase-dominant, common pyroxene, minor olivine); variable accessory mineral populations (zircon abundant in E-Geirvarta, present in V-Geirvarta, sparse in Nibba and Þórðarhyrna, absent in Pálsfjall and Húsbóndi; apatite common as a zircon inclusion); changing magma compositions (euhedral zoned plagioclase common at Þórðarhyrna); open system processes (e.g., small and sparse xenoliths in Nibba and E-Geirvarta, large and abundant glomerocrysts of plagioclase with disequilibrium textures hosting zircon in V-Geirvarta); variable cooling histories (e.g, microlites abundant at Nibba, less-abundant at E-Geirvarta, Pálsfjall, and Húsbóndi, and rare at V-Geirvarta and Þórðarhyrna), emplacement behavior (flow banding at E-Geirvarta and Pálsfjall) and post-emplacement alteration (glassy at Pálsfjall, glassy with abundant spherulites at Þórðarhyrna, increasingly devitrified from Nibba and V-Geirvarta to E-Geirvarta and Húsbóndi). Using evidence from accessory minerals, rock forming minerals, and whole rocks, we will build on these initial observations to pursue deeper understanding of silicic magmatism at Þórðarhyrna.