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Explosive Ocean Island Volcanism Explained by High Magmatic Water Content Determined Through Nominally Anhydrous Minerals

Harri Geiger 1,2, Franz Weis2,3,4, Valentin R. Troll2,4,5, Frances M. Deegan2,4, Henrik Skogby3 and Juan Carlos Carracedo5

  • Affiliations: 1Institute of Earth and Environmental Sciences, University of Freiburg, Freiburg im Breisgau, Germany; 2Department of Earth Sciences, Natural Resources & Sustainable Development (NRHU), Uppsala University, Sweden; 3Swedish Museum of Natural History, Department of Geosciences, Stockholm, Sweden; 4Centre of Natural Hazards and Disaster Sciences (CNDS), Uppsala University, Uppsala, Sweden; 5Universidad de Las Palmas de Gran Canaria, Instituto de Estudios Ambientales y Recursos Naturales (i-UNAT), Las Palmas de Gran Canaria, Spain

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 103

  • Programme No: 3.16.30

  • Theme 3 > Session 16

Abstract

Ocean island basalt (OIB) magmas typically contain less than 1 wt.% H₂O, making explosive eruption styles rare. When they do occur, such eruptions are thought to be driven either from volatile-enriched mantle sources or gas segregation processes during magma differentiation. In this study, we examine crystal- and water-rich porphyritic basanites and ankaramites from El Hierro in the Canary Islands, Spain, which erupted within the ≥39 ka El Golfo giant landslide collapse embayment. Combining rock and mineral chemistry with H₂O contents of nominally anhydrous minerals (olivine and clinopyroxene), we demonstrate that despite their relatively primitive composition, the post-collapse ankaramites are not primary mantle melts. Instead, they exhibit high crystal contents and unusually high water concentrations, reaching up to 3.20 ± 0.64 wt.% H₂O. These findings suggest that they represent a normally inaccessible snapshot of dense, crystal-rich magmas residing in the sub-island underplating zone. We hypothesize that their eruption was triggered by sudden decompression due to crustal unloading, implying that the El Golfo landslide may have influenced deeper parts of the magmatic plumbing system. This event likely facilitated the ascent of volatile-rich, crystal-laden magmas from the underplating zone. Based on these observations, we propose that some "wet" and explosive ocean island eruptions may result from the rise of deep-seated, water-rich magmas following vertical unloading and associated decompression.