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Volatiles, crustal storage depths, and degassing of alkalic, CO2-rich magmas at Nyiragongo and Nyamulagira Volcanoes, Democratic Republic of the Congo

Lissie Connors1, Paul Wallace 1, Kenneth Sims2, D. Matthew Sublett3, Robert Bodnar3

  • Affiliations: 1 Dept. of Earth Sciences, University of Oregon, OR 97403, USA; 2 Dept. of Geology and Geophysics, University of Wyoming, WY 82070, USA; 3 Dept. of Geosciences, Virginia Tech, VA 24061, USA

  • Presentation type: Talk

  • Presentation time: Friday 14:45 - 15:00, Room S150

  • Programme No: 3.17.7

  • Theme 3 > Session 17

Abstract

Nyiragongo and Nyamulagira are two of the most active volcanoes in the East African Rift System, producing some of the highest fluxes of volcanic CO₂ and SO₂ on Earth, yet pre-eruptive volatile constraints at these volcanoes remain sparse. Nyiragongo is well known for its semi-permanently active lava lake, and both volcanoes have large calderas, which formed several centuries ago (Nyiragongo) and ~300-500 years ago (Nyamulagira; Pouclet & Bram, 2021). We analyzed olivine-hosted melt inclusions (MI) from Mg-rich tephra erupted from flank cones of both volcanoes for volatile, major, and trace element compositions of MI glasses, CO₂ density in MI vapor bubbles, and Fe and S oxidation state. Lavas and tephra from Nyiragongo are highly silica-undersaturated (melilitite, nephelinite), whereas those from Nyamulagira are basanite and tephrite. Volatile solubility modeling yields magma crystallization depths of 10-16 km for primitive magmas and <5 km for evolved magmas. Although many flank vents are aligned along prominent fissures or fault systems radiating from the central edifices, our data suggest that the plumbing systems beneath the more distal flank cones are largely separated from the larger, more active magma reservoirs beneath the two edifices. Estimated CO₂ concentrations for primary melts based on equilibration with a lherzolite residue at mantle depths are very high: ~6.0 wt% for Nyiragongo and ~4.4 wt% for Nyamulagira. Degassing models based on our data show that volcanic gases released by summit activity at the two volcanoes can be explained by relatively evolved magmas feeding summit lava lakes through conduit convection and degassing.