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

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. Here, we report the geochemistry of melt inclusions (MI) from Mg-rich tephra erupted from flank cones of Nyamulagira and Nyiragongo. In our sample suite, CO₂ concentrations in bubble-corrected melt inclusions reach ∼1.3 and 0.9 wt% for Nyiragongo and Nyamulagira, respectively. Water concentrations are 0.8–1.6 wt% for primitive compositions and 0.2–0.4 wt% for more evolved compositions. Sulfur concentrations reach up to 3100 ppm at Nyiragongo and 2500 ppm at Nyamulagira. Major element ‘fingerprinting’ of MI shows that some tephra samples have MI with both Nyamulagira and Nyiragongo-type compositions, requiring mixing of olivine originally crystallized from multiple distinct magma types. Volatile solubility modeling yields a wide range of crystallization depths for more primitive magmas, with maximum values of ∼10–18 km, compared to <5 km for more evolved magmas erupted at the Nyamulagira summit. Estimated CO₂ concentrations for primary melts based on final equilibration with a lherzolite residue at mantle depths are 6.0 ± 2.5 wt% for Nyiragongo and 4.4 ± 2.5 wt% for Nyamulagira. Major element and volatile data are consistent with magma generation in metasomatized lithospheric mantle domains (metasomes) of amphibole+clinopyroxene+lesser phlogopite, with high initial CO₂ sourced from carbonate phase(s). Degassing models of CO₂, H₂O, and S show that gas compositions at the two volcanoes can be explained by evolved magmas feeding summit lava lakes by a process of conduit convection and degassing.