Magmatic differentiation and plumbing system beneath Nyamulagira volcano (Virunga Volcanic Province, East African Rift)

Nyamulagira volcano (eastern Democratic Republic of the Congo) is one of Africa's most active volcanoes and poses a significant threat to the nearby rapidly expanding population centers. Situated in the Virunga Volcanic Province in the western branch of the East African Rift System, Nyamulagira's frequent eruptions offer a valuable opportunity to study effusive rift volcanism. However, understanding its subsurface processes remains challenging, impeding monitoring efforts. Here, we report a comprehensive mineralogical, petrological, and geochemical study of rocks from historical eruptions at Nyamulagira. Mineral textures and compositions show strong spatial and temporal variations. We identify multiple active magma reservoirs in which magma compositions and textures evolve via crystal accumulation, fractional crystallization, magma recharge, mixing, and convection. We distinguish three distinct reservoirs: a deep reservoir at ∼ 22–30 km depth, the main storage region at ∼ 13–18 km depth, and a shallow reservoir at ∼ 2–9 km depth. We propose that differentiation at Nyamulagira integrates olivine and clinopyroxene crystallization and accumulation in the deep reservoir, and fractional crystallization and magma mixing/homogenization in the interconnected intermediate and shallow reservoirs. Primitive magmas from the deep reservoir are predominantly emitted via distal eruptions, whereas more evolved magmas from the shallower reservoirs have been erupted on the flanks or at the summit caldera in recent decades.