Mantle sources underlying Nyiragongo volcano

The Nyiragongo volcano, part of the Virunga Volcanic Province (VVP) in the Democratic Republic of Congo is a uniquely silica-undersaturated (32–49 wt% SiO2) and alkali-rich (7–17 wt% Na2O + K2O) volcanic system. Though alkaline volcanism is by no means rare in the western branch of the East African Rift System (EARS), compositions erupted by Nyiragongo are nevertheless notably alkaline, especially given that the other volcanoes in the VVP predominantly erupt silica-saturated compositions. In order to address the origin of the geochemical character of Nyiragongo it is imperative to understand the mantle source feeding its plumbing system. We approach this issue through the combined analysis of primitive whole-rock samples and early-crystallized olivine hosted by these samples. Major and trace element compositional data from olivine indicates a Ca-rich, but otherwise unremarkable geochemical signature indicative of melt derivation from a peridotite restite. However, whole-rock compositions require melting of a K-rich phlogopite bearing source in the presence of apatite, garnet, and clinopyroxene to produce Si-undersaturated, Ca-, and K- rich melts. Geochemical modelling using recently acquired partition coefficients suggests that such melting must occur primarily at high pressures (∼ 3 GPa), outside of the stability field of amphibole, in line with Nyiragongo residing on thick lithosphere bordering the Tanzanian craton.
Combining the peridotite restite-signature from olivine compositions with the phlogopite-pyroxenite melt-source inferred from whole-rock data results in a model of metasomatic vein-melting, wherein veins emplaced in the subcontinental lithospheric mantle during the Pan-African orogeny are preferentially molten through a breakdown reaction of phlogopite at temperatures of 1150 to 1350 °C. These melts subsequently equilibrate with the surrounding peridotite mantle, resulting in a peridotite-restite signature observed in olivine.