•The new Comoros volcano, Fani Maoré, has exceptionally high Ba and low Pb contents.•Fani Maoré mantle source includes a highly altered recycled oceanic crust.•The carbonated source is rich in Ba and sulfur.•The Comoros Archipelago and East African Rift share a common deep mantle source.•A link is established between carbonatites and HIMU islands. Intraplate volcanism provides remarkable insight into diverse sources in the mantle because its source can be quite shallow, or as deep as the core-mantle boundary, and its origin can be as diverse as recycled crustal material or undifferentiated mantle. While geophysical approaches can in some cases locate the source of magmas, a geochemical approach is necessary to characterize both the nature of the source and the way it melts to produce the erupted lavas. Here we present geochemical and isotopic data obtained on a new submarine volcano (Fani Maoré) that was discovered in 2019 next to Mayotte Island in the Comoros. The radiogenic isotope data are remarkably uniform at subdued values intermediate between HIMU and EM1 compositions but trace element contents are unusual with a marked enrichment in Ba (Ba/Th ≈ 370 compared with the ocean island basalt (OIB) average of 100) and depletion in Pb (Ce/Pb ≈ 70 versus 25 for average OIB). This unique data set suggests that the basanites formed by melting of a carbonated mantle source that was highly enriched in Ba and volatiles. A similar source is also present under the East African Rift where contemporaneous basanite and carbonatite eruptions are known. This establishes a possible link between the volcanic activity of the Comoros and the East African Rift zone. More generally, it demonstrates that carbonated sources are more common in the mantle than previously thought and can be traced using trace element geochemistry. Other volcanoes in the world carry similar characteristics and we suggest that carbonated mantle sources explain the geochemical peculiarities of not only Fani Maoré in the Comoros but also those of Cape Verde volcanics and more generally those of many HIMU-like OIBs, in particular the so-called ‘young HIMU’ OIBs.