Continental rifts emit mantle carbon dioxide (CO2) into Earth’s atmosphere. We studied sources and fluxes of CO2 over a 350-kilometer-long transect in the eastern branch of the East African rift zone. We found that displacement of carbon-enriched Tanzanian cratonic mantle concentrates deep carbon below parts of the East African Rift System. This creates a zone of highly concentrated deep carbon, and active carbonatite magmatism. Here is a link to our manuscript: