Changing volatile emissions and sources along the Ethiopian Rift

Rift volcanoes are sites of intense volatile emissions. However, major uncertainties remain about the magnitude of rift volatile fluxes, particularly for greenhouse gases (e.g. CO₂) and whether along-rift variations in degassing relate to changing mantle sources and/or rifting processes (i.e. melt production and lithospheric thinning). Here, we investigate CO₂ flux and gas isotopes in Ethiopia; a mature, plume-influenced continental rift. We focus on one of the largest caldera volcanoes, Bora-Baricha-Tulu Moye (BBTM), which is situated in a region of high mantle melt production in the Central Main Ethiopian rift (∼500 km south-west the putative plume head). BBTM gases are characterized by plume-like ³He/⁴He values up to 17 Ra (the highest ever observed in Ethiopian Rift fumaroles) and we calculate total magmatic CO₂ flux of 757–901 t d^-1 (making BBTM the largest volcanic CO₂ emitter in the Ethiopian Rift). Using our CO2 flux measurements, we estimate total CO2 emissions from Ethiopia’s volcanic systems to be 2500–9200 kt yr^-1 and reveal important along-rift variation in CO₂ flux. High CO₂ flux sectors are found in Central Afar, as well as the Northern and Central Main Ethiopian Rift. These sectors are all regions of high partial melt content (evidenced by low seismic velocity in the underlying mantle) and also show the greatest ³He/⁴He values (>14 R_a). Cooccurrence of high mantle melt production, elevated CO₂ emissions and high ³He/⁴He demonstrates that in mature continental rifts carbon emissions and plume volatile contributions are particularly elevated in regions of greatest lithospheric thinning and mantle melting.