Changing volatile emissions and sources along the Ethiopian Rift

• Bora-Baricha-Tulu Moye is one of the largest volcanic CO 2 emitters in the East African. • Bora-Baricha-Tulu Moye 3 He/ 4 He (∼17 R a ) is the largest in Ethiopian fumaroles. • Total CO 2 from Ethiopian Rift is 2500–9200 kt yr -1 , but shows along rift variations. • Rift segments with high CO 2 flux show high 3 He/ 4 He and low seismic velocity. • CO 2 flux and plume component enhanced in areas of deep and intense melting. 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. CO 2 ) 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 CO 2 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 3 He/ 4 He values up to 17 R a (the highest ever observed in Ethiopian Rift fumaroles) and we calculate total magmatic CO 2 flux of 757–901 t d -1 (making BBTM the largest volcanic CO 2 emitter in the Ethiopian Rift). Using our CO 2 flux measurements, we estimate total CO 2 emissions from Ethiopia’s volcanic systems to be 2500–9200 kt yr -1 and reveal important along-rift variation in CO 2 flux. High CO 2 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 3 He/ 4 He values (>14 R a ). Cooccurrence of high mantle melt production, elevated CO 2 emissions and high 3 He/ 4 He demonstrates that in mature continental rifts carbon emissions and plume volatile contributions are particularly elevated in regions of greatest lithospheric thinning and mantle melting.