The carbon content and CO2/Ba ratio of the Earth’s upper mantle

One of the fundamental constraints on the carbon content of the Earth’s upper mantle comes from using the measure CO2 in trace element depleted mid-ocean ridge basalts (D-MORB), which experiencing none or minimal CO2 degassing, and the ratios of CO2 to refractory elements of similar incompatibility during mantle melting (e.g., Rb, Ba, Th, Nb). The thus obtained CO2/Ba ratio of 120 ± 20 was not only measured in the D-MORB, but also estimated for the enriched MORB (E-MORB) derived from a chemically and isotopically distinct mantle. Based on these measurements and estimates, previous works concluded that the CO2/Ba in the Earth's mantle is homogeneous. In this study, we present and use new geochemical data on submarine glasses from the Southern East Pacific Rise, the Puka-Puka and Rano Rahi seamounts to define the CO2/Ba ratio of the E-MORB mantle. We suggest that the measured CO2/Ba ratio of 241 ± 30 (twice as high as previous estimates) in the most trace element depleted, but isotopically enriched E-MORB, is the most reliable pre-degas value for the E-MORB source. We use this CO2/Ba ratio for the Pacific D- and E-MORB to provide an estimate of the CO2 content of the Earth's upper mantle and its flux at mid-ocean ridges.