Melt inclusion evidence for CO₂-rich melts beneath the western branch of the East African Rift: implications for long-term storage of volatiles in the deep lithospheric mantle

We present new major element, trace element, and volatile (H₂O, CO₂, S, F, and Cl) concentrations of olivine-hosted melt inclusions from five high-K, low-silica basanites from the western branch of the East African Rift System and use these data to investigate the generation of H₂O- and CO₂-rich melts at up to ~150 km depth. Measured H₂O and CO₂ concentrations reach ~2.5 and ~1 wt%, respectively, representing some of the highest CO₂ concentrations measured in a melt inclusion to date. These measurements represent direct evidence of the high CO₂ and H₂O concentrations required to generate high-K alkaline lavas, and the CO₂ that has been previously inferred to be necessary for the low mantle potential temperatures in the area. Ratios of CO₂/Nb, CO₂/Ba, and CO₂/Cl are used to estimate an initial melt CO₂ concentration of 5–12 wt%. The measured CO₂ concentrations are consistent with CO₂ solubilities determined by molecular dynamics calculations and high-pressure experiments for melt generation at 3–6 GPa; the depth of melting suggested by previous studies in the area. These melt inclusions measurements represent direct evidence for the presence of H₂O- and CO₂-rich melts in the deep upper mantle that have been proposed based on experimental and seismic evidence. Primitive-mantle normalized trace element patterns more closely resemble those found in subduction settings rather than ocean island basalt, and ratios of slab fluid tracers such as Li/Dy and B/Be indicate that the measured volatile abundances may be related to Neoproterozoic subduction during the assembly of Gondwana, implying the storage of volatiles in the mantle by subduction-related metasomatism.