High CO2 in the mantle source of ocean island basanites

Some of the most CO₂-rich magmas on Earth are erupted by intraplate ocean island volcanoes. Here, we characterise olivine-hosted melt inclusions from recent (<10 ky) basanitic tephra erupted by Fogo, the only active volcano of the Cape Verde Archipelago in the eastern Atlantic Ocean. We determine H₂O, S, Cl, F in glassy melt inclusions and recalculate the total (glass + shrinkage bubbles) CO₂ budget by three independent methodologies. We find that the Fogo parental basanite, entrapped as melt inclusion in forsterite-rich (Fo_80-85) olivines, contains up to ~2.1 wt% CO₂, 3–47 % of which is partitioned in the shrinkage bubbles. This CO₂ content is among the highest ever measured in melt inclusions in OIBs. In combination with ~2 wt% H₂O content, our data constrain an entrapment pressure range for the most CO₂-rich melt inclusion of 648–1430 MPa, with a most conservative estimate at 773–1020 MPa. Our results therefore suggest the parental Fogo melt is stored in the lithospheric mantle at minimum depths of ~27 to ~36 km, and then injected into a vertically stacked magma ponding system. Overall, our results corroborate previous indications for a CO₂-rich nature of alkaline ocean island volcanism. We propose that the Fogo basanitic melt forms by low degrees of melting (F = 0.06–0.07) of a carbon enriched mantle source, containing up to 355–414 ppm C. If global OIB melts are dominantly as carbon-rich as our Fogo results suggest, then OIB volcanism may cumulatively outgas as high as ~16–21 Tg of carbon yearly, hence substantially contributing to the global deep carbon cycle.