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Some of the most CO 2 -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 H 2 O, S, Cl, F in glassy melt inclusions and recalculate the total (glass + shrinkage bubbles) CO 2 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% CO 2 , 3–47 % of which is partitioned in the shrinkage bubbles. This CO 2 content is among the highest ever measured in melt inclusions in OIBs. In combination with ∼2 wt% H 2 O content, our data constrain an entrapment pressure range for the most CO 2 -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 CO 2 -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. [ABSTRACT FROM AUTHOR]

From oxic atmosphere to metallic core, the Earth's components are broadly stratified with respect to oxygen fugacity. A simple picture of reducing oxygen fugacity with depth may be disrupted by the accumulation of oxidised crustal material in the deep lower mantle, entrained there as a result of subduction. While hotspot volcanoes are fed by regions of the mantle likely to have incorporated such recycled material, the oxygen fugacity of erupted hotspot basalts had long been considered comparable to or slightly more oxidised than that of mid-ocean ridge basalt (MORB) and more reduced than subduction zone basalts. Here we report measurements of the redox state of glassy crystal-hosted melt inclusions from tephra and quenched lava samples from the Canary and Cape Verde Islands, that we can independently show were entrapped prior to extensive sulphur degassing. We find high ferric iron to total iron ratios (Fe3+/∑Fe) of up to 0.27–0.30, indicating that mantle plume primary melts are significantly more oxidised than those associated with mid-ocean ridges and even subduction zone. These results, together with previous investigations from the Erebus, Hawaiian and Icelandic hotspots, confirm that mantle upwelling provides a return flow from the deep Earth for components of oxidised subducted lithosphere and suggest that highly oxidised material accumulates or is generated in the lower mantle. The oxidation state of the Earth's interior must therefore be highly heterogeneous and potentially locally inversely stratified. • Mantle plumes are two orders of magnitude more oxidised than MOR mantle. • Melt inclusions from five hotspots all show a reduction during S degassing. • Mantle plumes transport oxidized material out of the deep earth. [ABSTRACT FROM AUTHOR]