The helium and carbon isotopic signature of Ocean island basalts: Insights from Fogo volcano (Cape Verde archipelago)

Fluid inclusions (FI) entrapped in phenocrysts carried by Ocean Island Basalts (OIB) contain key information on volatiles' abundance and origin in their mantle sources. Here, we add new piece of knowledge to our understanding of volatile geochemistry in global OIB magmas, by presenting new noble gas (He-Ne-Ar) and carbon (C) isotope results for olivine- and clinopyroxene-hosted FI from enclaves, lavas, tephra and volcanic gas samples from Fogo, the only frequently active volcano at the Cape Verde archipelago (eastern Atlantic Ocean). FI, together with crater fumaroles, constrain the Fogo He-3/He-4 signature at 7.14-8.44 R-c/R-a (where R-C is the air-corrected He-3/He-4 isotope ratio, and R-a is the same ratio in air), which is within the typical MORB (Mid-Ocean Ridge Basalt) mantle. The carbon isotopic ratio (delta C-13 vs. Pee Dee Belemnite) of CO2 in FI and fumaroles range from-6.04 to-4.41 parts per thousand. We identify systematic variations of delta C-13 and He/Ar* with FI entrapment pressure (estimated from a combination of host mineral barometry and FI microthermometry), from which we develop a model for volatile degassing in the mantle-to-crustal magma storage system. The model predicts a crustal-like signature for carbon (delta C-13 of-0.4 +/- 1.0 parts per thousand) in primary melts formed by mantle melting at similar to 2200 MPa (similar to 77 km) and a source He/Ar* ratio of 0.90-0.24, which are indicative of variably depleted mantle metasomatized by carbon enriched melts/fluids from a crustal component. We also use our results to characterise regional (in the Cape Verde and Canary archipelagos) and global trends in C and He isotope composition from OIB. From a comparison with the few other OIB localities for which delta C-13 are available, we propose that a carbon enriched crustal component could be recurrent at a global scale in OIB magmatism, although often masked by isotope fractionation during magmatic degassing. We additionally find that, at regional scale, He isotopes in OIB scale inversely correlate with the degree of partial melting of the mantle beneath individual islands' (inferred from the La/Yb ratio of erupted basalts). More widely, our results corroborate previously established global relationships between OIB He isotopic signature, plume buoyancy flux and overlying plate velocity. In this interpretation, the MORB-like He-3/He-4 (8 +/- 1 Ra) at Fogo reflects a combination of (i) low to medium magma productivity, (ii) relatively low plume buoyancy flux (similar to 1.1 Mg/s), and (iii) slow average velocity (similar to 3 cm/yr) of the overlying plate.