Ocean island basalts (OIBs) serve as crucial proxies for studying the convective mantle geochemistry and heterogeneity, which vary between trace-element-depleted and varying distinct trace-element-enriched mantle compositions. The Cape Verde archipelago, and particularly Fogo Island, provides an optimal setting for investigating mantle heterogeneity and melting dynamics. The island exhibits pronounced geochemical variations over kilometer-scale distances, offering a unique framework to constrain spatial variations in mantle source composition, partial melting processes, and magma evolution within an intraplate volcanic system.
Previous geochemical studies on Fogo’s whole-rock basalts suggest a complex mantle source incorporating few components, varying between recycled oceanic crust, subcontinental lithospheric fragments, and lower mantle material. However, whole-rock trace elements and isotopic compositions alone cannot distinguish between different mantle lithologies, especially if they were subject to metasomatic enrichment events. For example, basalts with the same isotopic composition could result from the melting of either recycled crust or metasomatized peridotite. The distinction between such source components is even more ambiguous if carbonatitic metasomatic agents are involved.
Here, we utilize major and minor element ratios in olivine phenocrysts (e.g., Ni/Mg, Mn/Fe) to quantify the contributions of peridotite and pyroxenite in the mantle source of Fogo basalts. These data are integrated with whole-rock major and trace element abundances and Sr-Nd-Pb isotope compositions to provide a more detailed picture of the source of these basalts. Results show olivine compositions that are compatible with peridotite-derived melts. High Ca (up to 3,500 ppm) and low Al (~100-200 ppm) content in the olivine phenocrysts, resulting in an elevated Ca/Al ratio above MORB, suggests a source that was enriched with Ca, possibly by carbonatitic fluid/melt metasomatism. These findings imply a greater role for peridotite than previously inferred by some Cape Verde plume models, providing new insights into the lithological makeup and metasomatic processes occurring in the Cape Verde mantle source.