Buoyant ancient continental mantle embedded in oceanic lithosphere (Sal Island, Cape Verde Archipelago)

The Cape Verde Islands lie in the Atlantic Ocean off West Africa, in a clearly oceanic setting. The lavas of some islands carry mantle-derived xenoliths of depleted peridotites petrologically similar to those derived from cratonic lithospheric mantle. Oceanic lithospheric mantle, in contrast, consists mainly of less-depleted lherzolites and harzburgites formed by the extraction of mid-ocean ridge basalts. In situ Re-Os analyses of individual sulfide grains from the xenoliths yield Re-depletion model ages ranging mainly from Neoproterozoic to Archean. Their age distribution mirrors the tectonic history of the western margin of the West African Craton and the corresponding continental margin of Brazil. These data and seismic tomography suggest that part of the Cape Verde Archipelago is underlain by a fragment of ancient subcontinental lithospheric mantle, left stranded in the oceanic lithosphere during the opening of the Atlantic Ocean. Contamination of magmas by this ancient continental root can explain the unusual isotopic characteristics of some Cape Verde lavas without recourse to recycled continental material in the sources of mantle plumes.