Isotope geochemistry and FOZO mantle component of the alkaline–carbonatitic association of Fuerteventura, Canary Islands, Spain

Abstract

Carbonatites are very rare in oceanic environments, where they have been reported only at the Canary and Cape Verde islands in the Atlantic Ocean. In the Canary archipelago, calciocarbonatites occur only on the island of Fuerteventura, in clear spatial and temporal association to alkaline silicate rocks, ranging from ultramafic to salic, and comprising alkaline clinopyroxenites, melteigites–ijolites–urtites, nepheline-bearing gabbros and nepheline–syenites, together with their volcanic equivalents. Selected isotopic (Sr, Nd and Pb) data from representative samples of this alkaline–carbonatitic association show a very restricted range of isotopic compositions, with those of the calciocarbonatites overlapping those of the alkaline silicate rocks. Despite the fact that all these rocks display cumulate textures and cover a wide range of SiO 2 and Sr, Nd, Pb contents, initial isotopic compositions are approximately constant and mantle-like, indicating that evolution of the alkaline–carbonatitic association of Fuerteventura must have taken place in the absence of crustal contamination processes. Regarding the mantle source for the alkaline–carbonatitic association, its depleted initial Sr ( 87Sr/ 86Sr = 0.7032–0.7036) and Nd ( 143Nd/ 144Nd = 0.5128–0.5129) compositions, fall on the same trend as the East African young (< 40 Ma) carbonatites. In 143Nd/ 144Nd and 87Sr/ 86Sr vs. 206Pb/ 204Pb diagrams, the alkaline–carbonatitic association of Fuerteventura shows FOZO compositions for Sr and Nd, and slightly higher (towards HIMU) Pb compositions. Furthermore, these data fit the projection of the mantle plane defined by Sr, Nd and Pb isotopic compositions of the DMM, HIMU and EMI components, this fit not being found in sources containing crustal components. In view of this, we propose that the alkaline–carbonatitic association of Fuerteventura was derived mainly from a heterogeneous mantle plume with a deep-seated (FOZO) signature, mixed with HIMU component.