Os isotope systematics in Fogo Island: Evidence for lower continental crust fragments under the Cape Verde Southern Islands

Abstract

Os and Nd isotopic ratios as well as major and trace element compositions have been measured in 17 mafic lavas from Fogo Island, Cape Verde. These new data complement a previous archipelago-scale study (Southern and Northern Islands) by Doucelance et al. [Doucelance, R., Escrig, S., Moreira, M., Gariépy, C., Kurz, M., 2003. Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago. Geochim. Cosmochim. Acta, 67 (19), 3717-3733], in which major, trace element contents and Sr–Pb–He isotopic compositions of lavas were determined. We also report Os–Sr–Nd–Pb isotopic ratios of 2 carbonatites from Fogo. Fogo mafic lavas have Os–Sr–Nd–Pb isotopic ratios that define correlations similar to those previously observed for the Cape Verde Southern Islands and interpreted to reflect a mixture between a moderate HIMU end-member ( 206Pb / 204Pb ∼ 20), common to the Northern and Southern Islands of the archipelago, and an EM1-like end-member. Similar isotopic correlations are observed within the different lava flows which requires that the two end-members are mixed during the lava differentiation at shallow depth. The increasing contribution of the enriched end-member through time in Fogo lavas confirms the shallow origin for the EM1 signature. As with Pb, Sr and He, the Os and Nd isotopic compositions of the moderate HIMU end-member are explained by the mixing of 1.6 Ga recycled oceanic crust (ROC) and lower mantle material (LM) and interpreted to represent the Cape Verde mantle plume expression. For the EM1-like end-member, the high 187Os / 188Os ratios that are measured in mafic lavas preclude the sub-continental lithospheric mantle from being its origin, as previously proposed based on trace element and Pb, Sr and Nd isotopic compositions. Our new isotopic and trace element data still indicate that the enriched component is related to continental lithospheric material incorporated in the oceanic lithosphere during the opening of the Atlantic Ocean but requires the involvement of lower continental crust fragments.