PB-SR-ND ISOTOPIC DATA OF MESOZOIC OCEAN ISLAND BASALTS FROM THE EASTERN ATLANTIC-OCEAN CONTINENTAL-MARGIN

Carmenchu Peak forms a solitary volcano on the central Atlantic Ocean upper continental rise off West Africa. Extrusives dredged from the ce ntral and upper slopes of the seamount formed during alkalic capping p hase flank eruptions, and range in composition from picritic basalt to trachyte. K-Ar age determinations reveal that the entire alkalic sequ ence formed over a relatively short time interval of 6 Ma during the C enomanian, some 50 Ma after the mid-oceanic ridge formation of the N-M ORB foundation. The submarine volcanic edifice is likely a hot spot fl are product. Pb, Sr and Nd isotope signatures identify isotopically ma ntle reequilibrated ancient recycled subducted lithosphere (HIMU) as m ain source component of alkalic rocks; low Pb-206/Pb-204 and Pb-207-Pb -204 ratios, similar to the St. Helena Component (SHC), imply only a m inor admixture of highly Pb depleted mantle material (DM). Deviation o f data points plot from the established reference array for simple bin ary HIMU-DM mixing implies an additional low quantity input of a third portion, either a near-surface contaminant or variably enriched mantl e (EM). Model calculations reveal that <1% of pelagic sediment or sedi ment-derived fluids were incorporated in the OIB source generating the alkali portion of the seamount.