Trace element systematics and Nd, Sr and Pb isotopes of Pliocene flood basalt magmas (Ethiopian rift): A case for Afar plume-lithosphere interaction

We present high-precision trace element and Sr-Nd-Pb isotope data for Pliocene flood basalts located at Sire (eastern rift wall) and Chefe Donsa (western rift wall) sections from the main Ethiopian rift (MER). These basalts are tholeiitic to mildly alkaline and were derived by moderate degrees of melting (similar to 10-20%) of a spinel lherzolite containing minor amounts of amphibole within the lithospheric mantle. Strontium, Nd and Pb isotopic compositions (Sr-87/Sr-86: 0.7040-0.7057, Nd-143/Nd-144: 0.52164-0.51287, Pb-206/Pb-204: 17.75-19.20, Pb-207/Pb-204: 15.53-15.64 and Pb-208/Pb-204: 38.12-38.86) are between the inferred composition of the Afar mantle plume and an unspecified enriched lithospheric component, suggesting interaction of melts from asthenospheric and lithospheric mantle domains rather than melting of homogeneous mixed sources or magma mixing. It is proposed that in the Oligocene the impinging Afar plume creates zones of metasomatically enriched mantle. Later, lithospheric thinning as a result of incipient rifting leads to decompression and melting of this metasomatically enriched mantle due to its lower solidus temperature. The chemistry of the Pliocene lavas therefore reflect two associated scenarios namely (i) upwelling plume material from the advancing Afar plume may have induced metasomatism in the lithospheric mantle (ii) mobilization and melting of this enriched lithospheric mantle material resulted in melts characterized by variable mixtures of the two mantle reservoirs.