Upper Mantle Pollution during Afar Plume^Continental Rift Interaction

erupted from Debre Zeyit, Butajira, and the Wonji Fault Belt of the Main Ethiopian Rift, show systematic mixing relationships involving three distinct mantle sources.The Pb, Sr, Nd, and Hf iso-topic arrays converge in a specific region of isotopic multi-space where they define the composition of the Afar mantle plume (centered about 206Pb/204Pb19·5, 87Sr/86Sr 0·7035, eNdþ4·6, eHf þ9·3, 3He / 4He415 RA).This plume end-member has an identical composition to that observed previously in oceanic basalts.The dis-tinct isotopic arrays for the various volcanic areas in the Main Ethiopian Rift vary spatially in a systematic manner, and may be viewed as pseudo-binary mixing arrays. This further suggests that the Afar mantle plume interacts with the local continental lithosphere and upper mantle asthenosphere (mid-ocean ridge basalt-like source) through an ordered sequence of mixing events. Simple mixing models require that the mass proportions of continental litho