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Mantle Plume-Continental Lithosphere Interaction in the East African Rift

Mantle Plumes Figure 5

Click image to enlarge. Map of the MER study area, with Quaternary sample locations shown along the Wonji Fault Belt (WFB), Debre Zeyit (DZ) and Butajira (BJ) extensional zones. The zones contain 20-40 km spaced felsic volcanoes and calderas. Samples are from smaller basaltic cinder cones, associated flows and occasional fissure eruptions that are interspersed between these volcanoes. The inset shows the study area location in relation to the Afar triple junction. Backgrounds are reflectance images of East Africa and Yemen from the NASA/JPL Shuttle Radar Tomography Mission (USGS, 2004).

The Red Sea, Gulf of Aden and Main Ethiopian Rift (MER) meet in the Afar triangle, where they delineate three arms of a rift system that began to develop about 30 million years ago over the upwelling Afar mantle plume. The MER is the type example of modern continental rift in a plume-influenced environment, and provides a natural laboratory for studying the interaction of a mantle plume with continental lithosphere. Previous studies have described the isotopic diversity of magmas from this region in broad terms, via mixing between the Afar plume, shallow upper mantle and African lithosphere. New He isotope results, along with Pb, Sr, Nd and Hf isotopes, have been obtained for Quaternary basalts erupted in the Main Ethiopian Rift from the Wonji Fault Belt, Debre Zeyit and Butajira (see figure). These new results show that the Afar plume interacts with the continental lithosphere and shallow upper mantle following an ordered sequence of mixing events. Mixing models indicate that the mass proportions of continental lithosphere and upper mantle are constant within each individual volcanic belt, and that the proportion of plume material decreases regularly with distance south westward along the rift, away from the presumed center of the plume beneath Lake Abhe. The systematic relationships indicate that continental lithosphere becomes detached and mixed into the shallow mantle prior to the flow of upwelling plume material beneath the developing rift system. This mixing also produces large-scale pollution of the upper mantle by continental lithosphere that explains the "background" geochemical signatures of mid-ocean ridge basalts far away from mantle hotspots or plumes.

References

Rooney, T. O., B. B. Hanan, D. W. Graham, T. H. Furman, J. Blichert-Toft and J. G. Schilling (2012) Upper mantle pollution during Afar plume-continental rift interaction. J. Petrol. 53, 365-389. doi:10.1093/petrology/egr065 PDF

Projects

Mid-ocean Ridges and Upper Mantle Processes

Mantle Plumes and Geodynamics of Earth's Interior