Melting of an Hybrid Source Below the Danakil Region

The Danakil region is a modern example of rifting located atop a mantle plume. Along the rift system, a large number of shield volcanoes erupted large volumes of tholeiitic magmas with a wide compositional range, generally enriched in incompatible and trace elements, reflecting the source heterogeneity and the variability of melting processes that contributed to their generation. The Danakil lavas represent the combined product of continental rifting and ascending mantle plume processes. Major and trace elements were analyzed in modern lavas sampled from the Erta Ale Chain and the Asal region and compared to literature data. Although highly enriched in trace elements, our lavas are significantly different when compared to the Oligocene main lava suites generated in the earliest stages of mantle plume activity. Based on La/Sm, Rb/Sr and Zr/Nb ratios and REE abundances they are intermediate between the high-Ti primitive lavas and the low-Ti tholeiitic basalts erupted 30 Ma ago due to the arrival of the plumehead. Trace elements abundances and geochemical modelling indicate that our lavas derive from a “hybrid” source characterized by a great complexity, possibly a metasomatized sublitospheric-mantle component that includes hydrous phases and melting at depths lower than those that generated the Oligocene lavas. The wide compositional range of the Afar lavas suggests that those modern lavas erupted along the rift are not simply the product of melting of a deep mantle plume but derive from a composite source resulting from the interaction between the plume tail and the surrounding sublithospheric mantle previously metasomatized by the plume activity. As a consequence, this very complex and heterogeneous source undergoes extremely variable melting processes as testified by the characteristic chemistry of each volcanic complex. Further geochemical and isotopic investigations will help to better constrain the signature and contribution of each of the reservoirs and to what extent the mantle is metasomatized by hydrous phases below the Afar region.