The geochemistry and Sr-Nd-Pb isotopic ratios of high 3He/4He Afar and MER basalts indicate a significant role of the African Superplume in EARS magmatism

Abstract

Basaltic rocks from the Afar Depression and Main Ethiopian Rift at the northern end of the East African Rift System with 3He/4He from 6 to 17 times the atmospheric value (3He/4HeAir or RA) define two geographically and, to a lesser extent, geochemically distinct groups. These groups are inconsistent with the commonly held view that there is a distinct Afar mantle plume at the northern end of the rift system, and that the plume influence systematically decreases away from its center in Lake Abhe. The high (>9 RA) 3He/4He Afar and Ethiopian rift basaltic suites possess a common, high 3He/4He component. The ca. 30 Ma very high titanium transitional basalts and picrites from the Ethiopian-Yemeni plateau with the highest 3He/4He (ca. 20 RA) closely represent the common component. Combined with their OIB-like compositional features, the high 3He/4He basalts indicate a metasomatized or anomalous upper mantle sensu lato and African Superplume source. Similar to the mantle source of ocean island basalts, the superplume is heterogeneous as it consists of various enriched end-components with low 3He/4He (≤8 RA) and the common, depleted ‘focus zone’ component with high 3He/4He (>9 RA). Northern rift magmas are mainly pseudo-binary mixtures between the anomalous upper mantle and currently upwelling plumes from the superplume.