Mantle dynamics and secular variations beneath the East African Rift: Insights from peridotite xenoliths (Mega, Ethiopia)

Abstract

Segments of the Main Ethiopian Rift (MER), a key sector of the East African Rift (EAR) linking the Afar and Turkana depressions, record different stages of lithospheric evolution from initiation to continental break-up and incipient oceanic spreading. Although EAR tectonic and magmatic activity is generally ascribed to a manifestation of sublithospheric mantle processes, the number, depth of provenance and triggering mechanisms of the related mantle upwellings remain topics of debate. We present new Hf and Pb isotope data for EAR mantle xenolith clinopyroxenes from Southern Ethiopia (Mega, the Sidamo region) that compellingly testify to multiple episodes of mantle depletion and metasomatic enrichment. Radiogenic values (εHf up to +1076) suggest that present-day MER lithospheric mantle domains underwent melting, possibly in the presence of residual garnet, billions of years ago, thereby fractionating (increasing) the Lu/Hf ratio and ultimately leading to extremely high 176Hf/177Hf. Although the precise dating of these depletion episodes is hampered by possible metasomatic overprinting, positively correlated Lu/Hf and 176Hf/177Hf indicates apparent ingrowth at ca. 1.9Ga, providing a minimum age for the delineated Proterozoic melting events. Our findings of Early Proterozoic melting episodes are complementary to previously determined Os model ages on xenoliths from the same site indicating melt extraction between 2.4 and 2.8Ga (Reisberg et al., 2004; Chem. Geol. 208, 119–140). Taken together, the geochemical and isotopic characteristics of the Mega xenoliths therefore record melt extraction events and preserve memory of ancient mantle dynamics beneath this site, where important lithospheric discontinuities exist between the Archean/Early Proterozoic Tanzanian craton and the Late Proterozoic Panafrican mobile belt. Subsequent metasomatic reactions variably affected the Nd and Hf isotopic compositions of some samples and completely overprinted the Pb isotopic composition of the whole xenolith suite. The persistence of these geochemical heterogeneities within the investigated suites of xenoliths precludes pervasive melt refertilization, which would have homogenized existing compositions and obliterated the record of previous petrologic processes. This suggests that the MER segment considered here developed on a lithospheric section isolated by pre-existing tectonic structures, far from the influence of plumes originating deep in the convecting mantle.