Peridotite xenoliths from Ethiopia: inferences on mantle processes from Plume to Rift settings

A comprehensive petrological study has been carried out on Ethiopian mantle xenoliths entrained in Neogene-Quaternary alkaline lavas both overlying the Continental Flood Basalt (CFB) area (Dedessa River – Wollega Region, Injibara – Gojam Region) and from southern Main Ethiopian Rift (MER – Mega, Sidamo Region) in order to investigate the mantle evolution from plume to rift settings. Mantle xenoliths from the plateau area (Injibara, Dedessa River) range in composition from spinel lherzolite to harzburgite and olivine websterite, showing P-T equilibration conditions in the range 1.3–0.9GPa/1050–950°C. These xenoliths show flat chondrite(ch)-normalized bulk-rock REE patterns, with only few LREE-enriched samples (LaN/YbN up to 7) in the most refractory lithotypes. Clinopyroxene (cpx) REE patterns are mostly LREE depleted (LaN/YbN down to 0.2) or enriched (LaN/YbN up to 4.4). Sr-Nd isotopes on separated cpx mainly show compositions (87Sr/86Sr 0.5132) approaching the Depleted Mantle end-member, or displaced (87Sr/86Sr 0.7033-0.7034; 143Nd/144Nd 0.5129-0.5128) toward the Enriched Mantle components which characterise the Afar plume signature and the related Ethiopian Oligocene CFBs; 3He/4He ratios in olivines range from 6.6 to 8.9 R/Ra, slightly higher than those from other African mantle xenolith occurrences. These characteristics suggest that most xenoliths reflect complex asthenospere/lithosphere interactions due to refertilisation processes by mafic subalkaline melts that infiltrated and reacted with the pristine peridotite parageneses ultimately leading to the formation of olivine-websterite domains (Fig. 1a and b). On the other hand, mantle xenoliths from southern MER (Mega) consist of spinel lherzolite to harzburgites showing various degree of deformation and recrystallization, coupled with a wider range of P-T equilibration conditions, from 1.6±0.4 GPa/1040±80°C to 1.0±0.2 GPa/930±80°C. Bulk rock REE patterns show generally flat HREE ranging from 0.1 x ch in harzburgites up to 2 x ch in fertile lherzolites, and are variably enriched in LREE, with LaN/YbN up to 42 in the most refractory lithologies. The constituent clinopyroxenes have flat HREE distribution and LaN/YbN between 0.1 and 55, in general agreement with the respective bulk rock chemistry. Sr-Nd-Pb on separeted clinopyroxenes are 87Sr/86Sr 0.7022-0.7031, 143Nd/144Nd 0.5130- 0.5138, 206Pb/204Pb 18.38-19.34 for lherzolites, and 87Sr/86Sr 0.7027-0.7033, 143Nd/144Nd 0.5128-0.5130, 206Pb/204Pb 18.46- 18.52 for harzburgites, thus spanning in composition between the DM and HIMU mantle end-members; the helium isotopic composition varies between 7.1 to 8.0 R/Ra, closely comparable with the xenoliths from the plateau area. Regional comparison shows that HIMU-like alkali-silicate melt(s) were among the most effective metasomatising agent(s) in mantle sections beneath southern MER (Fig. 1c) as well as along the Arabian rifted continental margins and the whole East African Rift System (EARS). The decidedly different types of metasomatic agents recorded in Ethiopian mantle xenoliths from the CFB area and the Rift systems, clearly reflect distinct tectonomagmatic settings, i.e. the plume-related subalkaline magmatism (Fig. 1a and b) and the rif trelated alkaline volcanism which extends far beyond the influence of the Afar plume (Fig. 1c)