Evaluation of Mantle Processes in an Extensional Regime: Insight from In Situ O and Sr Isotope Systematics of Mantle Xenoliths from Ethiopia

New O and Sr isotope compositions of minerals from well-characterized and variably metasomatized mantle xenoliths in Cenozoic basalts from the northwestern plateau (Gundeweyn area) and southern rift zone (Dillo and Megado areas) of Ethiopia were acquired to further evaluate the nature and effects of metasomatic processes affecting the lithospheric mantle beneath the East African Rift System (EARS), where a transition from plume impingement to rifting is recorded. The studied samples are dominantly fresh spinel lherzolites with subordinate spinel harzburgites and clinopyroxenites. The in situ O-18 composition of olivine is largely homogeneous, with average values of 5.31 parts per thousand for Gundeweyn, 5.22 parts per thousand for Dillo, and 5.25 parts per thousand for Megado xenoliths. The values are within the range of normal mantle olivine (O-18 = 5.1 parts per thousand-5.4 parts per thousand), and there is no systematic variation of O-18 values with petrographic characteristics, elemental compositions, Sr isotopic ratios, or tectonic setting (i.e., plateau [Gundeweyn] vs. rift [Dillo and Megado]). This suggests the absence of major tectonic controls on the O-18 ratios of lithospheric mantle beneath the EARS and indicates buffering of mantle oxygen with respect to metasomatic fluids or melts. The average in situ Sr-87/Sr-86 ratios of clinopyroxenes vary from 0.70300 to 0.70371 for Gundeweyn, 0.70254 to 0.70327 for Dillo, and 0.70288 to 0.70330 for Megado mantle xenoliths. In contrast to O isotopes, radiogenic Sr isotope compositions combined with light rare earth element enrichment in clinopyroxenes in both tectonomagmatic settings betray interaction with metasomatic melts or fluids that may be related to the Afar plume, and neither precludes nor requires influence of a Pan-African subduction component to the continental lithospheric mantle beneath the area. However, slightly higher Sr-87/Sr-86 and a higher abundance of enriched peridotite types in the plateau xenolith suite suggest a stronger contribution from plume or aged metasomatized lithospheric mantle sources than in the rift peridotites.