Petrology of mantle xenoliths from the Northern Ethiopian plateau: Clues of plume related metasomatism?

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Beccaluva, Luigi۱; Bianchini, Gianluca۱; Ellam, Robert Marc۲; Natali, Claudio۱; Siena, Franca۱; Stuart, Finlay M.۲
Holding Date 08 October 2008

Mantle xenoliths entrained in alkaline lavas from Injibara (Gojam) and Nekemte (Wollega), at the western border of the Ethiopia-Yemen basaltic plateau, include prevalent spinel lherzolites and subordinate harzburgites (sometimes containing metasomatic amphibole) and olivine-websterites characterized by P-T equilibration conditions in the range of 1-2 GPa / 950-1050 °C.
They provide information on the lithospheric mantle evolution in an area where the possible existence of a deep mantle plume was suggested mainly on the basis of geophysical data (Afar plume; Courtillot et al., 2003).
Most xenoliths show flat chondrite-normalized bulk-rock REE patterns, with only few LREE-enriched samples (LaN/YbN up to 5).
Clinopyroxene (cpx) REE patterns are generally flat or LREE depleted (LaN/YbN down to 0.6). In a few samples a pristine equilibration in the garnet peridotite facies is suggested by peculiar convex-upward cpx REE patterns similar to those observed in garnet peridotite clinopyroxenes.
Sr-Nd isotopes on separated cpx mainly show compositions (87Sr/86Sr < 0.7030; 143Nd/144Nd > 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 also characterize the Ethiopian Oligocene plateau basalts.
The 3He/4He isotope ratios are slightly higher (R/Ra from 7 to 8) than those of other African subcontinental mantle xenoliths (Beccaluva et al., 2007 and 2008). These characteristics indicate that most xenoliths were variably affected by metasomatic processes, whose agents may be envisaged as mafic subalkaline melts that infiltrated and reacted at high melt/peridotite matrix ratio. The compositional evolution of the studied samples seems to reflect complex asthenospere/lithosphere interactions occurring in a plume region where lithospheric bulging and thinning by the uprising asthenosphere is accompanied by pervasive refertilization due to reactive percolation of sublithospheric subalkaline melts.