Petrology of mantle xenoliths from Megado and Dilo, Kenya rift, Southern Ethiopia.

A study has been made of ultramafic xenoliths embedded in the Quaternary basanitic lavas of Dilo and Megado areas, southern Ethiopia. This is the first time that xenoliths have been found at Dilo. In both areas, the xenoliths are spinel lherzolites and pyroxenites, garnet- and/or spinel-bearing pyroxenites occasionally containing amphibole. Temperature/pressure estimates indicate that the lherzolites and most of the pyroxenites last equilibrated in the upper mantle in similar P-T conditions (T ?950-1050°C at P ? 1.3-1.7 GPa), whereas some spinel pyroxenites equilibrated at higher temperatures (? 1150-1250°C). Based on mineral assemblage, four groups of pyroxenites were distinguished, two of which exhibit spectacular garnet coronas around spinels. Whole-rock and mineral composition (major and trace elements) indicates that garnet-free pyroxenites segregated from OIB-type melt. Successively, melt evolution caused garnet crystallisation (so that garnet bearing pyroxenites formed) and, eventually, pargasitic amphibole caused the generation of garnet- and amphibole- bearing pyroxenites with the consequent LILE-enrichment of residual melts. Melt evolution resembles that hypothesised by Bedini et al. (1997) to explain metasomatism of lherzolites based on reactive porous flow simulations. Thus, pyroxenites may represent the "counter-part" of the melts responsible of the metasomatism in this area. Furthermore, the OIB nature of parental permeating melts suggests that metasomatism (and pyroxenite genesis) was related to Quaternary magmatic activity responsible for the xenoliths ascent to the surface.