Evolution of the mantle beneath the Kenya Rift: Constraints from textural, P-T and geochemical studies on xenoliths from Marsabit.

A suite of basalt-hosted peridotites originating from the shallow lithospheric mantle beneath Marsabit volcanic field have been investigated in order to constrain the compositional characteristics and evolution of the mantle beneath this part of the Kenya Rift. Four types of peridotite xenoliths provide evidence of a cold, highly deformed, and heterogeneous upper mantle. Textures, thermobarometry and geochemical characteristics of minerals indicate that low temperatures in the spinel stability field (ca. 750-800°C at <1.5 GPa) were attained by decompression and cooling from initially high pressures and temperatures in the garnet stability field (970-1080°C at 2.3-2.9 GPa). Cooling, decompression and penetrative deformation are consistent with lithospheric thinning, probably related to the development of the Mesozoic to Paleogene Anza graben. Re-equilibrated and recrystallized peridotite xenoliths record heating up to 1100°C. Mineral trace element signatures indicate variable degrees of metasomatic overprint ranging from cryptic (LILE enrichment in cpx) up to strong modal metasomatism (formation of amphibole and phlogopite) by mafic silicic melts parental to the Quaternary host basanites and alkali basalts. Relationships between geochemistry, P-T conditions of equilibration and mineral texture can be explained by metasomatism and heating of the lithosphere related to the formation of the Kenya Rift above a zone of upwelling mantle (asthenospheric or plume-related).