THE EVOLUTION OF THE MANTLE BENEATH MARSABIT (KENYA): RESULTS FROM TEXTURAL, P-T AND GEOCHEMICAL ANALYSIS

A suite of mantle derived xenoliths from Marsabit (Northern Kenya) was investigated in order to constrain the textural and compositional characteristics of the lithospheric mantle beneath this part of the East African Rift. Most xenoliths originate from the shallow lithospheric mantle. The lithologies are grt websterite, extremely fertile, formerly garnetbearing spl lherzolite, metasomatically overprinted spl harzburgite and mylonitic spl lherzolite and harzburgite. Disequilibrium features testify to a complex P-T evolution. Formerly garnet-bearing lherzolites record an initial HP/HT stage (966-1075°C at 2.33-2.87 GPa) calculated from reintegrated exsolved pyroxenes and spl-opx-cpx symplectites (former garnet). Subsequently, they experienced decompression (i.e. grt breakdown and formation of symplectites; < 1.4 GPa) and cooling down to around 850°C (medium-T stage). This stage was found in all investigated lithologies. Subsequently, this shallow part of the lithosphere further cooled to very low final temperatures (~750-800°C). All rock types in this part of the lithosphere are highly deformed (porphyroclastic- mylonitic). Trace element analysis of clinopyroxene from formerly garnet-bearing spl lherzolite (in-situ LAICPMS) reveal strong core-rim zoning with respect to HREE’s, supporting former equilibrium with garnet, followed by decompression (i.e. grt-spl transition) and partial re-equilibration at lower pressures. Trace element characteristics in the more depleted lithologies (spl harzburgite) testify to variable degrees of metasomatic overprint (LILE- and LREE-enriched; HFSE-depleted), ranging from cryptic (i.e. incompatible element enrichment in cpx) up to strong modal metasomatism (formation of amphibole and phlogopite). A slightly deeper part of the lithosphere (around 1.60- 1.70 GPa) is represented by variably recrystallised and equilibrated, formerly garnet-bearing lherzolites. Several textural characteristics (e.g. relictic exsolution lamellae; presence of spl-opx-cpx symplectites) and compositional features (e.g. zoning patterns in pyroxene) support the idea that these rocks represent more deep-seated equivalents of the above mentioned shallow-seated, porphyroclastic former grt lherzolites. An isobaric heating event (from the medium-T stage of around 800°C up to ~1100°C), accompanied by metasomatic addition of incompatible elements is proposed to account for the transformation. The trace element signature of clinopyroxene (slight Fe-Ti + LILE, LREE and Nb-Ta enrichment; lack of negative Hf-Zr anomalies) favours an alkaline melt as metasomatising agent. The observed relationships between textures, P-T history and (major and trace element) composition are in line with a shallow mantle that is ‘cold’, highly deformed and very heterogeneous. It records a complex evolution including cooling and decompression, as well as modal metasomatism. In contrast, metasomatism in the slightly deeper part of the lithosphere was accompanied by considerable heating. This event is most likely related to the formation of the Tertiary- Quaternary East African Rift System. The heat source is most likely related to the seismically detected low velocity zone beneath the rift, due to upwelling of hot asthenospheric or plume-related material (e.g. Mechie et al., 1997).