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Geochemical evidence for plume metasomatism and old lithospheric mantle beneath the East African Rift in Tanzania



Geochemical evidence for plume metasomatism and old lithospheric mantle beneath the East African Rift in Tanzania



Abstracts with Programs - Geological Society of America 32(7): 164



We have applied the Re-Os system to a suite of mantle xenoliths from East African rift-related volcanism in Tanzania, in order to understand the effects of a mantle plume and rifting, on mantle roots. Peridotites span a wide range of compositions and mineralogies and include spinel facies harzburgites and lherzolites, chromite-bearing harzburgites, garnet harzburgites and lherzolites as well as Fe-rich dunites, pyroxenites and glimmerites. The Re-Os systematics of xenoliths from the Labait, Lashaine, Pello Hills, Olmani and Eladoi volcanos, yield TRD (time of Re depletion, which represents a minimum age) ages, which range from 2.9 to 0.3 Ga. These data show that ancient, refractory lithosphere is present to depths of at least 120 km from the edge of the Tanzanian craton out into the center of the East African rift. Several styles of pre-entrainment, metasomatic overprinting are recognized from petrography and whole rock geochemistry. All of the metasomatic samples have (super 187) Os/ (super 188) Os of between 0.132 and 0.139, higher than expected for chondritic mantle and overlapping the field of ocean island basalts. One sample that is both the highest temperature xenolith and most primitive, may be interpreted to represent a sample of upwelling asthenospheric mantle presently beneath the Tanzanian craton. Our data shows that wholesale lithospheric delamination resulting from present day rifting, has not occurred beneath the Tanzanian craton. Although significant lithospheric thinning is inferred to have occurred beneath the Kenyan portion of the east African rift, we observe 1.5 to 2.5 Ga mantle lithosphere beneath the Tanzanian portion of the East African Rift. The combination of our data with seismic tomographic results indicate that the old lithospheric mantle has not been removed beneath the east African rift and the presence of a deep thermal and geochemical anomaly that is consistent with the existence of a plume.

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