ND, SR, AND PB ISOTOPIC EVIDENCE FOR DIVERSE LITHOSPHERIC MANTLE SOURCES OF EAST-AFRICAN RIFT CARBONATITES

Carbonatites may provide valuable information on mantle source composi tions as their isotopic ratios are insensitive to crustal contaminatio n. In order to place constraints on mantle sources, nineteen samples f rom three Miocene to Quaternary carbonatite areas in the East African Rift were analysed for their Sr, Nd, and Pb isotopic compositions. The samples from Kerimasi (northern Tanzania), Homa Mountain, and Wasaki Peninsula (both Lake Victoria, Kenya) as a whole show considerable var iations in their isotope ratios (0.70327-0.70502 for Sr-87/Sr-86, 0.51 249-0.51283 for Nd-143/Nd-144, 18.72-20.41 for Pb-206/Pb-204, 15.52-15 .78 for Pb-207/Pb-204, and 39.22-40.47 for Pb-208/Pb-204) that lie bet ween the inferred compositions for HIMU (high U-238/Pb-204 mantle) and EM I (enriched mantle I) components in most isotope plots. The intern al isotopic variations of the three carbonatite areas define distinct arrays and diverse trends in isotope diagrams. Although the isotope da ta define linear arrays in Sr-Nd and Pb-Pb diagrams, which suggest bin ary mixing between HIMU and EM I mantle components, neither the isotop ic compositions of the carbonatites as a whole nor the compositional r anges for individual carbonatite occurrences can be explained by such a process. This clearly emerges from the absence of linear data trends in Sr-Pb and Nd-Pb isotope plots and from the lack of consistent endm ember compositions. These features are also displayed by previously pu blished isotope data for East African carbonatites. It is therefore su ggested that carbonatite complexes within the East African Rift have i sotopically distinct and small mantle sources that are probably not ad equately described in terms of the mantle components defined for ocean ic basalts. Most likely, these sources are located in a heterogeneous lithospheric mantle and were produced by enrichment and depletion proc esses at different times and degrees.