Carbonatite magmatism and plume activity: Implications from the Nd, Pb and Sr isotope systematics of Oldoinyo Lengai

New Nd (0.51261-0.51268), Pb (Pb-206/Pb-204: 19.24-19.26), and Sr (0.70437-0.70146) isotopic compositions from ten natrocarbonatite lavas, collected in June 1993 from Oldoinyo Lengai, the only known active carbonatite volcano, are relatively uniform, and are similar to data from the 1960 and 1988 flows. Three of the samples contain silicate spheroids, one of which has Nd and Sr isotopic ratios similar to host natrocarbanatite, consistent with an origin by liquid immiscibility or the mixing of melts with similar isotopic compositions Pb isotope data for two samples of trona are inconsistent with its involvement in. the genesis of natrocarbonatite. New Pb isotope data from silicate volcanic and plutonic blacks (ijolite, nephelinite, phonolite, syenite) from Oldoinyo Lengai are highly variable (Pb-206/Pb-204, 17.75-19.34; Pb-207/Pb-204, 15.41-15.67; Pb-208/Pb-204, 37.79-39.67), and define near-linear arrays in Pb-Pb diagrams. The isotopic data for the silicate rocks from Oldoinyo Lengai are best explained by invoking discrete partial melting events which generate undersaturated alkaline silicate magmas with distinct isotopic ratios. Pb isotope ratios from most ijolites and phonolites are predominantly lower and more variable than from the natrocarbonatites, and are attributed to interaction between silicate melts involving HIMU and EMI source components and an additional component, such as lower-crustal granulites, DMM or PREMA (prevalent mantle). Variations in Nd, Pb and Sr isotope ratios from Oldoinyo Lengai, among the largest yet documented from a single volcano, are attributed to mantle source heterogeneity involving mainly the mixing of HIMU and EMI mantle components Based on the new isotopic data from Oldoinyo Lengai and data from other East African carbonatites, and mantle xenoliths, we propose a two-stage model in an attempt to explain the isotope variations shown by carbonatites in this area. The model involves (1) the release of metasomatizing agents with HIMU-like signatures from upwelling mantle ('plume') source, which in turn metasomatize the sub-continental (old, isotopically enriched EMI-like) lithosphere, and (2) variable degrees and discrete partial melting of the resulting heterogeneous, metasomatized lithosphere.