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 natrocarbonatite, consistent with an origin by liquid immiscibility or the mixing of melts with similar isotopic compositions. Pb isotope data for two samples oftrona are inconsistent with its involvement in the genesis of natrocarbonatite. New Pb isotope data

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... om silicate volcanic and plutonic blocks (ijolite, nephelinite, phonolite, syenite) from Oldoinyo Lengai are highly variable 'Pb, ^Pb/^Pb, b" 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 lowercrustal 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 HIMUlike signatures from upwelling mantle ('plume') source, which in turn metasomatizfi the sub-continental (old, isotopically enriched, EMIAike) Ixthosphere, and (2) variable degrees and discrete partial melting of the resulting heterogeneous, metasomatized lithosphere. JOURNAL OF PETROLOGY VOLUME 37 NUMBER 6 DECEMBER 1996 208p b/ 2(Hp b vg 2<fcp b/ 2O4p b and 207 pb/ 204 pb 1 322 Downloaded from https://academic.oup.com