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 (0.7 0437-0.70146) isotopic compositions from ten natrocarbonatite lavas, c ollected in June 1993 from Oldoinyo Lengai, the only known active carb onatite volcano, are relatively uniform, and are similar to data from the 1960 and 1988 flows. Three of the samples contain silicate spheroi ds, one of which has Nd and Sr isotopic ratios similar to host natroca rbanatite, consistent with an origin by liquid immiscibility or the mi xing of melts with similar isotopic compositions Pb isotope data for t wo samples of trona are inconsistent with its involvement in. the gene sis of natrocarbonatite. New Pb isotope data from silicate volcanic an d plutonic blacks (ijolite, nephelinite, phonolite, syenite) from Oldo inyo 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 fr om Oldoinyo Lengai are best explained by invoking discrete partial mel ting events which generate undersaturated alkaline silicate magmas wit h distinct isotopic ratios. Pb isotope ratios from most ijolites and p honolites are predominantly lower and more variable than from the natr ocarbonatites, and are attributed to interaction between silicate melt s involving HIMU and EMI source components and an additional component , such as lower-crustal granulites, DMM or PREMA (prevalent mantle). V ariations in Nd, Pb and Sr isotope ratios from Oldoinyo Lengai, among the largest yet documented from a single volcano, are attributed to ma ntle 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, w e propose a two-stage model in an attempt to explain the isotope varia tions shown by carbonatites in this area. The model involves (1) the r elease of metasomatizing agents with HIMU-like signatures from upwelli ng mantle ('plume') source, which in turn metasomatize the sub-contine ntal (old, isotopically enriched EMI-like) lithosphere, and (2) variab le degrees and discrete partial melting of the resulting heterogeneous , metasomatized lithosphere.