Nd, Pb and Sr isotope data from carbonatites have shown to be effective in monitoring the nature and the temporal evolution of the sub-continental mantle (e.g. Bell and Blenkinsop, 1987a,b; Nelson et al., 1988; Tilton and Bell, 1994). Situated within the East African Rift Valley in northern Tanzania, Oldoinyo Lengai is the youngest of a group of nephelinite-phonolite-carbonatite volcanoes. As the only known active carbonatite volcano, Oldoinyo Lengai plays a key role in unraveling the difficulties involved in understanding the evolution and genesis of carbonatitic magmas.
We report new Nd, Pb and Sr isotope data for natrocarbonatite eruptives collected in June 1993, and Pb isotope ratios for older, associated silicate rocks in an attempt to:
1- compare the isotope compositions of the 1993 natrocarbonatite flows with those from older eruptions; and 2- evaluate the relationship between the associated silicate rocks and the natrocarbonatite lavas. On the basis of the isotope data from Oldoinyo Lengai, other East African carbonatites, and mantle xenoliths (Cohen et al., 1984; Rudnick et al., 1993), we propose a model to explain the large isotopic variation shown by carbonatites from the eastern segment of the East African Rift involving plume-lithosphere interaction.
New initial Nd (0.51261 - 0.51268), Pb (206Pb/204Pb: 19.24 -19.26), and Sr (0.70437 - 0.70446) isotopic ratios from ten natrocarbonatite lavas from Oldoinyo Lengai, are relatively uniform, and are similar to data from the 1960 and 1988 flows. The Nd and Sr data from the natrocarbonatite lavas fall close to the East African Carbonatite Line (EACL- Bell and Blenkinsop, 1987a), a line that mimics the "LoNd" array of Hart et al. (1986), and between HIMU and EMI mantle components. In marked contrast, Nd and Sr data for older, associated silicate flows and plutonic blocks (Bell and Dawson, 1995) are highly variable and define a linear array almost identical to the EACL. New Pb isotopic data from these older, associated silicate volcanic and plutonic blocks are also highly variable (206Pb/204Pb, 17.75-19.34; 207Pb/204Pb, 15.41-15.67; 208Pb/204Pb, 37.79-39.67), and form near-linear arrays in Pb-Pb diagrams similar in slope to the oceanic regression line.
The variations in Nd, Pb and Sr isotope ratios from Oldoinyo Lengai (in particular for the silicate rocks), 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. 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. This is consistent with the models of Bell and Dawson (1995), and Simonetti and Bell (1994, 1995) for the generation of discrete, small volume, mantle melts, such as primitive nephelinites, by the mixing of the same two mantle end members with HIMU and EMI isotope compositions. Pb isotopic ratios for most ijolites and phonolites from Oldoinyo Lengai, however, are predominantly lower and more variable than from the natrocarbonatites and these we attribute to interaction between silicate melts involving HIMU and EMI and an additional component, such as lower crustal granulites or PREMA (prevalent mantle).
We propose a two-stage model for carbonatite genesis in East Africa that involves (i) the release of metasomatizing agents with HIMU signatures from a plume source, which in turn metasomatize the sub-continental (old, isotopically enriched, EMI) lithosphere, and (ii) variable degrees of melting of the resulting metasomatized lithosphere.
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