Abstract
Morogan and Martin (1985) have proposed that the alkalic carbonatite lavas (natrocar-bonatite) of Oldoinyo Lengai were produced by partial melting of a regionally fenitized basement to generate two conjugate immiscible liquids: the carbonatite and the associated silicate lavas. The proposed origin is not viable. The existence of a regionally fenitized basement has not been established nor is it likely, certainly not by the method proposed. Immiscibility is restricted to systems with alkali carbonates, and so even if a regionally developed fenite composed of nepheline + alkali feldspar + clinopyroxene + calcite did exist, it would not produce alkali-rich carbonatite melt and phonolite magma as conjugate liquids. The high F and Cl contents of the carbonatite lava have no obvious source in the alleged fenite basement, nor can the high concentrations of Nb, REEs, Sr, Ba, Th, Pb, or U be explained in this way. Sr- and Nd-isotope values are generally consistent with a mantle origin and offer little support for derivation from Archean basement. The alkalic carbonatite lavas of Oldoinyo Lengai cannot have been formed by partial melting of fenite.
