The Trace Element Composition and Petrogenesis of Natrocarbonatites

Trace element data, including the REE, are reported for natrocarbonatites collected during the 1988 effusive period of Oldoinyo Lengai, Tanzania. These data, together with trace element data for the phenocryst phases nyerereite and gregoryite and for a related peralkaline nephe-linite, provide the basis for discussing petrogenetic hypotheses related to alkalic carbonatites. Natrocarbonatites are characterized, compared to the more common calciocarbonatites, dolomitic carbonatites, and ferrocarbonatites, by extreme fractionation of the REE, with (La/Sm)_N ratios >40, high concentrations for Sr, Ba, Rb, K, unusually high contents of Mo, W, U, As and Sb and strong depletion in Ta, Nb, Hf, Zr, Ti, Y, Yb and Lu. The trace element signature of natrocarbonatite as compared with the more common alkali-poor carbonatites shows that Lengai lavas represent exotic compositions so far restricted to this volcano, and cannot be used to model petrogenetic schemes for carbonatites in general. Liquid immiscibility is a viable model for the derivation of natrocarbonatite from an exceptionally peralkaline nephelinite.