Cuspidine-sodalite natrocarbonatite from Oldoinyo Lengai, Tanzania: a novel hybrid carbonatite formed by assimilation of ijolite

A unique hybrid natrocarbonatite, collected from the new ash cone of the volcano Oldoinyo Lengai, Tanzania in July 2008, consists of phenocrysts of nyerereite and gregoryite together with xenocrysts of clinopyroxene, nepheline and Ti-andradite set in a groundmass of cuspidine, sodalite, ferroan manganoan monticellite, K-Fe sulphide and manganoan titanian magnetite and gregoryite. The xenocrysts were not in equilibrium with the melt which formed their current host, as clinopyroxenes and Ti-andradite are mantled by cuspidine, and nepheline by sodalite and phlogopite-potassian kinoshitalite solid solutions. A microxenolith of ijolite exhibits similar reaction phenomena. The minerals of the xenocryst suite have similar compositions to plutonic ijolites found at Oldoinyo Lengai, and are thus considered to be derived by the fragmentation of such material in a previously contaminated natrocarbonatite melt. The latter, which has cuspidine, sodalite and monticellite as primary liquidus phases, is considered to have been formed by the complete assimilation of ijolitic material in a natrocarbonatite magma at depth in the volcano conduit. The occurrence of trace amounts of cuspidine, Fe-Mn-monticellite, K-Fe sulphide and Mn-Ti-spinel in recently erupted natrocarbonatites is ascribed to similar, but less extensive, assimilation of silicate material prior to their eruption.