Peralkaline Nephelinite-Natrocarbonatite Relationships at Oldoinyo Lengai, Tanzania

Abstract

The younger (<2000 a) nephelinites at Oldoinyo Lengai are peralkaline, with alkalinity indices ([Na + K]/Al) ranging from 1.43 in wollastonite nephelinite to 2.15 in combeite nephelinite. They have a common set of phenocrysts (nepheline, clinopyroxene, Ti-andradite, combeite and apatite) but some phenocrysts and groundmass phases are different (e.g. wollastonite phenocrysts and groundmass melilite in wollastonite nephelinite, and phenocrystal sodalite and groundmass lamprophyllite, ‘delhayelite’ and aegirine-rich pyroxene in combeite nephelinite). Examination of silicate-carbonate spherules from the 1993 eruption showed that natrocarbonatite unmixed from (formerly) carbonate-saturated wollastonite nephelinite, with strong partitioning of alkalis, Ba, Sr and halogens into the carbonate melt. Compared with wollastonite nephelinite, combeite nephelinite is enriched in these elements, and the model proposed here i that their enhancement in combeite nephelinite is due to CO2 loss from the parental melt, precluding the exsolution of natrocarbonatite; the alkalis, Ba, Sr and halogens perforce remain in the silicate melt, enhancing its peralkalinity and leading to precipitation of alkali-, Ba-, Sr- and halogen-rich phases.