Peralkaline nephelinite-natrocarbonatite relationships at Oldoinyo Lengai,Tanzania

The younger (<2000 a) nephelinites at Oldoinyo Lengai are peralkaline, with alkalinity indices ([Na + K]/Al) ranging from 1.43 in wollastonite nepheli nite to 2.15 in combeite nephelinite. They have a common set of phenocrysts (nepheline, clinopyroxene, Ti-andradite, combeite and apatite) but some ph enocrysts and groundmass phases are different (e.g. wollastonite phenocryst s and groundmass melilite in wollastonite nephelinite, and phenocrystal sod alite and groundmass lamprophyllite, 'delhayelite' and aegirine-rich pyroxe ne in combeite nephelinite). Examination of silicate-carbonate spherules fr om the 1993 eruption showed that natrocarbonatite unmixed from (formerly) c arbonate-saturated wollastonite nephelinite, with strong partitioning of al kalis, Ba, Sr and halogens into the carbonate melt. Compared with wollaston ite nephelinite, combeite nephelinite is enriched in these elements, and th e model proposed here is that their enhancement in combeite nephelinite is due to CO2 loss from the parental melt, precluding the exsolution of natroc arbonatite; the alkalis, Ba, Sr and halogens perforce remain in the silicat e melt, enhancing its peralkalinity and leading to precipitation of alkali- , Ba-, Sr- and halogen-rich phases.