Radiogenic isotope constraints on relationships between carbonatites and associated silicate rocks - a brief review

The diversity of silicate rocks in alkaline-carbonatite complexes cannot be attributed to differentiation of parent magmas operating under closed chemical conditions. Constraints imposed by Nd, Pb and Sr isotope data require discrete partial melting events coupled in some cases, with open-system behaviour that involves mixing either with other mantle melts or sources, or with lower continental crust. Patterns shown by isotope ratio diagrams for carbonatites and some nephelinites from East Africa indicate mixing dominated by two mantle end-members (broadly similar to HIMU and EMI), first recognised in oceanic basalts. Mixing is on a scale sufficient to generate: (1) coherent but variable ratios in carbonatites from much of East Africa, and (2) variable isotope ratios among some of the silicate rocks from the same eruptive centre (e.g. Oldoinyo Lengai, Shombole). Overlapping isotope ratios between carbonatites and some nephelinites from complexes from East Africa and elsewhere are consistent with magmatic differentiation (crystal fractionation, liquid immiscibility), or the melting of the same or isotopically similar sources. The wide isotopic variation shown by the ijolites, phonolites, syenites mid even some of the nephelinites requires rite involvement of other mantle components and/or continental crust.