Kovdor to Oldoinyo Lengai; the missing link in carbonatitic magma evolution

Experiments were conducted to test the hypothesis that interaction of a carbonatitic magma with quartz-rich rocks plays a key role in shaping carbonatite complexes. The host rocks were represented by quartz, and the magma was represented by synthetic mixtures of CaCO (sub 3) , MgCO (sub 3) , and Na (sub 2) CO (sub 3) . With increasing distance from the quartz, the reaction between the carbonate liquid and quartz produced a domain of Na(Ca)-rich silicate glass, a domain of metasomatic wollastonite, diopside, and forsterite, and a carbonate-rich domain. This zonation reproduces that observed in many carbonatite complexes, e.g., Kovdor, Russia. The experiments provide strong evidence that carbonatitic magma/host-rock interaction controls the evolution of carbonatite complexes and explains how Mg-Ca-carbonatitic magmas from the mantle can evolve to produce the natrocarbonatites and associated alkaline silicate rocks observed at Oldoinyo Lengai, Tanzania.