Three-liquid immiscibility and the origin of carbonatites

Abstract

THE relationship of rock types involved in carbonatite complexes has provoked much debate^1–4. One favoured theory of origin involves the immiscible separation of carbonate-and silicate-rich liquids to produce the bimodal distribution of rock types typical of many complexes. Previous investigations of liquid immiscibility at crustal pressures in both natural^5,6 and synthetic³ systems have demonstrated that immiscibility can produce much of the chemical diversity (in both major^3,5,6 and trace elements⁷) of this group of rocks; however, arguments persist regarding the relative importance and genetic relationship of alkali-rich and alkali-poor carbonatites^1,4,8–10. Here we report the first results of new experiments at upper-mantle pressures, which show unexpected features indicating the existence of three immiscible liquids, one silicate-and two carbonate-rich, with surprising density relationships. These two carbonate liquids and the associated vapour could account for many carbonatite characteristics, while avoiding some problems encountered in deriving magmas of variable alkali content from a single carbonatite parent.