IGNEOUS ROCKS/Carbonatites 227 Figure Nd and Sr isotopic data from young East African carbonatites Note the near linear array, that suggests mixing between two distinct mantle sources, shown by the red dots, similar to those found in oceanic island basalts (HIMU and EMI) The data from Oldoinyo Lengai cluster close to the cross that marks the position of values for the undifferentiated silicate part of the Earth Although the natrocarbonatites from Oldoinyo Lengai are quite different in chemical composition to other carbonatites, their isotopic data follow the same trend as other carbonatites from East Africa The absence of the high alkali contents in all other types of carbonatite raises doubts about the seminal role that Oldoinyo Lengai plays in understanding carbonatite genesis It was once thought that all carbonatitic melts were alkali rich, and that alkalies were lost during fluid migration from the melt during fenitisation Fascinating as the findings from Oldoinyo Lengai may be, in an introduction to a collection of papers dedicated solely to Oldoinyo Lengai, the consensus was that ‘‘natrocarbonatites should be considered the result of an extreme process rarely encountered in nature’’ even within one igneous province, such as the Kola Alkaline Province, or even within an individual complex, it is likely that carbonatites are produced in more than one way As to how individual carbonatites form is a difficult question to answer, since we still lack good criteria to separate one mechanism from another If carbonatitic melts are primary liquids, they are probably dolomitic in composition, have a Ca/Ca ỵ Mg ratio of between 0.5 and 0.7 and a silica content of 4–5 wt% However, many dolomite carbonatites can be found as late stage veins, dykes, and stringers cutting older calcite carbonatites Mode of Origin There has been a great deal of debate about the origin of carbonatites, and this is still a matter of ongoing concern On the basis of evidence from phase equilibrium studies, field relationships, and geochemistry, there are three possible ways of producing carbonatitic melts Firstly, they can be direct partial melts of a mantle containing carbonate phases Secondly, they can be produced by liquid immiscibility from a carbonated alkali-rich melt And finally, they can be generated by extreme crystal fractionation of a silicate magma Studies involving geochemistry, mineralogy, petrography, and field relationships show that The Source(s) One of the more important questions concerning carbonatites is the site of generation of their parental liquids At first it was thought that the carbonated melts were generated by melting metasomatized lithosphere, but this is not supported by the radiogenic isotopic data, which would require the involvement of more than one mantle source similar to those reflected in oceanic island basalts (Figure 8) The mantle source for carbonatites must contain carbonate phases The stability fields of Mg-rich calcite, dolomite (CaMg(CO3)2), and magnesite