Source of parental melts to carbonatites–critical isotopic constraints

Abstract

The restriction of most carbonatites to continental areas raises questions as to whether the parental liquids to carbonatites are generated within the continental lithosphere, or whether they are derived from deeper parts of the mantle with the lithosphere playing a subsidiary, but important, role in trapping volatile-rich melts/fluids. The constraints imposed by both radiogenic and stable isotopic data from carbonatites world-wide are consistent with a sub-lithospheric source for the parental melts, associated with either asthenospheric ‘upwellings’ or more deep-seated, plume-related activity. Crucial evidences that support the generation of carbonated melts from sub-lithospheric mantle are: the petrogenetic and temporal association of carbonatites with large igneous provinces (LIPs; e.g., Deccan, Parana), carbonatites with primitive noble gas isotopic signatures, radiogenic isotope ratios similar to OIBs (i.e. involving HIMU, EM1 and FOZO mantle components), and the uniform, time-integrated Rb/Sr and Sm/Nd development lines for Sr and Nd isotopic data for carbonatites from the Superior Province, Canada, and the Kola Peninsula, Russia. Such findings are difficult to reconcile with a lithospheric origin. Even if a metasomatized lithospheric mantle is considered to be the sole source of carbonated melts, an unknown mantle process is required, large enough to generate widespread, synchronous metasomatism spanning regions in excess of 1 × 106 km2.