Boron Abundances and Isotope Systematics of Carbonatites from Worldwide Sources

The origin of carbonatites and their associated alkaline silicate rocks has been linked to metasomatized upper mantle sources and related to mantle plume activity. However, the lithospheric or asthenospheric origin of the carbon contained within mantle-derived carbonatite melts is still under debate. In order to investigate this, carbonatite samples from worldwide sources have been analyzed for their stable C and O isotope ratios and for the first time, boron (B) isotope compositions. It is well documented that the global boron geochemical cycle is closely tied to recycling of geological material via subduction processes. Therefore, it is hypothesized that investigation of boron isotope systematics of carbonatites will aid in testing for a recycled component present in carbonatite magmas; this creates a unique way of evaluating their mantle sources. Carbonatites investigated here span a wide range of emplacement ages (~40 Ma to ~2600 Ma old), and therefore provide insight into the temporal evolution of their mantle sources. Results show that B abundances in carbonatites range between 0.04 and 2.0 parts per million (ppm), well within the range reported for mantle-derived rocks. δ11B values for the carbonatite samples varybetween +5.5‰ and -8.6‰. These seem to define a bimodal distribution: one group is characterized by more negative δ11B values (< -4.0‰), which suggests derivation from typical asthenospheric (depleted) upper mantle source, whereas samples belonging to the second group record more positive δ11B values (> -4.0‰) and indicates involvement of subducted (crustal) material. The latter may be sampled by carbonatite melts associated with mantle plume activity, and their corresponding emplacement ages coincide with major tectonic events in Earth’s history; these are linked to episodes of supercontinent formation and consequently significant periods of subduction at a global scale.