Recycling of subducted crustal components into carbonatite melts revealed by boron isotopes

The global boron geochemical cycle is closely linked to recycling of geologic material via subduction processes that have occurred over billions of years of Earth's history. The origin of carbonatites, unique melts derived from carbon-rich and carbonate-rich regions of the upper mantle, has been linked to a variety of mantle-related processes, including subduction and plume-lithosphere interaction. Here we present boron isotope (Î 11 B) compositions for carbonatites from locations worldwide that span a wide range of emplacement ages (between â '1/440 and â 1/42,600 Ma). Hence, they provide insight into the temporal evolution of their mantle sources for â 1/42.6 billion years of Earth's history. Boron isotope values are highly variable and range between â '8.6‰ and +5.5‰, with all of the young (<300 Ma) carbonatites characterized by more positive Î 11 B values (>â '4.0‰), whereas most of the older carbonatite samples record lower B isotope values. Given the Î 11 B value for asthenospheric mantle of â '7 ± 1‰, the B isotope compositions for young carbonatites require the involvement of an enriched (crustal) component. Recycled crustal components may be sampled by carbonatite melts associated with mantle plume activity coincident with major tectonic events, and linked to past episodes of significant subduction associated with supercontinent formation.