Constraints from noble-gas contents on the origin of carbonado diamonds

Abstract

CARBONADOS are porous aggregates of micrometre-size diamond crystals^1,2. Although they are not rare, they have not been found in kimberlites³, and they contain inclusion minerals typical of the Earth's crust, rather than the upper-mantle assemblages commonly found in kimberlite diamonds. Carbonados also have lower ¹³C/¹²C ratios than kimberlite diamonds^1,2,4. Although these observations strongly suggest that carbonados were formed in the crust, crustal conditions are generally unlikely to provide pressures and temperatures in the diamond stability field (but see ref. 5 for a possible exception). Other suggestions for the origin of carbonado include the transformation of subducted carbon in the mantle⁶, impact metamorphism of crustal rocks containing organic carbon⁷, and (for very fine-grained carbonado) the irradiation of organic matter by decaying uranium and thorium in uranium-rich phases^8–10. Here we present further evidence for a crustal connection, in the form of noble-gas data for four carbonados from Brazil and Africa. We find that all four contain large amounts of implanted xenon and krypton from ²³⁸U fission, nucleogenic neon and ⁴He, and tightly trapped atmospheric noble gases, from which we conclude that these carbonados formed in a uranium-rich crustal environment.