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

Abstract

CARBONADOS are porous aggregates of micrometre-size diamond crystals1,2. 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 diamonds1,2,4. 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 phases8–10. 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.