A carbon and nitrogen isotope study of diamond from primitive chondrites

Diamonds isolated from primitive chondrites of the carbonaceous, ordinary and enstatite groups have been analysed by high-resolution stepped combustion, followed by measurement of their C and N isotopes using a newly adapted technique that allows quantitative measurements of C/N ratios. The delta(13)C of the diamond is shown to vary between meteorite groups from -32 to -38 parts per thousand, and the measured C/N ratios suggest that the N concentration of diamond ranges over a factor of 7 from 1800 ppm (Tieschitz) to 13,000 ppm (Adrar 003). The delta(15)N of N released from diamond is constrained to -348 +/- 7 parts per thousand. The complexity of the C release pattern and C/N ratio during combustion implies the presence of more than one component, which suggests that either more than one type of diamond is present in the samples, or unidentified additional phases are located in the acid-resistant residue. The components are present in varying proportions between meteorite groups. The data are compatible with a model of a mix of different diamond populations (some probably presolar and some possibly solar) existing in the early solar nebula, where each population originally contributed a roughly equal amount to chondrites of every class. Subsequent metamorphism has resulted in overall variations in delta(13)C and C/N ratios in diamond isolated from meteorites of differing petrologic grade without significantly altering the N isotopic composition. Possible ways for this to be achieved are explored.