A study of carbon and nitrogen isotopes from the Earth's mantle

A technique for the routine determination of the nitrogen contents and the δ15N and δ13C values of diamonds has been developed. δ13C and δ15N measurements have been performed on coated, cubic and octahedral diamonds from Australia, Africa, Siberia and N. America. Amongst the octahedral diamonds two broad groups can be distinguished. Group O1 diamonds have δ13C values between -3 and -7v (similar to most other mantle samples) and contain nitrogen generally depleted in 15 relative to AIR; δ15N ca.-5‰ (range +5 to -13‰). Group O2 diamonds have variable δ13C values (-21 to -2‰) and contain N generally enriched in 15 relative to AIR; ca. +5‰ (range -5 to +16‰). The cubic coatings on diamonds from Zaire, Sierra Leone, Siberia, Angola and Botswana and cubic diamonds from Zaire have a restricted range in isotopic composition (δ13C -5 to -1.5‰, δ15N -2 to -8‰). Both group O1 and O2 diamonds were present as cores within these samples. The coats of coated diamonds have been interpreted as being phenocrystic overgrowths. The isotopic uniformity of diamond coats, on a regional scale, suggests the existence of a homogeneous C and N reservoir underlying the continental lithosphere. This reservoir supplies the volatiles associated with kimberlite eruptions. Coated diamonds occur in Phanerozoic kimberlites which implies that, by this time, the extreme carbon isotope heterogeneity within the mantle (+3 to -35‰) was restricted to the continental lithosphere. The isotopic characteristics of the coats are similar to the O1 diamonds. Since diamonds from Finsch and Premier (both O1) have been dated at 3.3 Ga and 1.2 Ga respectively there appears to have been little change in the δ13C and δ15N value of the uniform mantle reservoir since the mid-Archean. The origin of the isotopic characteristics of the group O2 diamonds is uncertain, however it is unlikely that they are reflecting relict primordial heterogeneity