The application of C isotope measurements to the identification of the sources of C in diamonds: a review

In the past decade, isotopic analyses of C in > 600 inclusion-bearing diamonds have revealed the following isotopic characteristics: (1) peridotitic diamonds, which typically contain garnet, chromite, olivine and/or orthopyroxene inclusions with refractory compositions (high Mg, Cr), have δ13C values predominantly between -10 and -1per mille, with a sharp peak in the distribution near -5per mille; (2) eclogitic diamonds, which commonly contain inclusions of omphacitic clinopyroxene, Cr-poor pyrope, and/or eclogitic accessory minerals such as rutile, kyanite, coesite or sanidine, have δ13C values between -34 and +3per mille, with a smaller peak near -5per mille; (3) the isotopic compositions obtained for suites of diamonds from individual occurrences are, in general, unique and do not resemble the range and distribution obtained by amalgamating the diamond isotope data from a number of localities; (4) isotopic zoning patterns and heterogeneities are found in some diamonds; cores of coated diamonds tend to be depleted in 13C relative to the rims, and within single octahedral diamonds δ13C variations of nearly 6per mille have been reported. This paper reviews the C isotope characteristics of diamonds and compares the models which have been proposed to explain the origins of these characteristics. (author)