THE APPLICATION OF C ISOTOPE MEASUREMENTS TO THE IDENTIFICATION OF THE SOURCES OF C IN DIAMONDS - A REVIEW

In the past decade, the isotopic compositions of C in > 600 inclusion-bearing diamonds have been determined. Such analyses have revealed the following isotopic characteristics: (1) peridotitic diamonds, which typically contain garnet, chromite, olivine and/of orthopyroxene inclusions with refractory compositions (high Mg, Cr), have delta-C-13 values predominantly between -10 and -1 parts-per-thousand, with a sharp peak in the distribution near -5 parts-per-thousand; (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 delta-C-13 values between -34 and +3 parts-per-thousand, with a smaller peak near -5 parts-per-thousand; (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 C-13 relative to the rims, and within single octahedral diamonds delta-C-13 variations of nearly 6 parts-per-thousand have been reported. Because expected C isotope fractionations at mantle temperatures are small, attempts to model the full range of diamond isotope values through fractionating a homogeneous mantle C source have been unsuccessful. Nevertheless, fractionation is probably responsible for some of the observed variation in delta-C-13 values. Two other models have also been proposed to account for the diamond characteristics outlined above. The "primordial model" suggests that the range and distribution of C isotope compositions are inherited from primordial C in the mantle which has an inhomogencous isotopic composition, such as that found in meteorites. The "subduction model" suggests that subducted, crustal C is the source of C in diamonds, as organic and inorganic C compounds in the crust exhibit a range of delta-C-13 values similar to that observed in diamonds. This paper reviews the C isotope characteristics of diamonds and compares the models which have been proposed to explain the origins of these characteristics.