Evidence for the involvement of a subduction component in diamond formation is analyzed based on literature data and our studies. Examination of xenoliths of diamondiferous eclogites, including X-ray tomography analysis, testifies to the superposed character of most diamonds. Diamond generation is accompanied by the serious modification of eclogite substratum. Isotope-geochemical data show that the eclogites originated from oceanic-crust rocks. The oxygen isotope compositions of garnets and clinopyroxenes from websterite xenoliths are similar to the mantle average (5.3-5.6‰). The eclogite minerals vary considerably in oxygen isotope composition (δ18O of 5.3 to 12.4‰). Diamonds of eclogitic paragenesis predominate dramatically in the placers of the northeastern Yakutian diamondiferous province. In placer eclogitic diamonds, δ13C varies from -27.2 to -3‰ (= 28). In diamonds of ultrabasic paragenesis, the range of δ13C values is much narrower (from -7.1 to -0.5‰). All diamonds of variety V have a lighter carbon isotope composition (from -24.1 to -17.4‰). In a wide range of crystals with a contrasting carbon isotope composition, the isotope composition of the rim tends toward the average mantle value. This suggests that the eclogitic diamonds grew first with the participation of carbon from subducted continental crust and finally with the involvement of mantle carbon.
- Upper mantle