ISOTOPE FRACTIONATION RELATED TO KIMBERLITE MAGMATISM AND DIAMOND FORMATION

This paper deals with a model of carbon isotope fractionation presumed to accompany the movement of mantle fluids. In the first part of the article, the experimental data and the relationships revealed are generalized and discussed; the remainder of the paper describes the model. The isotope compositions of different forms of carbon related to kimberlite magmatism vary widely. In diamonds, delta-C-13 values range from -34.5 to +2.8 parts per thousand. Carbonate-bearing autholiths in kimberlites occur enriched in C-13 up to +35 parts per thousand. Organic matter, including that occurring in fluid inclusions of magmatic minerals of kimberlites, is depleted in C-13 down to -30 parts per thousand. It is concluded that the delta-C-13-distribution for diamonds is specific for a particular occurrence (different kimberlite pipes, different regions, etc.). Principal differences in isotopic distribution patterns for diamonds of ultrabasic and basic paragenesis exist. Isotopically light diamonds are related only to the latter. When a diamond consists of several sequential generations of growth (coated diamonds, diamond-in-diamond), delta-C-13 values of the early generation vary widely, whereas the isotopic composition of the latter is more uniform. The intention of the model is to explain the observed variations of carbon isotope composition of diamond and other carbonaceous substances related to kimberlite magmatism. The model is based on the interaction of reduced sub-asthenospheric fluid with the relatively oxidized lithosphere. It is suggested that diamonds of ultrabasic paragenesis (lherzolite and harzburgite mineral assemblages) are produced during interaction of the fluid with sheared garnet lherzolite which is considered to be primitive mantle rock. During contact with the more oxidized mantle, reduced carbon (CH4) may partially be converted to CO2. Isotope exchange in CO2-CH4 system, combined with Rayleigh distillation, may provide a significant isotope fractionation. Diamonds of the basic (eclogitic) paragenesis are considered to be related to this fractionated carbon. Also, occurrence of carbonate material highly enriched in C-13 is explained by the model.