Re-Os isotope measurements of single sulfide inclusions in a Siberian diamond and its nitrogen aggregation systematics

We have measured the Re-Os isotopic compositions of individual syngenetic s ulfide inclusions from three different growth zones within a central cross section plate cut from a single Siberian diamond. Individual sulfides in th eir diamond host were isolated by laser cutting. The sulfides, and hence th e different growth zones of the diamond have been suggested to differ in ag e by up to 2 Ga on the basis of their Pb isotope compositions. Re-Os model ages of the four inclusions range from 3.1 +/- 0.3 to 3.5 +/- 0.3 Ga and su ggest a Middle Archaean age for the diamond. A sulfide inclusion in the rim of the diamond is very different in elemental composition from those of th e core and intermediate zones. It is enriched in Os, Re, Pb, and Zn and has more radiogenic Os and Pb isotopes. The inclusion is connected to the surf ace of the diamond by a healed crack, revealed by cathodoluminescence. The compositional distinction may be caused either by postformational interacti on between an ancient sulfide and a fluid, possibly at the time of kimberli te eruption, or later stage growth of new diamond plus sulfide. Such chemic al complexities, and the presence of healed fractures within the host diamo nd, emphasize the desirability of analyzing individual inclusions from well -characterized diamonds if isotope data for inclusions are to be better und erstood. Nitrogen contents and aggregation state in the core and intermediate zone o f the host diamond closely approximate theoretically calculated isotherms b ased on consideration of experimentally determined nitrogen aggregation kin etics. The nitrogen content of the rim diamond is too low to obtain spectra that allow accurate deconvolution of relative aggregation levels for use i n residence time calculations. The aggregation state of nitrogen in the cor e and intermediate growth zones is compatible with a long, ca. 3 Ga mantle residence time at normal lithospheric temperatures. The similarity of the s ulfide inclusion Re-Os model ages to the oldest Re-Os ages from Siberian pe ridotite xenoliths confirms an ancient age for the Siberian lithospheric ma ntle and indicates that some diamonds formed closely after lithosphere stab ilization. Copyright (C) 1999 Elsevier Science Ltd.