A PRELIMINARY-STUDY OF N-15 N-14 IN OCTAHEDRAL GROWTH FORM DIAMONDS

The deltaC-13, deltaN-15 and nitrogen concentrations have been determi ned in 43 octahedral diamonds from southern Africa, Australia and Nort h America. The sample set included locations such as Finsch (South Afr ica), a mine which produces diamonds having deltaC-13-values close to -5%0, and Argyle (Australia), a mine which produces C-13-depleted diam onds. The nitrogen within ''high-deltaC-13'' diamonds (deltaC-13 = -6. 4 to - 2.9%0) was generally depleted in N-15 relative to atmospheric n itrogen (deltaN-15 = - 12.3 to + 5.9%0: mode approximately - 4%0), sim ilar to the results which have been obtained previously from fibrous d iamond. It is believed that the volatiles from which these diamonds gr ew are primitive, being derived from a source located beneath the cont inental lithosphere, which may have changed little (in terms of deltaC -13 and deltaN-15) since the mid-Archean. The ''low-deltaC-13'' diamon ds (deltaC-13 = -19.4 to -9.5%0) contained nitrogen generally enriched in N-15 relative to air (deltaN-15 = - 3.0 to + 16.4%0: mode approxim ately + 5%0). It is suggested that the major carbon isotope heterogene ity within the mantle, as represented by diamonds such as these, is lo cated in deep, mechanically unstable regions of the continental lithos phere. The isotopic compositions of both C and N are consistent with t his heterogeneity resulting from the subduction of crustal material; h owever, isotope fractionation related directly to diamond growth canno t be ruled out.