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

The deltaC-13, deltaN-15 and nitrogen concentrations have been determined in 43 octahedral diamonds from southern Africa, Australia and North America. The sample set included locations such as Finsch (South Africa), a mine which produces diamonds having deltaC-13-values close to -5%0, and Argyle (Australia), a mine which produces C-13-depleted diamonds. The nitrogen within ''high-deltaC-13'' diamonds (deltaC-13 = -6.4 to - 2.9%0) was generally depleted in N-15 relative to atmospheric nitrogen (deltaN-15 = - 12.3 to + 5.9%0: mode approximately - 4%0), similar to the results which have been obtained previously from fibrous diamond. It is believed that the volatiles from which these diamonds grew are primitive, being derived from a source located beneath the continental lithosphere, which may have changed little (in terms of deltaC-13 and deltaN-15) since the mid-Archean. The ''low-deltaC-13'' diamonds (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 approximately + 5%0). It is suggested that the major carbon isotope heterogeneity within the mantle, as represented by diamonds such as these, is located in deep, mechanically unstable regions of the continental lithosphere. The isotopic compositions of both C and N are consistent with this heterogeneity resulting from the subduction of crustal material; however, isotope fractionation related directly to diamond growth cannot be ruled out.