CARBON AND NITROGEN ISOTOPES IN TYPE-II SUPERNOVA DIAMONDS

We construct a model for the origin of the abundant nanometer diamonds found in meteorites. We interpret them as interstellar particles that were grown during the expansions of supernova interiors. The physical setting and the chemical-vapor-deposition process that we describe present clear reasons both for the small sizes (nm) of the diamonds and for their mean isotopic composition being not greatly different from solar. To delineate the isotopic requirements for collections of diamonds (which are too small for analysis individually) we present measurements of carbon and nitrogen isotopes obtained by stepped combustion of diamond collections. Our model for the growth of supernova diamonds is motivated by a series of postulates, unremarkable as single postulates, that together produce a successful calculation. The computed growth occurs in the continuously mixing envelopes of expanding Type II supernova remnants. It provides a good characterization of these facts: (1) the C isotopic composition is not far from solar; (2) both carbon and nitrogen become isotopically lighter as the diamonds are combusted; (3) the C/N ratio changes during combustion; (4) the diamonds are individually tiny; (5) collections of diamonds are carriers of Xe-HL. We show that the isotopic gradient during combustion may be interpreted in this model as either an isotopic gradient within each diamond or as a correlation between isotopic composition and size of individual diamonds contained in the bulk collections.