SEPARATED GRAIN-SIZE FRACTIONS OF PRESOLAR DIAMONDS FROM EFREMOVKA - INSIGHT INTO THEIR ORIGIN FROM NITROGEN, CARBON AND NOBLE-GAS ISOTOPE DATA

The concentrations and isotopic compositions of carbon, nitrogen and n oble gases have been determined simultaneously in different size fract ions of nanometre-sized presolar diamonds isolated from the Efremovka CV3 chondrite. The fractions were prepared using ultracentrifugation; four different fractions were recovered, the finest and coarsest end m embers being ca. 2 and 4 nm in size, respectively. The noble gases dec rease in concentration with grain size, which strongly suggests that t hese species were implanted into the diamonds. To explain this effect we postulate that irradiating ions have sufficient energy to pass comp letely through the smaller diamond grains without being captured. Thus , it is likely that the crystals were in free space when they were irr adiated. However, it is certain that implantation of noble gases did n ot take place in a single event, since variations in noble gas concent rations vs, grain size do not follow the expected sequence, and there are other unconformities. In contrast to the noble gas patterns, nitro gen concentrations show only small variations with grain size, indicat ing a completely different mechanism of introduction into the diamonds . Rather than implantation, it is considered that most nitrogen was bo nded into the mineral lattice at the time and site of formation. Never theless, it is envisaged that some nitrogen, and even carbon, implanta tion takes place and if this occurred from a type II supernova source it could have an effect on isotopic composition because the tiny diamo nd contains so few atoms. We note in this respect that the diamond hav ing the highest noble gas abundances i.e. the most irradiated samples, are richest in the light isotopes of C and N. Taken together, the var iations in noble gas concentrations with grain size, the relative cons tancy of nitrogen content, and the variations in isotopic compositions of noble gases, nitrogen and carbon requires a complex irradiation hi story for the diamonds and/or several different diamond populations.