Separated grain size fractions of presolar diamonds from Efremovka Insight into their origin from nitrogen, carbon and noble gas isotope data

Abstract

The concentrations and isotopic compositions of carbon, nitrogen and noble gases have been determined simultaneously in different size fractions of nanometre-sized presolar diamonds isolated from the Efremovka CV3 chondrite. The fractions were prepared using ultracentrifugation; four differentfractions were recovered, the finest and coarsest end members being ca. 2and 4 nm in size, respectively. The noble gases decrease in concentration with grain size, which strongly suggests that these species were implanted into the diamonds. To explain this effect we postulate that irradiating ions have sufficient energy to pass completely through the smaller diamond grains without being captured. Thus, it is likely that the crystals were in free space when they were irradiated. However, it is certain that implantation of noble gases did not take place in a single event, since variations in noble gas concentrations vs. grain size do not follow the expected sequence, and there are other unconformities. In contrast to the noble gas patterns, nitrogen concentrations show only small variations with grain size, indicating a completely different mechanism of introduction into the diamonds. Rather than implantation, it is considered that most nitrogen was bonded into the mineral lattice at the time and site of formation. Nevertheless, it is envisaged that some nitrogen, and even carbon, implantation takes place and if this occurred from a type II supernova source it could have an effect on isotopic composition because the tiny diamond contains so few atoms. We note in this respect that the diamond having the highest noble gas abundances i.e. the most irradiated samples, are richest in the light isotopes of C and N. Taken together, the variations in noble gas concentrations with grain size, the relative constancy of nitrogen content, and the variations in isotopic compositions of noble gases, nitrogen and carbon requires a complex irradiation history for the diamonds and/or several different diamond populations.

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