C, N, AND NOBLE-GAS ISOTOPES IN GRAIN-SIZE SEPARATES OF PRESOLAR DIAMONDS FROM EFREMOVKA

Nanometer-size presolar diamonds from the Efremovka CV3 chondrite were physically separated into several grain size fractions by uttracentri fugation. The coarsest size fraction is the most enriched in carbon-12 ; the others have broadly similar carbon isotopic compositions. Measur ement of noble gases shows that their concentration decreases with dec reasing grain size. This effect is attributed to ion implantation, Suc h an episode could occur in the envelope of a supernova that produced the diamonds, or in interstellar space; in either case, ions with ener gies above a certain threshold pass completely through the smaller dia mond grains without being captured. Concentrations of nitrogen show on ly minor variations with grain size, indicating a different mechanism of incorporation into the diamonds.