Populations of nanodiamond grains in meteorites from the data on isotopic composition and content of nitrogen

The present study has shown that the dependence of the isotopic composition of nitrogen on the N/C ratio, revealed from the data for bulk samples of meteoritic nanodiamond, can be obtained within the framework of the following model of the composition of populations of nanodiamond grains: (a) initial nanodiamond, i.e., the nanodiamond in the protoplanetary cloud before the accretion of the meteorite parent bodies, was composed mainly of grains of two populations (denoted as C N and C F ), the ratio of which changed in meteorites depending on the degree of hydrothermal metamorphism; (b) only the grains of one of these populations (C N ) contain volume-bound nitrogen with δ 15 N = −350‰; (c) the grains of both populations contain surface-bound nitrogen (δ 15 N ≡ 0). The calculations revealed the following properties of population grains in this model. (1) The grains of the C N and C F populations are most likely the same in isotopic composition of carbon and heterogeneous in distribution of its isotopes: the central part of grains is enriched with the δ 12 C isotope relative to the remainder of the grain. While the value of δ 13 C is −37.3 ± 1.1‰ for carbon in the central part, it is −32.8 ± 1.5‰ for the whole volume of the grains. (2) The noble gases of the HL component, specifically Xe-HL, are anomalous in isotopic composition and are most likely contained in the third population of nanodiamond grains (denoted as C HL ), the mass fraction of which is negligible relative to that for other grain populations. Only the grains of the C HL population have an undoubtedly presolar origin, while the grains of the other nanodiamond populations could have formed at the early stages of the evolution of the protoplanetary cloud material before the accretion of the meteoritic parent bodies.