Heterogeneity of grain-size fractions of nanodiamonds from the Orgueil C1 meteorite

详细

A model for the composition of meteoritic nanodiamonds is suggested based on analysis of the concentrations and isotopic compositions of C, N, and Xe in the nanodiamond-rich grain-size fractions, which were separated for the first time from the Orgueil CI chondrite. According to the model, meteoritic nanodiamond consists of two populations of grains (denoted C_HL and C_N). The size distributions of grains in populations in the C_HL and C_N populations are different: the C_HL population is finer grained than C_N. The grains of the C_HL population are characterized by a radial gradient in the carbon isotopic composition, and they contain implanted anomalous noble gases (HL component) and the heavy nitrogen isotope ¹⁵N. Following (Clayton et al., 1995), the probable astrophysical source of this population of nanodiamond grains is thought to be the mixing helium and hydrogen shells of a Type-II supernova, and the mechanism that produced these grains was the slow CVD process. The C_N population grains have homogeneous isotopic compositions of carbon (δ¹³C ≡–100‰) and nitrogen (δ¹⁵N ≡–400‰) and contain almost all nitrogen of the nanodiamond-rich fractions. This population of nanodiamond grains was likely formed by a fast unequilibrated process, when shock waves affected organic compounds or gas rich in C- and N-bearing compounds during the early evolution of the protosolar nebula. Calculations within the framework of the model show that the nanodiamond-rich fractions separated from the Orgueil meteorite have the C_N/C_HL ratios varying from 1 in the finest grained fraction to 10 in the coarse-grained one. At these proportions of the populations, weighted mean δ¹³C values of C_HL grains in the fractions lie within the range of 42 to 394‰, and the concentrations of ¹³²Xe-HL and ¹⁵N are (49–563) × 10^–8 cm³/gC and (1.1–6.2) × 10^–5 cm³/gC, respectively.