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Verchovsky, A. B.; Fisenko, A. V.; Semjonova, L. F. and Wright, I. P.
(2009).
URL: http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1908...
Abstract
Introduction: In this paper we present new results of our long-term project on investigations of meteoritic nanodiamonds using grain-size analysis. The main goal of the project is to identify and characterize nanodiamond populations with different origin. The idea that meteoritic nanodiamonds can be represented by more than one population has been discussed for long time, almost since their discovery. The pointers to multiple populations include variations in carbon isotopic composition during stepped combustion of the nanodiamonds and variations between samples from meteorites with different metamorphic histories. The latter suggests that the different nanodiamond populations have varying level of resistance to the parent body metamorphic alterations (grades and environment where it takes place). In other words, notwithstanding what could have been an initial homogeneity in the diamond ensemble, a certain separation between populations occurred in different parent bodies (or at different places within individual parent bodies) during metamorphism under natural conditions. Laboratory grains-size separation of the nanodiamonds by means of centrifugation show that carbon isotopic composition varies systematically with size of the diamond grains. The total range of carbon isotope variations significantly increased and the effect of metamorphism considerably enhanced in the size fractions compared to unseparated samples so that it became possible to create a model from which carbon isotopic compositions of the pure populations can be found. In an early stage of the investigations we identified only two populations for the bulk grain-size fractions of Efremovka, Krymka and Boriskino, though the stepped combustion data suggested that a third populations may also be present. Recently we developed centrifugation procedures that have allowed us to produce the coarsest fractions. Applying these procedures to nanodiamonds from Orgueil we were able to identify a third high-temperature population with relatively heavy carbon isotopic composition (concentrated mostly in the coarsest fractions) which is associated with the P6 noble gas component. The other two populations we tentatively identified with the other known noble gas components: HL and P3. The new data we are going to discuss here are obtained for nanodiamonds from Kainsaz. In contrast to Orgueil this meteorite experienced a much stronger metamorphism under oxidising conditions, resulting in an extremely low SiC content relative to that for nanodiamonds. Since SiC (with a relatively heavy carbon isotopic composition) always represents a possible source of contamination of nanodiamonds, this sample gives us an opportunity to constrain the nature of the heavy carbon already identified in Orgueil (i.e. and rule out the presence of SiC as a factor).
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