Recondite interstellar carbon components in the Allende meteorite revealed by preparative precombustion

Abstract

Prolonged combustion (preparative precombustion) at low temperatures (390–450°C and 450–510°C) removes large quantities of isotopically normal carbon from an acid (HF/HCl) residue of the Allende meteorite to reveal two isotopically heavy carbon components. One of them, called C ℵ (carbon aleph) because of its possible relationship to C α the host of Ne-E(L), has δ 13 C of+345ℵ. and a combustion temperature of 700–750°C. It is believed that the presence of C ℵ is masked in the bulk acid residue because of a “tailing effect” in the combustion of less stable forms of carbon. The second heavy carbon component burns over the temperature regime 900–1000°C and has a δ 13 C of >527ℵ. This component called C κ (carbon kappa) is distinguishable from the well known types of heavy carbon called C β and C ϵ the hosts of s-Xe and NeE(H), respectively, and now recognised as forms of SiC. C κ , however, could also be a form of very poorly crystalline SiC; it appears to be released from the Allende acid residue because the precombustion slowly degrades a secondary host mineral, which is possibly a variety of spinel. Likewise, a component of light carbon, possibly C θ , poorly crystalline graphite, is also liberated by the precombustion reaction after between 52 and 108 h of treatment. Presumably C θ too is located within a specific type of spinel. The discovery of C %, C κ , and C θ , together with C λ , a component very rich in 12C (ASH et al. 1988a), confirms that primitive meteorites are a prolific source of interstellar grains. Preparative precombustion demonstrates that the legacy to be found in Allende is far more complicated than at first envisaged from the noble gas record of this meteorite.