Constraints on stellar grain formation from presolar graphite in the murchison meteorite

Abstract

We report the results of isotopic, chemical, structural, and crystallographic microanalyses of graphitic spherules (0.3-9 μm) extracted from the Murchison meteorite. The spherules have ¹²C/¹³C ratios ranging over 3 orders of magnitude (from 0.02 to 80 times solar), clearly establishing their presolar origin as stellar condensates. These and other isotopic constraints point to a variety of stellar types as sources of the carbon, including low-mass asymptotic giant branch (AGB) stars and supernovae. Transmission electron microscopy (TEM) of ultrathin sections of the spherules revealed that many have a composite structure consisting of a core of nanocrystalline carbon surrounded by a mantle of well-graphitized carbon. The nanocrystalline cores are compact masses consisting of randomly oriented graphene sheets, from PAH-sized units up to sheets 3-4 nm in diameter, with little graphitic layering order. These sheets probably condensed as isolated particles that subsequently coalesced to form the cores, after which the surrounding graphitic mantles were added by vapor deposition. We also detected internal crystals of metal carbides in one-third of the spherules. These crystals (5-200 nm) have compositions ranging from nearly pure TiC to nearly pure Zr-Mo carbide. Some of these carbides occur at the centers of the spherules and are surrounded by well-graphitized carbon, having evidently served as heterogeneous nucleation centers for condensation of carbon. Others were entrained by carbon as the spherules grew. The chemical and textural evidence indicates that these carbides formed prior to carbon condensation, which indicates that the C/O ratios in the stellar sources were very close to unity. Only one of the 67 spherules studied in the TEM contained SiC, from which we infer that carbon condensation nearly always preceded SiC formation. This observation places stringent limits on the possible delay of graphite formation and is consistent with the predictions of equilibrium thermodynamics in the inferred range of pressure and C/O ratios. We model the formation of the observed refractory carbides under equilibrium conditions, both with and without s-process enrichment of Zr and Mo, and show that the chemical variation among internal crystals is consistent with the predicted equilibrium condensation sequence. The compositions of most of the Zr-Mo-Ti carbides require an s-process enrichment of both Zr and Mo to at least 30 times their solar abundances relative to Ti. However, to account for crystals in which Mo is also enriched relative to Zr, it is necessary to suppose that Zr is removed by separation of the earliest formed ZrC crystals from their parent gas. We also explore the formation constraints imposed by kinetics, equilibrium thermodynamics, and the observation of clusters of carbide crystals in some spherules, and conclude that relatively high formation pressures (≳ 0.1 dynes cm^-2), and/or condensable carbon number densities (≳10⁸ cm^-3) are required. The graphite spherules with ¹²C/¹³C ratios less than the solar value may have originated in AGB stellar winds. However, in the spherically symmetric AGB atmospheres customarily assumed in models of stellar grain formation, pressures are much too low (by factors of ≳10²) to produce carbide crystals or graphite spherules of the sizes observed within plausible timescales. If some of the graphite spherules formed in the winds from such stars, it thus appears necessary to assume that the regions of grain formation are density concentrations with length scales less than a stellar radius. Some of the spherules with both¹²C/¹³C ratios greater than the solar value and 28Si excesses probably grew in the ejecta of super-novae. The isotopic compositions and growth constraints imply that they must have formed at high densities (e.g., with p≳10^-12 g cm^-3) from mixtures of inner-shell material with material from the C-rich outer zones.