Now showing items 21-26 of 26

    • The White Angel: A unique wollastonite-bearing, mass-fractionated refractory inclusion from the Leoville CV3 carbonaceous chondrite

      Caillet Komorowski, C. L. V.; Zinner, E. K.; McKeegan, K. D.; Hervig, R.; Buseck, P. R. (The Meteoritical Society, 2007-01-01)
      We report the study of an unusual compact type A refractory inclusion, named the White Angel, from the Leoville CV3 meteorite. The petrologic, mineral chemical, isotopic, and traceelement signatures of this once-molten Ca-Al-rich inclusion (CAI), which contains large, equant wollastonite crystals, indicate a short multistage history that occurred very early, before substantial decay of 26Al. Magnesium in the inclusion is isotopically heavy, with FMg reaching 18/amu, in the range of fractionated and with unidentified nuclear effects (FUN) inclusions. However, the absence of any nuclear anomalies in Ca and Ti and an inferred 26Al/27Al ratio of (5.5 +/- 0.9) 10^(5) indicate that the White Angel belongs to the F inclusions. Silicon and oxygen are also mass fractionated in favor of the heavy isotopes, but to a lesser extent. The O isotopes show a range in 16O excesses. On an O three-isotope plot, data points lie on a line parallel and to the right of the carbonaceous chondrite anhydrous mineral mixing line, with wollastonite being the most 16O-rich phase. The chondritenormalized rare earth and trace-element pattern of the whole inclusion is the complement of an ultrarefractory pattern indicating that precursor phases of the CAI must have condensed in an Al-, heavy rare earth element (HREE)-depleted reservoir. Melting of those precursor phases in an 16O-rich environment and evaporation led to mass-dependent isotopic fractionation of Mg, Si, and O. Partial isotopic exchange with a reservoir containing unfractionated Mg took place at a later stage but before any measurable decay of 26Al. Some minerals (melilite and perovskite) in the White Angel equilibrated oxygen isotopes with a relatively 16O-poor reservoir that was also mass-fractionated toward the heavy isotopes, different from that with which the normal or FUN inclusions interacted.
    • The origin of presolar nova grains

      José, J.; Hernanz, M. (The Meteoritical Society, 2007-01-01)
      Infrared observations reveal that classical novae often form dust in their expanding shells ejected into the interstellar medium as a consequence of violent outbursts. Recent experimental efforts have led to the identification of presolar nova candidate grains from the Acfer 094 and Murchison meteorites. Recently, however, concerns have been raised about the stellar paternity of these grains by new measurements on another sample of SiC grains: these grains are characterized by 12C/13C and 14N/15N ratios similar to the ones reported for the nova grains, but a number of different imprints suggest that a possible supernova origin cannot be excluded. Here we review the predicted nucleosynthetic imprints accompanying nova explosions and discuss the chances to synthesize heavier species, such as titanium, in nova-like events.
    • 3-D elemental and isotopic composition of presolar silicon carbides

      Henkel, T.; Stephan, T.; Jessberger, E. K.; Hoppe, P.; Strebel, R.; Amari, S.; Lewis, R. S. (The Meteoritical Society, 2007-01-01)
      Thirteen presolar silicon carbide grains—three of supernova (SN) origin and ten of asymptotic giant branch (AGB) star origin—were examined with time-of-flightsecondary ion mass spectrometry (TOF-SIMS). The grains had been extracted from two different meteorites—Murchison and Tieschitz—using different acid residue methods. At high lateral resolution of ~300 nm, isotopic and elemental heterogeneities within the micrometer-sized grains were detected. The trace elemental abundances, when displayed in two-element correlation plots, of Li, Mg, K, and Ca show a clear distinction between the two different meteoritic sources. The different concentrations might be attributed to differences of the host meteorites and/or of extraction methods whereas the stellar source seems to be less decisive. In one SN grain with 26Mg-enrichment from extinct 26Al, the acid treatment, as part of the grain separation procedure, affected the Mg/Al ratio in the outer rim and therefore the inferred initial 26Al/27Al ratio. A second SN grain exhibits a lateral heterogeneity in 26Al/27Al, which either is due to residual Al-rich contamination on the grain surface or to the condensation chemistry in the SN ejecta.
    • Constraining the 13C neutron source in AGB stars through isotopic analysis of trace elements in presolar SiC

      Barzyk, J. G.; Savina, M. R.; Davis, A. M.; Gallino, R.; Gyngard, F.; Amari, S.; Zinner, E.; Pellin, M. J.; Lewis, R. S.; Clayton, R. N. (The Meteoritical Society, 2007-01-01)
      Analyses of the isotopic compositions of multiple elements (Mo, Zr, and Ba) in individual mainstream presolar SiC grains were done by resonant ionization mass spectrometry (RIMS). While most heavy element compositions were consistent with model predictions forthe slow neutron capture process (s-process) in low-mass (1.5-3 M(sun)) asymptotic giant branch stars of solar metallicity when viewed on single-element three-isotope plots, grains with compositions deviating from model predictions were identified on multi-element plots. These grains have compositions that cannot result from any neutron capture process but can be explained by contamination in some elements with solar system material. Previous work in which only one heavy element per grain was examined has been unable to identify contaminated grains. The multi-element analyses of this study detected contaminated grains which were subsequently eliminated from consideration. The uncontaminated grains form a data set with a greatly reduced spread on the three-isotope plots of each element measured, corresponding to a smaller range of 13C pocket efficiencies in parent AGB stars. Furthermore, due to this reduced spread, the nature of the stellar starting material, previously interpreted as having solar isotopic composition, is uncertain. The constraint on 13C pocket efficiencies in parent stars of these grains may help uncover the mechanism responsible for formation of 13C, the primary neutron source for s-process nucleosynthesis in low-mass stars.
    • Ernst Zinner, lithic astronomer

      McKeegan, K. D. (The Meteoritical Society, 2007-01-01)
    • Editorial

      Floss, C.; Nittler, L. (The Meteoritical Society, 2007-01-01)