Iron and Nickel Isotopic Ratios in Presolar SiC Grains

We report the first Fe isotopic anomalies and the first Ni isotopic ratio measurements in presolar SiC grains of separate KJG from the Murchison meteorite. With NanoSIMS, we analyzed Fe and Ni in 37 X grains from Type II supernovae and 53 SiC grains of other types. The Ni/Fe and Co/Fe ratios in grains of all types are much higher than in the gas from which the grains are believed to have condensed. A majority of the X grains and a couple of mainstream grains contain Fe-rich subgrains. Most X grains have large excesses in 57Fe,61Ni, and 62Ni.60Ni excesses are small and the 54Fe/56Fe ratios of almost all X grains are normal. These isotopic compositions are best explained by mixing of material from the He/N zone of Type II supernovae with material from the He/C zone. The lack of any 54Fe excesses is puzzling in view of the fact that the Si/S zone, whose contribution resulted in the 28Si excesses in X grains, is very rich in 54Fe. It has yet to be seen whether elemental fractionation between Si and Fe is an explanation. The 57Fe deficits observed in a few X grains remain unexplained. In comparison to the X grains, fewer mainstream and AB grains have anomalies. Observed 62Ni excesses in some mainstream grains are larger than predicted for AGB stars of solar metallicity and are not accompanied by corresponding 61Ni excesses. A Y grain and a Z grain have excesses in 54Fe and 62Ni, but close to normal 57Fe/56Fe and 60,61Ni/58Ni ratios. These isotopic compositions are not expected for grains from low-metallicity AGB stars

New stellar sources for high-density, presolar graphite grains

WepresentC,N,O,Si,Al-Mg,K,Ca,andTiisotopicanalyses ofsevenhigh-density(ORG1f, � � 2:02 2:04 gcm � 3 ) graphite grains from Orgueil with 12 C/ 13 C ratios smaller than 20. The presence of 44Ti in three of these grains indicates an origin in Type II supernovae (SNe). The 13 C excesses in these SNe grains, however, remain enigmatic. The remaining grains have extremely large Ca and Ti isotopic anomalies, some of which are much larger than those predicted for envelopes of asymptotic giant branch (AGB) stars. These anomalies in conjunction with low 12 C/ 13 C ratios can only be explained by pure nucleosynthetic He-shell components of AGB stars. Born-again AGB stars that experience a late He flash are able to explain the low 12 C/ 13 C ratios of some of the grains along with the presence of extreme enrichments in the Ca and Ti isotopes. This study indicates that high-density graphite grains havemultiple stellar sources: SNe and born-again AGB stars, in addition to the previously established low-metallicity AGB stars. Subject headingg dust, extinction — meteors, meteoroids — nuclear reactions, nucleosynthesis, abundances — stars: abundances — stars: AGB and post-AGB — supernovae: genera

Evidence of Metasomatism in the Lowest Petrographic Types Inferred from A Na(-), K, Rich Rim Around A LEW 86018 (L3.1) Chondrule

Ordinary chondrites (OCs) represent the most abundant extraterrestrial materials and also record the widest range of alteration of primary, pristine minerals of early Solar system material available for study. Relatively few investigations, however, address: (1) the role of fluid alteration, and (2) the relationship between thermal metamorphism and metasomatism in OCs, issues that have been extensively studied in many other meteorite groups e.g., CV, CO, CR, and enstatite chondrites. Detailed elemental abundances profiles across individual chondrules, and mineralogical studies of Lewis Hills (LEW) 86018 (L3.1), an unequilibrated ordinary chondrite (UOC) of low petrographic type of 3.1 returned from Antarctica, provide evidence of extensive alteration of primary minerals. Some chondrules have Na(-), K(-), rich rims surrounded by nepheline, albite, and sodalite-like Na(-), Cl(-), Al-rich secondary minerals in the near vicinity within the matrices. Although, limited evidences of low temperature (approximately 250 C) fluid-assisted alteration of primary minerals to phyllosilicates, ferroanolivine, magnetite, and scapolite have been reported in the lowest grades (less than 3.2) Semarkona (LL3.00) and Bishunpur (LL3.10), alkali-rich secondary mineralization has previously only been seen in higher grade greater than 3.4 UOCs. This preliminary result suggests highly localized metamorphism in UOCs and widens the range of alteration in UOCs and complicates classification of petrographic type and extent of thermal metamorphism or metasomatism. The work in progress will document the micro-textures, geochemistry (Ba, Ca, REE), and isotopic composition (oxygen, Al(-)- 26 Mg-26) of mineral phases in chondrules and adjoining objects to help us understand the formation scenario and delineate possible modes of metamorphism in UOCs

SIMS Studies of Allende Projectiles Fired into Stardust-type Aluminum Foils at 6 km/s

We have explored the feasibility of C-, N-, and O-isotopic measurements by NanoSIMS and of elemental abundance determinations by TOF-SIMS on residues of Allende projectiles that impacted Stardust-type aluminum foils in the laboratory at 6 km/s. These investigations are part of a consortium study aimed at providing the foundation for the characterization of matter associated with micro-craters that were produced during the encounter of the Stardust space probe with comet 81P/Wild 2. Eleven experimental impact craters were studied by NanoSIMS and eighteen by TOF-SIMS. Crater sizes were between 3 and 190 microns. The NanoSIMS measurements have shown that the crater morphology has only a minor effect on spatial resolution and on instrumental mass fractionation. The achievable spatial resolution is always better than 200 nm, and C- and O-isotopic ratios can be measured with a precision of several percent at a scale of several 100 nm, the typical size of presolar grains. This clearly demonstrates that presolar matter, provided it survives the impact into the aluminum foil partly intact, is recognizable even if embedded in material of Solar System origin. TOF-SIMS studies are restricted to materials from the crater rim. The element ratios of the major rockforming elements in the Allende projectiles are well characterized by the TOF-SIMS measurements, indicating that fractionation of those elements during impact can be expected to be negligible. This permits information on the type of impactor material to be obtained. For any more detailed assignments to specific chondrite groups, however, information on the abundances of the light elements, especially C, is crucial

Stardust in STARDUST - the C, N, and O Isotopic Compositions of Wild 2 Cometary Matter in Al foil Impacts

In January 2006, the STARDUST mission successfully returned dust samples from the tail of comet 81P/Wild 2 in two principal collection media, low density silica aerogel and Al foil. While hypervelocity impacts at 6.1 km/s, the encounter velocity of STARDUST, into Al foils are generally highly disruptive for natural, silicate-dominated impactors, previous studies have shown that many craters retain sufficient residue to allow a determination of the elemental and isotopic compositions of the original projectile. We have used the NanoSIMS to perform C, N, and O isotope imaging measurements on four large (59-370 microns diameter) and on 47 small (0.32-1.9 microns diameter) Al foil impact craters as part of the STARDUST Preliminary Examination. Most analyzed residues in and around these craters are isotopically normal (solar) in their C, N, and O isotopic compositions. However, the debris in one large crater shows an average 15N enrichment of approx. 450 %, which is similar to the bulk composition of some isotopically primitive interplanetary dust particles. A 250 nm grain in another large crater has an O-17 enrichment with approx. 2.65 times the solar O-17/O-16 ratio. Such an O isotopic composition is typical for circumstellar oxide or silicate grains from red giant or asymptotic giant branch stars. The discovery of this circumstellar grain clearly establishes that there is authentic stardust in the cometary samples returned by the STARDUST mission. However, the low apparent abundance of circumstellar grains in Wild 2 samples and the preponderance of isotopically normal material indicates that the cometary matter is a diverse assemblage of presolar and solar system materials

Presolar He and Ne Isotopes in Single Circumstellar SiC Grains

Noble gas isotopes in presolar silicon carbide (SiC) dust grains from primitive meteorites provide, together with major element isotopic compositions, insight into the nucleosynthetic output of different types of evolved stars >4.5 Gyr ago. We report here new results from helium and neon isotopic analyses of single presolar SiC grains with sizes between 0.6 and 6.3 μm using an ultrahigh sensitivity mass spectrometer. These noble gas studies were complemented by an ion microprobe study (NanoSIMS) of Si, C, and N isotopic compositions of the same grains. About 40%, or 46 of the 110 grains analyzed, contain nucleosynthetic 22Ne and/or 4He from their parent stars above our mass spectrometer's detection limit. We discuss the possible stellar sources using isotopic ratios as constraints combined with new model predictions for low- to intermediate-mass (1.5, 2, 3, and 5 M☉) asymptotic giant branch (AGB) stars of different metallicities (1, 1/2, 1/3, and 1/6 Z☉). Most SiC grains are of the mainstream type and originated in low-mass AGB stars. We find a higher-than-expected percentage of A/B type grains, with some containing 22Ne and/or 4He. In addition, we find one noble gas-rich nova grain candidate, one supernova grain (X-type grain), and one 22Ne-rich X- or Z-type grain candidate

Isotopic records in CM hibonites: Implications for timescales of mixing of isotope reservoirs in the solar nebula

The magnesium isotopic compositions of 26 hibonite-bearing inclusions from the CM chondrite Murchison, as well as isotopic measurements on a subset of these samples for oxygen, titanium, and lithium-beryllium-boron are reported along with oxygen isotopi