^1,2Amaury Thiabaud, ^1,2Ulysse Marboeuf, ^1,2,4Yann Alibert, ^1,2Ingo Leya, ^1,3Klaus Mezger
¹Center for Space and Habitability, Universität Bern, 3012 Bern, Switzerland
e-mail: amaury.thiabaud@csh.unibe.ch
²Physikalisches Institut, Universität Bern, 3012 Bern, Switzerland
³Institut für Geologie, Universität Bern, 3012 Bern, Switzerland
⁴On leave from CNRS, Observatoire de Besançon, 25000 Besançon, France

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Reference
Thiabaud A, Marboeuf U, Alibert Y, Leya I, Mezger K (2015) Elemental ratios in stars vs planets. Astronomy & Astrophysics 580, A30
Link to Article [dx.doi.org/10.1051/0004-6361/201525963]

¹Daniel Carrera, ¹Anders Johansen, ¹Melvyn B. Davies
¹Lund ObservatoryDepartment of Astronomy and Theoretical Physics, Lund University, Box 43, 22100 Lund, Sweden

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Reference
Carrera D, Johansen A, Davies MB (2015) How to form planetesimals from mm-sized chondrules and chondrule aggregates. Astronomy & Astrophysics 579, A43
Link to Article [dx.doi.org/10.1051/0004-6361/201425120]

¹Peter Hoppe, ¹Jan Leitner, ¹János Kodolányi
¹Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, D-55128 Mainz, Germany

We studied about 5000 μm2 of fine-grained matrix material in the Acfer 094 meteorite by high-resolution (nominal 50 nm) NanoSIMS ion imaging for the presence of O-rich presolar (stardust) grains. This approach permits identifying presolar grains down to 150 nm in lower-resolution (nominal 100 nm) ion imaging surveys. The number density of identified presolar grains is a about a factor of two to three higher than what was found by lower-resolution ion imaging studies. The abundances of grains of O isotope Group 3 and 4 are higher than previously found. None of the presolar grains shows the strong enrichments in 16O expected from model predictions for the majority of supernova (SN) grains. Other potential O-rich SN grains, the Group 4 and some of the Group 3 grains, make up 33% by number and 19% by mass. This is clearly higher than the ~10% (by number) inferred before and the 5% (by mass) estimated by a model for stellar dust in the interstellar medium. Our work shows that O-rich SN grains might be more abundant among the population of presolar grains in primitive solar system materials than currently thought, even without the 16O-rich grains as predominantly expected from SN models.

Reference
Hoppe P, Leitner J, Kodolányi J (2015) New Constraints on the Abundances of Silicate and Oxide Stardust from Supernovae in the Acfer 094 Meteorite. Astrophysical Journal 808 L9.
Link to Article [doi:10.1088/2041-8205/808/1/L9]

¹Noriyuki Kawasaki, ¹Chizu Kato, ¹Shoichi Itoh, ¹Shigeyuki Wakaki, ²Motoo Ito, ¹Hisayoshi Yurimoto
¹Department of Natural History Sciences, Hokkaido University, Sapporo, 060-0810, Japan
²Kochi Institute for Core Sample Research, JAMSTEC B200, Monobe, Nankoku, Kochi, 783-8502, Japan

Disequilibrium oxygen isotopic distributions of Ca-Al-rich inclusions (CAIs) correspond to multiple melting events in the solar nebula. 26Al-26Mg systematics may be applicable for age differences among such melting events. We have carried out a coordinated study of detailed petrographic observations and in-situ oxygen and magnesium isotope measurements for a Type C CAI, EK1-04-2, from the Allende CV3 meteorite to determine the melting events and their ages. The CAI consists mainly of spinel, anorthite, olivine, and pyroxene, and has a core and mantle structure. Petrography of the core suggests that the crystallization sequence of the core minerals is from spinel, anorthite, olivine, and to pyroxene. The mantle has the same mineral assemblage as the core, and shows incomplete melting and solidification textures. Oxygen isotopic compositions of the minerals are distributed along the carbonaceous chondrite anhydrous mineral (CCAM) line (δ18O = −44 to +9‰), which indicates to preserve a chemical disequilibrium status in the CAI. Spinel shows a 16O-rich signature (δ18O ∼ −43‰), while anorthite is 16O-poor (δ18O ∼ +8‰). Olivine and pyroxene in the core have the same oxygen isotopic composition (δ18O ∼ −15‰), which indicates their equilibrium. Olivine and pyroxene in the mantle have variable oxygen isotopic compositions and are slightly depleted in 16O (δ18O = −13 to −4‰) compared with the same minerals in the core. The 26Al-26Mg systematics is consistent with the disequilibrium status observed according to the petrography and oxygen isotopes. Spinel is plotted on a line of (26Al/27Al)0 = (3.5 ± 0.2) × 10−5, anorthite is plotted on a line of (−1 ± 5) × 10−7, and olivine and pyroxene in the core are plotted on a line of (−1 ± 7) × 10−6. Plots of olivine and pyroxene in the mantle are scattered below the isochron of these minerals in the core. This study indicates that the EK1-04-2 Type C CAI underwent multiple heating events after the formation of its CAI precursor. The precursor CAI was formed ∼0.4 Myr after the formation of the Solar System defined by canonical CAI formation. At least 1.6 Myr after the precursor CAI formation, the CAI was partially melted and the melt exchanged oxygen isotopes with surrounding 16O-poor nebular gas. 16O-poor olivine and pyroxene in the core crystallized from the melt. Subsequently, Al-rich chondrules accreted onto the CAI, and the CAI experienced partial melting again and recrystallized to form the mantle. The oxygen and magnesium isotopes in anorthite were redistributed during thermal metamorphism in the Allende parent body. Our study reveals that the CAI had been retained in the solar nebula for at least 1.6 Myr and underwent multiple melting events in the nebula, and oxygen and 26Al-26Mg systematics has been partially disturbed depending on crystal sizes by metamorphism on the parent body.

Reference
Kawasaki N, Kato C, Itoh S, Wakaki S, Ito M, Yurimotoa H (2015) 26Al-26Mg chronology and oxygen isotope distributions of multiple melting for a Type C CAI from Allende. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.07.037]

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