Dominik C. Hezel^a,b,c, Johanna S. Wilden^a, Daniel Becker^a, Sonja Steinbach^d, Frank Wombacher^a,c, Markus Harak^a
Earth and Planetary Science Letters 490, 31-39 Link to Article [https://doi.org/10.1016/j.epsl.2018.03.013]
^aUniversity of Cologne, Department of Geology and Mineralogy, Zülpicher Str. 49b, 50674 Köln, Germany
^bNatural History Museum, Department of Mineralogy, Cromwell Road, SW7 5BD, London, UK
^cSteinmann-Institut, Poppelsdorfer Schloss, Meckenheimer Allee 169, 53115 Bonn, Germany
^dDeutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Materialphysik im Weltraum, Linder Höhe, 51147 Köln, Germany
Copyright Elsevier

Chondrules are a major constituent of primitive meteorites. The formation of chondrules is one of the most elusive problems in cosmochemistry. We use Fe isotope compositions of chondrules and bulk chondrites to constrain the conditions of chondrule formation. Iron isotope compositions of bulk chondrules are so far only known from few studies on CV and some ordinary chondrites. We studied 37 chondrules from the CM chondrite Murchison. This is particularly challenging, as CM chondrites contain the smallest chondrules of all chondrite groups, except for CH chondrites. Bulk chondrules have δ⁵⁶Fe between −0.62 and +0.24‰ relative to the IRMM-014 standard. Bulk Murchison has as all chondrites a δ⁵⁶Fe of 0.00‰ within error. The δ⁵⁶Fe distribution of the Murchison chondrule population is continuous and close to normal. The width of the δ⁵⁶Fe distribution is narrower than that of the Allende chondrule population. Opaque modal abundances in Murchison chondrules is in about 67% of the chondrules close to 0 vol.%, and in 33% typically up to 6.5 vol.%. Chondrule Al/Mg and Fe/Mg ratios are sub-chondritic, while bulk Murchison has chondritic ratios. We suggest that the variable bulk chondrule Fe isotope compositions were established during evaporation and recondensation prior to accretion in the Murchison parent body. This range in isotope composition was likely reduced during aqueous alteration on the parent body. Murchison has a chondritic Fe isotope composition and a number of chondritic element ratios. Chondrules, however, have variable Fe isotope compositions and chondrules and matrix have complementary Al/Mg and Fe/Mg ratios. In combination, this supports the idea that chondrules and matrix formed from a single reservoir and were then accreted in the parent body. The formation in a single region also explains the compositional distribution of the chondrule population in Murchison.

Keisuke Sakuma¹, Hiroshi Hidaka¹, and Shigekazu Yoneda²

Astrophysical Journal 853, 92 Link to Article [DOI: 10.3847/1538-4357/aaa1e3]
¹Department of Earth and Planetary Sciences, Nagoya University Nagoya 464-8601, Japan
²Department of Science and Engineering, National Museum of Nature and Science Tsukuba 305-0005, Japan

Aqueous alteration is one of the primitive activities that occurred on meteorite parent bodies in the early solar system. The Tagish Lake meteorite is known to show an intense parent body aqueous alteration signature. In this study, quantitative analyses of the alkaline elements and isotopic analyses of Sr and Ba from acid leachates of TL (C2-ungrouped) were performed to investigate effects of aqueous alteration. The main purpose of this study is to search for isotopic evidence of extinct ¹³⁵Cs from the Ba isotopic analyses in the chemical separates from the Tagish Lake meteorite. Barium isotopic data from the leachates show variable ¹³⁵Ba isotopic anomalies (ε = −2.6 ~ +3.6) which correlatewith ¹³⁷Ba and ¹³⁸Ba suggesting a heterogeneous distribution of s– and r-rich nucleosynthetic components in the early solar system. The ⁸⁷Rb–⁸⁷Sr and ¹³⁵Cs–¹³⁵Ba decay systems on TL in this study do not provide any chronological information. The disturbance of the TL chronometers is likely a reflection of the selective dissolution of Cs and Rb given the relatively higher mobility of Cs and Rb compared to Ba and Sr, respectively, during fluid mineral interactions.

Edward R. D. Scott¹, Alexander N. Krot¹, and Ian S. Sanders²

Astrophysical Journal 854, 164 Link to Article [DOI: 10.3847/1538-4357/aaa5a5]
¹Hawai’i Institute of Geophysics and Planetology, University of Hawai’i, Honolulu, HI 96822, USA
²Department of Geology, Trinity College, Dublin 2, Ireland

Whole rock Δ¹⁷O and nucleosynthetic isotopic variations for chromium, titanium, nickel, and molybdenum in meteorites define two isotopically distinct populations: carbonaceous chondrites (CCs) and some achondrites, pallasites, and irons in one and all other chondrites and differentiated meteorites in the other. Since differentiated bodies accreted 1–3 Myr before the chondrites, the isotopic dichotomy cannot be attributed to temporal variations in the disk. Instead, the two populations were most likely separated in space, plausibly by proto-Jupiter. Formation of CCs outside Jupiter could account for their characteristic chemical and isotopic composition. The abundance of refractory inclusions in CCs can be explained if they were ejected by disk winds from near the Sun to the disk periphery where they spiraled inward due to gas drag. Once proto-Jupiter reached 10–20 M_⊕, its external pressure bump could have prevented millimeter- and centimeter-sized particles from reaching the inner disk. This scenario would account for the enrichment in CCs of refractory inclusions, refractory elements, and water. Chondrules in CCs show wide ranges in Δ¹⁷O as they formed in the presence of abundant ¹⁶O-rich refractory grains and ¹⁶O-poor ice particles. Chondrules in other chondrites (ordinary, E, R, and K groups) show relatively uniform, near-zero Δ¹⁷O values as refractory inclusions and ice were much less abundant in the inner solar system. The two populations were plausibly mixed together by the Grand Tack when Jupiter and Saturn migrated inward emptying and then repopulating the asteroid belt with roughly equal masses of planetesimals from inside and outside Jupiter’s orbit (S- and C-type asteroids).

Motohiko Kusakabe and Grant J. Mathews

Astrophysical Journal 854, 183 Link to Article [DOI: 10.3847/1538-4357/aaa125]
Center for Astrophysics, Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA

We investigate the cosmic-ray nucleosynthesis (CRN) of proton-rich stable nuclides (p-nuclides). We calculate the cosmic-ray (CR) energy spectra of heavy nuclides with mass number , taking into account the detailed nuclear spallation, decay, energy loss, and escape from the Galaxy during the CR propagation. We adopt the latest semiempirical formula SPACS for the spallation cross sections and the latest data on nuclear decay. Effective electron-capture decay rates are calculated using the proper cross sections for recombination and ionization in the whole CR energy region. Calculated CR spectral shapes vary for different nuclides. Abundances of proton-rich unstable nuclides increase in CRs with increasing energy relative to those of other nuclides. Yields of the primary and secondary spallation processes and differential yields from respective seed nuclides are calculated. We find that the CR energy region of MeV/nucleon predominantly contributes to the total yields. The atomic cross sections in the low-energy range adopted in this study are then necessary. Effects of CRN on the Galactic chemical evolution of p-nuclides are calculated. Important seed nuclides are identified for respective p-nuclides. The contribution of CRN is significant for ^180mTa, accounting for about 20% of the solar abundance. About 87% of the ^180m Ta CRN yield can be attributed to the primary process. The most important production routes are reactions of ¹⁸¹Ta, ¹⁸⁰Hf, and ¹⁸²W. CRN yields of other p-nuclides are typically about (10⁻⁴–10⁻²) of solar abundances.

Hongu Yang^1,2 and Masateru Ishiguro¹

Astrophysical Journal 854, 137 Link to Article [DOI: 10.3847/1538-4357/aaab59]
¹Department of Physics and Astronomy, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea
²Korea Astronomy and Space Science Institute (KASI), Republic of Korea

In this study, we numerically investigated the orbital evolution of cometary dust particles, with special consideration of the initial size–frequency distribution (SFD) and different evolutionary tracks according to the initial orbit and particle shape. We found that close encounters with planets (mostly Jupiter) are the dominating factor determining the orbital evolution of dust particles. Therefore, the lifetimes of cometary dust particles (~250,000 yr) are shorter than the Poynting–Robertson lifetime, and only a small fraction of large cometary dust particles can be transferred into orbits with small semimajor axes. The exceptions are dust particles from 2P/Encke and, potentially, active asteroids that have little interaction with Jupiter. We also found that the effects of dust shape, mass density, and SFD were not critical in the total mass supply rate to the interplanetary dust particle (IDP) cloud complex when these quantities are confined by observations of zodiacal light brightness and SFD around the Earth’s orbit. When we incorporate a population of fluffy aggregates discovered in the Earth’s stratosphere and the coma of 67P/Churyumov–Gerasimenko within the initial ejection, the initial SFD measured at the comae of comets (67P and 81P/Wild 2) can produce the observed SFD around the Earth’s orbit. Considering the above effects, we derived the probability of mutual collisions among dust particles within the IDP cloud for the first time in a direct manner via numerical simulation and concluded that mutual collisions can mostly be ignored.

Shinya Wanajo^1,2, Bernhard Müller^3,4, Hans-Thomas Janka⁵, and Alexander Heger^4,6,7

Astrophysical Journal 852, 40 Link to Article [DOI: 10.3847/1538-4357/aa9d97]
¹Department of Engineering and Applied Sciences, Sophia University, Chiyoda-ku, Tokyo 102-8554, Japan
²iTHES Research Group, RIKEN, Wako, Saitama 351-0198, Japan
³Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast, Belfast, BT7 1NN, UK
⁴Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, VIC 3800, Australia
⁵Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85748 Garching, Germany
⁶University of Minnesota, School of Physics and Astronomy, Minneapolis, MN 55455, USA
⁷Shanghai Jiao-Tong University, Department of Physics and Astronomy, Shanghai 200240, P. R. China

We examine nucleosynthesis in the innermost neutrino-processed ejecta (a few ) of self-consistent two-dimensional explosion models of core-collapse supernovae (CCSNe) for six progenitor stars with different initial masses. Three models have initial masses near the low-mass end of the SN range of (e8.8; electron-capture SN), (z9.6), and (u8.1), with initial metallicities of 1, 0, and 10⁻⁴ times the solar metallicity, respectively. The other three are solar-metallicity models with initial masses of (s11), (s15), and (s27). The low-mass models e8.8, z9.6, and u8.1 exhibit high production factors (nucleosynthetic abundances relative to the solar abundances) of 100–200 for light trans-Fe elements from Zn to Zr. This is associated with an appreciable ejection of neutron-rich matter in these models. Remarkably, the nucleosynthetic outcomes for the progenitors e8.8 and z9.6 are almost identical, including interesting productions of ⁴⁸Ca and ⁶⁰Fe, irrespective of their quite different (O–Ne–Mg and Fe) cores prior to collapse. In the more massive models s11, s15, and s27, several proton-rich isotopes of light trans-Fe elements including the p-isotope ⁹²Mo (for s27) are made, up to production factors of ~30. Both electron-capture SNe and CCSNe near the low-mass end can therefore be dominant contributors to the Galactic inventory of light trans-Fe elements from Zn to Zr and probably ⁴⁸Ca and live ⁶⁰Fe. The innermost ejecta of more massive SNe may have only subdominant contributions to the chemical enrichment of the Galaxy except for ⁹²Mo.