Jing Yang^a, Chi Zhang^a, Masaaki Miyahara^a, Xu Tang^a, LixinGu^a, Yangting Lin^a,c
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.03.009]
^aKey Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
^bDepartment of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Hiroshima 739-8526, Japan
^cCollege of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100029, China
Copyright Elsevier

The ungrouped achondrite NWA 7325 is a cumulate olivine gabbro (Irving et al., 2013). It contains abundant and unique micro-inclusions of Ca-pyroxene (Bischoff et al., 2013) and spinel-like Al-Mg oxide (Goodrich et al., 2017) in plagioclase, indicating a remelting event induced either by impact (Goodrich et al., 2017) or by magma intrusion (Bischoff et al., 2013, Weber et al., 2016). In this work, a combined FIB-TEM study has been conducted on these micro-inclusions to address their petrogenesis and the related history of NWA 7325. TEM study revealed that micro-inclusions in the interiors of large plagioclase grains are Al-enriched spinel (Mg/Al atomic ratio: 0.03-0.4) with minor needle-like α-corundum, whereas those in the margins are predominantly Al-rich diopside (En_44.5-46.6Fs_1.2-1.5Wo_31.2-36.7CaTs_17.6-22.4) with minor forsterite (Fo_94.6-94.7). The Mg/Al atomic ratios of the spinel micro-inclusions are negatively correlated with the distance away from the interface of plagioclase-pyroxene. Large plagioclase grains also exhibit a decrease in the Mg/Al atomic ratio from the rims to the cores. Based on the reaction texture at the interfaces of plagioclase-pyroxene, we infer that the Mg concentration gradient in large plagioclase grains could have resulted from Mg diffusion from the remelted rims of pyroxene into plagioclase. In addition, TEM observations showed that large plagioclase grains are not single crystals, but assemblages of submicron to micron-sized crystals. The preservation of Mg concentration gradients, submicron-sized polycrystalline plagioclases, and the consistent presence of micro-inclusions within large plagioclase grains likely indicate complete remelting of plagioclase and partial remelting of pyroxene (only rims of pyroxene with plagioclase) followed by fast cooling. We propose that micro-inclusions of diopside, forsterite, Al-rich spinel and corundum crystallized from the melt, which developed a Mg concentration gradient during the remelting of NWA 7325.

The heating temperatures of pyroxene and plagioclase in the remelting event were estimated to be 1274-1327 °C and Σ1530 °C, respectively. A subsequent cooling rate of Σ500-650 °C/h at 1300 °C was found by fitting the measured Mg concentration gradient in large plagioclase grains with a Fick’s second law model that incorporated the diffusion coefficients of Mg in plagioclase-melt. These results are better explained by a shock event; a magmatic intrusion process is ruled out. To achieve the coexistence of shock-induced high temperature (Σ1274 °C) in-situmelting and only undulatory extinction of forsterite grains, an ambient temperature of 1000-1100 °C in the surrounding, parent rock of NWA 7325 was required prior to impact. This work suggests a very early shock event when NWA 7325 was hot and buried in the crust of its parental planetesimal, which is a scenario consistent with its magma crystallization age (∼4.3 Ma after CAIs, e.g., Koefoed et al., 2016). This work also implies that impacts are a potential heat source for melting hot planetesimals in the early Solar System.

^aLukáš Ackerman^a, RomanSkála^a, ŠárkaKřížová^a,b, KarelŽák^a, TomášMagna^c
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.03.009]
^aInstitute of Geology, The Czech Academy of Sciences, Rozvojová 269, CZ-165 00 Prague 6 – Lysolaje, Czech Republic
^bInstitute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, CZ-128 43 Prague 2, Czech Republic
^cCzech Geological Survey, Klárov 3, CZ-118 21 Prague 1, Czech Republic
Copyright Elsevier

Extremely low and variable concentrations of osmium (Os) and other highly siderophile elements (HSE) in most tektites make it challenging to establish direct links between these impact-related materials and their possible extraterrestrial contribution. New Os concentrations (2–43 ppt) and ¹⁸⁷Os/¹⁸⁸Os ratios (0.131–0.68) in a suite of fifteen well-characterized Australasian tektites from Laos (Muong Nong and splash-form types) with variable Ni enrichment indicate a maximum of ∼0.005% addition of a chondritic impactor. This is similar to some Australasian tektites from Vietnam with similarly low siderophile contents, but significantly lower than found in previous studies of more Ni-rich Australasian splash-form tektites and microtektites from different parts of the Australasian strewn field (e.g., Indonesia, South China Sea). The contents of HSE and Re–Os isotopic compositions of layered Muong Nong-type Australasian tektites are highly variable, suggesting mingling of crustal-derived (siderophile element-poor) and extraterrestrial (siderophile element-rich) materials. The absence of a direct correlation between HSE and Ni contents is interpreted to result from a fractionation process related to their different vaporization/condensation temperatures. The low Os abundance in most analyzed Australasian tektites, combined with non-radiogenic ¹⁸⁷Os/¹⁸⁸Os far below average upper continental crust, may provide a direct test to distinguish continental versus seawater impact scenario. In the absence of any specific low-Os target, a particular process of Os loss following impact is required. We envisage a scenario where evaporative loss of >>90% Os in the form of Os oxides from the overheated tektite melt is aided by volatile species derived from dissociated seawater and/or saline pore water embedded in sediments off-shore Indochina, consistent with elevated contents of halogens in Australasian tektites. This water-assisted Os loss could also play significant role for Central European tektites, while the continental surface with limited amount of water would prevent from more efficient HSE loss as could be the case for Ivory Coast tektites.

Ke Zhu (朱柯)^1,2, Jia Liu¹, Frédéric Moynier^2,3, Liping Qin^1,4, Conel M. O’D. Alexander⁵, and Yongsheng He⁴
Astrophysical Journal 873, 82 Link to Article [DOI: 10.3847/1538-4357/aafe79 ]
¹CAS Key Laboratory of Crust–Mantle Materials and Environment and CAS Center for Excellence in Comparative Planetology, School of Earth and Space Science, University of Science and Technology of China, Hefei 230026, People’s Republic of China
²Institut de Physique du Globe de Paris, Université Sorbonne Paris Cité, CNRS, 1 rue Jussieu, Paris F-75005, France
³Institut Universitaire de France, Paris, France
⁴State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Beijing), Beijing 100083, People’s Republic of China
⁵Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road, Washington, DC 20015, USA

Chondrules are the main components of primitive meteorites and possibly the building blocks of planetary embryos and terrestrial planets. However, their ages and modes of formation are still highly debated. Here, we present high-precision Cr isotope data of nine chondrules from one of the more primitive chondrites, the CO3 chondrite Ornans. These chondrules define an external ⁵³Mn–⁵³Cr isochron, with an initial ⁵³Mn/⁵⁵Mn of (7.1 ± 1.6) × 10⁻⁶, corresponding to an age of 4567.6 ± 1.3 Ma when anchored to the angrite D’Orbigny (U-corrected). This age is within error of the age of formation of calcium-aluminum-rich inclusions (CAIs). All chondrules show a wide range of ε ⁵⁴Cr values (+0.20 to +1.22) and a positive correlation between ε ⁵³Cr and ε ⁵⁴Cr values, suggesting mixing of different isotopic sources in the protoplanetary disk. This could reflect that silicate materials from the CAI-forming region (with complementary compositions to CAIs, i.e., low Mn/Cr and ε ⁵⁴Cr) were transported to the accretion region of the CO chondrite parent body and mixed with CI-like material (high-Mn/Cr and ε ⁵⁴Cr) during chondrule formation. Such mixing must have occurred prior to the formation of chondrule precursors. Furthermore, chondrules from chondrites with more CAIs (CV and CO) exhibit greater variability in ε ⁵⁴Cr than chondrules from chondrites formed later with fewer CAIs (e.g., CB and CR), suggesting that the accretion regions of the former received more material transported from the inner solar system than the latter. This dichotomy may indicate the CB and CR chondrites accreted at greater orbital distances than other chondrites.

Maitrayee Bose^1,2 and Sumner Starrfield¹
Astrophysical Journal 873, 14 Link to Article [DOI: 10.3847/1538-4357/aafc2f ]
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA
²Center for Isotope Analysis (CIA), Arizona State University.

This study on presolar grains compares high-precision isotopic compositions of individual SiC grains with low ¹²C/¹³C ratios, low ¹⁴N/¹⁵N ratios, large ³⁰Si excesses, and high ²⁶Al/²⁷Al ratios, all available in the presolar grain database, to new CO nova models with white dwarf (WD) masses from 0.6 to 1.35 M _⊙. The models were designed to match the Large Binocular Telescope high-dispersion spectra acquired for nova V5668 Sgr. These CO nova models provide elemental abundances up to calcium and include mixing of WD material into the accreted material in a binary star system under several scenarios, including one where mixing occurs only after temperatures >7 × 10⁷ K are achieved during a thermonuclear runaway (TNR). The 0.8–1.35 M _⊙ simulations where 25% of the WD core matter mixes with 75% of the accreted material (assumed solar) from its binary companion after the TNR has begun provide the best fits to the measured isotopic data in four presolar grains. One grain matches the 50% accreted 50% solar 1.35 M _⊙ simulation. For these five presolar grains, less than 25% of solar system material is required to be mixed with the CO nova ejecta to account for the grains’ compositions. Thus, our study reports evidence of pure CO nova ejecta material in meteorites. Finally, we speculate that SiC grains can form in the winds of cool and dense CO novae, where the criterion C > O may not be locally imposed, and thus nova winds can be chemically inhomogeneous.

Alexis Bouquet^1,2, Christopher R. Glein¹, and J. Hunter Waite Jr.^1,3
Astrophysical Journal 873, 28 Link to Article [DOI: 10.3847/1538-4357/ab0100 ]
¹Space Science and Engineering Division, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX, 78238, USA
²Aix Marseille Université, CNRS, CNES, LAM, Marseille, France
³Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, USA

We study the effect of adsorption of volatile organic compounds (VOCs) in Enceladus’ geysers, both onto the ice grains ejected in the plumes, and onto the ice walls of the cracks connecting Enceladus’ internal ocean to its surface. We use a model of adsorption/desorption based on the Polanyi–Wiegner equation and experimental values of binding energies (energy of desorption E _des) of the adsorbed compounds to water ice. We find that under conditions expected at Enceladus, the process of adsorption tends to ensure that the VOCs with the highest binding energy are over-represented on the ice surface, even if their abundance is comparatively lower than those of other compounds. We find that VOCs with E _des ≤ 0.5 eV are insignificantly affected by adsorption while compounds with E _des ≥ 0.7 eV are readily retained on the surface and compete to occupy most of the adsorption sites. We also deduce that ice grains falling back onto the surface are likely to retain most of the molecules adsorbed on their surface. The implication is that remote observation or sampling of the ice in the cracks or of the surface around it would show a mixture of VOCs that would not be representative of the gas phase of the plumes, with the high E _des VOCs dominating the adsorbed phase.

Wei Zhu (祝伟)
Astrophysical Journal 873, 8 Link to Article [DOI: 10.3847/1538-4357/ab0205 ]
Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada

We study the influence of stellar metallicity on the fraction of stars with planets (i.e., the occurrence rate of planetary systems) and the average number of planets per star (i.e., the occurrence rate of planets). The former directly reveals the planet formation efficiency, whereas the latter reveals the final product of formation and evolution. We show that these two occurrence rates have different dependences on stellar metallicity. Specifically, the fraction of stars with planets rises gradually with metallicity, from ~25% to ~36% for 0.4 dex of [Fe/H] for all Kepler-like planets (period P < 400 days and radius ). The average number of planets per star reaches a plateau (or possibly starts declining) at [Fe/H]  0.1. This is plausibly caused by the emergence of distant giant planets at high metallicities, given that the close-in small planets and the distant giants preferentially coexist in the same system.

¹Allan H. Treiman,²Michael J. Kulis,³Allen F. Glazner
American Mineralogist 104, 370-384 Link to Article [https://doi.org/10.2138/am-2019-6652]
¹Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Boulevard, Houston, Texas 77058, U.S.A. Orcid 0000-0002-8073-2839
²Extreme Environments Laboratory, NASA Glenn Research Center, 2100 Brookpark Road, Cleveland, Ohio 44135, U.S.A.
³Department of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599, U.S.A. Orcid 0000-0002-3111-5885
Copyright: The Mineralogical Society of America

Magnesium aluminate spinel, (Mg,Fe)Al₂O₄, is uncommon in lunar rocks but petrologically significant. Recent near-infrared spectra of the Moon have delineated regions where spinel is the only ferromagnesian mineral; the rock is inferred to be spinel anorthosite. One hypothesis is that significant pressure is required for spinel formation; another is that spinel-bearing rocks form by low-pressure assimilation of highlands anorthosite into olivine-rich basaltic (i.e., picritic) magmas. Here, we evaluate the heat (i.e., enthalpy) required for this assimilation process. Magma compositions are the picritic Apollo 14 B green glass and an estimation of the magma parental to Mg-suite cumulate rocks. From calculated enthalpy-composition phase diagrams, assimilation of anorthite into either magma cannot produce spinel anorthosite unless the anorthite is already hotter than ~1300 °C. For cooler anortho-site, assimilation will produce olivine- and/or pyroxene-bearing rocks. Such hot anorthite could be produced by the nearby passage of large volumes of magma, but this is not obviously consistent with occurrences of spinel far from outcrops of basaltic rocks. Hot anorthosite could also be produced by global tidal flexure; that mechanism could have only been efficient early in lunar history when a solid anorthosite crust floated above an evolved magma ocean, and it is not clear how picritic magma could pass through the magma ocean to interact with anorthite in the crust. On the other hand, spinel-bearing anorthosite can form directly upon cooling of superliquidus melts of anorthite-rich composition. Such superliquidus melts can be generated by impact events; this mechanism seems likely, given the Moon’s ubiquity of impact craters, abundance of impact-metamorphosed lunar rocks, and common presence in lunar regolith of impact glasses (quenched superliquidus impact melts) of appropriate compositions. High pressure does stabilize spinel in basaltic and peridotitic systems, but available models do not permit quantitative evaluation of the effects of pressure on the enthalpy required for assimilation. Near the lunar surface, the most likely process of spinel formation is rapid crystallization of impact melts of anorthosite + picrite or peridotite compositions. The presence of spinel anorthosite on the walls and central peaks of impact craters results from rapid cooling and partial crystallization of superliquidus melts produced in the impacts, and not from uplift of deep material to the Moon’s surface.

Motohiko Kusakabe^1,2, Myung-Ki Cheoun^1,2,3, K. S. Kim⁴, Masa-aki Hashimoto⁵, Masaomi Ono⁶, Ken’ichi Nomoto⁷, Toshio Suzuki^2,8, Toshitaka Kajino^1,2,9, and Grant J. Mathews^2,10
Astrophysical Journal 872, 164 Link to Article [DOI: 10.3847/1538-4357/aafc35 ]
¹School of Physics, and International Research Center for Big-Bang Cosmology and Element Genesis, Beihang University, 37, Xueyuan Rd., Haidian-qu, Beijing 100083, People’s Republic of China
²National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³Department of Physics and Origin of Matter and Evolution of Galaxy (OMEG) Institute, Soongsil University, Seoul 156-743, Republic of Korea
⁴School of Liberal Arts and Science, Korea Aerospace University, Goyang 412-791, Republic of Korea
⁵Kyushu University, Hakozaki, Fukuoka 812-8581, Japan
⁶RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan
⁷Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan
⁸Department of Physics, College of Humanities and Science, Nihon University, Sakurajosui 3-25-40, Setagaya-ku, Tokyo 156-8550, Japan
⁹Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁰Center for Astrophysics, Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA

We reinvestigate effects of neutrino oscillations on the production of ⁷Li and ¹¹B in core-collapse supernovae (SNe). During the propagation of neutrinos from the proto–neutron star, their flavors change, and the neutrino reaction rates for spallation of ¹²C and ⁴He are affected. In this work, corrected neutrino spallation cross sections for ⁴He and ¹²C are adopted. Initial abundances involving heavy s-nuclei and other physical conditions are derived in a new calculation of the SN 1987A progenitor in which the effects of the progenitor metallicity are included. A dependence of the SN nucleosynthesis and final yields of ⁷Li and ¹¹B on the neutrino mass hierarchy are shown in several stellar locations. In the normal hierarchy case, the charged-current (CC) reaction rates of are enhanced, and yields of proton-rich nuclei, along with ⁷Be and ¹¹C, are increased. In the inverted hierarchy case, the CC reaction rates of are enhanced, and yields of neutron-rich nuclei, along with ⁷Li and ¹¹B, are increased. We find that variation of the metallicity modifies the yields of ⁷Li, ⁷Be, ¹¹B, and ¹¹C. This effect is caused by changes in the neutron abundance during SN nucleosynthesis. Therefore, accurate calculations of Li and B production in SNe should take into account the metallicity of progenitor stars.

Yudong Luo^1,2,3, Toshitaka Kajino^1,2,3, Motohiko Kusakabe^1,3, and Grant J. Mathews^1,4
Astrophysical Journal 872, 172 Link to Article [DOI: 10.3847/1538-4357/ab0088 ]
¹National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²Department of Astronomy, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
³School of Physics and Nuclear Energy Engineering, and International Research Center for Big-Bang Cosmology and Element Genesis, Beihang University 37, Xueyuan Road, Haidian-qu, Beijing 100083, People’s Republic of China
⁴Center for Astrophysics, Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA

We investigate the effect on the Big Bang nucleosynthesis (BBN) from the presence of a stochastic primordial magnetic field (PMF) whose strength is spatially inhomogeneous. We assume a uniform total energy density and a Gaussian distribution of field strength. In this case, domains of different temperatures exist in the BBN epoch due to variations in the local PMF. We show that in such a case, the effective distribution function of particle velocities averaged over domains of different temperatures deviates from the Maxwell–Boltzmann distribution. This deviation is related to the scale invariant strength of the PMF energy density ρ _Bc and the fluctuation parameter σ _B. We perform BBN network calculations taking into account the PMF strength distribution and deduce the element abundances as functions of the baryon-to-photon ratio η, ρ _Bc, and σ _B. We find that the fluctuations of the PMF reduce the ⁷Be production and enhance D production. We analyze the averaged thermonuclear reaction rates compared with those of a single temperature and find that the averaged charged-particle reaction rates are very different. Finally, we constrain the parameters ρ _Bcand σ _B from observed abundances of ⁴He and D and find that the ⁷Li abundance is significantly reduced. We also find that if the η value during BBN was larger than the present-day value due to a dissipation of the PMF or a radiative decay of exotic particles after BBN or if the stellar depletion of ⁷Li occurred, abundances of all light elements can be consistent with observational constraints.

¹Takashi Yoshizaki, ¹Daisuke Nakashima, ¹Tomoki Nakamura,²Changkun Park, ³Naoya Sakamoto, ¹Hatsumi Ishida, ⁴Shoichi Itoh
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.02.034]
¹Department of Earth Science, Graduate School of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan
²Division of Earth-System Sciences, Korea Polar Research Institute, Incheon 21990, South Korea
³Creative Research Institution Sousei, Hokkaido University, Kita, Sapporo 001-0021, Japan
⁴Department of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan
Copyright Elsevier

Ultrarefractory (UR) phases in calcium-aluminum-rich inclusions (CAIs) could have formed at higher temperature compared to common CAI minerals and thus they potentially provide constraints on very high temperature processes in the solar nebula. We report a detailed characterization of the mineralogy, petrology and oxygen isotopic composition of an UR phase davisite (CaScAlSiO₆) bearing CAI from a reduced type CV chondrite. The CAI is an irregular-shaped, compound inclusion that consists of five units that are composed of melilite + spinel + Al,Ti-rich pyroxene ±perovskite with various modal abundances of minerals and lithologies, and surrounded by the Wark-Lovering (WL) rim. Davisite occurs only in one lithological unit that consists of three chemically and isotopically distinct parts: i) 16O-poor (-20‰⩽δ18O ⩽0‰) regions with reversely-zoned melilite and davisite; ii) 16O-rich (-50‰⩽δ18O ⩽-40‰) regions consisting of unzoned, gehlenitic melilite, Al,Ti-rich diopside and spinel; and iii) spinel framboids composed of 16O-rich spinel and 16O-poor melilite. Absence of secondary iron- and/or alkali-rich phases, occurrence of low-iron, manganese-enriched (LIME) olivine, and random distribution of the oxygen isotopic heterogeneity indicate that primitive chemical and isotopic compositions are preserved in the inclusion. The occurrence of chemical and isotopic heterogeneities with sharp boundaries in the CAI indicates formation of the inclusion by an aggregation of mineral assemblages formed and processed separately at different time and/or space in the solar nebula. Although isotope exchange between 16O-rich solids and 16O-poor gases prior to the final agglomeration of the CAI cannot be ruled out, we suggest that modification of chemical and isotopic composition of porous CAI precursors or aggregation of isotopically distinct mineral assemblages are alternative scenarios for the origin of oxygen isotopic heterogeneity in CAIs. In either case, coexistence of spatially and/or temporally distinct 16O-rich and 16O-poor gaseous reservoirs at the earliest stage of the solar system formation is required. The grain-scale oxygen isotopic disequilibrium in the CAI indicate that post-formation heating of the inclusion (i.e., the WL rim formation event) was short (e.g., ≲103hours at 1400 K; ≲105hours at 1100 K), which can be achieved by rapid outward transport of the CAI. High Ti³⁺/Ti^tot ratios of pyroxene from CAI interior and the rim and LIME composition of the olivine rim document that the entire CAI formation process took place under highly reducing conditions.