¹Bidong Zhang, ¹Sean R. Shieh, ¹Anthony C. Withers, ¹Audrey Bouvier
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13106]
¹Department of Earth Sciences, Centre for Planetary Science and Exploration, University of Western Ontario, London, Ontario, Canada
Published by arrangement with John Wiley & Sons

We present Raman patterns of enstatite in different classes of enstatite‐rich chondrites and achondrites of various shock levels as previously reported from petrographic observations and X‐ray diffraction analyses. Thin sections or mineral separates of four enstatite chondrites (LaPaz Icefield [LAP] 02225, MacAlpine Hills [MAC] 02837, Pecora Escarpment [PCA] 91020, and Itqiy), three aubrites (Larkman Nunatak [LAR] 04316, Khor Temiki, and Allan Hills [ALH] 84008), and a ureilite (Sayh al Uhaymir [SaU] 559) were examined by laser Raman spectroscopy. We find that the frequencies of fundamental Raman peaks of enstatites from the chondrites and aubrites deviate by ≤2 cm⁻¹ from the values for unshocked enstatite. This small difference implies a negligible effect of shock metamorphism on peak positions. Significant differences (<6 cm⁻¹) for peak positions are found for the pyroxenes of SaU 559 and may be attributed to minor substitution of Fe and Ca for Mg. Linear regressions of peak widths of enstatite chondrites against their established shock stages show a strong positive correlation for each mode (r² > 0.94). From this linear relationship, the 343 and 1014 cm⁻¹ peaks of the aubrites coincide with S4 determined from petrography. For Itqiy, we find S4–5, while the shock levels of SaU 559 exceed the petrologic scheme (S1–6), suggesting that the ureilite might have sustained multiple shock events or have been deformed in a high‐pressure environment. Alternatively, for Itqiy (peak 343 cm⁻¹) and SaU 559 (all peaks) enstatites, minor substitutions of Fe and Ca for Mg may have further broadened their peak widths.

¹S. A. Singerling, ¹A. J. Brearley
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13108]
¹Department of Earth & Planetary Sciences, MSC‐03 2040 1 University of New Mexico, Albuquerque, New Mexico, USA
Published by arrangement with John Wiley & Sons

We have carried out a systematic study involving SEM, EPMA, and TEM analyses to determine the textures and compositions of sulfides and sulfide–metal assemblages in a suite of minimally to weakly altered CM and CR carbonaceous chondrites. We have attempted to constrain the distribution and origin of primary sulfides that formed in the solar nebula, rather than by secondary asteroidal alteration processes. Our study focused primarily on sulfide assemblages associated with chondrules, but also examined some occurrences of sulfides within the matrices of these meteorites. Although sulfides are a minor phase in carbonaceous chondrites, we have determined that primary sulfide grains are actually a major proportion of the sulfide grains in weakly altered CM chondrites and have survived aqueous alteration relatively unscathed. In minimally altered CR chondrites, we have determined that essentially all of the sulfides are of primary origin, confirming the observations of Schrader et al. (2015). The pyrrhotite–pentlandite intergrowth (PPI) grains formed from crystallization of monosulfide solid solution (mss) melts, while sulfide‐rimmed metal (SRM) grains formed from sulfidization of Fe,Ni metal. Micron‐sized metal inclusions in some PPI grains may have formed by co‐crystallization of metal and sulfide from a sulfide melt that experienced S volatilization during the chondrule formation event, or alternatively, may be a remnant of sulfidization of Fe,Ni metal that also occurred during chondrule formation. Sulfur fugacity for SRM grains ranged from −18 to −10 (log units) largely in agreement with predicted solar nebular values. Our observations show that understanding the formation mechanisms of primary sulfide grains provides clues to solar nebular conditions, such as the sulfur fugacity during chondrule formation.

^1,2T. Mohr‐Westheide, ¹A. Greshake, ³R. Wirth, ^1,4,5W. U. Reimold
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13109]
¹Museum für Naturkunde—Leibniz Institute for Evolution and Biodiversity Science, , Berlin, Germany
²Institut für Geologische Wissenschaften, Freie Universität Berlin, , Berlin, Germany
³Helmholtz‐Zentrum Potsdam, Deutsches GeoForschungsZentrum, , Potsdam, Germany
⁴Geochronology Laboratory, University of Brasília, Brasília, Brazil
⁵Humboldt Universität zu Berlin, , Berlin, Germany
Published by arrangement with John Wiley & Sons

The oldest known large bolide impacts onto Earth are represented by approximately 3.47–3.2 Ga old Archean spherule layers of the Barberton Greenstone Belt (BGB) in South Africa and the Pilbara craton in West Australia. These layers were recognized as impact deposits by their excessively high platinum group element (PGE) contents that are indicative of an extraterrestrial component. This was followed by measurements of extraterrestrial Cr isotopic ratios, in some cases. Recently, the extraterrestrial PGE signature in Archean spherule layers from the BGB was localized and positively associated with the presence of submicrometer PGE alloy micronuggets associated with Ni,Cr‐rich spinel. The actual formation of these platinum group mineral (PGM) phases has, however, not yet been resolved. Primary meteoritic particles from the impacting body, the products of impact melting, or condensation from impact vapor plumes have all been proposed as possible genetic process. Resolving this requires detailed microanalytical investigation of the internal microchemical and microstructural compositions, textural characteristics, and crystallographic relationships between the different phases. Here, we report the results of a first transmission electron microscopy (TEM) study of six such PGE microparticles enclosed in Ni‐Cr spinel or occurring in groundmass of Barberton spherule layers from the BARB5 ICDP drill core and from the CT3 exploration core. Results include a variety of chemical and structural PGM compositions that are difficult to explain by a single process, leading to the conclusion that several processes may have been involved in the formation of PGMs in Archean spherule layers from the BGB. There is evidence supporting formation of these PGMs by exsolution from the spinel host phase, precipitation from a melt phase, and condensation from a gas phase (of the impact vapor plume).

¹Brandon Mahan, ^1,2Julien Siebert, ¹Ingrid Blanchard, ¹Stephan Borensztajn, ¹James Badro, ^1,2Frédéric Moynier
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.04.032]
¹Institut de Physique du Globe de Paris, Université Paris Diderot, Sorbonne Paris Cité, CNRS UMR 7154, 1 rue Jussieu, 75238 Paris Cedex 05
²Institut Universitaire de France, Paris, France
Copyright Elsevier

Zinc is a moderately volatile and slightly siderophile element, and therefore provides information into the timing and conditions of volatile element delivery to Earth as well as the redistribution of these elements during planetary differentiation. Specifically, due to its similar volatility with S, it has been assumed that the Earth and its source material maintain the same relative abundances of these elements, and therefore the same S/Zn ratio. In this study, we have conducted Zn metal-silicate partitioning experiments at pressures up to 81 GPa and temperatures up to 4100 K in diamond anvil cells, for two distinct silicate compositions (one pyrolitic, one basaltic) and varying S contents in order to characterize Zn metal-silicate partitioning as a function of these variables. These results have been input into numerous Earth formation models where various parametric controls have been evaluated, namely source material, impactor size and volatile delivery, to determine plausible sets of conditions that can generate present-day bulk silicate Earth (BSE) Zn and S abundances. Modelling results indicate that to arrive at present-day BSE contents for Zn and S, and with core S contents of ∼2 wt% or less, the Earth likely accreted heterogeneously – initially from a volatile-depleted source material chemically akin to the metal and silicate chondrules of CH chondrites, with later delivery of more volatile-rich material.

Reto Trappitsch¹, Patrick Boehnke^2,3, Thomas Stephan^2,3, Myriam Telus⁴, Michael R. Savina¹, Olivia Pardo^2,3, Andrew M. Davis^2,3,5, Nicolas Dauphas^2,3,5, Michael J. Pellin^2,3,5,6, and Gary R. Huss⁷
Astrophysical Journal Letters 857, L2 Link to Article [DOI: 10.3847/2041-8213/aabba9]
¹Lawrence Livermore National Laboratory, Nuclear and Chemical Sciences Division, 7000 East Avenue, L-231, Livermore, CA 94550, USA
²The University of Chicago, Department of the Geophysical Sciences, 5734 South Ellis Avenue, Chicago, IL 60637, USA
³Chicago Center for Cosmochemistry, Chicago, IL, USA
⁴University of California Santa Cruz, Earth and Planetary Sciences, 1156 High Street, Santa Cruz, CA 95064, USA
⁵The University of Chicago, Enrico Fermi Institute, Chicago, IL 60637, USA
⁶Argonne National Laboratory, Materials Science Division, 9700 South Cass Avenue, Argonne, IL 60439, USA
⁷Hawai’i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai’i at Mānoa, 1680 East-West Road, POST 602, Honolulu, HI 96822, USA

Establishing the abundance of the extinct radionuclide ⁶⁰Fe (half-life 2.62 Ma) in the early solar system is important for understanding the astrophysical context of solar system formation. While bulk measurements of early solar system phases show a low abundance consistent with galactic background, some in situ measurements by secondary ion mass spectrometry (SIMS) imply a higher abundance, which would require injection from a nearby supernova (SN). Here we present in situ nickel isotopic analyses by resonance ionization mass spectrometry (RIMS) in a chondrule from the primitive meteorite Semarkona (LL3.00). The same chondrule had been previously analyzed by SIMS. Despite improved precision compared to SIMS, the RIMS nickel isotopic data do not reveal any resolved excesses of ⁶⁰Ni that could be unambiguously ascribed to in situ ⁶⁰Fe decay. Linear regression of ⁶⁰Ni/⁵⁸Ni versus ⁵⁶Fe/⁵⁸Ni yields an initial ⁶⁰Fe/⁵⁶Fe ratio for this chondrule of (3.8 ± 6.9) × 10⁻⁸, which is consistent with both the low initial value found by bulk measurements and the low end of the range of initial ratios inferred from some in situ work. The same regression also gives a solar initial ⁶⁰Ni/⁵⁸Ni ratio, which shows that this sample was not disturbed by nickel mobilization, thus agreeing with a low initial ⁶⁰Fe/⁵⁶Fe ratio. These findings agree with a re-evaluation of previous SIMS measurements of the same sample. Supernova injection of ⁶⁰Fe into the solar system or its parental cloud material is therefore not necessary to account for the measured solar system’s initial amount of ⁶⁰Fe.

Larry R. Nittler, Conel M. O’D. Alexander, Nan Liu¹, and Jianhua Wang
Astrophysical Journal Letters 856, L24 Link to Article [DOI: 10.3847/2041-8213/aab61f]
Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015, USA
¹Present address: Dept. of Physics, Washington University, 1 Brookings Drive, St. Louis, MO 63130, USA.

We report the identification of 19 presolar oxide grains from the Orgueil CI meteorite with substantial enrichments in ⁵⁴Cr, with ⁵⁴Cr/⁵²Cr ratios ranging from 1.2 to 56 times the solar value. The most enriched grains also exhibit enrichments at mass-50, most likely due in part to ⁵⁰Ti, but close-to-normal or depleted ⁵³Cr/⁵²Cr ratios. There is a strong inverse relationship between ⁵⁴Cr enrichment and grain size; the most extreme grains are all <80 nm in diameter. Comparison of the isotopic data with predictions of nucleosynthesis calculations indicate that these grains most likely originated in either rare, high-density Type Ia supernovae (SN Ia), or in electron-capture supernovae (ECSN), which may occur as the end stage of evolution for stars of mass 8–10 M _⊙. This is the first evidence for preserved presolar grains from either type of supernova. An ECSN origin is attractive, as these likely occur much more frequently than high-density SN Ia, and their evolutionary timescales (~20 Myr) are comparable to those of molecular clouds. Self-pollution of the Sun’s parent cloud from an ECSN may explain the heterogeneous distribution of n-rich isotopic anomalies in planetary materials, including a recently reported dichotomy in Mo isotopes in the solar system. The stellar origins of three grains with solar ⁵⁴Cr/⁵²Cr, but anomalies in ⁵⁰Cr or ⁵³Cr, as well as of a grain enriched in ⁵⁷Fe, are unclear.

Ryuki Hyodo and Hidenori Genda
Astrophysical Journal Letters 856, L36 Link to Article [DOI: 10.3847/2041-8213/aab7f0]
Earth-Life Science Institute/Tokyo Institute of Technology, 2-12-1 Tokyo, Japan

Observations and meteorites indicate that the Martian materials are enigmatically distributed within the inner solar system. A mega impact on Mars creating a Martian hemispheric dichotomy and the Martian moons can potentially eject Martian materials. A recent work has shown that the mega-impact-induced debris is potentially captured as the Martian Trojans and implanted in the asteroid belt. However, the amount, distribution, and composition of the debris has not been studied. Here, using hydrodynamic simulations, we report that a large amount of debris (~1% of Mars’ mass), including Martian crust/mantle and the impactor’s materials (~20:80), are ejected by a dichotomy-forming impact, and distributed between ~0.5–3.0 au. Our result indicates that unmelted Martian mantle debris (~0.02% of Mars’ mass) can be the source of Martian Trojans, olivine-rich asteroids in the Hungarian region and the main asteroid belt, and some even hit the early Earth. The evidence of a mega impact on Mars would be recorded as a spike of ⁴⁰Ar–³⁹Ar ages in meteorites. A mega impact can naturally implant Martian mantle materials within the inner solar system.

Yassir A. Abdu¹, Frank C. Hawthorne², and Maria E. Varela³
Astrophysical Journal Letters 856, L9 Link to Article [DOI: 10.3847/2041-8213/aab433]
¹Department of Applied Physics and Astronomy, University of Sharjah, P.O. Box 27272, Sharjah, United Arab Emirates
²Department of Geological Sciences, University of Manitoba, Winnipeg, MB R3T 2N2, Canada
³Instituto de Ciencias Astronómicas de la Tierra y del Espacio (ICATE) Avenida España 1512 sur, J5402DSP, San Juan, Argentina

We report the finding of nanodiamonds, coexisting with amorphous carbon, in carbonaceous-chondrite (CC) material from the Kapoeta achondritic meteorite by Fourier-transform infrared (FTIR) spectroscopy and micro-Raman spectroscopy. In the C–H stretching region (3100–2600 cm⁻¹), the FTIR spectrum of the Kapoeta CC material (KBr pellet) shows bands attributable to aliphatic CH₂ and CH₃ groups, and is very similar to IR spectra of organic matter in carbonaceous chondrites and the diffuse interstellar medium. Nanodiamonds, as evidenced by micro-Raman spectroscopy, were found in a dark region (~400 μm in size) in the KBr pellet. Micro-FTIR spectra collected from this region are dramatically different from the KBr-pellet spectrum, and their C–H stretching region is dominated by a strong and broad absorption band centered at ~2886 cm⁻¹ (3.47 μm), very similar to that observed in IR absorption spectra of hydrocarbon dust in dense interstellar clouds. Micro-FTIR spectroscopy also indicates the presence of an aldehyde and a nitrile, and both of the molecules are ubiquitous in dense interstellar clouds. In addition, IR peaks in the 1500–800 cm⁻¹ region are also observed, which may be attributed to different levels of nitrogen aggregation in diamonds. This is the first evidence for the presence of the 3.47 μm interstellar IR band in meteorites. Our results further support the assignment of this band to tertiary CH groups on the surfaces of nanodiamonds. The presence of the above interstellar bands and the absence of shock features in the Kapoeta nanodiamonds, as indicated by Raman spectroscopy, suggest formation by a nebular-condensation process similar to chemical-vapor deposition.

Oleksiy Golubov^1,2,3, Vladyslav Unukovych², and Daniel J. Scheeres¹
Astrophysical Journal Letters 857, L5 Link to Article [DOI: 10.3847/2041-8213/aaba15]
¹Department of Aerospace Engineering Sciences, University of Colorado at Boulder, 429 UCB, Boulder, CO 80309, USA
²School of Physics and Technology, V. N. Karazin Kharkiv National University, 4 Svobody Square, Kharkiv, 61022, Ukraine
³Institute of Astronomy of V. N. Karazin Kharkiv National University, 35 Sumska Street, Kharkiv, 61022, Ukraine

The evolution of rotation states of small asteroids is governed by the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, nonetheless some asteroids can stop their YORP evolution by attaining a stable equilibrium. The same is true for binary asteroids subjected to the binary YORP (BYORP) effect. Here we discuss a new type of equilibrium that combines these two, which is possible in a singly synchronous binary system. This equilibrium occurs when the normal YORP, the tangential YORP, and the BYORP compensate each other, and tidal torques distribute the angular momentum between the components of the system and dissipate energy. If unperturbed, such a system would remain singly synchronous in perpetuity with constant spin and orbit rates, as the tidal torques dissipate the incoming energy from impinging sunlight at the same rate. The probability of the existence of this kind of equilibrium in a binary system is found to be on the order of a few percent.

Andrew McNeill, David E. Trilling, and Michael Mommert
Astrophysical Journal Letters 857, L1 Link to Article [DOI: 10.3847/2041-8213/aab9ab]
Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ 86011, USA

1I/’Oumuamua was discovered by the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS 1) on 2017 October 19. Unlike all previously discovered minor planets, this object was determined to have eccentricity e > 1.0, suggesting an interstellar origin. Since this discovery and within the limited window of opportunity, several photometric and spectroscopic studies of the object have been made. Using the measured light curve amplitudes and rotation periods we find that, under the assumption of a triaxial ellipsoid, a density range 1500 < ρ < 2800 kg m⁻³ matches the observations and no significant cohesive strength is required. We also determine that an aspect ratio of 6 ± 1:1 is most likely after accounting for phase-angle effects and considering the potential effect of surface properties. This elongation is still remarkable, but less than some other estimates.