¹Hirakawa, N.,¹Kebukawa, Y.,²Furukawa, Y.,³Kondo, M.,⁴Nakano, H.,¹Kobayashi, K.
Earth, Planets and Space 73, 16 Link to Article [DOI: 10.1186/s40623-020-01352-6]
¹Graduate School of Engineering Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan
²Department of Earth Science, Tohoku University, 6-3 Aza-aoba, Aramaki, Aoba-ku, Sendai, 980-8578, Japan
³Instrumental Analysis Center, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan
⁴Faculty of Culture and Sport Policy, Toin University of Yokohama, 1614 Kurogane-cho, Aoba-ku, Yokohama, 225-8503, Japan

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¹Kim Müsing,¹Henner Busemann,¹Liliane Huber,¹Colin Maden,¹My E. I. Riebe,¹Rainer Wieler,²Knut Metzler
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13644]
¹Department of Earth Sciences, Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, CH‐8092 Zürich, Switzerland
²Institut für Planetologie, University of Münster, Wilhelm‐Klemm‐Str. 10, 48149 Münster, Germany
Copyright Elsevier

We measured noble gases in “cluster chondrite clasts” from nine unequilibrated ordinary chondrites (UOCs). For five meteorites, we also present data for so‐called “clastic matrix,” the impact‐brecciated material in which the angular to subrounded cluster chondrite clasts are often embedded. Cluster chondrite clasts are characterized by close‐fit texture of deformed and indented chondrules with lower amounts of fine‐grained interchondrule matrix than in other UOCs (Metzler 2012). They are ubiquitous in UOCs and may indicate accretion and compaction of hot and deformable chondrules within hours or days after formation. Clastic matrix of four of the five meteorites contains He and Ne implanted by the solar wind (SW), indicating that they are regolith breccias. In contrast, cluster chondrite clasts are essentially devoid of SW, confirming that they are fragments of “primary accretionary rocks” (Metzler 2012). Trapped Kr and Xe in all samples are essentially primordial (type “Q”). Trapped Xe concentrations in cluster chondrite clasts are similar to values in other UOCs of similar metamorphic grade despite their low fractions of primordial gas‐bearing fine‐grained materials. This possibly indicates that the interchondrule matrix in cluster chondrite clasts is more pristine than matrix of regular UOCs. Later loss of primordial gases during parent body metamorphism is mirrored in the decreasing concentrations of primordial noble gases with increasing petrologic type. Relative to cluster chondrite lithologies, clastic matrix often contains excesses of cosmogenic noble gases, most likely due to precompaction exposure in the parent body regolith.

^1,2Michael A. Velbel,³Michael E. Zolensky
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13636]
¹Department of Earth and Environmental Sciences, Michigan State University, 288 Farm Lane, Room 207, Natural Sciences Building, East Lansing, Michigan, 48824–1115 USA
²Division of Meteorites, Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia, 20013–7012 USA
³X12 Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, Texas, 77058 USA
Published by arrangement with John Wiley & Sons

The target bodies of C‐complex asteroid sample return missions are carbonaceous chondrite‐like near‐Earth asteroids (NEAs), chosen for the abundance and scientific importance of their organic compounds and “hydrous” (including hydroxylated) minerals, such as serpentine‐group phyllosilicates. Science objectives include returning samples of pristine carbonaceous regolith from asteroids for study of the nature, history, and distribution of its constituent minerals, organic material, and other volatiles. Heating after the natural aqueous alteration that formed the abundant phyllosilicates in CM and similar carbonaceous chondrites dehydroxylated them and altered or decomposed other volumetrically minor constituents (e.g., carbonates, sulfides, organic molecules; Tonui et al. 2003, 2014). We propose a peak‐temperature thermometer based on dehydroxylation as measured by analytical totals from electron probe microanalysis (EPMA) of matrices in a number of heated and aqueously altered (but not further heated) CM chondrites. Some CM lithologies in Maribo and Sutter’s Mill do not exhibit the matrix dehydroxylation expected for surface temperatures expected from insolation of meteoroids with their known orbital perihelia. This suggests that insolated‐heated meteoroid surfaces were lost by ablation during passage through Earth’s atmosphere, and that insolation‐heated material is more likely to be encountered among returned asteroid regolith samples than in meteorites. More generally, several published lines of evidence suggest that episodic heating of some CM material, most likely by impacts, continued intermittently and locally up to billions of years after assembly and early heating of ancestral CM chondrite bodies. Mission spectroscopic measures of hydration can be used to estimate the extent of dehydroxylation, and the new dehydroxylation thermometer can be used directly to select fragments of returned samples most likely to contain less thermally altered inventories of primitive organic molecules.

¹Sébastien Charnoz,²Paolo A.Sossi,³Yueh-Ning Lee,¹Julien Siebert,⁴Ryuki Hyodo,¹Laetitia Allibert,¹Francesco C.Pignatale,¹Maylis Landeau,⁵Apurva V.Oza,¹Frédéric Moynier
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114451]
¹Université de Paris, Institut de Physique du Globe de Paris, CNRS, F-75005 Paris, France
²Institute of Geochemistry and Petrology, ETH Zürich, CH-8092 Zürich, Switzerland
³Department of Earth Sciences, National Taiwan Normal University, 88, Sec. 4, Ting-Chou Road, Taipei City 11677, Taiwan
⁴ISAS, JAXA, Sagamihara, Japan
⁵Physikalisches Institut, Universität Bern, Bern, Switzerland
Copyright Elsevier

Prevailing models for the formation of the Moon invoke a giant impact between a planetary embryo and the proto-Earth (Canup, 2004; Ćuk et al., 2016). Despite similarities in the isotopic and chemical abundances of refractory elements compared to Earth’s mantle, the Moon is depleted in volatiles (Wolf and Anders, 1980). Current models favour devolatilisation via incomplete condensation of the proto-Moon in an Earth-Moon debris-disk (Charnoz and Michaut, 2015; Canup et al., 2015; Lock et al., 2018). However the physics of this protolunar disk is poorly understood and thermal escape of gas is inhibited by the Earth’s strong gravitational field (Nakajima and Stevenson, 2014). Here we investigate a simple process, wherein the Earth’s tidal pull promotes intense hydrodynamic escape from the liquid surface of a molten proto-Moon assembling at 3–6 Earth radii. Such tidally-driven atmospheric escape persisting for less than 1 Kyr at temperatures ∼1600 − 1700 K reproduces the measured lunar depletion in K and Na, assuming the escape starts just above the liquid surface. These results are also in accord with timescales for the rapid solidification of a plagioclase lid at the surface of a lunar magma ocean (Elkins-Tanton et al., 2011). We find that hydrodynamic escape, both in an adiabatic or isothermal regime, with or without condensation, induces advective transport of gas away from the lunar surface, causing a decrease in the partial pressures of gas species (Ps) with respect to their equilibrium values (Psat). The observed enrichment in heavy stable isotopes of Zn and K (Paniello et al., 2012; Wang and Jacobsen, 2016) constrain Ps/Psat > 0.99, favouring a scenario in which volatile loss occurred at low hydrodynamic wind velocities (<1% of the sound velocity) and thus low temperatures. We conclude that tidally-driven atmospheric escape is an unavoidable consequence of the Moon’s assembly under the gravitational influence of the Earth, and provides new pathways toward understanding lunar formation.

¹Mathilde Faure et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114462]
¹Université Grenoble Alpes, CNRS, Institut de Planétologie et Astrophysique de Grenoble (IPAG), UMR 5274, Grenoble F-38041, France
Copyright Elsevier

We question here the radiolytic origin of (i) polyaromatic insoluble organic matter (IOM) recovered from primitive chondrites, and (ii) organics at the surface of reddish Trans-Neptunian Objects (TNOs), some minor planets and icy satellites. Organic synthesis by ion irradiation was investigated through experiments on a variety of targets: Polyethylene glycol 1450, lignin, cellulose and sucrose, exposed to low (C 40 keV and Ne 170 keV) and high energy (C 12 MeV, Ni 17 MeV, 78Kr 59 MeV) ions. These experiments show that all carbonaceous precursors evolve towards a sp2-rich amorphous carbon (a-C) above a critical nuclear dose of 10−7+10 eV.atom−1. A thorough review of the literature shows that this value applies for a large range of carbonaceous materials, including C-rich simple ices. Below this critical dose, irradiated targets are carbonized and transformed into cross-linked polymeric disordered solids, with abundant olefinic and acetylenic bonds, but devoid of aromatic or polyaromatic species. Ion irradiation of simple compounds, e.g. ices, is thereby not a viable process to synthesize IOM. However, in the case of aromatic-rich precursors, swift heavy ions irradiation leads to polyaromatic materials, by bridging existing aromatic or polyaromatic units. In the context of Early Solar System, i.e. Galactic Cosmic Rays (GCR) irradiation during 10–20 Myr, the formation of chondritic IOM from simple ices mixed with interstellar Polycyclic Aromatic Hydrocarbons (PAHs) appears as a plausible mechanism. This scenario, based on the recycling of existing carbonaceous interstellar grains under low-temperature conditions, would account for the heterogeneity of the D, 15N and 13C isotopic fractionations at the molecular scale, and the preservation of deuterium hot spots that are highly sensitive to high-temperature conditions (> 300 °C). At the surface of TNOs, sp2-rich amorphous carbons are formed by the implantation of GCRs and Solar wind ions. The electronic dose is also very high for an irradiation time of several Gyr (> 100 eV.atom−1), leading to the formation of reddish disordered solids, provided that the surface contains a minimum abundance of carbonaceous species. Finally, sp2-rich amorphous carbons produced in the laboratory (e.g. the ACAR compound from Zubko et al., 1996) are fair analogues of the darkening agent produced by radiolysis.

¹Craig R. Walton,²Ioannis Baziotis,³Ana Černok,⁴Ludovic Ferrière,⁵Paul D. Asimow,^1,6Oliver Shorttle,^3,7Mahesh Anand
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13648]
¹Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ UK
²Department of Natural Resources Management and Agricultural Engineering, Agricultural University of Athens, IeraOdos 75, 11855 Athens, Greece
³Department of Physical Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA UK
⁴Natural History Museum, Burgring 7, A‐1010 Vienna, Austria
⁵Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E California Blvd, Pasadena, California, 91125 USA
⁶Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 OHA UK
⁷Department of Earth Sciences, The Natural History Museum, London, SW7 5BD UK
Published by arrangement with John Wiley & Sons

The geochemistry and textures of phosphate minerals can provide insights into the geological histories of parental asteroids, but the processes governing their formation and deformation remain poorly constrained. We assessed phosphorus‐bearing minerals in the three lithologies (light, dark, and melt) of the Chelyabinsk (LL5) ordinary chondrite using scanning electron microscope, electron microprobe, cathodoluminescence, and electron backscatter diffraction techniques. The majority of studied phosphate grains appear intergrown with olivine. However, microtextures of phosphates (apatite [Ca5(PO4)3(OH,Cl,F)] and merrillite [Ca9NaMg(PO4)7]) are extremely variable within and between the differently shocked lithologies investigated. We observe continuously strained as well as recrystallized strain‐free merrillite populations. Grains with strain‐free subdomains are present only in the more intensely shocked dark lithology, indicating that phosphate growth predates the development of primary shock‐metamorphic features. Complete melting of portions of the meteorite is recorded by the shock‐melt lithology, which contains a population of phosphorus‐rich olivine grains. The response of phosphorus‐bearing minerals to shock is therefore hugely variable throughout this monomict impact breccia. We propose a paragenetic history for P‐bearing phases in Chelyabinsk involving initial phosphate growth via P‐rich olivine replacement, followed by phosphate deformation during an early impact event. This event was also responsible for the local development of shock melt that lacks phosphate grains and instead contains P‐enriched olivine. We generalize our findings to propose a new classification scheme for Phosphorus‐Olivine‐Assemblages (Type I–III POAs). We highlight how POAs can be used to trace radiogenic metamorphism and shock metamorphic events that together span the entire geological history of chondritic asteroids.

^1,2Caroline E. Caplan,²Gary R. Huss,²Hope A. Ishii,²John P. Bradley,^2,3Birger Schmitz,¹Kazuhide Nagashima
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13649]
¹Department of Earth Sciences, University of Hawai‘i at Mānoa, 1680 East‐West Road, Honolulu, Hawai‘i, 96822 USA
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, 1680 East‐West Road, Honolulu, Hawai‘i, 96822 USA
³Astrogeobiology Laboratory, Department of Physics, Lund University, Lund, Sweden
Published by arrangement with John Wiley & Sons

Remnant extraterrestrial chrome spinels from terrestrial sediments provide information on how the mixture of meteoritic materials falling to Earth has changed over Earth’s history. The parent meteorite type of each grain can be identified by characteristic elemental and oxygen‐isotope abundances. Some meteorite types can be difficult to classify because their chrome‐spinel compositional ranges overlap. Silicate inclusions within chrome spinels of modern ordinary chondrites have been shown to have discriminating power among meteorite subclasses. We employed energy‐dispersive X‐ray spectroscopy in a scanning electron microscope (SEM) and in a (scanning) transmission electron microscope (S/TEM) to investigate inclusions in chrome‐spinel grains from Ordovician and Jurassic sediments. Unaltered Ordovician inclusions allowed us to establish the size limits for reliable SEM analysis of inclusions. The Jurassic grains were more altered, but the use of STEM techniques on small inclusions (<3 μm diameter at their polished surfaces) allowed us to determine chemical compositions and mineral structures of inclusions in three chrome spinels. The parent meteorite type was determined for one Jurassic grain based on its inclusion compositions. Our study confirms that silicate inclusions can be used to classify parent meteorite types of chrome‐spinel grains, but the size of the inclusions and the complex effects of terrestrial alteration must be taken into account. During our study, we also found some interesting exsolution phenomena in the host chrome‐spinel grains.

¹Akimasa Suzumura,²Noriyuki Kawasaki,³Yusuke Seto,²Hisayoshi Yurimoto,¹Shoichi Itoh
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.03.030]
¹Department of Earth and Planetary Sciences, Kyoto University, Kyoto 606-8502, Japan
²Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
³Department of Planetology, Kobe University, Kobe 657-8501, Japan
Copyright Elsevier

We report the in-situ oxygen isotopic distributions corresponding to the petrographic-mineralogical observation on a compact Type A (CTA) Ca-Al-rich inclusion (CAI), KU-N-02, from a reduced CV3 chondrite, Northwest Africa 7865. The CTA has an igneous texture and mainly consists of spinel, melilite, and Al-Ti-rich clinopyroxene (fassaite). Oxygen isotopic compositions of the constituent minerals plot along the carbonaceous chondrite anhydrous mineral line. The spinel grains are poikilitically enclosed in the melilite and fassaite and are uniformly 16O-rich (Δ17O = approximately −23‰). The fassaite is texturally classified into two types: blocky fassaite and intergranular fassaite. The blocky fassaite crystals exhibit growth zoning as they change from Ti-rich to Ti-poor along the inferred directions of crystal growth from core to rim, while the oxygen isotopic compositions change from 16O-poor (Δ17O = approximately −6‰) to 16O-rich (Δ17O = approximately −23‰) with crystal growth. The intergranular fassaite crystals exist between the melilite crystals and exhibit variable Ti abundance and oxygen isotopic compositions. Additionally, their relationships between Ti contents and oxygen isotopic composition are similar to those of the blocky fassaite. The melilite grains are homogeneously 16O-poor (Δ17O = approximately −2‰), irrespective of their åkermanite (Åk) content. Each melilite grain generally exhibits growth zoning with increasing Åk contents from core to rim, although the melilite contains Åk-rich patches within single crystal. Åk-rich patches often include two types of fassaite: small blebby crystals attached to spinel crystals and round crystals. The oxygen isotopic compositions of the Åk-rich patch and blebby fassaite are 16O-poor (Δ17O = approximately −2‰), similar to that of the host melilite. On the other hand, the round fassaite exhibits significant variation in oxygen isotopic compositions ranging from Δ17O = −23‰ to −4‰, which are different from those of the host melilite. These petrographic textures and oxygen isotopic variations indicate the presence of a solid precursor with variable oxygen isotopic compositions for the CTA. The spinel and round fassaite grains are relicts of the precursor that melted in the 16O-poor nebular gas, resulting in the crystallization of the host melilite from the 16O-poor melt. The Åk-rich patches and blebby fassaite crystallized from melts trapped by the growing host melilite crystals. The blocky and intergranular fassaite crystallized after the melilite did, and the oxygen isotopic composition of the melt changed to 16O-rich during the crystallization process, suggesting that the oxygen isotopic composition of the surrounding nebular gas could be varied. The inferred oxygen isotopic evolution for CTA is consistent with those inferred for Type B CAIs, suggesting that coarse-grained igneous CAIs formed in a similar nebular environment regardless of precursor chemistry.

¹F. N. Lindsay et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13637]
¹Department of Chemistry & Chemical Biology, Rutgers University, New Brunswick, New Jersey, 08854 USA
Published by arrangement with John Wiley & Sons

The Martian breccias NWA 7034, NWA 7533, and paired meteorites record events ranging in age from 4.47 Ga to <200 Ma. Published ages indicate a period of major disturbance at ~1.4 Ga, examined in detail here through 40Ar/39Ar dating of handpicked grains and two small chips. Argon diffusion parameters were obtained for six samples. Also presented are He, Ne, Ar, Kr, and Xe contents of two small (<100 µg), handpicked mineral separates, a felsic “Light” sample and a mafic/pyroxene‐rich “Dark” sample. The 40Ar/39Ar ages of five samples, four containing >1 wt% K and thought to be rich in feldspar and one containing <~1 wt% K, cluster near 1.4 Ga. The 40Ar/39Ar ages of nine grains with low K contents have a wide range of apparent ages from 0.3 ± 0.1 Ga to 2.9 ± 0.1 Ga for individual temperature steps, and from 0.74 ± 0.06 Ga to ~2.1 Ga for plateau ages. Isochron ages are less precise, but generally agree with plateau ages. Only two isochrons have the significantly positive intercepts expected in the presence of terrestrial or Martian atmospheric argon. At higher release temperatures, activation energies for diffusion obtained from 39Ar data for six samples are generally 160–200 kJ mol−1, consistent with published values for feldspathic minerals. For three of these samples, lower temperature data on Arrhenius plots are best fit with a much lower activation energy of <100 kJ mol−1. We attribute the low values to the effects of varying degrees of shock on feldspathic minerals and/or the presence of phases in vitrophyric spherules produced by hydrothermal alteration. The low activation energies place an upper limit of ~14 ka on the terrestrial age of NWA 7034. Much lower concentrations of cosmogenic (c) 3He and 21Ne in the Light than in the Dark separate indicate substantial losses concurrent with or postdating cosmic ray irradiation. A one‐stage, cosmic ray exposure (CRE) age for the Dark separate from NWA 7034 is estimated to be between 7 and 10 Ma from the concentrations of 3Hec and 38Arc, and of close to 15 Ma from the concentration of 21Nec. Most of the 40Ar/39Ar and noble gas data are compatible with (1) a heating and alteration event ~1.40 Ga caused by contact metamorphism, an impact, and/or the infiltration of hydrothermal fluids; and (2) at least one later event at lower temperatures that led to either loss of He and Ar from phases with low activation energies, or to gain of K. Most of the 40Ar/39Ar ages are consistent with the assembly of NWA 7034 1.4 Ga ago or perhaps earlier followed more recently by selective alteration. A more recent time of assembly is also consistent with these ages provided that the temperature stayed low. The five most precise 40Ar/39Ar ages of the samples analyzed are all ~1.4 Ga, a value seen frequently in other NWA 7034 chronometers and very similar to crystallization ages of nakhlites and chassignites (NC). Some CRE ages based on noble gases in NWA 7034 agree within their considerable uncertainties with those of NC. These two chronometric coincidences suggest that the NWA 7034 clan and the NC share a launch date on Mars. We propose that K‐rich fluids derived from the nakhlite source area interacted with proto‐NWA 7034 and modified the K/Ar ratios and ages of previously shocked feldspar grains, with the degree of modification depending on the degree of shock. The NWA 7034 clan may therefore be considered components from a metamorphic aureole around a nakhlite massif.

^1,2F. Jourdan,^2,3L. Forman,^1,2T. Kennedy,¹G. K. Benedix,⁴E. Eroglu,¹C. Mayers
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13640]
¹Western Australian Argon Isotope Facility, John de Laeter Centre, TIGeR, Curtin University, Perth, 6845 Western Australia, Australia
²Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, Perth, 6845 Western Australia, Australia
³Department of Earth & Planetary Sciences, Western Australian Museum, Locked Bag 49, Welshpool DC, Perth, 6986 Western Australia, Australia
⁴School of Molecular and Life Sciences, Curtin University, Perth, 6845 Western Australia, Australia
Published by arrangement with John Wiley & Sons

Asteroid 4 Vesta is the only largely preserved differentiated asteroid and thus is an excellent proxy to study early magmatism occurring on planets and moons. In this study, we focus on eucrite Pecora Escarpment (PCA) 82502, a medium‐ to fine‐grained eucrite which chemical analyses suggest belongs to the main howardite–eucrite–diogenite clan, albeit with some peculiarities. We carried out backscattered electron and electron backscattered diffraction microscopy analyses of the meteorite along with step‐heating 40Ar/39Ar dating analyses of various types of groundmass aliquots. We show that Pecora Escarpment 82502 is composed of medium‐grained igneous crystalline clasts and smaller fractured satellite clasts surrounded by approximately 50 µm wide impact melt veins of the same composition. Our results show that the large crystalline clasts and fine‐grained veins both display little evidence of shock processes. Six groundmass aliquots from large crystalline clasts returned concordant plateau (>70% of 39Ar) or mini‐plateau (50–70% of 39Ar) 40Ar/39Ar ages with a weighted mean of 4531 ± 6 Ma (P = 0.67). Thermodynamic cooling and 40Ar diffusion models suggest that the K/Ar system recorded and preserved the igneous age despite subsequent infiltration of hot and quickly quenched melt veins. Our new igneous age, combined with evidence for four other young volcanic and plutonic eucrites of similar age, shows that Vesta was still magmatically active around 4531 Ma. The lack of younger ages suggests that this age might well represent the end of the magmatic activity in the upper crust of Vesta. When combined with existing paleomagnetic constraints, our data suggest that 4 Vesta had an active dynamo that was still active ~35 Ma after accretion.