^1,2MingenPan(潘明恩)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113851]
¹Department of Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA
²Department of Computer Science, Columbia University, 500 West 120th Street, New York, NY 10027, USA
Copyright Elsevier

Refractory Metal Nuggets (RMNs; submicrometer highly siderophile element rich metal alloys) are observed in Ca, Al-rich inclusions (CAIs) and other components of primitive meteorites, and some RMNs could have condensed from the Solar Nebula. In order to study the condensation of RMNs in the Solar Nebula, NUCON – a kinetic condensation model – has been developed to simulate the nucleation and condensation of refractory siderophile metal phases. NUCON treats RMNs as solid solutions where multiple elements can accrete onto one RMN. To achieve this goal, the homogeneous nucleation theory is modified to compute the nucleation of solid solutions. Also, a numerical method is developed to compute the integration of condensation and evaporation rates of an RMN. Equilibrium among gaseous phases is also considered, including monatomic gases and oxides. The oxygen fugacity of the simulated Solar Nebula can also be modified by adjusting carbon abundance. NUCON shows that the nucleation of RMNs was inhibited even when the cooling rate of the Solar Nebula was below 0.1 K/year, and RMNs experienced kinetic condensation largely deviated from the equilibrium condensation.

This study modeled the condensation of RMNs in the RMN-forming regions with different cooling rates, total pressures, and oxygen fugacities, and explored how these parameters affect the radii and Ni/Fe ratios of RMNs. To reproduce the RMNs reported in literature, most of which have radii from 100 to 1000 nm, the cooling rate during the accretion of refractory siderophile metals in RMN-forming regions should be in the order of 1 K/year. The timescale of refractory-metal condensation is in the order of 10² years. In addition, RMNs have been measured to have Ni/Fe ratios from almost zero to over unity, and NUCON shows that the cooling rate during FeNi accretion in RMN-forming regions should be in the order of 10 K/h so that the observed Ni/Fe ratios of RMNs can be reproduced. The timescale of FeNi condensation is in the order of 10 h. Thus, NUCON predicts a transition from slow cooling to rapid cooling that is likely to have occurred during RMN condensation.

¹Sarah Lentfort,¹Addi Bischoff,^1,2Samuel Ebert,¹Markus Patzek
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13486]
¹Institut für Planetologie, Westfälische Wilhelms‐Universität Münster, Wilhelm‐Klemm Str. 10, D‐48149 Münster, Germany
²SOEST/HIGP, University of Hawaii at Manoa, University of Hawai’i, 1680 East‐West Road, POST 516B, Honolulu, HI, 96822 USA
Published by arrangement with John Wiley & Sons

CM chondrites are complex impact (mostly regolith) breccias, in which lithic clasts show various degrees of aqueous alteration. Here, we investigated the degree of alteration of individual clasts within 19 different CM chondrites and CM‐like clasts in three achondrites by chemical analysis of the tochilinite‐cronstedtite‐intergrowths (TCIs; formerly named “poorly characterized phases”). To identify TCIs in various chondritic lithologies, we used backscattered electron (BSE) overview images of polished thin sections, after which appropriate samples underwent electron microprobe measurements. Thus, 75 lithic clasts were classified. In general, the excellent work and specific criteria of Rubin et al. (2007) were used and considered to classify CM breccias in a similar way as ordinary chondrite breccias (e.g., CM2.2‐2.7). In BSE images, TCIs in strongly altered fragments in CM chondrites (CM2.0‐CM2.2) appear dark grayish and show a low contrast to the surrounding material (typically clastic matrix), and can be distinguished from TCIs in moderately (CM2.4‐CM2.6) or less altered fragments (CM2.7‐CM2.9); the latter are bright and have high contrast to the surroundings. We found that an accurate subclassification can be obtained by considering only the “FeO”/SiO₂ ratio of the TCI chemistry. One could also consider the TCIs’ S/SiO₂ ratio and the metal abundance, but these were not used for classification due to several disadvantages. Most of the CM chondrites are finds that have suffered terrestrial weathering in hot and cold deserts. Thus, the observed abundance of metal is susceptible to weathering and may not be a reliable indicator of subtype classification. This study proposes an extended classification scheme based on Rubin’s scale from subtypes CM2.0‐CM2.9 that takes the brecciation into account and includes the minimum to maximum degree of alteration of individual clasts. The range of aqueous alteration in CM chondrites and small spatial scale of mixing of clasts with different alteration histories will be important for interpreting returned samples from the OSIRIS‐REx and Hayabusa 2 missions in the future.

¹Devin L.Schrader,²Kazuhide Nagashima,¹Jemma Davidson,³Timothy J.McCoy,⁴Ryan C.Ogliore,⁵Roger R.Fu
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.05.014]
¹Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, 781 East Terrace Road, Tempe, AZ 85287, USA
²HIGP/SOEST, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
³Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th & Constitution Avenue NW, Washington, DC 20560-0119, USA
⁴Department of Physics, Washington University in St. Louis, St. Louis, MO 63130, USA
⁵Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138, USA
Copyright Elsevier

Determining the origins of chondrule precursors is key to constraining how material migrated in the early Solar System. Chondrules that were only partially melted during their formation retain portions of their solid precursors, termed relict grains. By measuring the chemical and O-isotopic compositions of relict grains in chondrules from an unequilibrated ordinary chondrite (UOC), and Renazzo-like carbonaceous (CR) and Mighei-like carbonaceous (CM) chondrites we constrain their origins and discuss implications for disk transport within the first 4 million years of the Solar System. For all three chondrite groups, the chemical and O-isotopic compositions of dusty olivine grains are sometimes consistent with the reduction of type I (FeO-poor) and/or type II (FeO-rich) chondrules from the same meteorite group. However, other dusty olivine grains from the CM chondrites and the UOC are found to be xenocrysts that require an origin from a source distinct from the host meteorite. This material plausibly originated as fragments of earlier-formed chondrules from another chondrite group or of partially or fully differentiated planetesimals that migrated into an active chondrule-forming region. Multiple CM chondrite dusty olivine chondrules have O-isotope compositions that match those of UOC chondrule olivine (Δ¹⁷O ∼ 0‰), suggesting an origin from an UOC source. This implies that UOC chondrules and/or chondrule fragments migrated from the inner Solar System outwards to CM chondrite chondrule-forming region, likely beyond the orbit of Jupiter. These UOC chondrules or chondrule fragments could have migrated outwards in the protoplanetary disk before the formation of the Jupiter Gap, or <300 μm diameter fragments could have migrated outwards after Gap formation as CM chondrite chondrule dusty olivine grains with Δ¹⁷O ∼ 0‰ were small enough to pass through Jupiter Gap. The identification of xenocrysts in each meteorite group studied here argues for widespread migration of material in the early Solar System, potentially crossing the Jupiter Gap.

^1,2Zoltán Váci,¹Carl B. Agee,²Christopher D. K. Herd,^2,3Erin Walton,⁴Oliver Tschauner,¹Karen Ziegler,⁵Vitali B. Prakapenka,⁵Eran Greenberg,⁴Sylvia Monique‐Thomas
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13479]
¹Department of Earth and Planetary Sciences, Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, 87106 USA
²Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3 Canada
³Department of Earth and Planetary Sciences, MacEwan University, Edmonton, Alberta, T5J 4S2 Canada
⁴Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, Nevada, 89154 USA
⁵GSECARS, Argonne National Laboratory, Lemont, Illinois, 60439 USA
Published by arrangement with John Wiley & Sons

Hydrous alteration of olivine macrocrysts in a Martian olivine phyric basalt, NWA 10416, and a terrestrial basalt from southern Colorado are examined using SEM, EPMA, TEM, and µXRD techniques. The olivines in the meteorite contain linear nanotubes of hydrous material, amorphous areas, and fluid dissolution textures quite distinct from alteration identified in other Martian meteorites. Instead, they bear resemblance to terrestrial deuteric alteration features. The presence of the hydrous alteration phase Mg‐laihunite within the olivines has been confirmed by µXRD analysis. The cores of the olivines in both Martian and terrestrial samples are overgrown by unaltered rims whose compositions match those of a separate population of groundmass olivines, suggesting that the core olivines are xenocrysts whose alteration preceded crystallization of the groundmass. The terrestrial sample is linked to deep crustal metasomatism and the “ignimbrite flare‐up” of the Oligocene epoch. The comparison of the two samples suggests the existence of an analogous relatively water‐rich magmatic reservoir on Mars.

^1,2Naoya Imae,¹Makoto Kimura
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13491]
¹National Institute of Polar Research, 10‐3 Midori‐cho, Tachikawa‐shi, Tokyo, 190‐8518 Japan²Department of Polar Science, the Graduate University for Advanced Studies, SOKENDAI, 10‐3 Midori‐cho, Tachikawa‐shi, Tokyo, 190‐8518 Japan
Published by arrangement with John Wiley & Sons

Identification and characterization of small extraterrestrial samples, such as small Antarctic meteorites <~1 cm, require the development of convenient laboratory‐based nondestructive analytical techniques using X‐ray diffraction (XRD). We explore the characterization criteria using an X‐ray diffractometer with a Gandolfi attachment using sub‐mm small fragments and powder aggregates for various kinds of stony meteorites and develop a new analytical technique. We primarily focus on olivine and pyroxene because they are the most abundant and important minerals for stony meteorite classification. A new calibration is performed to estimate the FeO content of the olivine in unequilibrated ordinary chondrites, which is useful for determining the meteorite chemical group irrespective of powder aggregate diameter but dependent on fragment grain diameter. This is because X‐ray intensity absorption is more effective for grains than for powders. Clinoenstatite (Cen) and orthoenstatite (Oen) were distinguished using the presence or absence of the isolated Oen 511 index peak. The method is also applied to other stony meteorites including carbonaceous chondrites and achondrites. The XRD results are consistent with studies based on polished sections involving textural observations by scanning microscope and chemical compositions of the constituent minerals. The new measurement technique presented here is convenient because of its use in air by the laboratory‐based X‐ray diffractometer, which makes it useful for the initial analyses of restricted extraterrestrial sample characterization.

¹N.M.Abreu,^2,3J.C.Aponte,⁴E.A.Cloutis,⁵A.N.Nguyen
Geochemistry (Chemie der Erde) (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2020.125631]
¹Earth Science, Pennsylvania State University – DuBois Campus, DuBois, PA, 15801, USA
²Solar System Exploration Division, Code 691, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
³Department of Chemistry, Catholic University of America, Washington, DC 20064, USA
⁴Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, MB R3B 2E9, Canada
⁵Jacobs Engineering Group Inc., NASA Johnson Space Center, Houston, Texas 77058, USA
Copyright Elsevier

We present here a review of the characteristics of CR carbonaceous chondrite meteorites. Over the past three decades, our knowledge and understanding of the scientific value of the CR chondrites have increased dramatically, as more samples from cold and hot deserts have become available for analysis. Based on a variety of compositional, mineralogical, isotopic, and spectroscopic studies, we have come to understand that CR chondrites are excellent samples of asteroidal meteorites to look for virtually unaltered solar nebula material and to observe asteroidal processes in progress. This paper summarizes these investigations, their similarities, and differences with other chondritic groups, their relationships to asteroids, and the questions yet to be addressed.

^1,2Samuel Ebert,¹Kazuhide Nagashima,¹Alexander N.Krot,²Addi Bischoff
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.05.012]
¹School of Ocean, Earth Science and Technology, Hawai’i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, USA
²Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Germany
Copyright Elsevier

The nature of oxygen-isotope heterogeneity in refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)] from weakly metamorphosed chondrites is one of the outstanding problems in cosmochemistry. To obtain insights into possible processes resulting in O-isotope heterogeneity of refractory inclusions, we investigated the mineralogy, petrology, and oxygen isotopic compositions of six CAIs and two AOAs and aqueously formed fayalite grains within the matrix of the H3.1 chondrite Northwest Africa (NWA) 3358. Most of the refractory inclusions studied appear to be unmolten solar nebula condensates; some may have experienced partial melting and/or high-temperature annealing. The NWA 3358 refractory inclusions nearly completely avoided metasomatic alteration on the H-chondrite parent body: nepheline grains replacing anorthite and/or melilite are either very minor or absent. Five out of eight refractory inclusions studied have heterogeneous O-isotope composition: Δ17O ranges from ∼ −25‰ to ∼ 3.5±2‰ (2σ). This O-isotope heterogeneity appears to be mineralogically controlled with melilite and anorthite being systematically 16O-depleted compared to hibonite, spinel, Al,Ti-diopside, and forsterite all having similar solar-like Δ17O of ∼ −24±2‰. In contrast to NWA 3358 refractory inclusions, the previously studied AOAs and a fine-grained CAI from the LL3.00 chondrite Semarkona have uniform Δ17O of ∼ −25‰ (McKeegan et al., 1998; Itoh et al., 2007). Because the mineralogically-controlled O-isotope heterogeneity in refractory inclusions from ordinary chondrites appears to correlate with petrologic type of a host meteorite experienced by aqueous alteration, we suggest O-isotope exchange in NWA 3358 CAIs and AOAs resulted from aqueous fluid-rock interaction on the H-chondrite parent asteroids. This is supported by the presence of 16O-depleted anorthite (Δ17O ∼ 3.5±2‰) and aqueously formed fayalite similar depleted in 16O (Δ17O ∼ 4±2‰). The Δ17O of NWA 3358 fayalite is comparable to that of magnetite and fayalite in Semarkona and other weakly metamorphosed L3 and LL3 chondrites (Choi et al., 1998; Doyle et al., 2015) suggesting similar Δ17O of aqueous fluids on the H, L, and LL chondrite parent asteroids.

¹Yuri Shimaki et al. (>10)
Icarus (in Press) Link to article [https://doi.org/10.1016/j.icarus.2020.113835]
¹Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan
Copyright Elsevier

TIR, the thermal infrared imager on Hayabusa2, acquired high-resolution thermal images of the asteroid 162173 Ryugu for one asteroid rotation period on August 1, 2018 to investigate the thermophysical properties of the asteroid. The surface temperatures of Ryugu suggest that the surface has a low thermal inertia, indicating the presence of porous materials. Thermophysical models that neglect or oversimplify surface roughness cannot reproduce the flat diurnal temperature profiles observed during daytime. We performed numerical simulations of a thermophysical model, including the effects of roughness on the diurnal brightness temperature, the predictions of which successfully reproduced the observed diurnal variation of temperature. The global thermal inertia was obtained with a standard deviation of 225 ± 45 J m⁻² s^−0.5 K⁻¹, which is relatively low but still within the range of the value estimated in our previous study (Okada et al., Nature 579, 518–522, 2020), confirming that the boulders on Ryugu are more porous in nature than typical carbonaceous chondrites. The global surface roughness (the ratio of the variance of the height relative to a local horizontal surface length) was determined as 0.41 ± 0.08, corresponding to a RMS surface slope of 47 ± 5°. We identified a slightly lower roughness distributed along the equatorial ridge, implying a mass movement of boulders from the equatorial ridge to the mid-latitudes.

^1,2L. F. White,^1,2,3A. Černok,⁴J. R. Darling,⁵M. J. Whitehouse,⁶K. H. Joy,⁷C. Cayron,⁴J. Dunlop,^1,2 K. T. Tait,^3,8M. Anand
Nature Astronomy (in Print) Link to Article [DOIhttps://doi.org/10.1038/s41550-020-1092-5]
¹Centre of Applied Planetary Mineralogy, Department of Natural History, Royal Ontario Museum, Toronto, Ontario, Canada
²Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada
³School of Physical Sciences, The Open University, Milton Keynes, UK
⁴School of Earth and Environmental Sciences, University of Portsmouth, Portsmouth, UK
⁵Swedish Museum of Natural History, Stockholm, Sweden
⁶Department of Earth and Environmental Science, University of Manchester, Manchester, UK
⁷Laboratory of ThermoMechanical Metallurgy (LMTM), PX Group Chair, École Polytechnique Fédérale de Lausanne (EPFL), Neuchâtel, Switzerland
⁸Department of Earth Sciences, The Natural History Museum, London, UK

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^1,2Michaël Marsset et al. (>10)
Nature Astronomy (in Press) Link to Article [DOIhttps://doi.org/10.1038/s41550-019-1007-5]
¹Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, MA, USA
²Astrophysics Research Centre, Queen’s University Belfast, Belfast, UK

We currently do not have a copyright agreement with this publisher and cannot display the abstract here