¹John T. Wasson
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13205]
¹Institute of Geophysics, University of California, Los Angeles, California, USA
Published by arrangement with John Wiley & Sons

A sample of Campo del Cielo with any other name would have the same composition. During the last three decades, our instrumental neutron activation analyses (INAA) of many supposedly new iron meteorites have shown an anomalously large fraction to have compositions within the compositional field of the IAB‐MG iron Campo del Cielo. A plot of Ir versus Au provides the best discrimination; only two independent‐fall irons found after 1980 with good recovery documentation fall within the 90% contour ellipse around the centroid of this Campo field, and one of these is from Antarctica. Now (early 2018) a total of 36 other irons attributed to other geographical locations have compositions that cannot be resolved from the Campo compositional field. Because it is possible that some of these are actually independent falls, the Meteoritical Society Nomenclature Committee has chosen to assign about half these meteorites Nova XXX names used for meteorites whose discovery localities are not adequately documented. However, for Campo‐like irons with too little information (e.g., total weight not known) or for which no adequately large type specimens are available, the decision is to call them Campos with the working name used during the UCLA analysis. In the UCLA Meteorite Collection, they are cataloged together with the documented Campos.

¹M. Shyam Prasad, ¹N. G. Rudraswami, ¹Agnelo Alexandre De Araujo, ¹V. D. Khedekar
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13206]
¹Geological Oceanography Division, CSIR–National Institute of Oceanography, Dona Paula, Goa, India
Published by arrangement with John Wiley & Sons

Metal in various forms is common in almost all meteorites but considerably rare among micrometeorites. We report here the discovery of two metal micrometeorites, i.e., (1) an awaruite grain similar to those found in the metal nodules of CV chondrites and (2) a metal micrometeorite of kamacite composition enclosing inclusions of chromite and merrillite. This micrometeorite appears to be a fragment of H5/L5 chondrite. These metal micrometeorites add to the inventory of solar system materials that are accreted by the Earth in microscopic form. They also strengthen the argument that a large proportion of material accreted by the Earth that survives atmospheric entry is from asteroidal sources.

¹Giulio F.D.Solferino, ²Gregor J.Golabek
Earth and Planetary Science Letters 504, 38-52 Link to Article [https://doi.org/10.1016/j.epsl.2018.09.027]
¹Department of Earth Sciences, Royal Holloway University of London, TW20 0EX Egham, United Kingdom
²Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
Copyright Elsevier

The origin of pallasites has been the focus of a number of recent studies. Yet, their formation process remains elusive, while the mechanism leading to the genesis of the sub-group termed ‘mixed type’ pallasites (containing polygonal, rounded, and fragmental olivines simultaneously) is unclear. Here we test the hypothesis of mixing of olivine fragments with Fe–Ni–S after a non-destructive impact followed by annealing employing both experimental analogues and numerical models.
The experimental series evidenced that the addition of sulfur to olivine + Fe–Ni accelerates olivine grain growth, though the growth rate is reduced when Fe–Ni–S is not fully molten. This is shown to be the consequence of competing growth of olivine and Fe–Ni grains.
Numerical models satisfying available formation constraints from natural samples indicate that planetesimals with radii ≥200 km are favorable for the genesis of rounded olivine-bearing pallasites by annealing of fragments in partially molten Fe–Ni–S. Moreover, early mixing in the planetesimal can form regions containing olivine grains with different grain sizes that could explain the formation of mixed-type pallasites.

¹B. J. Tkalcec, ¹F. E. Brenker
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13204]
¹Institute of Geoscience, Goethe University, Frankfurt am Main, Germany
Published by arrangement with John Wiley & Sons

Shock is often given as the cause for many observations in meteorites due to the assumed previous exposure of most meteorites to at least one impact event that ultimately led to their ejection from their parent body. Here we present electron backscatter diffraction (EBSD) results on a substantially shocked dunitic achondrite, chassignite Northwest Africa (NWA) 8694, and question the general culpability of shock exposure for the formation of preferred orientation fabrics of meteoritic olivine crystals. Despite the ubiquitous presence of substantial shock indicators, the EBSD results for NWA 8694 reveal an absence of preferred orientation of olivine crystals, displaying instead an overall random fabric. We propose that the passage of shock waves through olivine crystals within a solid, crystalline, dunitic rock does not produce an overall preferred orientation, nor is it likely to actively form a whole‐rock random fabric but instead has likely no bearing on the formation of olivine orientation fabrics.

¹Laurette Piani, ¹Yves Marrocchi
Earth & Planetary Science Letters 504, 64-71 Link to Article [https://doi.org/10.1016/j.epsl.2018.09.031]
¹CRPG, UMR 7358 CNRS, Université de Lorraine, 54500 Vandoeuvre-lès-Nancy, France
Copyright Elsevier

Among the different groups of carbonaceous chondrites, variable concentrations of hydrous minerals and organic matter are observed that might be related to the time and/or place of formation of their asteroidal parent bodies. However, the precise distribution of these volatile-bearing components between chondrite groups and their chemical and isotopic compositions remain fairly unknown. In this study, we used a novel secondary ion mass spectrometry analytical protocol to determine the hydrogen isotopic composition of water-bearing minerals in CV-type carbonaceous chondrites. This protocol allows for the first time the D/H ratio of CV chondrite hydrous minerals to be determined without hindrance by hydrogen contributions from adjacent organic material. We found that water in the altered CV chondrites Kaba, Bali, and Grosnaja has an average D/H ratio of D/HCV-water = [144−21+8] × 10−6 (or δDCV-water = ‰−77−131+54‰, 2σ), significantly higher than water in most CM-type carbonaceous chondrites (D/HCM-water = [101 ± 6] × 10−6 or δDCM-water = −350 ± 40‰, 2σ). We show that because organic matter in CV chondrites is depleted in deuterium compared to that in CM chondrites, such differences could result from isotopic exchange between water and organics. Another possibility is that the CM and CV parent bodies sampled different reservoirs of water ice and organics characterized by variable isotopic compositions due to their different time and/or place of accretion.

^1,2Myriam Lemelin, ²Paul G.Lucey, ³Katarina Miljković, ⁴Lisa R.Gaddis, ⁴Trent Hare, ⁵Makiko Ohtake
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2018.10.003]
¹Lassonde School of Engineering, Earth and Space Science and Engineering Department, York University, 4700 Keele St, Toronto, ON, M3J 1P3, Canada
²Hawai‘i Institute of Geophysics and Planetology, Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI, 96822, USA
³Department of Applied Geology, Curtin University, Perth, WA, 6845, Australia
⁴Astrogeology Science Center, United States Geological Survey, Flagstaff, AZ, 86001, USA
⁵Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 252-5210, Japan

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

¹M.Salvatore, ²K.Truitt, ²K.Roszell, ³N.Lanza, ⁴E.Rampe, ⁵N.Mangold, ⁶E.Dehouck, ³R.Wiens, ³S.Clegg
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.10.007]
¹Northern Arizona University, Flagstaff, AZ
²University of Michigan-Dearborn, Dearborn, MI
³Los Alamos National Laboratory, Los Alamos, NM
⁴NASA Johnson Space Center, Houston, TX
⁵LPG-Nantes, Université de Nantes, France
⁶Université de Lyon, UCBL, ENSL, CNRS, LGL-TPE, 69622 Villeurbanne, France
Copyright Elsevier

Alteration of the uppermost surfaces of geologic materials is a pervasive process on planetary surfaces that is dependent upon factors including parent composition and the environment under which alteration is occurring. While rapid and pervasive in hot and humid climates on Earth, chemical weathering of rock surfaces has also been found to dominate in some of Earth’s coldest and driest landscapes as well. Specifically, surfaces dominated by resistant fine-grained igneous rocks in the Antarctic preserve evidence of oxidative weathering processes, which represent the initial immature surface alteration processes that stagnate due to the lack of available water and kinetics necessary for the production of more mature alteration phases. In this study, we test the hypothesis that oxidative weathering also dominates the surfaces of sedimentary rocks throughout the Antarctic. We investigated the chemistry and mineralogy of a suite of sedimentary rocks from the Transantarctic Mountains ranging from fine-grained tuffs to coarse-grained sandstones and conglomerates. Our results show that, like the previously studied fine-grained igneous rocks in the Antarctic, sedimentary rocks generally showed only minor chemical weathering signatures at their surfaces relative to their interiors. However, unlike the igneous rocks in this earlier study, the sedimentary rocks exhibited a wide variety of non-systematic differences between surface and interior compositions. This variability of surface weathering signatures is equally as complex as the physical properties and compositions inherently present within these different sedimentary lithologies. Based on these analyses, it is apparent that oxidative weathering products do not dominate the surfaces of sedimentary rocks throughout the Transantarctic Mountains, which instead exhibit a wide array of weathering signatures that are likely dependent on both lithological and environmental factors. Considering that sedimentary lithologies are widespread across a significant fraction of the martian surface, our results suggest that observed alteration signatures limited to the surfaces of martian sedimentary rocks are most likely to be minor and to vary as a result of the lithological properties of the specific rock unit and not as a result of the widespread influences of the modern cold and dry climatic conditions.

¹Matthew E.Sanborn, ¹Josh Wimpenny, ¹Curtis D.Williams, ¹Akane Yamakawa, ²Yuri Amelin, ³Anthony J.Irving, ¹Qing-ZhuYin
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.10.004]
¹Department of Earth and Planetary Sciences, University of California-Davis, One Shields Avenue, Davis, CA 95616 USA
²Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601 Australia
³Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195 USA
Copyright Elsevier

Northwest Africa (NWA) 6704/6693 are medium- to coarse-grained achondrites with unique petrologic and geochemical traits that are distinct from the currently established meteorite groups. Here, we report on the extinct 26Al-26Mg and 53Mn-53Cr systems to establish fine-scale chronology of its formation and Cr and Ti isotopic anomalies to constrain the composition of the source reservoir of NWA 6704/6693. Excesses in the neutron-rich 54Cr and 50Ti isotopes, due to nucleosynthetic anomalies, separate NWA 6704/6693 from the vast majority of established achondrites and instead resemble the excesses seen among the carbonaceous chondrites; specifically, the CR-type chondrites. The excesses in these isotopes indicate a common feeding zone during accretion in the protoplanetary disk between the source of NWA 6704/6693 and that of the carbonaceous chondrites. The 26Al-26Mg data for pyroxene and plagioclase from NWA 6704 yield a (26Al/27Al)0 = (3.15 ± 0.38)×10-7 (MSWD = 0.49) and δ26Mg∗ = -0.004 ± 0.005 at the time of isotopic closure. This initial (26Al/27Al)0 translates to an absolute age of 4563.14 ± 0.38 Ma, relative to the D’Orbigny angrite. However, given the potential heterogeneity of 26Al, the D’Orbigny angrite might not be a good age anchor for the purpose of calculating 26Al-26Mg ages. The 26Al-26Mg age relative to another carbonaceous achondrite NWA 2976 is 4562.66 ± 0.60 Ma. The 53Mn-53Cr systematics of NWA 6704/6693 indicate a (53Mn/55Mn)0 of (2.59 ± 0.34)×10-6 (MSWD = 1.2) with an evolved initial ε53Cr of +0.14 ± 0.03. The (53Mn/55Mn)0 yields an 53Mn-53Cr age of 4562.17 ± 0.76 Ma relative to the D’Orbigny angrite. Concordant ages determined using the short-lived 26Al-26Mg and 53Mn-53Cr systems and extant 207Pb-206Pb system (4562.60±0.30 Ma for NWA 6704/6693; Amelin et al., 2018) indicate rapid cooling and nearly contemporaneous closing of multiple isotope systems. The ancient crystallization ages and positive 54Cr and 50Ti anomalies of NWA 6704/6693 indicate widespread melting and differentiation processes occurring in both non-carbonaceous (NC) and carbonaceous chondrite (CC) regions of the protoplanetary disk. Additionally, we report the Cr and Ti isotopic composition for a petrologic range of CR-type materials (CR2, CR6, and achondrites). The additional Cr and Ti isotopic data for these CR-type materials indicates a range in isotopic composition not previously observed based on CR2 chondrites alone.

¹Josh Wimpenny, ¹Matthew E.Sanborn, ²Piers Koefoed, ³Ilsa R.Cooke, ³Claudine Stirling, ²Yuri Amelin, ¹Qing-ZhuYin
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.10.006]
¹Department of Earth and Planetary Sciences, University of California at Davis, One Shields Avenue, Davis, CA 95616, USA
²Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia
³Department of Chemistry, University of Otago, Dunedin 9016, New Zealand
Copyright Elsevier

The achondrite Asuka 881394 has mineralogy broadly similar to that of eucrites but is isotopically, chemically and texturally distinct from them. Previous U-Pb chronology shows that it is very old; forming within the first 0.8 Ma of the formation of calcium-aluminum rich inclusions (CAIs). However, the age difference between Asuka 881394 and other very old Solar System materials (CAIs, quenched angrites) measured with the 26Al-26Mg and 53Mn-53Cr extinct radionuclide chronometers, and 207Pb/206Pb chronometer, is not the same. This could be interpreted to reflect heterogeneity in the distribution of 26Al and 53Mn in the early Solar System. Given the significant implications for the early Solar System chronology if 26Al and 53Mn are indeed heterogeneously distributed, in this study we further investigate the origin of Asuka 881394 and the apparent age discordance between short-lived and absolute chronometers, by focusing on measurement of its ε54Cr composition, renewed measurements of the absolute Pb-Pb age and new, high precision measurements of its 26Al-26Mg systematics.
New Cr isotope data places additional constraints on the origin of Asuka 881394; its ε 54Cr value of -0.37 ± 0.10ε is resolvable outside of uncertainty from HED meteorites (-0.72 ± 0.10ε), reinforcing evidence from oxygen isotopic analyses that suggest it originated from a distinct parent body, unlikely to be 4 Vesta. New Pb-Pb analyses, combined with using a directly measured 238U/235U ratio for age calculation, result in a recalculated Pb-Pb age of 4564.95 ± 0.53Ma, ∼1.5Ma younger than previously reported age. With this age Asuka 881394 remains one of the oldest known achondrites in our Solar System. New high precision 26Al-26Mg data produce an initial 26Al/27Al ratio of 1.48 ± 0.12 × 10-6, within error of previous data. This ratio corresponds to an Al-Mg age of 4563.69 ± 0.36 Ma or 4564.83 ± 0.21 Ma relative to CAIs and the D’Orbigny angrite, respectively. Thus, depending on which age anchor is used, the 26Al-26Mg age is either 1.26 Ma or 0.12 Ma younger than the new Pb-Pb age, the latter being unresolved within analytical uncertainty.
Though a potential age discrepancy between 26Al-26Mg and Pb-Pb could be a result of heterogeneous distribution of 26Al, we demonstrate with our new high precision Mg isotope data, in conjunction with petrographic evidence, that the Mg isotope system has been disturbed in Asuka 881394. We suggest that the 26Al-26Mg system closed to diffusion after the U-Pb system, either due to slow cooling on the parent body or low-grade metamorphic re-equilibration of Mg. Thus, we can satisfactorily explain the observed age discrepancy between 26Al-26Mg and U-Pb systems in Asuka 881394 without invoking heterogeneous distribution of 26Al/27Al ratio in the early Solar System.
Comparison of the Asuka 881394 data with that of other anomalous achondrites from distinct parent bodies suggests that these could also have evolved from a source region with a canonical 26Al/27Al ratio. Because these achondrites have significant differences in their ε54Cr-Δ17O systematics, which could be indicative of location within the early protoplanetary disk, it is consistent with homogeneous distribution of 26Al in the early Solar System. Angrites remain an outlier; either because they evolved from a source with a lower 26Al/27Al ratio or because their 26Al-26Mg or Pb-Pb data are problematic. In either case, this suggests that basaltic angrites are questionable as the age anchor for the entire Solar System as a whole, and other very old, well preserved achondrites should be considered for that role.

¹Satoki Okabayashi, ¹Tetsuya Yokoyama, ¹Nao Nakanishi, ^1,2Hikaru Iwamori
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.10.003]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
²Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa 237-0061, Japan
Copyright Elsevier

To investigate the formation processes of metal grains in chondrites, we measured the abundances of highly siderophile elements (HSEs: Re, Os, Ir, Ru, Pt, Rh, Pd, and Au) using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) on individual Fe-Ni metal grains from four petrologic type 3 ordinary chondrites: NWA 6910 (L3.3), NWA 4910 (LL3.1), Richfield (LL3.7), and SAH 97210 (L/LL3.2). Among HSEs, the abundances of Pd and Au in the metal grains had positive correlations with the measured Ni abundances, indicating equilibrium partitioning of Pd and Au between kamacite and taenite via thermal metamorphism. In contrast, the other HSEs (Re, Os, Ir, Ru, Pt, Rh) showed large variations in concentrations spanning nearly three orders of magnitude without evidence of redistribution between kamacite and taenite, suggesting that these elements preserved the initial compositions before kamacite-taenite segregation. The CI-normalized HSE patterns presented large depletions in Os and Ir with relatively large Os/Ir variations (0.29–3.2) and the Ru/Ir ratios also varied significantly (0.27–40). In addition, HSE abundances in fine metal grains showed wide variations compared to those of coarse metal grains. We suggest that the variation of refractory HSE compositions in Fe-Ni metal grains with characteristic Os-Ir depletions were most likely caused by solid metal-liquid metal partitioning during crystallization of a Fe-Ni metal melt containing 2 wt.% of C. The liquid metal is considered to be generated during multiple heating events related to chondrule formation. The lack of Fe-Ni metal grains exhibiting coexistence of liquid metal and solid metal composition within a single metal grain would suggest that solid metal grains were physically segregated from the liquid metal during the crystallization of Fe-Ni metals. Droplets of the segregated liquid metal collided and merged with other liquid metal droplets and solid metal grains to form coarser metal grains. The resultant larger metal grains have relatively homogeneous HSE abundances that are close to the bulk metal composition as a result of the mixing of liquid metal with solid metal. In contrast, molten metal droplets and solid metal grains that did not collide and merge formed finer metal grains formed finer metal grains with more variable HSE abundances.