Andreas T. Hertwig^a, Kimura Makoto^b, Takayuki Ushikubo^c, Céline Defouilloy, ^aNoriko T.Kita^a
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.02.020]
^aWiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA
^bNational Institute of Polar Research, Meteorite Research Center, Midoricho 10-3, Tachikawa, Tokyo 190-8518, Japan
^cKochi Institute for Core Sample Research, JAMSTEC, 200 Monobe-otsu, Nankoku, Kochi 783-8502 Japan
Copyright Elsevier

The ²⁶Al-²⁶Mg ages of FeO-rich (type II) chondrules from Acfer 094, one of the least thermally metamorphosed carbonaceous chondrites, were determined by SIMS analysis of plagioclase and olivine/pyroxene using a radio frequency (RF) plasma oxygen ion source. In combination with preexisting ²⁶Al-²⁶Mg ages of FeO-poor (type I) chondrules, the maximum range of formation ages recorded in chondrules from a single meteorite is determined to help provide constraints on models of material transport in the proto-planetary disk. We also report new SIMS oxygen three-isotope analyses of type II chondrules in Acfer 094. All but one of the plagioclase analyses show resolvable excesses in ²⁶Mg and isochron regressions yield initial ²⁶Al/²⁷Al ratios of type II chondrules that range from (3.62 ± 0.86) × 10^–6 to (9.3 ± 1.1) × 10^–6_, which translates to formation ages between 2.71 ^–0.22/_+0.28 Ma and 1.75 ^–0.11/_+0.12 Ma after CAI. This overall range is indistinguishable from that determined for type I chondrules in Acfer 094. The initial ²⁶Al/²⁷Al ratio of the oldest type II chondrule is resolved from that of all other type II chondrules in Acfer 094. Importantly, the oldest type I chondrule and the oldest type II chondrule in Acfer 094 possess within analytical error indistinguishable initial ²⁶Al/²⁷Al ratios and Δ¹⁷O values of ∼0‰. Ages and oxygen isotope ratios clearly set these two chondrules apart from all other chondrules in Acfer 094. It is therefore conceivable that the formation region of these two chondrules differs from that of other chondrules and in turn suggests that Acfer 094 contains two distinct chondrule generations.

Conel M. O’D.Alexander
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.02.008]
Dept. Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington DC 20015, USA
Copyright Elsevier

A quantitative understanding of the elemental and isotopic fractionations recorded in the compositions of the chondritic meteorites would provide fundamental constraints for astrophysical models of early Solar System evolution. Here it is shown through least squares fitting that almost all features of the bulk elemental and isotopic compositions of the main carbonaceous chondrite (CC) groups, as well as the ungrouped Tagish Lake (C2) meteorite, can be reproduced using mixtures of the same four components. The fractionations amongst the non-CCs (ordinary, Rumuruti and enstatite chondrites) are distinctly different to those in the CCs and are the subject of a separate study (Alexander, 2019). The four CC components are: (1) a ‘chondrule’ (or chondrule precursor) component that partially lost Fe,Ni metal and volatiles, but is otherwise CI-like, (2) the CC-RI component that has a refractory inclusion-like bulk composition and is largely responsible for the refractory element enrichments and nucleosynthetic isotope anomalies in the bulk CCs, (3) anhydrous and reduced but otherwise CI-like matrix that accounts for almost all of the most volatile element (e.g., Zn, Se and C) contents of the CCs, and (4) water with relatively high Δ¹⁷O and δ¹⁸O values. Comparison of the inferred component compositions to additional meteoritic constraints produces some notable results. The ε⁴⁸Ca≈8 and ε⁵⁰Ti≈8 values for the CC-RI component are consistent with the average value for refractory inclusions. On the other hand, the ε⁵⁴Cr≈-10 is not, but is required by the negative correlation between ε⁵⁰Ti and ε⁵⁴Cr amongst the bulk CCs. The CC-RI component may be comprised of a more CAI-like sub-component that carries the ε⁴⁸Ca and ε⁵⁰Ti anomalies, and a more ferromagnesian sub-component that carries the negative ε⁵⁴Cr anomalies. The compositions of the volatile and metal subcomponents lost from the ‘chondrule’ component are consistent with condensation models, suggesting that the fractionations predated chondrule formation. The isotopic compositions of chondrules from the more CC-RI-rich CC groups (e.g., CV, CO and CM) seem to require the addition of some of the CC-RI component to their precursors. The assumption that matrix is CI-like is inconsistent with chondrule-matrix complementarity, but is justified by the success of the fits and the relatively uniform and CI-like abundances of organics and presolar grains in the matrices of the most primitive CCs. The inferred Δ¹⁷O=3.5 ‰ for the water component is consistent with most constraints from secondary phases in the CCs. The large O isotopic mass fractionation (δ¹⁸O≈18-21 ‰) of the water is consistent with ∼89-95 % condensation of ice from a vapor under Rayleigh conditions at 150-170 K. The water was entirely accreted with the matrix with fairly constant (0.32±0.06 by wt.) and CI-like (∼0.38 by wt.) water/matrix ratios. These water/matrix ratios are much less than the water/rock ratio of one that is often cited for a nebula of solar composition, but can be explained if much of the C in the CC formation regions was present as CO and CO₂, and the abundance of CH₄ was low.

¹David Baratoux et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13253]
¹Geosciences Environnement Toulouse, UMR5563, CNRS, Universite de Toulouse & IRD, 14, Avenue Edouard Belin, 31400Toulouse, France
Published by arrangement with John Wiley & Sons

The about 10.5 km diameter Bosumtwi impact crater is one of the youngest large impact structures on Earth. The crater rim is readily noticed on topographic maps or in satellite imagery. It defines a circular basin filled by water (Lake Bosumtwi) and lacustrine sediments. The morphology of this impact structure is also characterized by a circular plateau extending beyond the rim and up to 9–10 km from the center of the crater (about 2 crater radii). This feature comprises a shallow ring depression, also described as an annular moat, and a subdued circular ridge at its outer edge. The origin of this outermost feature could so far not be elucidated based on remote sensing data only. Our approach combines detailed topographic analysis, including roughness mapping, with airborne radiometric surveys (mapping near‐surface K, Th, U concentrations) and field observations. This provides evidence that the moat and outer ring are features inherited from the impact event and represent the partially eroded ejecta layer of the Bosumtwi impact structure. The characteristics of the outer ridge indicate that ejecta emplacement was not purely ballistic but requires ejecta fluidization and surface flow. The setting of Bosumtwi ejecta can therefore be considered as a terrestrial analog for rampart craters, which are common on Mars and Venus, and also found on icy bodies of the outer solar system (e.g., Ganymede, Europa, Dione, Tethys, and Charon). Future studies at Bosumtwi may therefore help to elucidate the mechanism of formation of rampart craters.

¹Andreas Morlok,²Stephan Klemme,¹Iris Weber,¹Aleksandra Stojic,³Martin Sohn, ¹Harald Hiesinger,⁴Joern Helbert
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.02.010]
¹Institut für Planetologie, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Institut für Mineralogie, Corrensstraße 24, 48149 Münster, Germany
³Hochschule Emden/Leer, Constantiaplatz 4, 26723 Emden, Germany
⁴Institute for Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany
Copyright Elsevier

The MERTIS (MErcury Radiometer and Thermal Infrared Spectrometer) instrument onboard the ESA/JAXA BepiColombo mission will provide mid-infrared data, which will be crucial to characterize the surface mineralogy of Mercury. In order to interpret the results, we are creating a database of mid-infrared spectra. As part of a study of synthetic glasses which are to serve as analog materials for the interpretation of remote sensing and modeling data, we present mid-infrared data for analog materials of Mercury regolith, surface and mantle compositions. In addition, we provide data for similar analogs of Earth, Moon, Venus, and Mars rocks for a coherent picture.
The analog samples have been first characterized by optical microscopy, Raman spectroscopy and EMPA. Powdered size fractions (0–25 μm, 25–63 μm, 63–125 μm, and 125–250 μm) were studied in reflectance in the mid-infrared range from 2.5 to 18 μm (550 to 2000 cm−1), additional micro-FTIR analyses were also obtained.
Results for the size fractions of the surface and regolith analogs for Mercury show typical features for amorphous material with Christiansen Features (CF) at 8–8.1 μm, Reststrahlen Bands (RB) at 9.8–9.9 μm, and the Transparency Feature (TF) at 12 μm. The six bulk silicate Mercury analogs have varying CF positions from 8.1 to 9 μm, with RB crystalline features of various olivines dominating in most samples. Similarly, bulk silicate analogs of the other planetary bodies show glassy features for the surface analogs with CF from 7.9 μm (Earth Continental Crust) to 8.3 μm (Lunar Mare), strong RB from 9.5 μm (Earth Continental Crust) to 10.6 μm (Lunar Mare and Highlands). TF are usually very weak for the glassy analogs.
Bulk silicate analogs for the other planetary bodies are again dominated by crystalline olivine features. Trends between SCFM (SiO2/(SiO2 + CaO + FeO + MgO)) index reflecting polymerization, CF and RB positions, and the SiO2 contents in previous studies are basically confirmed, but there is indication that several samples (Moon Mare and Highlands) do not follow the trend observed for low SiO2 samples and the RB position in earlier studies. Comparison with a ground based mid-IR spectrum of Mercury demonstrates general similarity in band positions with glassy Mercury surface and regolith material, and a chondrite based model of the planet. However, no mix so far can explain all spectral features.

^1,2Burlakovs, J.,³Krievans, M.,³Seglins, V.,⁴Berzins, K.,⁵Stiebrins, O.
International Multidisciplinary Scientific GeoConference Surveying Geology and Mining Ecology Management, SGEM 18, 75-81 Link to Article[DOI: 10.5593/sgem2018/1.1/S01.010]
¹Linnaeus University, Sweden
²Russian Geographical Society,
Russian Federation
³University of Latvia, Latvia
⁴Astronomical Society of Latvia and Meteoriti.LV, Latvia
⁵Geological Society of Latvia, Latvia

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¹Aonishi, T.,²Hirata, T.,^3,4Kuwatani, T.,⁵Fujimoto, M.,³Chang, Q.,³Kimura, J.-I.
Journal of Analytical Atomic Spectroscopy 33, 2210-2218 Link to Article [DOI: 10.1039/c7ja00334j]
¹School of Computing, Tokyo Institute of Technology, Nagatsuda 4259, Yokohama, 226-8502, Japan
²Geochemical Research Center, University of Tokyo, Hongo, 7-3-1, Tokyo, 113-0033, Japan
³Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology, Natsushima-cho 2-15, Yokosuka, 237-0061, Japan
⁴PRESTO, Japan Science and Technology Agency (JST), Honcho 4-1-8, Kawaguchi, 332-0012, Japan
⁵Department of Geoscience, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan

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^1,2Ananya Mallik,^1,3Tariq Ejaz,¹Svyatoslav Shchek,⁴Gordana Garapic
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.02.014]
¹Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany
²Department of Geosciences, University of Rhode Island, 9 E. Alumni Avenue, Kingston RI 02881, United States of America
³National Centre for Antarctic and Ocean Research, Headland Sada, Vasco-da-Gama, Goa 403804, India
⁴State University of New York at New Paltz, 1 Hawk Drive, New Paltz, NY 12561, United States of America
Copyright Elsevier

Lunar mantle overturn caused by gravitational instability of the Fe-Ti rich KREEP layer (formed as the last 5% of a crystallizing magma ocean, and emplaced between the overlying anorthitic crust and the underlying lunar mantle) is a process that would introduce Fe-Ti enriched bodies deep inside the lunar interior. These chemical heterogeneities in the lunar mantle may be the source of the very Fe-Ti enriched near-primary Apollo basalts. Also, the Fe-Ti and KREEP enriched layer deep inside the Moon may be responsible for the 5-30% partial melt seismically detected close to the core-mantle boundary (CMB). This is assuming that that the partial melt is neutrally buoyant at P-T conditions of the CMB. Here, we experimentally investigate the phase equilibria of the overturned Fe-Ti rich layer mixed with the mantle, at P-T conditions deep inside the lunar interior, focusing on the partial melt compositions formed. Our aim is to test (a) whether potential partial melt compositions formed near the CMB are neutrally buoyant with respect to the surrounding mantle, hence, stable; (b) if the partial melts formed within the lunar interior are positively buoyant and ascend, whether they can reproduce chemical characteristics of Apollo basalts. The densities calculated for the Fe-Ti rich partial melts from this study, using the physical parameters from previous studies, range from lower to higher values compared to that of the lunar mantle. This provides a basis for future investigations to experimentally constrain better the densities of these partial melts. Depending on the buoyancy of the partial melts, the following two scenarios are likely to happen. Firstly, if the partial melts are neutrally buoyant at the CMB, 5-30% partial melt would constrain the CMB temperatures between 1330(±1) – 1470(±19) °C. This can be used by future studies to derive the selenotherm better. Secondly, if the partial melts are positively buoyant, they should ascend and react with the mantle along their path. Upon reaching shallow depths below the crust, they may likely assimilate any Fe-Ti rich layer that was left over from the gravitational overturn, as well as undergo olivine fractionation upon pooling in a shallow magma chamber. We modeled assimilation-fractional crystallization of the partial melts using the Gibb’s-free minimization algorithm alphaMELTS. Our results show that reactive ascent of Fe-Ti rich partial melts through the lunar mantle and subsequent olivine fractionation in a shallow magma chamber is a promising way to evolve the melt compositions to converge with the lunar basalts better. Shallow level assimilation of Fe-Ti rich lithology post reactive-ascent through the mantle is also feasible, but only for low degrees of assimilation.

¹Hikari Hasegawa,^1,2Takashi Mikouchi,^3,4Akira Yamaguchi,⁵Masahiro Yasutake,⁶Richard C. Greenwood,⁶Ian A. Franchi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13249]
¹Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, , Tokyo, 113‐0033 Japan
²The University Museum, The University of Tokyo, , Tokyo, 113‐0033 Japan
³Antarctic Meteorite Research Center, National Institute of Polar Research, Tachikawa, Tokyo, 190‐8518 Japan
⁴Department of Polar Science, School of Multidisciplinary Science, Graduate University for Advanced Sciences, Tachikawa, Tokyo, 190‐8518 Japan
⁵Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto, 606‐8502 Japan
⁶ and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA UK
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 6112, Miller Range (MIL) 090206 (plus its pairs: MIL 090340 and MIL 090405), and Divnoe are olivine‐rich ungrouped achondrites. We investigated and compared their petrography, mineralogy, and olivine fabrics. We additionally measured the oxygen isotopic compositions of NWA 6112. They show similar petrography, mineralogy, and oxygen isotopic compositions and we concluded that these five meteorites are brachinite clan meteorites. We found that NWA 6112 and Divnoe had a c axis concentration pattern of olivine fabrics using electron backscattered diffraction (EBSD). NWA 6112 and Divnoe are suggested to have been exposed to magmatic melt flows during their crystallization on their parent body. On the other hand, the three MIL meteorites have b axis concentration patterns of olivine fabrics. This indicates that the three MIL meteorites may be cumulates where compaction of olivine grains was dominant. Alternatively, they formed as residues and were exposed to olivine compaction. The presence of two different olivine fabric patterns implies that the parent body(s) of brachinite clan meteorites experienced diverse igneous processes.

¹P. Rochette, ²R. Alaç³P. Beck,²G. Brocard,⁴A. J. Cavosie,⁵V. Debaille,¹B. Devouard,⁴F. Jourdan,^6,7B. Mougel,¹F. Moustard,⁶F. Moynier,⁸S. Nomade,⁹G. R. Osinski,¹⁰B. Reynard,¹¹J. Cornec
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13244]
¹Aix‐Marseille Univ., CNRS, INRA, IRD, Coll. France, CEREGE, 13545 Aix‐en‐Provence, France
²Basin Genesis Hub, School of Geosciences, University of Sydney, Sydney, Australia
³Univ Grenoble Alpes, CNRS, IPAG, , 38041 Grenoble, France
⁴Space Science and Technology Centre and The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Science, Curtin University, Perth, Western Australia, Australia
⁵Laboratoire G‐Time, Université Libre de Bruxelles, Brussels, Belgium
⁶Institut de Physique du Globe de Paris, Université Sorbonne Paris Cité, CNRS UMR 7154, Paris, France
⁷Centro de geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro
⁸LSCE, CEA, , 91190 Gif sur Yvette, France
⁹Centre for Planetary Science and Exploration and Department of Earth Science, University of Western Ontario, London, Canada
¹⁰University of Lyon, ENS de Lyon, Université Claude Bernard Lyon 1, , 69007 Lyon, France
¹¹Geologist, Denver, USA
Published by arrangement with John Wiley & Sons

The circa 14 km diameter Pantasma circular structure in Oligocene volcanic rocks in Nicaragua is here studied for the first time to understand its origin. Geomorphology, field mapping, and petrographic and geochemical investigations all are consistent with an impact origin for the Pantasma structure. Observations supporting an impact origin include outward‐dipping volcanic flows, the presence of former melt‐bearing polymict breccia, impact glass (with lechatelierite and low H₂O, <300 ppm), and also a possible ejecta layer containing Paleozoic rocks which originated from hundreds of meters below the surface. Diagnostic evidence for impact is provided by detection in impact glass of the former presence of reidite in granular zircon as well as coesite, and extraterrestrial ε⁵⁴Cr value in polymict breccia. Two ⁴⁰Ar/³⁹Ar plateau ages with a combined weighted mean age of 815 ± 11 ka (2 σ; P = 0.17) were obtained on impact glass. This age is consistent with geomorphological data and erosion modeling, which all suggest a rather young crater. Pantasma is only the fourth exposed crater >10 km found in the Americas south of N30 latitude, and provides further evidence that a significant number of impact craters may remain to be discovered in Central and South America.

¹J. Semprich,²S. P. Schwenzer,¹A.H. Treiman,¹J. Filiberto
Journal of Geophysica research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005869]
¹Lunar and Planetary Institute, USRA, Houston, TX, USA
²School of Environment, Technology, Engineering and Mathematics, The Open University, Milton Keynes, UK
Published by arrangement with John Wiley & Sons

Hydrous phases have been identified to be a significant component of martian mineralogy. Particularly prehnite, zeolites, and serpentine are evidence for low‐grade metamorphic reactions at elevated temperatures in mafic and ultramafic protoliths. Their presence suggests that at least part of the martian crust is sufficiently hydrated for low‐grade metamorphic reactions to occur. A detailed analysis of changes in mineralogy with variations in fluid content and composition along possible martian geotherms can contribute to determine the conditions required for subsurface hydrous alteration, fluid availability and rock properties in the martian crust. In this study, we use phase equilibria models to explore low‐grade metamorphic reactions covering a pressure‐temperature range of 0‐0.5 GPa and 150‐450 °C for several martian protolith compositions and varying fluid content. Our models replicate the detected low‐grade metamorphic/hydrothermal mineral phases like prehnite, chlorite, analcime, unspecified zeolites, and serpentine. Our results also suggest that actinolite should be a part of lower‐grade metamorphic assemblages, but actinolite may not be detected in reflectance spectra for several reasons. By gradually increasing the water content in the modeled whole rock composition, we can estimate the amount of water required to precipitate low‐grade metamorphic phases. Mineralogical constraints do not necessarily require an elevated geothermal gradient for the formation of prehnite. However, restricted crater excavation depths even for large impact craters are not likely sampling prehnite along colder gradients, either suggesting a geotherm of ~ 20 °C/km in the Noachian or an additional heat source such as hydrothermal or magmatic activity.