^1,2Paolo A. Sossi, ³Oliver Nebel, ¹Hugh St.C.O’Neill, ²FrédéricMoynier
Chemical Geology 477, 73-84 Link to Article [https://doi.org/10.1016/j.chemgeo.2017.12.006]
¹Research School of Earth Sciences, Australian National University, Acton 2601, ACT, Australia
²Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, CNRS, F-75005 Paris, France
³School of Earth, Atmosphere and Environment, Monash University, Clayton 3800, VIC, Australia

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^1,2Maurizio Pajola, ²Ted Roush, ^2,3Cristina Dalle Ore, ⁴Giuseppe A. Marzo, ⁵Emanuele Simioni
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2018.02.016]
¹Universities Space Research Association, NASA NPP Program1, USA
²NASA Ames Research Center, Moffett Field, CA, 94035, USA
³Carl Sagan Center, SETI Institute, Mountain View, CA, 94043, USA
⁴ENEA C. R. Casaccia, 00123, Roma, Italy
⁵INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122, Padova, Italy

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^1,2,3,4M. Van Ginneken, ¹M.J. Genge, ⁵R.P. Harvey
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.02.041]
¹IARC, Department of Earth Science and Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK
²Department of Earth Science, The Natural History Museum, London SW7 2BT, UK
³Department of Analytical, Environmental and Geo-Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussel, Belgium1
⁴Laboratoire G-Time, Université Libre de Bruxelles, Franklin Rooseveltlaan 50, 1050 Brussel, Belgium1
⁵Department of Geological Sciences, 112 A. W. Smith Building, Case Western Reserve University, Cleveland, OH 44106-7216, USA
Copyright Elsevier

We report on the discovery of microtektites (microscopic impact glass spherules) in a glacial moraine near Larkman Nunatak in the Transantarctic Mountains, Antarctica. The microtektites were identified based on their physical and chemical properties. Major and trace element compositions of the particles suggest that they may be related to the Australasian strewn field. This would further extend the current strewn field ∼800 km southward. Depletion in volatiles and enrichment in refractory elements in Larkman Nunatak microtektites fit the volatilization trend defined by Australasian microtektites, suggesting that they may represent a new highly vapor fractionated end-member thereof. This observation is supported by their low vesicularity and absence of mineral inclusions. This discovery has significant implications for the formation of microtektites (i.e. their evolution with respect to the distance from the source crater). Finally, the discovery of potentially old (i.e. 0.8 Ma) microtektites in moraine has implications for the stability of the East Antarctic Ice Sheet in the Larkman Nunatak area over the last ∼1 Ma and, as a consequence, the high efficiency of such moraines as traps for other extraterrestrial materials (e.g. micrometeorites and meteoritic ablation debris).

¹Helen E. Maynard-Casely, ²Morgan L. Cable, ²Michael J. Malaska, ²Tuan H. Vu, ²Mathieu Choukroun,  ²Robert Hodyss
American Mineralogist 103, 343-349 Link to Article [DOI: https://doi.org/10.2138/am-2018-6259]
¹Australian Nuclear Science and Technology Organisation, Kirrawee DC, New South Wales 2232, Australia
²Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109, U.S.A.
Copyright: The Mineralogical Society of America

Saturn’s moon Titan has a surface that is dominated by molecular materials, much of which are photochemically produced in the moon’s atmosphere. This outlook reviews the potential minerals that would be expected to form on the surface and subsurface of Titan from these molecular solids. We seek to classify them and look toward how the future study of these minerals will enhance our understanding of this planetary body. The classification uses the basis of intermolecular interactions, with the materials grouped into “Molecular solids,” “Molecular co-crystals,” and “Hydrates” classes alongside speculation on other possible classes of potential Titan minerals.

¹Noël Chaumard,¹Céline Defouilloy, ¹Noriko T. Kita
Geochimica et Cosmochmica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.02.040]
¹WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706-1692, USA
Copyright Elsevier

High-precision oxygen three-isotope measurements of olivine and pyroxene were performed on 29 chondrules in the Murchison CM2 chondrite by secondary ion mass spectrometry (SIMS). The oxygen isotope ratios of analyzed chondrules all plot very close to the primitive chondrule minerals (PCM) line. In each of 24 chondrules, the olivine and/or pyroxene grains analyzed show indistinguishable oxygen isotope ratios. Exceptions are minor occurrences of isotopically distinguished relict olivine grains, which were found in nine chondrules. The isotope homogeneity of these phenocrysts is consistent with a co-magmatic crystallization of olivine and pyroxene from the final chondrule melts and a significant oxygen isotope exchange between the ambient gas and the melts. Homogeneous type I chondrules with Mg#’s of 98.9–99.5 have host chondrule Δ17O values ranging from –6.0‰ to –4.1‰, with one exception (Δ17O: –1.2‰; Mg#: 99.6). Homogeneous chondrules with Mg#’s <96, including four type II chondrules (Mg# ∼65–70), have Δ17O values of around –2.5‰. Five type I chondrules (Mg# ≥99) have internally heterogeneous oxygen isotope ratios with Δ17O values ranging from –6.5‰ to –4.0‰, similar to those of host chondrule values. These heterogeneous chondrules have granular or porphyritic textures, convoluted outlines, and contain numerous metal grains dispersed within fine-grained silicates. This is consistent with a low degree of melting of the chondrule precursors, possibly because of a low temperature of the melting event and/or a shorter duration of melting. The Δ17O values of relict olivine grains in nine chondrules range from –17.9‰ to –3.4‰, while most of them overlap the range of the host chondrule values.

Similar to those reported from multiple carbonaceous chondrites (Acfer 094, Y-82094, CO, CR, and CV), the Δ17O ∼–5‰ and high Mg# (≥99) chondrules, which might derive from a reduced reservoir with limited dust enrichments (∼50× Solar System), dominate the population of chondrules in Murchison. Other chondrules in Murchison formed in more oxidizing environment (Mg#<96) with higher Δ17O values of –2.5‰, in agreement with the low Mg# chondrules in Acfer 094 and CO chondrites and some chondrules in CV and CR chondrites. They might form in environments containing the same anhydrous precursors as for the Δ17O ∼–5‰ and Mg# ∼99 chondrules, but enriched in 16O-poor H2O ice (∼0.3–0.4× the CI dust; Δ17O>0‰) and at dust enrichments of ∼300–2000×.

Regarding the Mg# and oxygen isotope ratios, the chondrule populations sampled by CM and CO chondrites are similar and indistinguishable. The similarity of these 16O-rich components in CO and CM chondrites is also supported by the common Fe/Mn ratio of olivine in type II chondrules. Although they accreted similar high-temperature silicates, CO chondrites are anhydrous compared to CM chondrites, suggesting they derived from different parent bodies formed inside and outside the snow line, respectively. If chondrules in CO and CM chondrites formed at the same disk locations but the CM parent body accreted later than the CO parent body, the snow line might have crossed the the common chondrule-forming region towards the Sun between the time of the CO and CM parent bodies accretion.

¹Andrea Patzer, and ¹Harry Y. McSween Jr.
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13064]
¹Department of Applied Geology, University of Goettingen, Goldschmidtstr. 3, 37077 Goettingen, Germany
²Department of Earth and Planetary Sciences and Planetary Geoscience Institute, University of Tennessee, Knoxville, Tennessee37996–1410, USA
Published by arrangement with John Wiley & Sons

We investigated several olivine-bearing, medium-grained, ophitic to subophitic eucritic clasts from three different Antarctic howardites. Based on grain size (0.5–2 mm), these clasts could represent intrusive igneous units. Based on mineral composition (pyroxene and plagioclase), they are similar to basaltic eucrites. Elemental concentrations of the major silicates and bulk mg#, however, range from those known for basaltic eucrites to those found in cumulate eucrites. Recognizable cumulus phases are absent. Conservatively speaking, the clasts examined may simply classify as relatively coarse-grained unequilibrated basaltic eucrites. Alternatively, at least one of the clasts showing intermediate grain size and a relatively primitive chemical composition (mg# 50) may sample a rock type that could be genetically placed between the basaltic and cumulate eucrite lines of origin. A minor, yet genetically meaningful common feature of the clasts studied is the occurrence of fayalitic olivine. Two distinct categories exist. They are (1) fine veinlets exclusively percolating through pyroxene and (2) more substantial (up to 100 μm wide) veins and/or interstitial deposits. Only the fine veinlets also contain variable amounts of anorthite, ilmenite, and troilite. Although both types of olivine are ferroan, textural aspects suggest distinct paths of generation. The fine veinlets are best explained by decomposition of relatively FeO-rich, heterogeneous, and locally metastable pyroxene, caused in situ by impact heating and subsequent fast cooling. The wider, often very ragged-looking monomineralic olivine fillings, on the other hand, may represent the iron-enriched portion of a highly fractionated magma.

¹A.Raponi et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.02.001]
INAF-IAPS Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere, 100, Rome I-00133, Italy
Copyright Elsevier

Occator Crater on dwarf planet Ceres hosts the so-called faculae, several areas with material 5 to 10 times the albedo of the average Ceres surface: Cerealia Facula, the brightest and largest, and several smaller faculae, Vinalia Faculae, located on the crater floor. The mineralogy of the whole crater is analyzed in this work. Spectral analysis is performed from data of the VIR instrument on board the Dawn spacecraft. We analyse spectral parameters of all main absorption bands, photometry, and continuum slope. Because most of the absorption features are located in a spectral range affected by thermal emission, we developed a procedure for thermal removal. Moreover, quantitative modeling of the measured spectra is performed with a radiative transfer model in order to retrieve abundance and grain size of the identified minerals. Unlike the average Ceres surface that contains a dark component, Mg–Ca-carbonate, Mg-phyllosilicates, and NH4-phyllosilicates, the faculae contain mainly Na-carbonate, Al-phyllosilicates, and NH4-chloride. The present work establishes unambiguously the presence of NH4-chloride thanks to the high-spatial resolution data. Vinalia and Cerealia Faculae show significant differences in the concentrations of these minerals, which have been analyzed. Moreover, heterogeneities are also found within Cerealia Facula that might reflect different deposition events of bright material. An interesting contrast in grain size is found between the center (10–60 µm) and the crater floor/peripheral part of the faculae (100–130 µm), pointing to different cooling time of the grains, respectively faster and slower, and thus to different times of emplacement. This implies the faculae formation is more recent than the crater impact event, consistent with other observations reported in this special issue. For some ejecta, we derived larger concentrations of minerals producing the absorption bands, and smaller grains with respect to the surrounding terrain. This may be related to heterogeneities in the material pre-existent to the impact event.

^1,2Quinn R. Shollenberger, ²Lars E. Borg, ¹Jan Render, ¹Samuel Ebert, ¹Addi Bischoff, ³Sara S. Russell, ¹Gregory A. Brennecka
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.02.006]
¹Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Straße 10, Münster 48149 Germany
²Nuclear & Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue L-231, Livermore, CA 94550 USA
³Department of Earth Sciences, Natural History Museum, Cromwell Road, Kensington, London SW7 5BD, UK
Copyright Elsevier

Calcium-aluminum-rich inclusions (CAIs) are the oldest dated materials in the Solar System and numerous previous studies have revealed nucleosynthetic anomalies relative to terrestrial rock standards in many isotopic systems. However, most of the isotopic data from CAIs has been limited to the Allende meteorite and a handful of other CV3 chondrites. To better constrain the isotopic composition of the CAI-forming region, we report the first Sr, Mo, Ba, Nd, and Sm isotopic compositions of two CAIs hosted in the CK3 desert meteorites NWA 4964 and NWA 6254 along with two CAIs from the CV3 desert meteorites NWA 6619 and NWA 6991. After consideration of neutron capture processes and the effects of hot-desert weathering, the Sr, Mo, Ba, Nd, and Sm stable isotopic compositions of the samples show clearly resolvable nucleosynthetic anomalies that are in agreement with previous results from Allende and other CV meteorites. The extent of neutron capture, as manifested by shifts in the observed ¹⁴⁹Sm-¹⁵⁰Sm isotopic composition of the CAIs is used to estimate the neutron fluence experienced by some of these samples and ranges from 8.40×10¹³ to 2.11×10¹⁵ n/cm². Overall, regardless of CAI type or host meteorite, CAIs from CV and CK chondrites have similar nucleosynthetic anomalies within analytical uncertainty. We suggest the region that CV and CK CAIs formed was largely uniform with respect to Sr, Mo, Ba, Nd, and Sm isotopes when CAIs condensed and that CAIs hosted in CV and CK meteorites are derived from the same isotopic reservoir.

¹Abigail A. Fraeman
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2018JE005535]
¹Jet Propulsion Laboratory, California Institute of Technology
Published by arrangement with John Wiley & Sons

Mittlefehldt et al. [2018] synthesize APXS chemical measurements along more than 4.5 km of Endeavour crater’s rim. Their analyses clarify details of Endeavour’s geologic history, including evidence for three to four distinct episodes of aqueous alteration. Fracture driven aqueous systems and Mn mobility are particularly important both here and at Curiosity’s landing site on the opposite side of the planet. The detailed documentation of APXS data products within this paper will be a key reference for researchers who want to perform future work on questions related to Mars aqueous geochemistry, impact processes, and Martian crustal and atmospheric evolution.

¹David W. Mittlefehldt et al. (>10)
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005474]
¹Mail code XI3, Astromaterials Research Office, NASA/Johnson Space Center, Houston, TX, USA
Published by arrangement with John Wiley & Sons

We report the results of geological studies by the Opportunity Mars rover on the Endeavour Crater rim. Four major units occur in the region (oldest to youngest): the Matijevic, Shoemaker, Grasberg and Burns formations. The Matijevic formation, consisting of fine-grained clastic sediments, is the only pre-Endeavour-impact unit and might be part of the Noachian etched units of Meridiani Planum. The Shoemaker formation is a heterogeneous polymict impact breccia; its lowermost member incorporates material eroded from the underlying Matijevic formation. The Shoemaker formation is a close analog to the Bunte Breccia of the Ries Crater, although the average clast sizes are substantially larger in the latter. The Grasberg formation is a thin, fine-grained, homogeneous sediment unconformably overlying the Shoemaker formation, and likely formed as an airfall deposit of unknown areal extent. The Burns formation sandstone overlies the Grasberg, but compositions of the two units are distinct; there is no evidence that the Grasberg formation is a fine-grained subfacies of the Burns formation. The rocks along the Endeavour Crater rim were affected by at least four episodes of alteration in the Noachian and Early Hesperian: (i) vein formation and alteration of pre-impact Matijevic formation rocks; (ii) low-water/rock alteration along the disconformity between the Matijevic and Shoemaker formations; (iii) alteration of the Shoemaker formation along fracture zones; and (iv) differential mobilization of Fe and Mn, and CaSO4-vein formation in the Grasberg and Shoemaker formations. Episodes (ii) and (iii) possibly occurred together, but (i) and (iv) are distinct from either of these.