¹Olivier Mousis, ²Jean-Marc Simon, ²Jean-Pierre Bellat, ³Frédéric Schmidt, ³Sylvain Bouley, ³Eric Chassefière, ⁴Violaine Sautter, ⁵Yoann Quesnel, ⁶Sylvain Picaud, ⁷Sébastien Lectez
Icarus (in Press) Link to Article [doi:10.1016/j.icarus.2016.05.035]
¹Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388, Marseille, France
²Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303, CNRS-Université de Bourgogne Franche Comté, Dijon, France
³Laboratoire GEOPS (Géosciences Paris Sud), Bat. 509, Université Paris Sud, 91405 Orsay Cedex, France
⁴Muséum d’Histoire Naturelle, Paris, France
⁵Aix-Marseille Université, CNRS, IRD, CEREGE UM34, 13545 Aix-en-Provence, France
⁶Université de Franche-Comté, Institut UTINAM, CNRS/INSU, UMR 6213, Besançon Cedex, France
⁷Leeds University, School of Earth and Environment, Leeds, United Kingdom
Copyright Elsevier

The origin of the martian methane is still poorly understood. A plausible explanation is that methane could have been produced either by hydrothermal alteration of basaltic crust or by serpentinization of ultramafic rocks producing hydrogen and reducing crustal carbon into methane. Once formed, methane storage on Mars is commonly associated with the presence of hidden clathrate reservoirs. Here, we alternatively suggest that chabazite and clinoptilolite, which belong to the family of zeolites, may form a plausible storage reservoir of methane in the martian subsurface. Because of the existence of many volcanic terrains, zeolites are expected to be widespread on Mars and their Global Equivalent Layer may range up to more than ∼ 1 km, according to the most optimistic estimates. If the martian methane present in chabazite and clinoptilolite is directly sourced from an abiotic source in the subsurface, the destabilization of a localized layer of a few millimeters per year may be sufficient to explain the current observations. The sporadic release of methane from these zeolites requires that they also remained isolated from the atmosphere during its evolution. The methane release over the ages could be due to several mechanisms such as impacts, seismic activity or erosion. If the methane outgassing from excavated chabazite and/or clinoptilolite prevails on Mars, then the presence of these zeolites around Gale Crater could explain the variation of methane level observed by Mars Science Laboratory.

¹Steven B. Simon, ^1,2Stephen R. Sutton, ^{1, 3}Lawrence Grossman
Geochimica et Cosmochmica Acta (in Press)  Link to Article [doi:10.1016/j.gca.2016.06.013]
¹Dept. of the Geophysical Sciences, 5734 S. Ellis Ave, The University of Chicago, Chicago, IL 60637
²Center for Advanced Radiation Sources (CARS), 5640 S. Ellis Ave, The University of Chicago, Chicago, IL 60637
³The Enrico Fermi Institute, 5640 S. Ellis Ave, The University of Chicago, Chicago, IL 60637
Copyright Elsevier

One way to better understand processes related to chondrite metamorphism is to evaluate changes in chondrite features as a function of petrologic type. Toward this end the valence and coordination of Ti in olivine and pyroxene in suites of ordinary (H, L, and LL) and enstatite (EH and EL) chondrites of types 3 through 6 have been determined with XANES spectroscopy. Trivalent Ti, typically 10-40% of the Ti in the analytical volumes, was found in ordinary chondrites of all types, despite the stability of oxidized iron in the samples. Average valences and the proportions of Ti that are in tetrahedral coordination generally decrease with increasing grade between types 3.0 and 3.5, increase from 3.5 to 4, and then level off. These trends are consistent with previous studies of chondrite oxidation states using other methods, except here the onset of oxidation is observed at a lower type, ∼3.5, than previously indicated (4). These results are also consistent with previous suggestions that oxidation of higher-grade ordinary chondrite samples involved exposure to aqueous fluids from melting of accreted ice. In the enstatite chondrites, typically 20-90% of the Ti is trivalent Ti, so it is reduced compared to Ti in the ordinary chondrites. Valence decreases slightly from petrologic type 3 to 4 and increases from 4 to 6, but no increases in tetrahedral coordination with petrologic type are observed, indicating a redox environment or process distinct from that of ordinary chondrite metamorphism. The presence of Ti4+ in the E chondrites supports previous suggestions that they formed from oxidized precursors that underwent reduction. Unlike ordinary chondrites, enstatite chondrites are thought to have been derived from a body or bodies that did not accrete ice, which could account for their different valence-coordination-petrologic type relationships. The hypothesis, based on observations of unmetamorphosed chondrules and supported by laboratory experiments, that equilibration of Ti valence is sluggish compared to that of Fe could account for the coexistence of reduced Ti and oxidized Fe seen in chondrites of all petrologic types.

¹Devin L. Schrader, ²Jemma Davidson, ¹Timothy J. McCoy
Geochimica et Cosmochimica Acta (in Press) Link to Article [doi:10.1016/j.gca.2016.06.012]
¹Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10[th] & Constitution Avenue NW, Washington, D.C. 20560-0119, USA
²Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015-1305, USA
Copyright Elsevier

By investigating the compositional and textural evolution of sulfides within a wide range of relatively pristine, aqueously altered, and thermally metamorphosed chondrites we constrain the equilibration temperatures of sulfide minerals and compare them to the metamorphic history of their host meteorite. Sulfides in Mighei-like carbonaceous chondrites are complex as they equilibrated mostly between 100 and 135°C, but some may have equilibrated at temperatures up to 600°C. This is consistent with some CM chondrite sulfides forming at high temperature during chondrule cooling and others during low-temperature aqueous alteration and/or annealing. Karoonda-like carbonaceous chondrite sulfides equilibrated between 500 and 230°C, which is consistent with formation during cooling and annealing after thermal metamorphism. Sulfides in the LL chondrites equilibrated between 600 and 230°C, and are consistent with formation during chondrule cooling for Semarkona (LL3.00) and during cooling after thermal metamorphism for the equilibrated samples (types 4–6). Sulfides in the Rumuruti-like (R) chondrites equilibrated between 600 and 500°C, and are consistent with formation after thermal metamorphism. The sulfides within the brachinite equilibrated between 600 and 400°C, consistent with formation during cooling after thermal metamorphism.

Contrary to the assertion that pentlandite is solely the product of low-temperature aqueous alteration in many chondrite groups, this study suggests that most sulfides in chondrites are formed at or upon cooling from high-temperature. The evaluation of a single mineral system within samples that retain petrographic context is vital to the interpretation of formation and alteration processes recorded by small extraterrestrial samples, such as those that have been returned by the spacecraft missions Stardust and Hayabusa and will be returned by OSIRIS-REx and Hayabusa2.

¹Richard V. Morris et al. (>10)*
Proceedings for the National Academy of Sciences (in Press) Link to Article [doi: 10.1073/pnas.1607098113]
¹NASA Johnson Space Center, Houston, TX 77058
*Find the extensive, full author and affiliation list on the publishers website

Tridymite, a low-pressure, high-temperature (>870 °C) SiO2 polymorph, was detected in a drill sample of laminated mudstone (Buckskin) at Marias Pass in Gale crater, Mars, by the Chemistry and Mineralogy X-ray diffraction instrument onboard the Mars Science Laboratory rover Curiosity. The tridymitic mudstone has ∼40 wt.% crystalline and ∼60 wt.% X-ray amorphous material and a bulk composition with ∼74 wt.% SiO2 (Alpha Particle X-Ray Spectrometer analysis). Plagioclase (∼17 wt.% of bulk sample), tridymite (∼14 wt.%), sanidine (∼3 wt.%), cation-deficient magnetite (∼3 wt.%), cristobalite (∼2 wt.%), and anhydrite (∼1 wt.%) are the mudstone crystalline minerals. Amorphous material is silica-rich (∼39 wt.% opal-A and/or high-SiO2 glass and opal-CT), volatile-bearing (16 wt.% mixed cation sulfates, phosphates, and chlorides−perchlorates−chlorates), and has minor TiO2 and Fe2O3T oxides (∼5 wt.%). Rietveld refinement yielded a monoclinic structural model for a well-crystalline tridymite, consistent with high formation temperatures. Terrestrial tridymite is commonly associated with silicic volcanism, and detritus from such volcanism in a “Lake Gale” catchment environment can account for Buckskin’s tridymite, cristobalite, feldspar, and any residual high-SiO2 glass. These cogenetic detrital phases are possibly sourced from the Gale crater wall/rim/central peak. Opaline silica could form during diagenesis from high-SiO2 glass, as amorphous precipitated silica, or as a residue of acidic leaching in the sediment source region or at Marias Pass. The amorphous mixed-cation salts and oxides and possibly the crystalline magnetite (otherwise detrital) are primary precipitates and/or their diagenesis products derived from multiple infiltrations of aqueous solutions having variable compositions, temperatures, and acidities. Anhydrite is post lithification fracture/vein fill.

^1,2B. Schmitz, ³Q. -Z. Yin, ³M. E. Sanborn, ¹M. Tassinari, ²C. E. Caplan, ²G. R. Huss
Nature Communications 7,11851      Link to Article [doi:10.1038/ncomms11851]
¹Astrogeobiology Laboratory, Department of Physics, Lund University, 221 00 Lund, Sweden
²Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Manoa, Honolulu, Hawaii 96822, USA
³Department of Earth and Planetary Sciences, University of California at Davis, Davis, California 95616, USA

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¹McCollom Thomas M., ¹Donaldson Christopher.
Astrobiology June 2016, 16(6): 389-406. Link to Article [doi:10.1089/ast.2015.1382]
¹Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, CO 80309

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¹A.B. Sanina et al. (>10)*
Icarus (in Press)    Link to Article [doi:10.1016/j.icarus.2016.06.002]
¹Institute for Space Research of Russian Academy of Sciences, Moscow 117997, Russian Federation
*Find the extensive, full author and affiliation list on the publishers website

We present a method of conversion of the lunar neutron counting rate measured by the Lunar Reconnaissance Orbiter (LRO) Lunar Exploration Neutron Detector (LEND) instrument collimated neutron detectors, to water equivalent hydrogen (WEH) in the top ∼1 meter layer of lunar regolith. Polar maps of the Moon’s inferred hydrogen abundance are presented and discussed.

Copyright Elsevier

¹Hasnaa Chennaoui Aoudjehane, ¹Houda El Kerni,^2,3Wolf Uwe Reimold, ^4,5David Baratoux, ^6,7Christian Koeberl, ⁸Sylvain Bouley,⁹Mohamed Aoudjehane
Meteoritics & Planetary Science (in Press)  Link to Article [DOI: 10.1111/maps.12661]
¹Hassan II University Casablanca, Faculty of Sciences Ain Chock, GAIA Laboratory, Casablanca, Morocco
²Museum für Naturkunde Berlin—Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
³Humboldt-Universität zu Berlin, Berlin, Germany
⁴Géosciences-Environnement-Toulouse, Université Paul Sabatier CNRS & IRD UMR 5563, Toulouse, France
⁵Institut Fondamental d’Afrique Noire Cheikh Anta Diop, Dakar, Senegal
⁶Natural History Museum, Vienna, Austria
⁷Department of Lithospheric Research, University of Vienna, Vienna, Austria
⁸GEOPS—Géosciences Paris Sud—Université Paris Sud—Bât, Orsay Cedex, France
⁹Casablanca, Morocco
Published by arrangement with John Wiley & Sons

Associations between impact structures and meteorite occurrences are rare and restricted to very young structures. Meteorite fragments are often disrupted in the atmosphere, and in most cases, meteorite falls that have been decelerated by atmospheric drag do not form a crater. Furthermore, meteorites are rapidly weathered. In this context, the finding of shatter cones in Jurassic marly limestone in the same location as a recent (105 ± 40 ka) iron meteorite fall near the village of Agoudal (High Atlas Mountains, Morocco) is enigmatic. The shatter cones are the only piece of evidence of a meteorite impact in the area.

The overlap of a meteorite strewn field with the area of occurrence of shatter cones led previous researchers to consider that the meteorite fall was responsible for the formation of shatter cones in the context of formation of one or several small (<100 m) impact craters that had since been eroded. Shatter cones are generally not reported in association with subkilometer-diameter impact craters. Here, we present new field observations and an analysis of the distribution and characteristics of shatter cones, breccia, and meteorites in the Agoudal area. Evidence for local deformation not related to the structural High Atlas tectonics has been observed, such as a vertical to overturned stratum trending N150-N160. New outcrops with exposures of shatter cones are reported and extend the previously known area of occurrence. The area of in situ shatter cones (~0.15 km²) and the strewn field of meteorites are distinct, although they show some overlap. The alleged impact breccia is revealed as calcrete formations. No evidence for a genetic relationship between the shatter cones and the meteorites can be inferred from field observations. The extent of the area where in situ shatter cones and macrodeformation not corresponding to Atlas tectonic deformation are observed suggest that the original diameter of an impact structure could have been between at least 1–3 km. For typical erosion rates in the Atlas region (~0.08 cm yr⁻¹), the period of time required for the erosion of such a structure (1.25–3.75 Ma) is much larger than the age of the meteorite fall. This line of reasoning excludes a genetic link between the shatter cones and the meteorite fall and indicates that the observed shatter cones belong to an ancient impact structure that has been almost entirely eroded.

¹Patrick N. Peplowski,¹Andrew W. Beck,¹David J. Lawrence
Journal of Geophysical Research Planets (in Press)  Link to Article [DOI: 10.1002/2015JE004950]
¹The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
Published by arrangement with John Wiley & Sons

Thermal neutron emissions from the lunar surface provide a direct measure of bulk elemental composition that can be used to constrain the chemical properties of near-surface (depth <1 m) lunar materials. We present a new calibration of the Lunar Prospector thermal neutron map, providing a direct link between measured count rates and bulk elemental composition. The data are used to examine the chemical and mineralogical composition of the lunar surface, with an emphasis on constraining the plagioclase concentration across the highlands. We observe that the regions of lowest neutron absorption, which correspond to estimated plagioclase concentrations of >85%, are generally associated with large impact basins and are colocated with clusters of nearly pure plagioclase identified with spectral reflectance data.

¹Dominik Loroch, ^1,2Alexander Deutsch, ¹Jasper Berndt,³André Bornemann
Meteoritics & Planetary Science (in Press)    Link to Article [DOI: 10.1111/maps.12667]
¹Institut für Mineralogie, Westfälische Wilhelms-Universität Münster (WWU), Muenster, Germany
²Institut für Planetologie, WWU Münster, Muenster, Germany
³Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Hannover, Germany
Published by arrangement with John Wiley & Sons

We present results of an in-situ geochemical study using laser-ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS) analyses along a ~4.3 cm long section across the K-Pg event bed, drilled during IODP Expedition 342 at J Anomaly Ridge south of St. John’s, Newfoundland. This section comprises the Maastrichtian with a sharp boundary to the graded, between 1.5 and 1.8 cm thick ejecta layer with totally altered impact glass spherules, which in turn is topped by Danian sediments. The porous and clayey material required elaborate preparation in order to yield reliable data. The ejecta bed shows a highly variable depletion in rare earth elements that even results in strongly subchondritic concentrations. The Ce/Ce* varies strongly (0.81–34), Ni/Cr ranges from 0.38 to 2.79. The maximum platinum group elements (PGE) concentrations are located in one LA-spot exactly at the basis of the ejecta layer; they amount (in μg g−1) to 0.35 (Rh), 1.64 (Pd), 2.79 (Pt), and 0.86 (Au). The Nb/Ta ratio increases in the Ma from ~10 to 35.9 toward the ejecta horizon, which itself has higher Nb, Ta, Zr, and Hf concentrations than the background sedimentation, combined with low Nb/Ta (~5–10), and low Zr/Hf (~20–30). The overall result is that alteration processes changed totally the original geochemical characteristics of this K-Pg spherule bed. To explain the exorbitant element mobility at distances of hundreds of μm, we discuss a combination of mostly reducing redox processes and interaction with organic compounds. This study demonstrates the high potential of in-situ analyses with high spatial resolution at complex geological materials. Moreover, our results indicate that some caution is necessary in determining the projectile type in impactites via PGE ratios.