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.

¹Joseph A. Nuth,²Natasha M. Johnson,^2,3Frank T. Ferguson,²Alicia Carayon
Meteoritics & Planetary Science (in Press)   Link to Article [DOI: 10.1111/maps.12666]
¹Solar System Exploration Division, Code 690, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
²Astrochemistry Laboratory, Code 691, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
³Chemistry Department, The Catholic University of America, Washington, D.C. 20064, USA
⁴International Space University, Strasbourg Central Campus, France
Published by arrangement with John Wiley & Sons

We report the ratio of the initial carbon available as CO that forms gas-phase compounds compared to the fraction that deposits as a carbonaceous solid (the gas/solid branching ratio) as a function of time and temperature for iron, magnetite, and amorphous iron silicate smoke catalysts during surface-mediated reactions in an excess of hydrogen and in the presence of N2. This fraction varies from more than 99% for an amorphous iron silicate smoke at 673 K to less than 40% for a magnetite catalyst at 873 K. The CO not converted into solids primarily forms methane, ethane, water, and CO2, as well as a very wide range of organic molecules at very low concentration. Carbon deposits do not form continuous coatings on the catalytic surfaces, but instead form extremely high surface area per unit volume “filamentous” structures. While these structures will likely form more slowly but over much longer times in protostellar nebulae than in our experiments due to the much lower partial pressure of CO, such fluffy coatings on the surfaces of chondrules or calcium aluminum inclusions could promote grain–grain sticking during low-velocity collisions.

¹Hu, J., ¹Sharp, T. G
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA

Northwest Africa 757 is unique in the LL chondrite group because of its abundant shock-induced melt and high-pressure minerals. Olivine fragments entrained in the melt transform partially and completely into ringwoodite. Plagioclase and Ca-phosphate transform to maskelynite, lingunite, and tuite. Two distinct shock-melt crystallization assemblages were studied by FIB-TEM analysis. The first melt assemblage, which includes majoritic garnet, ringwoodite plus magnetite-magnesiowüstite, crystallized at pressures of 20–25 GPa. The other melt assemblage, which consists of clinopyroxene and wadsleyite, solidified at ~15 GPa, suggesting a second veining event under lower pressure conditions. These shock features are similar to those in S6 L chondrites and indicate that NWA 757 experienced an intense impact event, comparable to the impact event that disrupted the L chondrite parent body at 470 Ma.

Reference
Hu J, Sharp TG (2016) High-pressure phases in shock-induced melt of the unique highly shocked LL6 chondrite Northwest Africa 757.
Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12672]
Published by arrangement with John Wiley & Sons

¹Rebecca J. Thomas, ^1,2Brian M. Hynek, ³David A. Rothery, ⁴Susan J. Conway
¹Laboratory for Atmospheric and Space Physics, University of Colorado, 3665 Discovery Drive, Boulder, CO 80303, USA
²Department of Geological Sciences, University of Colorado, 399 UCB, Boulder, CO 80309, USA
³Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
⁴Laboratoire de Planétologie et Géodynamique – UMR CNRS 6112, 2 rue de la Houssinière – BP 92208, 44322 Nantes Cedex 3, France

Unusually low reflectance material, within which depressions known as hollows appear to be actively forming by sublimation, is a major component of Mercury’s surface geology. The observation that this material is exhumed from depth by large impacts has the intriguing implication that the planet’s lower crust or upper mantle contains a significant volatile–rich, low–reflectance layer, the composition of which will be key for developing our understanding of Mercury’s geochemical evolution and bulk composition. Hollows provide a means by which the composition of both the volatile and non–volatile components of the low–reflectance material (LRM) can be constrained, as they result from the loss of the volatile component, and any remaining lag can be expected to be formed of the non–volatile component. However, previous work has approached this by investigating the spectral character of hollows as a whole, including that of bright deposits surrounding the hollows, a unit of uncertain character. Here we use high–resolution multispectral images, obtained as the MESSENGER spacecraft approached Mercury at lower altitudes in the latter part of its mission, to investigate reflectance spectra of inactive hollow floors where sublimation appears to have ceased, and compare this to those of the bright surrounding products and the parent material. This analysis reveals that the final lag after hollow–formation has a flatter spectral slope than that of any other unit on the planet and reflectance approaching that of more space–weathered parent material. This indicates firstly that the volatile material lost has a steeper spectral slope and higher reflectance than the parent material, consistent with (Ca,Mg) sulfides, and secondly, that the low–reflectance component of LRM is non–volatile and may be graphite.

Reference
Thomas RJ, Hynek BM, Rothery DA, Conway SJ (2016) Mercury’s Low-Reflectance Material: Constraints from Hollows. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.05.036]
Copyright Elsevier

¹Daniel M. Applin, ^1,2,3Matthew R.M. Izawa,¹Edward A. Cloutis
¹Hyperspectral Optical Sensing for Extraterrestrial Reconnaissance Laboratory, Dept. Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba, Canada R3B 2E9
²Dept. Earth Sciences, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario, Canada L2S 3A1
³Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395

The diversity of oxalate formation mechanisms suggests that significant concentrations of oxalic acid and oxalate minerals could be widely distributed in the solar system. We have carried out a systematic study of the reflectance spectra of oxalate minerals and oxalic acid, covering the 0.2-16 µm wavelength region.. Our analyses show that oxalates exhibit unique spectral features that enable discrimination between oxalate phases and from other commonly occurring compounds, including carbonates, in all regions of the spectrum except for the visible. Using these spectral data, we consider the possible contribution of oxalate minerals to previously observed reflectance spectra of many objects throughout the solar system, including satellites, comets, and asteroids. We find that polycarboxylic acid dimers and their salts may explain the reflectance spectra of many carbonaceous asteroids in the 3 µm spectral region.. We suggest surface concentration of these compounds may be a type of space weathering from the photochemical and oxidative decomposition of the organic polymer found in carbonaceous chondrites. The stability and ubiquity of these minerals on Earth, in extraterrestrial materials, and in association with biological processes make them useful for many applications in Earth and planetary sciences.

Reference
Applin DM, Izawa MRM, Cloutis EA (2016) Reflectance spectroscopy of oxalate minerals and relevance to solar system carbon inventories. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.05.005]
Copyright Elsevier

¹Paolo A. Sossi, ^1,2Oliver Nebel, ^3,4Mahesh Anand, ⁵Franck Poitrasson
¹Research School of Earth Sciences, Australian National University, Canberra 2601, ACT, Australia
²School of Earth, Atmosphere and Environment, Monash University, Melbourne 3800, VIC, Australia
³Department of Physical Sciences, Open University, Milton Keynes, MK76AA, UK
⁴Department of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK
⁵Laboratoire Géosciences Environnement Toulouse, CNRS UMR 5563 – UPS – IRD, 14-16, Avenue Edouard Belin, 31400, Toulouse, France

Iron is the most abundant multivalent element in planetary reservoirs, meaning its isotope composition (expressed as δ57Fe) may record signatures of processes that occurred during the formation and subsequent differentiation of the terrestrial planets. Chondritic meteorites, putative constituents of the planets and remnants of undifferentiated inner solar system bodies, have View the MathML sourceδFe57≈0‰; an isotopic signature shared with the Martian Shergottite–Nakhlite–Chassignite (SNC) suite of meteorites. The silicate Earth and Moon, as represented by basaltic rocks, are distinctly heavier, View the MathML sourceδFe57≈+0.1‰. However, some authors have recently argued, on the basis of iron isotope measurements of abyssal peridotites, that the composition of the Earth’s mantle is View the MathML sourceδFe57=+0.04±0.04‰, indistinguishable from the mean Martian value. To provide a more robust estimate for Mars, we present new high-precision iron isotope data on 17 SNC meteorites and 5 mineral separates. We find that the iron isotope compositions of Martian meteorites reflect igneous processes, with nakhlites and evolved shergottites displaying heavier View the MathML sourceδFe57(+0.05±0.03‰), whereas MgO-rich rocks are lighter (View the MathML sourceδFe57≈−0.01±0.02‰). These systematics are controlled by the fractionation of olivine and pyroxene, attested to by the lighter isotope composition of pyroxene compared to whole rock nakhlites. Extrapolation of the View the MathML sourceδFe57 SNC liquid line of descent to a putative Martian mantle yields a δ57Fe value lighter than its terrestrial counterpart, but indistinguishable from chondrites. Iron isotopes in planetary basalts of the inner solar system correlate positively with Fe/Mn and silicon isotopes. While Mars and IV-Vesta are undepleted in iron and accordingly have chondritic δ57Fe, the Earth experienced volatile depletion at low (1300 K) temperatures, likely at an early stage in the solar nebula, whereas additional post-nebular Fe loss is possible for the Moon and angrites.

Reference
Sossi PA, Nebel O, Anand A, Poitrasson F (2016) On the iron isotope composition of Mars and volatile depletion in the terrestrial planets. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2016.05.030]
Copyright Elsevier

¹S. Marchi, ²B.A. Black, ³L.T. Elkins-Tanton, ¹W.F. Bottke
¹Southwest Research Institute, Boulder, CO, United States
²City College, City University of New York, New York, NY, United States
³Arizona State University, Tempe, AZ, United States

Recent revisions to our understanding of the collisional history of the Hadean and early-Archean Earth indicate that large collisions may have been an important geophysical process. In this work we show that the early bombardment flux of large impactors (>100 km) facilitated the atmospheric release of greenhouse gases (particularly CO2) from Earth’s mantle. Depending on the timescale for the drawdown of atmospheric CO2, the Earth’s surface could have been subject to prolonged clement surface conditions or multiple freeze-thaw cycles. The bombardment also delivered and redistributed to the surface large quantities of sulfur, one of the most important elements for life. The stochastic occurrence of large collisions could provide insights on why the Earth and Venus, considered Earth’s twin planet, exhibit radically different atmospheres.

Reference
Marchi S, Black BA, Elkins-Tanton LT, Bottke WF (2016) Massive impact-induced release of carbon and sulfur gases in the early Earth’s atmosphere. Earth and Planetary Science Letters 449, 96–104.
Link to Article [doi:10.1016/j.epsl.2016.05.032]
Copyright Elsevier

¹T.S. Peretyazhko, ²A. Fox, ¹B. Sutter, ³P.B. Niles, ⁴M. Adams, ³R.V. Morris, ³D.W. Ming
¹Jacobs, NASA Johnson Space Center, Houston, TX 77058
^{2/sup>Indiana University, Bloomington, IN 47406
3NASA Johnson Space Center, Houston, TX 77058
4University of Hawaii at Hilo, Hilo, HI 96720}

Akaganeite, a Cl-bearing Fe(III) (hydr)oxide, has been recently discovered in Yellowknife Bay in Gale crater on Mars by the Mars Science Laboratory (MSL) Curiosity Rover. Akaganeite was associated with sulfate and sulfide minerals at Yellowknife Bay indicating that sulfate ions could be present in solution during akaganeite formation. The mechanism and conditions of akaganeite formation in the Yellowknife Bay mudstone are unknown. We investigated formation of akaganeite through hydrolysis of ferric chloride solution in the presence of 0, 0.01, 0.05, 0.1 and 0.2 M sulfate and at initial pH of 1.5, 2 and 4 at 90 °C. Mineralogy of the precipitated Fe(III) phases was characterized by X-ray diffraction and infrared spectroscopy. The precipitates were also acid digested to determine total sulfate and chloride contents. Akaganeite and natrojarosite formed at initial solution pH of 1.5; akaganeite, goethite and natrojarosite precipitated in initial pH 2 solutions and goethite, hematite and 2-line ferrihydrite precipitated at initial solution pH of 4. Sulfate addition did not inhibit akaganeite formation. Increasing initial solution sulfate concentrations resulted in increasing sulfate to chloride ratio in the precipitated akaganeite. Infrared spectroscopy revealed akaganeite bands at ∼2 μm (H2O combination band) and at ∼2.46 μm (OH combination band). The H2O combination band position linearly correlated with total chloride content in akaganeite. Overall, laboratory studies demonstrated formation of akaganeite at initial sulfate concentration ⩽ 0.2 M (sulfate to chloride molar ratio ⩽0.3) and pH ⩽ 2, implying that those conditions might prevail (perhaps as micro-environments) during akaganeite formation in Yellowknife Bay mudstone. The occurrence of Fe(II) sulfides (pyrite and pyrrhotite) in Yellowknife Bay mudstone is a potential acidity source. Dissolution of sulfide minerals might occur under localized oxidizing water-limiting Cl-rich conditions creating favorable environments for akaganeite formation.

Reference
Peretyazhko TS, Fox A, Sutter B, Niles PB, Adams M, Morris RV, Ming DW (2016)
Synthesis of akaganeite in the presence of sulfate:Implications for akaganeite formation in Yellowknife Bay, Gale Crater, Mars. Geochmica et Cosmochmica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.06.002]
Copyright Elsevier

¹Toni Schulz, ²Ambre Luguet, ¹Wencke Wegner, ²David van Acken,^1,3Christian Koeberl
¹Department of Lithospheric Research, University Vienna, Vienna, Austria
²Steinmann Institut of Geology, Mineralogy and Palaeontology, University of Bonn, Bonn, Germany
³Natural History Museum, Vienna, Austria

The Lonar crater is a ~0.57-Myr-old impact structure located in the Deccan Traps of the Indian peninsula. It probably represents the best-preserved impact structure hosted in continental flood basalts, providing unique opportunities to study processes of impact cratering in basaltic targets. Here we present highly siderophile element (HSE) abundances and Sr-Nd and Os isotope data for target basalts and impactites (impact glasses and impact melt rocks) from the Lonar area. These tools may enable us to better constrain the interplay of a variety of impact-related processes such as mixing, volatilization, and contamination. Strontium and Nd isotopic compositions of impactites confirm and extend earlier suggestions about the incorporation of ancient basement rocks in Lonar impactites. In the Re-Os isochron plot, target basalts exhibit considerable scatter around a 65.6 Myr Re-Os reference isochron, most likely reflecting weathering and/or magma replenishment processes. Most impactites plot at distinctly lower 187Re/188Os and 187Os/188Os ratios compared to the target rocks and exhibit up to two orders of magnitude higher abundances of Ir, Os, and Ru. Moreover, the impactites show near-chondritic interelement ratios of HSE. We interpret our results in terms of an addition of up to 0.03% of a chondritc component to most impact glasses and impact melt rocks. The magnitude of the admixture is significantly lower than the earlier reported 12–20 wt% of extraterrestrial component for Lonar impact spherules, reflecting the typical difference in the distribution of projectile component between impact glass spherules and bulk impactites.

Reference
Schulz T, Luguet A, Wegner W, van Acken D, Koeberl C (2016) Target rocks, impact glasses, and melt rocks from the Lonar crater, India: Highly siderophile element systematics and Sr-Nd-Os isotopic signatures. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12665]
Published by arrangement with John Wiley & Sons