¹M.R. Lee, ^1,2T. Tomkinson, ¹L.J. Hallis, ²D.F. Mark
¹School of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow G12 8QQ, U.K.
²Scottish Universitites Environmental Research Centre, Rankine Avenue, Scottish Enterprise Technology Park, East Kilbride, G75 0QF, U.K.

The Lafayette meteorite is an olivine clinopyroxenite that crystallized on Mars ∼1300 million years ago within a lava flow or shallow sill. Liquid water entered this igneous rock ∼700 million years later to produce a suite of secondary minerals, collectively called ‘iddingsite’, that occur as veins within grains of augite and olivine. The deuterium/hydrogen ratio of water within these secondary minerals shows that the aqueous solutions were sourced from one or more near-surface reservoirs. Several petrographically distinct types of veins can be recognised by differences in their width, shape, and crystallographic orientation. Augite and olivine both contain veins of a very fine grained hydrous Fe- and Mg-rich silicate that are ∼1-2 micrometres in width and lack any preferred crystallographic orientation. These narrow veins formed by cementation of pore spaces that had been opened by fracturing and probably in response to shock. The subset of olivine-hosted veins whose axes lie parallel to (001) have serrated walls, and formed by widening of the narrow veins by interface coupled dissolution-precipitation. Widening started by replacement of the walls of the narrow precursor veins by Fe-Mg silicate, and a crystallographic control on the trajectory of the dissolution-precipitation front created micrometre-scale {111} serrations. The walls of many of the finely serrated veins were subsequently replaced by siderite, and the solutions responsible for carbonation of olivine also partially recrystallized the Fe-Mg silicate. Smectite was the last mineral to form and grew by replacement of siderite. This mineralization sequence shows that Lafayette was exposed to two discrete pulses of aqueous solutions, the first of which formed the Fe-Mg silicate, and the second mediated replacement of vein walls by siderite and smectite. The similarity in size, shape and crystallographic orientation of iddingsite veins in the Lafayette meteorite and in terrestrial basalts demonstrates a common microstructural control on water-mineral interaction between Mars and Earth, and indicates that prior shock deformation was not a prerequisite for aqueous alteration of the martian crust.

Reference
Lee MR, Tomkinson T, Hallis LJ, Mark DF (2015) Formation of iddingsite veins in the martian crust by centripetal replacement of olivine: Evidence from the nakhlite meteorite Lafayette. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.01.022]

Copyright Elsevier

¹David J. Lawrence, ¹Patrick N. Peplowski, ¹Jeffrey B. Plescia, ²Benjamin T. Greenhagen, ³Sylvestre Maurice, ⁴Thomas H. Prettyman
¹The Johns Hopkins University, Applied Physics Laboratory, Laurel, Maryland, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
³Institut de Recherche en Astrophysique et Planétologie, Toulouse, France
⁴Planetary Science Institute, Tucson, Arizona, USA

A map of bulk hydrogen concentrations in the lunar highlands region is reported. This map is derived using data from the Lunar Prospector Neutron Spectrometer (LP-NS). We resolve prior ambiguities in the interpretation of LP-NS data with respect to non-polar hydrogen concentrations by comparing the LP-NS data with maps of the 750 nanometer albedo reflectance, optical maturity, and the wavelength position of the thermal infrared Christiansen Feature. The best explanation for the variations of LP-NS epithermal neutron data in the lunar highlands is variable amounts of solar-wind-implanted hydrogen. The average hydrogen concentration across the lunar highlands and away from the lunar poles is 65 ppm. The highest hydrogen values range from 120 ppm to just over 150 ppm. These values are consistent with the range of hydrogen concentrations from soils and regolith breccias at the Apollo 16 highlands landing site. Based on a moderate-to-strong correlation of epithermal neutrons and orbit-based measures of surface maturity, the map of highlands hydrogen concentration represents a new global maturity index that can be used for studies of the lunar soil maturation process. We interpret these hydrogen concentrations to represent a bulk soil property related to the long-term impact of the space environment on the lunar surface. Consequently, the derived hydrogen concentrations are not likely related to the surficial enhancements (top tens to hundreds of microns) or local time variations of OH/H2O measured with spectral reflectance data.

Reference
Lawrence DJ, Peplowskia, Plescia JB, Greenhagen BT, Maurice S, Prettyman TH (2015) Bulk Hydrogen Abundances in the Lunar Highlands: Measurements from Orbital Neutron Data. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.01.005]

Copyright Elsevier

¹C. M. O’D. Alexander, ²R. Bowden, ²M. L. Fogel,³K. T. Howard
¹DTM, Carnegie Institution of Washington, Washington, District of Columbia, USA
²GL, Carnegie Institution of Washington, Washington, District of Columbia, USA
³Kingsborough Community College of the City University of New York, Brooklyn, New York, USA

We report the bulk C abundances, and C and O isotopic compositions of carbonates in 64 CM chondrites, 14 CR chondrites, 2 CI chondrites, LEW 85332 (C2), Kaba (CV3), and Semarkona (LL3.0). For the unheated CMs, the total ranges of carbonate isotopic compositions are δ13C ≈ 25–75‰ and δ18O ≈ 15–35‰, and bulk carbonate C contents range from 0.03 to 0.60 wt%. There is no simple correlation between carbonate abundance and isotopic composition, or between either of these parameters and the extent of alteration. Unless accretion was very heterogeneous, the uncorrelated variations in extent of alteration and carbonate abundance suggests that there was a period of open system behavior in the CM parent body, probably prior to or at the start of aqueous alteration. Most of the ranges in CM carbonate isotopic compositions can be explained by their formation at different temperatures (0–130 °C) from a single fluid in which the carbonate O isotopes were controlled by equilibrium with water (δ18O ≈ 5‰) and the C isotopes were controlled by equilibrium with CO and/or CH4 (δ13C ≈ −33‰ or −20‰ for CO- or CH4-dominated systems, respectively). However, carbonate formation would have to have been inefficient, otherwise carbonate compositions would have resembled those of the starting fluid. A quite similar fluid composition (δ18O ≈ −5.5‰, and δ13C ≈ −31‰ or −17‰ for CO- or CH4-dominated systems, respectively) can explain the carbonate compositions of the CIs, although the formation temperatures would have been lower (~10–40 °C) and the relative abundances of calcite and dolomite may play a more important role in determining bulk carbonate compositions than in the CMs. The CR carbonates exhibit a similar range of O isotopes, but an almost bimodal distribution of C isotopes between more (δ13C ≈ 65–80‰) and less altered samples (δ13C ≈ 30–40‰). This bimodality can still be explained by precipitation from fluids with the same isotopic composition (δ18O ≈ −9.25‰, and δ13C ≈ −21‰ or −8‰ for CO- or CH4-dominated systems, respectively) if the less altered CRs had higher mole fractions of CO2 in their fluids. Semarkona and Kaba carbonates have some of the lightest C isotopic compositions of the meteorites studied here, probably because they formed at higher temperatures and/or from more CO2-rich fluids. The fluids responsible for the alteration of chondrites and from which the carbonates formed were almost certainly accreted as ices. By analogy with cometary ices, CO2 and/or CO would have dominated the trapped volatile species in the ices. The chondrites studied are too oxidized for CO-dominated fluids to have formed in their parent bodies. If CH4 was the dominant C species in the fluids during carbonate formation, it would have to have been generated in the parent bodies from CO and/or CO2 when oxidation of metal by water created high partial pressures of H2. The fact that the chondrite carbonate C/H2O mole ratios are of the order predicted for CO/CO2-H2O ices that experienced temperatures of >50–100 K suggests that the chondrites formed at radial distances of <4–15 AU.

Reference
Alexander CMO’D, Bowden R, Fogel ML, Howard KT (2015) Carbonate abundances and isotopic compositions in chondrites. Meteoritics&Planetary Society (in Press)
Link to Article [DOI: 10.1111/maps.12410]

Published by arrangement with John Wiley&Sons

¹Gian Paolo Sighinolfi, ^2,3Emanuela Sibilia, ¹Gabriele Contini,²Marco Martini
¹Dipartimento di Scienze Chimiche e Geologiche, Università di Modena e Reggio E., Modena, Italy
²Dipartimento di Scienza dei Materiali, Università degli Studi di Milano Bicocca, Milano, Italy
³INFN, Sezione di Milano Bicocca, Milano, Italy

Thermoluminescence (TL) dating has been used to determine the age of the meteorite impact crater at Gebel Kamil (Egyptian Sahara). Previous studies suggested that the 45 m diameter structure was produced by a fall in recent times (less than 5000 years ago) of an iron meteorite impactor into quartz-arenites and siltstones belonging to the Lower Cretaceous Gilf Kebir Formation. The impact caused the complete fragmentation of the impactor, and the formation of a variety of impactites (e.g., partially vitrified dark and light materials) present as ejecta within the crater and in the surrounding area. After a series of tests to evaluate the TL properties of different materials including shocked intra-crater target rocks and different types of ejecta, we selected a suite of light-colored ejecta that showed evidence of strong thermal shock effects (e.g., partial vitrification and the presence of high-temperature and -pressure silica phases). The abundance of quartz in the target rocks, including the vitrified impactites, allowed TL dating to be undertaken. The variability of radioactivity of the intracrateric target rocks and the lack of direct in situ dosimetric evaluations prevented precise dating; it was, however, possible to constrain the impact in the 2000 BC–500 AD range. If, as we believe, the radioactivity measured in the fallback deposits is a reliable estimate of the mean radioactivity of the site, the narrower range 1600–400 BC (at the 2σ confidence level) can be realistically proposed.

Reference
Sighinolfi GP, Sibilia E, Contini G, Martini M (2015) Thermoluminescence dating of the Kamil impact crater (Egypt). Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12417]

Published by arrangement with John Wiley&Sons

¹Pavel P. Povinec et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website
¹Department of Nuclear Physics and Biophysics, Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia

On February 15, 2013, after the observation of a brilliant fireball and a spectacular airburst over the southern Ural region (Russia), thousands of stones fell and were rapidly recovered, bringing some extremely fresh material for scientific investigations. We undertook a multidisciplinary study of a dozen stones of the Chelyabinsk meteorite, including petrographic and microprobe investigations to unravel intrinsic characteristics of this meteorite. We also study the short and long-lived cosmogenic radionuclides to characterize the initial meteoroid size and exposure age. Petrographic observations, as well as the mineral compositions obtained by electron microprobe analyses, allow us to confirm the classification of the Chelyabinsk meteorite as an LL5 chondrite. The fragments studied, a few of which are impact melt rocks, contain abundant shock melt veins and melt pockets. It is likely that the catastrophic explosion and fragmentation of the Chelyabinsk meteoroid into thousands of stones was in part determined by the initial state of the meteoroid. The radionuclide results obtained show a wide range of concentrations of 14C, 22Na, 26Al, 54Mn, 57Co, 58Co, and 60Co, which indicate that the pre-atmospheric object had a radius >5 m, consistent with other size estimates based on the magnitude of the airburst caused by the atmospheric entry and breakup of the Chelyabinsk meteoroid. Considering the observed 26Al activities of the investigated samples, Monte Carlo simulations, and taking into account the 26Al half-life (0.717 Myr), the cosmic-ray exposure age of the Chelyabinsk meteorite is estimated to be 1.2 ± 0.2 Myr. In contrast to the other radionuclides, 14C showed a very large range only consistent with most samples having been exposed to anthropogenic sources of 14C, which we associate with radioactive contamination of the Chelyabinsk region by past nuclear accidents and waste disposal, which has also been confirmed by elevated levels of anthropogenic 137Cs and primordial 40K in some of the Chelyabinsk fragments.

Reference
Povinec PP et al. (2015) Cosmogenic radionuclides and mineralogical properties of the Chelyabinsk (LL5) meteorite: What do we learn about the meteoroid? Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12419]

Published by arrangement with John Wiley&Sons

^1,2A. Calzada-Diaz, ³K. H. Joy, ^1,2I. A. Crawford, ^2,4T. A. Nordheim
¹Department of Earth and Planetary Sciences, Birkbeck College, London, UK
²Centre for Planetary Sciences UCL/Birkbeck, London, UK
³School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK
⁴Mullard Space Science Laboratory, University College London, Dorking, UK

Lunar meteorites provide important new samples of the Moon remote from regions visited by the Apollo and Luna sample return missions. Petrologic and geochemical analysis of these meteorites, combined with orbital remote sensing measurements, have enabled additional discoveries about the composition and age of the lunar surface on a global scale. However, the interpretation of these samples is limited by the fact that we do not know the source region of any individual lunar meteorite. Here, we investigate the link between meteorite and source region on the Moon using the Lunar Prospector gamma ray spectrometer remote sensing data set for the elements Fe, Ti, and Th. The approach has been validated using Apollo and Luna bulk regolith samples, and we have applied it to 48 meteorites excluding paired stones. Our approach is able broadly to differentiate the best compositional matches as potential regions of origin for the various classes of lunar meteorites. Basaltic and intermediate Fe regolith breccia meteorites are found to have the best constrained potential launch sites, with some impact breccias and pristine mare basalts also having reasonably well-defined potential source regions. Launch areas for highland feldspathic meteorites are much less well constrained and the addition of another element, such as Mg, will probably be required to identify potential source regions for these.

Reference
Calzada-Diaz A, Joy KH, Crawford IA, Nordheim TA (2015) Constraining the source regions of lunar meteorites using orbital geochemical data. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12412]
Published by arrangement with John Wiley&Sons

^1,2,3H. Downes, ⁴F. A. J. Abernethy, ³C. L. Smith, ^2,3,5A. J. Ross, ⁴A. B. Verchovsky, ⁴M. M. Grady, ⁶P. Jenniskens,⁷M. H. Shaddad
¹Department of Earth and Planetary Sciences, Birkbeck University of London, London, UK
²UCL/Birkbeck Centre for Planetary Sciences, UCL, London, UK
³Department of Earth Sciences, Natural History Museum, London, UK
⁴Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, UK
⁵Department of Earth Sciences, University College London, London, UK
⁶SETI Institute, Carl Sagan Centre, Mountain View, California, USA
⁷Department of Physics, University of Khartoum, Khartoum, Sudan

This study characterizes carbon and nitrogen abundances and isotopic compositions in ureilitic fragments of Almahata Sitta. Ureilites are carbon-rich (containing up to 7 wt% C) and were formed early in solar system history, thus the origin of carbon in ureilites has significance for the origin of solar system carbon. These samples were collected soon after they fell, so they are among the freshest ureilite samples available and were analyzed using stepped combustion mass spectrometry. They contained 1.2–2.3 wt% carbon; most showed the major carbon release at temperatures of 600–700 °C with peak values of δ13C from −7.3 to +0.4‰, similar to literature values for unbrecciated (“monomict”) ureilites. They also contained a minor low temperature (≤500 °C) component (δ13C = ca −25‰). Bulk nitrogen contents (9.4–27 ppm) resemble those of unbrecciated ureilites, with major releases mostly occurring at 600–750 °C. A significant lower temperature release of nitrogen occurred in all samples. Main release δ15N values of −53 to −94‰ fall within the range reported for diamond separates and acid residues from ureilites, and identify an isotopically primordial nitrogen component. However, they differ from common polymict ureilites which are more nitrogen-rich and isotopically heavier. Thus, although the parent asteroid 2008TC3 was undoubtedly a polymict ureilite breccia, this cannot be deduced from an isotopic study of individual ureilite fragments. The combined main release δ13C and δ15N values do not overlap the fields for carbonaceous or enstatite chondrites, suggesting that carbon in ureilites was not derived from these sources.

Reference
Downes H, Abernethy FAJ, Smith CL, Ross AJ, Verchovsky AB, Grady MM, Jenniskens P, Shaddad MH (2015) Isotopic composition of carbon and nitrogen in ureilitic fragments of the Almahata Sitta Meteorite. Meteoritics and Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12413]
Published by arrangement with John Wiley&Sons

¹James F. J. Bryson, ¹Claire I. O. Nichols,^2,3Julia Herrero-Albillos,⁴Florian Kronast,⁵Takeshi Kasama,⁵Hossein Alimadadi,⁶Gerrit van der Laan,⁷Francis Nimmo¹Richard J. Harrison
¹Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
²Centro Universitario de la Defensa, Carretera de Huesca s/n, E-50090 Zaragoza, Spain
³Instituto de Ciencia de Materiales de Aragón, CSIC—Universidad de Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain
⁴Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, 12489 Berlin, Germany
⁵Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
⁶Diamond Light Source, Chilton, Didcot, Oxfordshire OX11 0DE, UK
⁷Department of Earth and Planetary Sciences, University of California, Santa Cruz, California 95064, USA

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

Bryson JFJ, Nichols CIO, Herrero-Albillos J, Kronast F, Kasama T, Alimadadi H, van der Laan G, Nimmo F, Harrison RJ (2015) Long-lived magnetism from solidification-driven convection on the pallasite parent Body. Nature 517, 472–475
Link to Article [doi:10.1038/nature14114]

¹M. Millot et al. (>10)*
¹Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.
*Find the extensive, full author and affiliation list on the publishers Website

Deep inside planets, extreme density, pressure, and temperature strongly modify the properties of the constituent materials. In particular, how much heat solids can sustain before melting under pressure is key to determining a planet’s internal structure and evolution. We report laser-driven shock experiments on fused silica, α-quartz, and stishovite yielding equation-of-state and electronic conductivity data at unprecedented conditions and showing that the melting temperature of SiO2 rises to 8300 K at a pressure of 500 gigapascals, comparable to the core-mantle boundary conditions for a 5–Earth mass super-Earth. We show that mantle silicates and core metal have comparable melting temperatures above 500 to 700 gigapascals, which could favor long-lived magma oceans for large terrestrial planets with implications for planetary magnetic-field generation in silicate magma layers deep inside such planets.

Reference
Millot M (2015) Shock compression of stishovite and melting of silica at planetary interior conditions. Science 347, 6220, 418-420
Link to Article [DOI: 10.1126/science.1261507]

Published with permission from AAAS

¹P. R. Mahaffy et al. (>10)*
¹Planetary Environments Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
*Find the extensive, full author and affiliation list on the publishers Website

The deuterium-to-hydrogen (D/H) ratio in strongly bound water or hydroxyl groups in ancient martian clays retains the imprint of the water of formation of these minerals. Curiosity’s Sample Analysis at Mars (SAM) experiment measured thermally evolved water and hydrogen gas released between 550° and 950°C from samples of Hesperian-era Gale crater smectite to determine this isotope ratio. The D/H value is 3.0 (±0.2) times the ratio in standard mean ocean water. The D/H ratio in this ~3-billion-year-old mudstone, which is half that of the present martian atmosphere but substantially higher than that expected in very early Mars, indicates an extended history of hydrogen escape and desiccation of the planet.

Reference
Mahaffy PR et al. (2015) The imprint of atmospheric evolution in the D/H of Hesperian clay minerals on Mars.
Science 347, 6220, 412-414
Link to Article [DOI: 10.1126/science.1260291]

Reprinted with permission from AAAS