The abundance, distribution, and isotopic composition of Hydrogen in the Moon as revealed by basaltic lunar samples: Implications for the volatile inventory of the Moon

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Romain Tartèsea,*, Mahesh Ananda,b, Jessica J. Barnesa,b, Natalie A. Starkeya, Ian A. Franchia, Yuji Sanoc

aPlanetary and Space Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
bDepartment of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom
cAtmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8564, Japan

In this study we have measured the OH contents and D/H ratios in apatite grains in lunar basalts. These new data considerably expand the limited dataset published so far. The data presented in this study also show that there is a major difference between high- and low-Ti mare basalts in terms of their OH and D/H systematics. Apatites in high-Ti basaltic samples display a relatively restricted range in OH contents (~1500–3000 ppm) with large δD variations (~600–1000‰) whereas apatites in low-Ti Apollo basalts and lunar meteorites display a comparatively larger range in OH contents (~500–15000 ppm), each sample displaying relatively restricted variations in their D/H ratios. Analyses of apatites in basaltic meteorites Miller Range 05035 and LaPaz Icefield 04841 substantially expand the lower bound for δD values measured in apatites from Apollo mare basalts, down to δD values of ~100‰. In these meteorites, high resolution mapping of the distribution of secondary ions of H and C was used to avoid cracks and hotspots. Together with mixing calculations for terrestrial contamination, this analytical protocol ensured that most of the values reported for MIL 05035 and LAP 04841 correspond to their actual lunar signatures. We interpret the large variations of apatite δD values in mare basalts between ~200‰ and 1000‰ as a result of different amounts of degassing of H-bearing species initially dissolved in the basaltic parental melts. Indeed, the average δD values measured in different low-Ti basalts are consistent with ~85–99% degassing of H as H2, starting from a δD value of 100‰. Degassing of H-bearing species essentially as H2 was favoured by the reduced nature of lunar magmas. In low-Ti mare basalts, apatite crystallisation occurred after degassing of the H-bearing species and the OH variations reflect different degrees of fractional crystallisation. In high-Ti mare basalts, large δD variations with relatively restricted range in OH contents imply that apatite crystallisation and degassing of H-bearing species were mostly coeval. Geochemical modelling integrating corrections for degassing and fractional crystallisation suggests that the mantle source regions of the different low-Ti mare basalts could have contained ~5–50 ppm H (equivalent to ~45–450 ppm H2O), which are similar to the estimated range of ~60–350 ppm water for the Earth’s upper mantle. Finally, the H isotopic composition of pre-degassed lunar hydrogen in mare basalts is consistent with a CI-chondrite-type value of ~100‰, which is consistent with the increasing evidence suggesting that the Earth, Mars and the Moon might have accreted similar water of chondritic origin.

Reference
Tartèse R, Mahesh Anand M, Barnes JJ, Starkey NA, Franchi IA and Sano Y (2013) The abundance, distribution, and isotopic composition of Hydrogen in the Moon as revealed by basaltic lunar samples: Implications for the volatile inventory of the Moon. Geochimica et Cosmochimica Acta 122:58–74.
[doi:dx.doi.org/10.1016/j.gca.2013.08.014]
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Fine-grained material encased in microtracks of Stardust samples

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Hugues Leroux and Damien Jacob

Unite Materiaux et Transformations, UMR 8207, Universite Lille 1 and CNRS, Villeneuve d’Ascq, F-59655 France

Dust from comet 81P/Wild 2 was captured at high speed in silica aerogel collectors during the Stardust mission. Studies of deceleration tracks in aerogel showed that a number of cometary particles were poorly cohesive and fragmented during impact. Fragments are now scattered along the walls of impact cavities. Here, we report a transmission electron microscope study of a piece of aerogel extracted from the wall of track 10. We focused on micron-sized secondary tracks along which fragments of a fine- grained material are disseminated. Two populations of fragments were identified. The first is made of polycrystalline silicate assemblages (olivine, pyroxene, and spinel) that appear to be chemically related to each other. The second corresponds to silica-rich glassy clumps characteristic of a mixture of melted cometary material and aerogel. A significant number of fragments have been found with a composition close to chondritic CI for the major elements Fe-Mg-S at a submicron scale. These fragments have thus never been chemically differentiated by high-temperature processes prior to the accretion on the comet, in contrast to terminal particles, which are dominated by larger, denser, and frequently monomineralic components.

Reference
Leroux H and Jacob D (2013) Fine-grained material encased in microtracks of Stardust samples. Meteoritics & Planetary Science (in press)
[doi:10.1111/maps.12185]
Published by arrangement with John Wiley & Sons

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Vestan lithologies mapped by the visual and infrared spectrometer on Dawn

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Eleonora Ammanito1, Maria C. De Sanctis1, Fabrizio Capaccioni1, M. Teresa Capria1, F. Carraro1, Jean-Philippe Combe2, Sergio Fonte1, Alessandro Frigeri1, Steven P. Joy3, Andrea Longobardo1, Gianfranco Magni1, Simone Marchi4, Thomas B. McCord2, Lucy A. McFaddens5, Harry Y. McSween6, Ernesto Palomba1, Carle M Pieters7, Carol A. Polanskey8, Carol A. Raymond8, Jessica M. Sunshine9, Federico Tosi1, Francesca Zambon1 and Christopher T. Russell3

1Istituto di Astrofisica e Planetologia Spaziali, INAF, Rome, Italy
2Bear Fight Institute, 22 Fiddler’s Road, Box 667, Winthrop, Washington 98862, USA
3Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095–1567, USA
4NASA Lunar Science Institute, Southwest Research Institute, 1050 Walnut St, Suite 300, Boulder, Colorado 80302, USA
5NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
6Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996–1410, USA
7Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA
8Jet Propulsion Laboratory, Pasadena, California 91109, USA
9University of Maryland, College Park, Maryland 20742–2421, USA

We present global lithological maps of the Vestan surface based on Dawn mission’s Visible InfraRed (VIR) Spectrometer acquisitions with a spatial sampling of 200 m. The maps confirm the results obtained with the data set acquired by VIR with a spatial sampling of 700 m, that the reflectance spectra of Vesta’s surface are dominated by pyroxene absorptions that can be interpreted within the context of the distribution of howardites, eucrites, and diogenites (HEDs). The maps also partially agree with the ground and Hubble Space Telescope observations: they confirm the background surface being an assemblage of howardite or polymict eucrite, as well as the location of a diogenitic-rich spot; however, there is no evidence of extended olivine-rich regions in the equatorial latitudes. Diogenite is revealed on the Rheasilvia basin floor, indicating that material of the lower crust/mantle was exposed. VIR also detected diogenites along the scarp of Matronalia Rupes, and the rims of Severina and a nearby, unnamed crater, and as ejecta of Antonia crater. The diogenite distribution is fully consistent with petrological constraints; although the mapped distribution does not provide unambiguous constraints, it favors the hypothesis of a magma ocean.

Reference
Ammanito E, Sanctis MC, Capaccioni F, Capria MT, Carraro F, Combe JP, Fonte S, Frigeri A, Joy SP, Longobardo A, Magni G, Marchi S, McCord TB, McFaddens LA, McSween HY, Palomba E, Pieters CM, Polanskey CA, Raymond CA, Sunshine JM, Tosi F, Zambon F and Russell CT (2013) Vestan lithologies mapped by the visual and infrared spectrometer on Dawn. Meteoritics & Planetary Science (in press)
Published by arrangement with John Wiley & Sons

[doi:10.1111/maps.12192]

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Olivine or impact melt: Nature of the ‘‘Orange’’ material on Vesta from Dawn

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Lucille Le Correa,b,*, Vishnu Reddya,b, Nico Schmedemannc, Kris J. Beckerd, David P. O’Briena, Naoyuki Yamashitaa, Patrick N. Peplowskie, Thomas H. Prettymana, Jian-Yang Lia, Edward A. Cloutisf, Brett W. Denevie, Thomas Kneisslc, Eric Palmera, Robert W. Gaskella, Andreas Nathuesb, Michael J. Gaffeyg, David W. Mittlefehldth, William B. Garryi, Holger Sierksb, Christopher T. Russellj, Carol A. Raymondk, Maria C. De Sanctisl, Eleonora Ammanitol

aPlanetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA
b Max-Planck-Institute for Solar System Research, 37191 Katlenburg-Lindau, Germany
c Institute of Geological Sciences, Freie Universitaet Berlin, 12249 Berlin, Germany
d Astrogeology Science Center, USGS, Flagstaff, AZ 86001, USA
e Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
f Department of Geography, University of Winnipeg, Manitoba, Canada
g Department of Space Studies, University of North Dakota, Room 518, Box 9008, Grand Forks, ND 58202, USA h Astromaterials Research Office, NASA Johnson Space Center, Houston, TX 77058, USA
i NASA Goddard Spaceflight Center, Greenbelt, MD 20771, USA
j Institute of Geophysics and Planetary Physics, University of California Los Angeles, Los Angeles, CA 90095, USA
k Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
l Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, Rome, Italy

NASA’s Dawn mission observed a great variety of colored terrains on asteroid (4) Vesta during its survey with the Framing Camera (FC). Here we present a detailed study of the orange material on Vesta, which was first observed in color ratio images obtained by the FC and presents a red spectral slope. The orange material deposits can be classified into three types: (a) diffuse ejecta deposited by recent medium-size impact craters (such as Oppia), (b) lobate patches with well-defined edges (nicknamed ‘‘pumpkin patches’’), and (c) ejecta rays from fresh-looking impact craters. The location of the orange diffuse ejecta from Oppia corresponds to the olivine spot nicknamed ‘‘Leslie feature’’ first identified by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157) from ground-based spectral observations. The distribution of the orange material in the FC mosaic is concentrated on the equatorial region and almost exclusively outside the Rheasilvia basin. Our in-depth analysis of the composition of this material uses complementary observations from FC, the visible and infrared spectrometer (VIR), and the Gamma Ray and Neutron Detector (GRaND). Several possible options for the composition of the orange material are investigated including, cumulate eucrite layer exposed during impact, metal delivered by impactor, olivine–orthopy- roxene mixture and impact melt. Based on our analysis, the orange material on Vesta is unlikely to be metal or olivine (originally proposed by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157)). Analysis of the elemental composition of Oppia ejecta blanket with GRaND suggests that its orange material has 25% cumulate eucrite component in a howarditic mixture, whereas two other craters with orange mate- rial in their ejecta, Octavia and Arruntia, show no sign of cumulate eucrites. Morphology and topography of the orange material in Oppia and Octavia ejecta and orange patches suggests an impact melt origin. A majority of the orange patches appear to be related to the formation of the Rheasilvia basin. Combining the interpretations from the topography, geomorphology, color and spectral parameters, and elemental abundances, the most probable analog for the orange material on Vesta is impact melt.

Reference
Le Corre L, Reddy V, Schmedemann N, Becker KJ, O’Brien DP, Yamashita N, Peplowski PN, Prettyman TH, Li JY, Cloutis EA, Denevi BW, Kneissl T, Palmer E, Gaskell RW, Nathues A, Gaffey MJ, Mittlefehldt DW, Garry WB, Sierks H, Russell CT, Raymond CA, Sanctis MC and Ammanito E (2013) Olivine or impact melt: Nature of the ‘‘Orange’’ material on Vesta from Dawn. Icarus 226:1568-1594.
Copyright Elsevier

[doi:dx.doi.org/10.1016/j.icarus.2013.08.013]

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A petrologic, thermodynamic and experimental study of brachinites: Partial melt residues of an R chondrite-like precursor

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Kathryn G. Gardner-Vandya,b,*, Dante S. Laurettaa, Timothy J. McCoyb

aLunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721, United States
bDepartment of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, PO Box 37012, MRC 119, Washington, DC 20013, United States

The primitive achondrites provide a window into the initial melting of asteroids in the early solar system. The brachinites are olivine-dominated meteorites with a recrystallized texture that we and others interpret as evidence of partial melting and melt removal on the brachinite parent body. We present a petrologic, thermodynamic and experimental study of the brachi- nites to evaluate the conditions under which they formed and test our hypothesis that the precursor material to the brachinites was FeO-rich compared to the precursors of other primitive achondrites. Petrologic analysis of six brachinites (Brachina, Allan Hills (ALH) 84025, Hughes 026, Elephant Moraine (EET) 99402, Northwest Africa (NWA) 3151, and NWA 4969) and one brachinite-like achondrite (NWA 5400) shows that they are meteorites with recrystallized texture that are enriched in olivine (≥80 vol.%) and depleted in other minerals with respect to a chondritic mineralogy. Silicates in the brachinites are FeO-rich (Fa32–36). Brachinite-like achondrite Northwest Africa 5400 is similar in mineralogy and texture to the brachinites but with a slightly lower FeO-content (Fa30). Thermodynamic calculations yield equilibration temperatures above the Fe,Ni–FeS cotectic temperature (~950 °C) for all meteorites studied here and temperatures above the silicate eutectic (~1050 °C) for all but two. Brachina formed at an fO2 of ~IW -1, and the other brachinites and NWA 5400 formed at IW -1. All the mete- orites show great evidence of formation by partial melting having approximately chondritic to depleted chondritic mineral- ogies, equilibrated mineral compositions, and recrystallized textures, and having reached temperatures above that required for melt generation. In an attempt to simulate the formation of the brachinite meteorites, we performed one-atmosphere, gas-mix- ing partial melting experiments of R4 chondrite LaPaz Ice Field 03639. Experiments at 1250 °C and an oxygen fugacity of IW -1 produce residual phases that are within the mineralogy and mineral compositions of the brachinites. These experi- ments provide further evidence for the formation of brachinites as a result of partial melting of a chondritic precursor similar in mineralogy and mineral compositions to the R chondrites.

Reference
Gardner-Vandy KG, Lauretta DS and McCoy, TJ (2013) A petrologic, thermodynamic and experimental study of brachinites: Partial melt residues of an R chondrite-like precursor. Geochimica et Cosmochimica Acta 122:36-57.
[doi: dx.doi.org/10.1016/j.gca.2013.07.035]
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Nanomagnetic intergrowths in Fe–Ni meteoritic metal: The potential for time-resolved records of planetesimal dynamo fields

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James F.J. Brysona, Nathan S. Churcha, Takeshi Kasamab, Richard J. Harrisona

aDepartment of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
bCenter for Electron Nanoscopy, Technical University of Denmark, Kongens Lyngby, Denmark

Nanoscale intergrowths unique to the cloudy zones (CZs) of meteoritic metal display novel magnetic behaviour with the potential to reveal new insight into the early development of magnetic fields on protoplanetary bodies. The nanomagnetic state of the CZ within the Tazewell IIICD iron meteorite has been imaged using off-axis electron holography. The CZ is revealed to be a natural nanocomposite of magnetically hard islands of tetrataenite (ordered FeNi) embedded in a magnetically soft matrix of ordered Fe3Ni. In the remanent state, each tetrataenite island acts as a uniaxial single domain particle with its [001] magnetic easy axis oriented along one of three 〈100〉 crystallographic directions of the parent taenite phase. Micromagnetic simulations demonstrate that switching occurs via the nucleation and propagation of domain walls through individual tetrataenite particles. The switching field (Hs) varies with the length scale of the matrix phase (Lm), with Hs > 1 T for Lm ∼10 nm (approaching the intrinsic switching field for isolated single domain tetrataenite) and 0.2<Hs<0.6 T for Lm ∼30 nm. The reduction in Hs with increasing Lc is caused by exchange coupling between the hard tetrataenite islands and the soft magnetic matrix, which lowers the critical field for domain wall nucleation, providing an explanation for previously observed coercivity variations throughout the CZ. Non-random distributions of the tetrataenite easy axes are observed locally throughout the CZ, suggesting a magnetic field could have been present during nanostructure formation. This observation demonstrates the potential for stable chemical transformation remanent magnetisation to be encoded by the nanostructure, with variations in the proportions of the six possible magnetisation states reflecting the intensity and relative direction of the magnetic fields present during cooling. According to recent cooling models, the cooling rate of meteoritic metal originating near the surface of differentiated planetesimals was such that the magnetic signal across the CZ could potentially record dynamo field intensity and direction variations over time (10–100 Ma), which would enable events such as magnetic reversals and the decay of an asteroid dynamo to be observed.

Reference
Bryson JFJ, Church NS, Kasama T and Harrison RJ (2014) Nanomagnetic intergrowths in Fe–Ni meteoritic metal: The potential for time-resolved records of planetesimal dynamo fields. Earth and Planetary Science Letters 388:237–248.
[doi:10.1016/j.epsl.2013.12.004]
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