¹K.A.Farley et al. (>10)*
¹Division of Geological and Planetary Sciences, Caltech, Pasadena, CA, USA
*Find the extensive, full author and affiliation list on the publishers website

Cl isotope ratios measured on HCl thermally evolved from as-yet-unknown phases in sedimentary rocks and sand in Gale Crater provide unexpected insights to the Martian surficial Cl cycle. The seven samples yield δ37Cl values ranging from −1±25‰−1±25‰ to −51±5‰−51±5‰. Five analyses from two samples of the Sheepbed mudstone (Yellowknife Bay study area) are analytically indistinguishable with a mean δ37Cl of −11±7‰−11±7‰(1σ)(1σ). In contrast, four mudstones/sandstones from the Kimberley and Pahrump study areas also yielded indistinguishable ratios, but with a mean δ37Cl of −43±6‰−43±6‰. The Rocknest sand deposit gave a highly uncertain δ37Cl value of −7±44‰−7±44‰.

These light and highly variable δ37Cl values are unique among known solar system materials. Two endmember models are offered to account for these observations, and in both, perchlorate, with its extreme ability to fractionate Cl isotopes, is critical. In the first model, SAM is detecting HCl from an oxychlorine compound (e.g., perchlorate) produced from volcanic gas emissions by atmospheric chemical reactions. Similar reactions in Earth’s atmosphere may be responsible for the isotopically lightest known Cl outside of this study, in perchlorate from the Atacama Desert. Some of the Gale Crater δ37Cl values are more negative than those in Atacama perchlorate, but because reaction mechanisms and associated fractionation factors are unknown, it is impossible to assess whether this difference is prohibitive. If the negative δ37Cl signal is produced in this fashion, the isotopic variability among samples could arise either from variations in the relative size of the reactant chloride and product perchlorate reservoirs, or from variations in the fraction of perchlorate reduced back to chloride after deposition. Such reduction strongly enriches 37Cl in the residual perchlorate.

Perchlorate reduction alone offers an alternative endmember model that can explain the observed data if SAM measured HCl derived from chloride. In this model isotopically normal perchlorate produced by an unspecified mechanism is reduced to chloride. Depending on the relative size of the reduced reservoir, the integrated product chloride can vary in isotopic composition from −70‰−70‰ in the first increment all the way to the starting composition if the perchlorate is fully reduced. Thus, variable degrees of perchlorate reduction can produce chloride with the appropriate δ37Cl range. Combination of the two endmember models, in which the perchlorate subject to post-deposition reduction is isotopically negative from atmospheric reactions, is also possible.

Determination of the phase hosting the Cl measured by SAM, an oxychlorine compound or chloride, is critical for selecting between these models, and for developing implications of the results for the Mars surficial Cl cycle. At present it is not possible to conclusively establish which phase is responsible (possibly both), but limited evidence favors the conclusion that the measured Cl derives mostly from an oxychlorine compound.

Reference
Farley KA et al. (2016) Light and variable 37Cl/35Cl ratios in rocks from Gale Crater, Mars: Possible signature of perchlorate. Earth and Planetary Science Letters 438, 14–24.
Link to Article [doi:10.1016/j.epsl.2015.12.013]
Copyright Elsevier

¹Ewa Słaby, ²Monika Koch-Müller, ²Hans-Jürgen Förster, ²Richard Wirth, ²Dieter Rhede, ²Anja Schreiber, ³Ulrich Schade
¹Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Warsaw, Warsaw, Poland
²Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, Potsdam, Germany
³Helmholtz-Zentrum Berlin, Elektronenspeicherring BESSY II, Berlin, Germany

We combined the focused ion beam sample preparation technique with polarized synchrotron-based FTIR (Fourier transform infrared) spectroscopy, laser-Raman spectroscopy, electron microprobe analysis (EMPA), and transmission electron microscope (TEM) analysis to identify and quantify structurally bound OH, F, Cl, and CO3 groups in fluorapatite from the Northwest Africa 2975 (NWA 2975) shergottite. In this study, the first FTIR spectra of the OH-stretching region from a Martian apatite are presented that show characteristic OH-bands of a F-rich, hydroxyl-bearing apatite. Depending on the method of apatite-formula calculation and whether charge balance is assumed or not, the FTIR-based quantification of the incorporated OH, expressed as wt% H2O, is in variably good agreement with the H2O concentration calculated from electron microprobe data. EMP analyses yielded between 0.35 and 0.54 wt% H2O, and IR data yielded an average H2O content of 0.31 ± 0.03 wt%, consistent with the lower range determined from EMP analyses. The TEM observations implied that the volatiles budget of fluorapatite is magmatic. The water content and the relative volatile ratios calculated for the NWA 2975 magma are similar to those established for other enriched or intermediate shergottites. It is difficult to define the source of enrichment: either Martian wet mantle or crustal assimilation. Comparing the environment of parental magma generation for NWA 2975 with the terrestrial mantle in terms of water content, it displays a composition intermediate between enriched and depleted MORB.

Reference
Słaby E, Monika Koch-Müller, Förster H-J, Wirth R, Rhede D, Schreiber A, Schade U (2016) Determination of volatile concentrations in fluorapatite of Martian shergottite NWA 2975 by combining synchrotron FTIR, Raman spectroscopy, EMPA, and TEM, and inferences on the volatile budget of the apatite host-magma. Meteoritics & Planetary Sciences (in Press)
Link to Article [DOI: 10.1111/maps.12598]
Published by arrangement with John Wiley & Sons

¹Knut Metzler, ²Andreas Pack
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, Germany
²Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Georg-August-Universität Göttingen, Göttingen, Germany

Cluster chondrites are characterized by close-fit textures of deformed and indented chondrules, taken as evidence for hot chondrule accretion (Metzler 2012). We investigated seven cluster chondrite clasts from six brecciated LL3 chondrites and measured their bulk oxygen isotopic and chemical composition, including REE, Zr, and Hf. The same parameters were measured in situ on 93 chondrules and 4 interchondrule matrix areas. The CI-normalized REE patterns of the clasts are flat, showing LL-chondritic concentrations. The mean chemical compositions of chondrules in clasts and other LL chondrites are indistinguishable and we conclude that cluster chondrite chondrules are representative of the normal LL chondrule population. Type II chondrules are depleted in MgO, Al2O3 and refractory lithophiles (REE, Zr, Hf) by factors between 0.65 and 0.79 compared to type I chondrules. The chondrule REE patterns are basically flat with slight LREE < HREE fractionations. Many chondrules exhibit negative Eu anomalies while matrix shows a complementary pattern. Chondrules scatter along a correlation line with a slope of 0.63 in the oxygen 3-isotope diagram, interpreted as the result of O-isotope exchange between chondrule melts and 18O-rich nebular components. In one clast, a distinct anticorrelation between chondrule size and δ18O is found, which may indicate a more intense oxygen isotope exchange by smaller chondrules. In some clasts the δ18O values of type I chondrules are correlated with concentrations of SiO2 and MnO and anticorrelated with MgO, possibly due to the admixture of a SiO2- and MnO-rich component to chondrule melts during oxygen isotope exchange. Two chondrules with negative anomalies in Sm, Eu, and Yb were found and may relate their precursors to refractory material known from group III CAIs. Furthermore, three chondrules with strong LREE > HREE and Zr/Hf fractionations were detected, whose formation history remains to be explained.

Reference
Metzler K, Pack A (2016) Chemistry and oxygen isotopic composition of cluster chondrite clasts and their components in LL3 chondrites. Meteoritics & Planetary Sciences (in Press)
Link to Article [DOI: 10.1111/maps.12592]
Published by arrangement with John Wiley & Sons

R.Wieler et al. (>10)*
¹Department of Earth Sciences, ETH Zürich, Zürich, Switzerland
*Find the extensive, full author and affiliation list on the publishers website

We present noble gas data for 16 shergottites, 2 nakhlites (NWA 5790, NWA 10153), and 1 angrite (NWA 7812). Noble gas exposure ages of the shergottites fall in the 1–6 Ma range found in previous studies. Three depleted olivine-phyric shergottites (Tissint, NWA 6162, NWA 7635) have exposure ages of ~1 Ma, in agreement with published data for similar specimens. The exposure age of NWA 10153 (~12.2 Ma) falls in the range of 9–13 Ma reported for other nakhlites. Our preferred age of ~7.3 Ma for NWA 5790 is lower than this range, and it is possible that NWA 5790 represents a distinct ejection event. A Tissint glass sample contains Xe from the Martian atmosphere. Several samples show a remarkably low (21Ne/22Ne)cos ratio < 0.80, as previously observed in a many shergottites and in various other rare achondrites. This was explained by solar cosmic ray-produced Ne (SCR Ne) in addition to the commonly found galactic cosmic ray-produced Ne, implying very low preatmospheric shielding and ablation loss. We revisit this by comparing measured (21Ne/22Ne)cos ratios with predictions by cosmogenic nuclide production models. Indeed, several shergottites, acalpulcoites/lodranites, angrites (including NWA 7812), and the Brachina-like meteorite LEW 88763 likely contain SCR Ne, as previously postulated for many of them. The SCR contribution may influence the calculation of exposure ages. One likely reason that SCR nuclides are predominantly detected in meteorites from rare classes is because they usually are analyzed for cosmogenic nuclides even if they had a very small (preatmospheric) mass and hence low ablation loss.

Reference
Wieler R et al. (2016) Noble gases in 18 Martian meteorites and angrite Northwest Africa 7812—Exposure ages, trapped gases, and a re-evaluation of the evidence for solar cosmic ray-produced neon in shergottites and other achondrites. Meteoritics & Planetary Sciences (in Press)
Link to Article [DOI: 10.1111/maps.12600]
Published by arrangement with John Wiley & Sons

¹Jonathan Oulton, ¹Munir Humayun, ²Alexei Fedkin, ^2,3Lawrence Grossman
¹National High Magnetic Field Laboratory & Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL 32310, USA
²Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA.
³Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA

The silicate and metal clasts in CB chondrites have been inferred to form as condensates from an impact-generated vapor plume between a metal-rich body and a silicate body. A detailed study of the condensation of impact-generated vapor plumes showed that the range of CB silicate clast compositions could not be successfully explained without invoking a chemically differentiated target. Here, we report the most comprehensive elemental study yet performed on CB silicates with 32 silicate clasts from nine slices of Gujba analyzed by laser ablation inductively coupled plasma mass spectrometry for 53 elements. Like in other studies of CBs, the silicate clasts are either barred olivine (BO) or cryptocrystalline (CC) in texture. In major elements, the Gujba silicate clasts ranged from chondritic to refractory enriched. Refractory element abundances ranged from 2-10xCI, with notable anomalies in Ba, Ce, Eu, and U abundances. The two most refractory-enriched BO clasts exhibited negative Ce anomalies and were depleted in U relative to Th, characteristic of volatilization residues, while other BO clasts and the CC clasts exhibited positive Ce anomalies with excess U (1-3xCI), and Ba (1-6xCI) anomalies indicating re-condensation of ultra-refractory element depleted vapor. The Rare Earth Elements (REE) also exhibit light REE (LREE) enrichment or depletion in several clasts with a range of (La/Sm)CI of 0.9-1.8. This variation in the LREE is essentially impossible to accomplish by processes involving vapor-liquid or vapor-solid exchange of REE, and appears to have been inherited from a differentiated target. The most distinctive evidence for inherited chemical differentiation is observed in highly refractory element (Sc, Zr, Nb, Hf, Ta, Th) systematics. The Gujba clasts exhibit fractionations in Nb/Ta that correlate positively with Zr/Hf and span the range known from lunar and Martian basalts, and exceed the range in Zr/Hf variation known from eucrites. Variations of highly incompatible refractory elements (e.g. Th) against less incompatible elements (e.g., Zr, Sr, Sc) are not chondritic, but exhibit distinctly higher Th abundances requiring a differentiated crust to be admixed with depleted mantle in ratios that are biased to higher crust/mantle ratios than in a chondritic body. The possibility that these variations are due to admixture of refractory inclusion-debris into normal chondritic matter is raised but cannot be definitively tested because existing “bulk” analyses of CAIs carry artifacts of unrepresentative sampling. The inferences drawn from the compositions of Gujba silicate clasts, here, complement what has been inferred from the compositions of metallic clasts, but provide surprisingly detailed insight into the structure of the target. Evidence that metal and silicate in CB chondrites both formed from impact-generated vapor plumes, taken together with recent work on metallic nodules in E chondrites, and on ordinary chondrites, indicates that chondrule formation occurs by this mechanism quite widely. However, the nature of the impact on the CB body is quite different than the popular conceptions of impact of partially or wholly molten chondritic bodies and the younger (5 Ma) age of CB chondrules is consistent with origin in a disk with more evolved targets and impactors gravitationally perturbed by nascent planets.

Reference
Oulton J, Humayun M, Fedkin A, Grossman L (2016) Chemical Evidence for Differentiation, Evaporation and Recondensation from Silicate Clasts in Gujba. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.01.008]
Copyright Elsevier

^1,2,3Philipp R. Heck, ^1,4Birger Schmitz, ^1,2Surya S. Rout, ⁵Travis Tenner, ^1,2,3Krysten Villalon, ⁴Anders Cronholm, ⁴Fredrik Terfelt, ⁵Noriko T. Kita
¹Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, 1400 S Lake Shore Dr, Chicago, IL 60605, USA
²Chicago Center for Cosmochemistry, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA
³Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA
⁴Astrogeobiology Laboratory, Department of Physics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden
⁵WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706-1692, USA

A large abundance of L-chondritic material, mainly in the form of fossil meteorites and chromite grains from micrometeorites, has been found in mid-Ordovician 470 Ma old sediments globally. The material has been determined to be ejecta from the L chondrite parent body breakup event, a major collision in the asteroid belt 470 Ma ago. In this study we search the same sediments for H-chondritic chromite grains in order to improve our understanding of the extraterrestrial flux to Earth after the asteroid breakup event. We have used SIMS in conjunction with quantitative SEM/EDS to determine the three oxygen isotopic and elemental compositions, respectively, of a total of 120 randomly selected, sediment-dispersed extraterrestrial chromite grains mainly representing micrometeorites from 470 Ma old post-breakup limestone from the Thorsberg quarry in Sweden and the Lynna River site in Russia. We show that 99% or more of the grains are L-chondritic, whereas the H-chondritic fraction is 1% or less. The L-/H-chondrite ratio after the breakup thus was >99 compared to 1.1 in today’s meteoritic flux. This represents independent evidence, in agreement with previous estimates based on sediment-dispersed extraterrestrial chromite grain abundances and sedimentation rates, of a two orders of magnitude higher post-breakup flux of L-chondritic material in the micrometeorite fraction. Finally, we confirm the usefulness of three oxygen isotopic SIMS analyses of individual extraterrestrial chromite grains for classification of equilibrated ordinary chondrites. The H- and L-chondritic chromites differ both in their three oxygen isotopic and elemental compositions, but there is some overlap between the groups. In chromite, TiO2 is the oxide most resistant to diagenesis, and the combined application of TiO2 and oxygen three-isotope analysis can resolve uncertainties arising from the compositional overlaps.

Reference
Heck PR, Schmitz B, Rout SS, Tenner T, Villalon K, Cronholm A, Terfelt F,Kitae NT (2016) A search for H-chondritic chromite grains in sediments that formed immediately after the breakup of the L-chondrite parent body 470 Ma ago. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.11.042]
Copyright Elsevier

^1,2Queenie Hoi Shan Chan, ¹Yoshito Chikaraishi, ¹Yoshinori Takano, ¹Nanako O. Ogawa, ¹Naohiko Ohkouchi
¹Department of Biogeochemistry, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka 237-0061, Kanagawa, Japan
²Present address: NASA Johnson Space Center, 2101 NASA Parkway, Houston 77058, TX, USA

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

Reference
Chan QHS, Chikaraishi Y, Takano Y, Ogawa NO, Ohkouchi N (2016) Amino acid compositions in heated carbonaceous chondrites and their compound-specific nitrogen isotopic ratios. Earth, Planets and Space 68, 7
Link to Article [doi:10.1186/s40623-016-0382-8]

¹Selby Cull-Hearth, ¹Alexis van Venrooy, ¹M. Caroline Clark,²Adriana Cvitkovic
¹Department of Geology, Bryn Mawr College, 101 N. Merion Avenue, Bryn Mawr, PA 19123, USA
²Haverford College, Haverford, PA, 19041, USA

Most sulfate minerals form only in specific pH conditions, making them useful markers of past environmental conditions on Mars. However, interpreting past environments requires a full understanding of the suite of minerals present, a task which is complicated by the fact that some minerals can spectrally mask others in the visible- to near-infrared (VNIR, 0.4 to 2.5 μm). Here, we report VNIR spectra of two-phase mineral combinations obtained from the Río Tinto acid mine drainage system of southern Spain. Our results show that in VNIR reflectance spectroscopy: 1) copiapite masks rhomboclase and partially masks melanterite; 2) coquimbite masks copiapite, jarosite, and rhomboclase; 3) at wavelengths 1.2 μm, gypsum masks these minerals; 4) unlike copiapite, jarosite, or melanterite, halotrichite masks gypsum completely; 5) in two-phase mixtures of copiapite and jarosite, both phases are evident. No consistent VNIR relationship is observed in two-phase mixtures of melanterite and halotrichite, suggesting that microtextures are likely more important than optical properties in determining VNIR reflectance. We also show that the shorter wavelengths are more sensitive to the presence of both phases: even in mixtures where one phase is masking another, both phases usually impact absorptions in the 0.75 – 0.95 μm region. This region may therefore be useful in remotely identifying mineral mixtures on Mars. These results have implications for several regions on Mars: most notably, they imply that the jarosite exposures reported at Mawrth Vallis may be jarosite-copiapite mixtures.

Reference
Cull-Hearth S, van Venrooy A, Clark MC, Cvitkovic A (2016) Acid-sulfate mixtures from Río Tinto, Spain: Spectral masking relationships and implications for Mars. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.01.001]
Copyright Elsevier

^1,2B. Carry, ^3,4E. Solano, ¹S. Eggl, ^5,6F.E. DeMeo
¹IMCCE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Uni- versit es, UPMC Univ Paris 06, Univ. Lille
^“European Space Astronomy Centre, ESA, P.O. Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
^§Centro de Astrobiologia (INTA-CSIC), Departamento de Astrofisica. P.O. Box 78, E-28691 Villanueva de la Cañada, Madrid, Spain
⁴Spanish Virtual Observatory
⁵Department of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA
⁶Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-16, Cambridge, MA, 02138, USA

The nature and origin of the asteroids orbiting in near-Earth space, including those on a potentially hazardous trajectory, is of both scientific interest and practical importance. We aim here at determining the taxonomy of a large sample of near-Earth and Mars-crosser asteroids and analyze the distribution of these classes with orbit. We use this distribution to identify the source regions of near-Earth objects and to study the strength of planetary encounters to refresh asteroid surfaces. We measure the photometry of these asteroids over four filters at visible wavelengths on images taken by the Sloan Digital Sky Survey (SDSS). These colors are used to classify the asteroids into a taxonomy consistent with the widely used Bus-DeMeo taxonomy (DeMeo et al., Icarus 202, 2009) based on visible and near-infrared spectroscopy. We report here on the taxonomic classification of 206 near-Earth and 776 Mars-crosser asteroids determined from SDSS photometry, representing an increase of 40% and 663% of known taxonomy classifications in these populations. Using the source region mapper by Greenstreet et al. (Icarus, 217, 2012), we compare for the first time the taxonomic distribution among near-Earth and main-belt asteroids of similar diameters. Both distributions agree at the few percent level for the inner part of the Main Belt and we confirm this region as a main source of near-Earth objects. The effect of planetary encounters on asteroid surfaces are also studied by developing a simple model of forces acting on a surface grain during planetary encounter, which provides the minimum distance at which a close approach should occur to trigger resurfacing events. By integrating numerically the orbit of the 519 S-type and 46 Q-type asteroids in our sample back in time for 500,000 years and monitoring their encounter distance with Venus, Earth, Mars, and Jupiter, we seek to understand the conditions for resurfacing events. The population of Q-type is found to present statistically more encounters with Venus and the Earth than S-types, although both S- and Q-types present the same amount of encounters with Mars.

Reference
Carry B, Solano E, Eggl S, DeMeo FE (2016) Spectral properties of near-Earth and Mars-crossing asteroids using Sloan photometry. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.12.047]
Copyright Elsevier

¹P. François et al. (>10)*
¹Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR 7583, CNRS, Univ. Denis Diderot & Univ. Paris-Est Créteil, Créteil Cedex, France
*Find the extensive, full author and affiliation list on the publishers website

Pyrolysis of soil or rock samples is the preferred preparation technique used on Mars to search for organic molecules up today. During pyrolysis, oxichlorines present in the soil of Mars release oxidant species that alter the organic molecules potentially contained in the samples collected by the space probes. This process can explain the difficulty experienced by in situ exploration probes to detect organic materials in Mars soil samples until recently. Within a few months, the Curiosity rover should reach and analyze for the first time soils rich in sulfates which could induce a different behavior of the organics during the pyrolysis compared with the types of soils analyzed up today. For this reason, we systematically studied the pyrolysis of organic molecules trapped in magnesium sulfate, in the presence or absence of calcium perchlorate. Our results show that organics trapped in magnesium sulfate can undergo some oxidation and sulfuration during the pyrolysis. But these sulfates are also shown to protect organics trapped inside the crystal lattice and/or present in fluid inclusions from the oxidation induced by the decomposition of calcium perchlorate, and probably other oxychlorine phases currently detected on Mars. Trapped organics may also be protected from degradation processes induced by other minerals present in the sample, at least until these organics are released from the pyrolyzed sulfate mineral (~700 °C in our experiment). Hence, we suggest magnesium sulfate as one of the minerals to target in priority for the search of organic molecules by the Curiosity and ExoMars 2018 rovers.

Reference
François P et al. (2016) Magnesium sulfate as a key mineral for the detection of organic molecules on Mars using pyrolysis. Journal of Geophysical Research Planets (in Press)
Link to Article [DOI: 10.1002/2015JE004884]
Published by arrangement with John Wiley&Sons