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

¹ S. Fornasier,¹C. Lantz, ¹D. Perna, ²H. Campins, ¹M.A. Barucci, ³D. Nesvorny
¹LESIA, Observatoire de Paris, PSL Research University, CNRS, Univ. Paris Diderot, Sorbonne Paris Cité, UPMC Univ., Paris 06, Sorbonne Universités, 5 Place J. Janssen, 92195 Meudon Pricipal Cedex, France
²Department of Physics and Astronomy, University of Central Florida, Orlando, Florida 32816-2385, USA
³Department of Space Studies, Southwest Research Institute, Boulder, CO 80302, USA

Themis is an old and statistically robust asteroid family populating the outer main belt, and resulting from a catastrophic collision that took place 2.5 ± 1.0 Gyr ago. Within the old Themis family a young sub-family, Beagle, formed less than 10 Myr ago, has been identified.We present the results of a spectroscopic survey in the visible and near infrared range of 22 Themis and 8 Beagle families members. The Themis members investigated exhibit a wide range of spectral behaviors, including asteroids with blue/neutral and moderately red spectra, while the younger Beagle family members look spectrally bluer than the Themis ones and they have a much smaller spectral slope variability. Four Themis members, including (24) Themis, have absorption bands centered at 0.68-0.73 μm indicating the presence of aqueously altered minerals.The best meteorite spectral analogues found for both Themis and Beagle families members are carbonaceous chondrites having experienced different degrees of aqueous alteration, prevalently CM2 but also CV3 and CI, and some of them are chondrite samples being unusual or heated. The presence of aqueous altered materials on the asteroids surfaces and the meteorite matches indicate that the parent body of the Themis family experienced mild thermal metamorphism in the past. We extended the spectral analysis including the data available in the literature on Themis and Beagle families members, and we looked for correlations between spectral behavior and physical parameters using the albedo and size values derived from the WISE data. The analysis of this larger sample confirm the spectral diversity within the Themis family and that Beagle members tend to be bluer and to have an higher albedo. The differences between the two family may be partially explained by space weathering processes, which act on these primitive surfaces in a similar way than on S-type asteroids, i.e. producing reddening and darkening. However we see several Themis members having albedos and spectral slopes similar to the young Beagle members. Alternative scenarios are proposed including heterogeneity in the parent body having a compositional gradient with depth, and/or the survival of projectile fragments having a different composition than the parent body.

Reference
Fornasier S, Lantz C, Perna D, Campins H, Barucci MA, Nesvorny D (2016) Spectral variability on primitive asteroids of the Themis and Beagle families: space weathering effects or parent body heterogeneity? Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.01.002]
Copyright Elsevier

^1,2Noël Chaumard,^1,3Bertrand Devouard
¹Laboratoire Magmas et Volcans, CNRS UMR 6524, Université Blaise Pascal, OPGC-IRD, Clermont-Ferrand Cedex, France
²Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Universités, Muséum national d’histoire naturelle, UPMC Univ. Paris 06, UMR CNRS 7590, IRD UMR 206, Paris Cedex 5, France
³Aix-Marseille Université, CNRS, IRD, Aix en Provence, France

CK chondrites are the only group of carbonaceous chondrites with petrologic types ranging from 3 to 6. It is commonly reported than ~15 vol% of CK4–6 samples are composed of chondrules. The modal abundance of chondrules estimated here for 18 CK3–6 (including five CK3s) ranges from zero (totally recrystallized) to 50.5%. Although almost all chemically re-equilibrated with the host matrix, we recognized in CK3s and Tanezrouft (Tnz) 057 (CK4) up to 85% of chondrules as former type I chondrules. Mean diameters of chondrules range from 0.22 to 1.05 mm for Karoonda (CK4) and Tnz 057 (CK4), respectively. Up to ~60% of chondrules in CK3–4 are surrounded by igneous rims (from ~20 μm to 2 mm width). Zoned olivines were found in unequilibrated chondrules from DaG 431 (CK3-an), NWA 4724 (CK3.8), NWA 4423 (CK3.9), and Tnz 057 (CK4). We modeled Fe/Mg interdiffusion profiles measured in zoned olivines to evaluate the peak metamorphic temperatures and time scales of the CK parent body metamorphism, and proposed a two-stage diffusion process in order to account for the position of inflection points situated within chondrules. Time scales inferred from Fe/Mg interdiffusion in olivine from unequilibrated chondrules are on the order of tens to a hundred thousand years (from 50 to 70,000 years for peak metamorphic temperatures of 1140 and 920 K, respectively). These durations are longer than what is commonly accepted for shock metamorphism and shorter than what is required for nuclide decay. Using the concept of a continuous CV–CK metamorphic series, which is reinforced by this study, we estimated peak metamorphic temperatures

Reference
Chaumard N, Devouard B (2016) Chondrules in CK carbonaceous chondrites and thermal history of the CV–CK parent body. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12599]
Published by arrangement with John Wiley&Sons

¹Allan H. Treiman et al. (>10)*
¹Lunar and Planetary Institute, Houston, TX, USA
*Find the extensive, full author and affiliation list on the publishers website

The Windjana drill sample, a sandstone of the Dillinger member (Kimberley formation, Gale Crater, Mars), was analyzed by CheMin X-ray diffraction (XRD) in the MSL Curiosity rover. From Rietveld refinements of its XRD pattern, Windjana contains: sanidine (21% weight, ~Or95); augite (20%); magnetite (12%); pigeonite; olivine; plagioclase; amorphous and smectitic material (~25%); and percent-levels of others including ilmenite, fluorapatite, and bassanite. From mass balance on the APXS chemical analysis, the amorphous material is Fe-rich with nearly no other cations — like ferrihydrite. The Windjana sample shows little alteration, and was likely cemented by its magnetite and ferrihydrite. From ChemCam LIBS chemical analyses, Windjana is representative of the Dillinger and Mt. Remarkable members of the Kimberley formation. LIBS data suggest that the Kimberley sediments include at least three chemical components. The most K-rich targets have 5.6% K2O, ~1.8 times that of Windjana, implying a sediment component with >40% sanidine, e.g., a trachyte. A second component is rich in mafic minerals, with little feldspar (like a shergottite). A third component is richer in plagioclase and in Na2O, and is likely to be basaltic. The K-rich sediment component is consistent with APXS and ChemCam observations of K-rich rocks elsewhere in Gale Crater. The source of this sediment component was likely volcanic. The presence of sediment from many igneous sources, in concert with Curiosity’s identifications of other igneous materials (e.g., mugearite), implies that the northern rim of Gale Crater exposes a diverse igneous complex, at least as diverse as found in similar-age terranes on Earth.

Reference
Treiman AH et al. (2016) Mineralogy, Provenance, and Diagenesis of a Potassic Basaltic Sandstone on Mars: CheMin X-ray Diffraction of the Windjana Sample (Kimberley Area, Gale Crater). Journal of Geophysical Research Planets (in Press)
Link to Article [DOI: 10.1002/2015JE004932]

Published by arrangement with John Wiley&Sons

^1,2N. van Roosbroek, ²L. Pittarello, ³A. Greshake,¹V. Debaille,²P. Claeys
¹Laboratoire G-Time, Université Libre de Bruxelles, Brussels, Belgium
²Analytical, Environmental, and Geo-Chemistry (AMGC), Vrije Universiteit Brussel, Brussels, Belgium
³Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Berlin, Germany

About half of the IIE nonmagmatic iron meteorites contain silicate inclusions with a primitive to differentiated nature. The presence of preserved chondrules has been reported for two IIE meteorites so far, Netschaëvo and Mont Dieu, which represent the most primitive silicate material within this group. In this study, silicate inclusions from two samples of Netschaëvo were examined. Both silicate inclusions are characterized by a porphyritic texture dominated by clusters of coarse-grained olivine and pyroxene, set in a fine-grained groundmass that consists of new crystals of olivine and a glassy appearing matrix. This texture does not correspond to the description of the previously examined pieces of Netschaëvo, which consist of primitive chondrule-bearing angular clasts. Detailed petrographic observations and geochemical analyses suggest that the investigated samples of Netschaëvo consist of quenched impact melt. This implies that Netschaëvo is a breccia containing metamorphosed and impact-melt rock (IMR) clasts and that collisions played a major role in the formation of the IIE group.

Reference
van Roosbroek N, Pittarello L, Greshake A, Debaille V, Claeys P (2016) First finding of impact melt in the IIE Netschaëvo meteorite. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12596]
Published by arrangement with John Wiley&Sons