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

¹Yoshitaka HOMMA, ²Yui KOUKETSU, ¹Hiroyuki KAGI, ³Takashi MIKOUCHI, ⁴Hikaru YABUTA
¹Geochemical Research Center, Graduate School of Science, The University of Tokyo
²Department of Earth and Planetary Science, Graduate School of Environmental Studies, Nagoya University
³Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo
⁴Department of Earth and Space Science, Graduate School of Science, Osaka University

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

HOMMA Y, KOUKETSU Y, KAGI H, MIKOUCHI T, YABUTA H (2015) Raman spectroscopic thermometry of carbonaceous material in chondrites: four–band fitting analysis and expansion of lower temperature limit. Journal of Mineralogical and Petrological Sciences 110, 276-282
Link to Article [http://doi.org/10.2465/jmps.150713a]

¹Zoe A. Landsman, ^2,3Javier Licandro, ¹Humberto Campins, ⁴Julie Ziffer, ⁵Mario de Prá, ⁶Dale P. Cruikshank
¹Department of Physics, University of Central Florida, 4111 Libra Drive, PS 430, Orlando, FL 32826, United States
²Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
³Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
⁴Department of Physics, University of Southern Maine, 96 Falmouth St, Portland, ME 04103, United States
⁵Observatório Nacional, R. General José Cristino, 77 – Imperial de São Cristóvão, Rio de Janeiro – RJ, 20921-400, Brazil
⁶NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035, United States

Spectroscopic investigations of primitive asteroid families constrain family evolution and composition and conditions in the solar nebula, and reveal information about past and present distributions of volatiles in the solar system. Visible and near-infrared studies of primitive asteroid families have shown spectral diversity between and within families. Here, we aim to better understand the composition and physical properties of two primitive families with vastly different ages: ancient Themis ( ∼ 2.5 Gyr) and young Veritas ( ∼ 8 Myr). We analyzed the 5 – 14μm Spitzer Space Telescope spectra of 11 Themis-family asteroids, including eight previously studied by Licandro et al. (2012), and nine Veritas-family asteroids, for a total of 20 asteroids in our sample. We detect a broad 10-μm emission feature, attributed to fine-grained and/or porous silicate regolith, in all 11 Themis-family spectra and six of nine Veritas-family asteroids, with 10-μm spectral contrast ranging from 1% ± 0.1% to 8.5% ± 0.9%. We used thermal modeling to derive diameters, beaming parameters and albedos for our sample. Asteroids in both families have beaming parameters near unity and geometric albedos in the range 0.03 – 0.14. Spectral contrast of the 10-μm silicate emission feature is correlated with beaming parameter and rotation period in the Themis family, and may be related to near-infrared spectral slope for both families. We see no correlations of 10-μm emission with diameter or albedo for either family. Comparison with laboratory spectra of primitive meteorites suggests these asteroids are similar to meteorites with relatively low abundances of phyllosilicates. Overall, our results suggest the Themis and Veritas families are primitive asteroids with variation in composition and/or regolith properties within both families.

Reference
Landsman ZA, Licandro J, Campins H, Ziffer J, Prá M, Cruikshank DP (2016)
The Veritas and Themis asteroid families: 5–14 μm spectra with the Spitzer Space Telescope. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.01.008]
Copyright Elsevier

¹D. P. Moriarty III, ¹C. M. Pieters
¹Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island, USA

We reexamine the relationship between pyroxene composition and near-infrared absorption bands, integrating measurements of diverse natural and synthetic samples. We test an algorithm (PLC) involving a two-part linear continuum removal and parabolic fits to the 1 and 2 μm bands—a computationally simple approach which can easily be automated and applied to remote sensing data. Employing a suite of synthetic pure pyroxenes, the PLC technique is shown to derive similar band centers to the modified Gaussian model. PLC analyses are extended to natural pyroxene-bearing materials, including (1) bulk lunar basalts and pyroxene separates, (2) diverse lunar soils, and (3) HED meteorites. For natural pyroxenes, the relationship between composition and absorption band center differs from that of synthetic pyroxenes. These differences arise from complexities inherent in natural materials such as exsolution, zoning, mixing, and space weathering. For these reasons, band center measurements of natural pyroxene-bearing materials are compositionally nonunique and could represent three distinct scenarios (1) pyroxene with a narrow compositional range, (2) complexly zoned pyroxene grains, or (3) a mixture of multiple pyroxene (or nonpyroxene) components. Therefore, a universal quantitative relationship between band centers and pyroxene composition cannot be uniquely derived for natural pyroxene-bearing materials without additional geologic context. Nevertheless, useful relative relationships between composition and band center persist in most cases. These relationships are used to interpret M3 data from the Humboldtianum Basin. Four distinct compositional units are identified (1) Mare Humboldtianum basalts, (2) distinct outer basalts, (3) low-Ca pyroxene-bearing materials, and (4) feldspathic materials.

References
Moriarty III DP, Pieters CM (2016) Complexities in pyroxene compositions derived from absorption band centers: Examples from Apollo samples, HED meteorites, synthetic pure pyroxenes, and remote sensing data. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12588]
Published by arrangement with John Wiley&Sons

¹E.S. Steenstra, ¹J.S. Knibbe, ^2,3N. Rai, ¹W. van Westrenen
¹Faculty of Earth and Life Sciences, VU University Amsterdam, Amsterdam, The Netherlands
²Centre for Planetary Sciences, Birkbeck – UCL, London, UK
³Department of Earth Sciences, Mineral and Planetary Sciences Division, Natural History Museum, London, UK

It is now widely accepted that the asteroid 4–Vesta has an Fe–rich metallic core, but the composition of the core and the conditions prevailing during core–mantle differentiation are poorly constrained. In light of new constraints on Vesta’s geophysical and geochemical properties obtained by the DAWN mission, we have re–examined the conditions at which core–mantle differentiation in Vesta may have occurred by linking the estimated mantle depletions of siderophile elements P, Co, Ni, Cu, Ga, Ge, Mo and W in the vestan mantle to newly derived predictive equations for core–mantle partitioning of these elements. We extend the number of elements previously considered in geochemical modeling of vestan core formation and use published metal–silicate partitioning data obtained at low pressures to characterize the dependence of metal/silicate partition coefficients (D) on pressure, temperature, oxygen fugacity and composition of the silicate and metallic melt. In our modeling we implement newly derived mantle depletions of P, Co, Ni and Ga through analysis of published HED meteorite analyses and assess two contrasting bulk compositional models for Vesta.

Modeling results using Monte Carlo simulations constrain vestan core formation to have occurred at mildly reducing conditions of approximately 2 log units below the iron–wüstite (IW) buffer (ΔIW = –2.05±0.20) if the two most likely bulk compositions (binary mixtures of H + CM or H + CV chondritic meteorites) are considered, assuming a temperature range between 1725–1850 K and a sulfur–free pure Fe core. If the core is assumed to be sulfur–rich (15 wt.% S) as predicted by the latter bulk compositional models, observed depletions for all eight siderophile elements can be simultaneously satisfied at ΔIW = –2.35±0.10 and 1725–1850 K for the H + CV bulk composition and ΔIW = –2.30±0.15 and 1725–1850 K for the H + CM bulk composition. More reducing conditions are not consistent with the observed siderophile element depletions in the vestan mantle, independent of the sulfur content of the vestan core and of the bulk compositional model chosen.

Our analysis shows that a previously proposed shallow magma ocean on Vesta during core formation is not consistent with the observed mantle depletion of Ga for the two considered bulk compositions irrespective of core composition. Instead, our results are consistent with the existence of a deep magma ocean during core formation on Vesta, requiring >50 to 100 per cent mantle melting.

References
Steenstra ES, Knibbe JS, Rai N, van Westrenen W (2016) Constraints on core formation in Vesta from metal–silicate partitioning of siderophile elements. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.01.002]
Copyright Elsevier

^1,2,3John N. Bigolski,^1,2,3Michael K. Weisberg,^1,2,3Harold C. Connolly Jr.,^1,3Denton S. Ebel
¹Earth and Environmental Sciences, The Graduate Center of the City University of New York, New York, New York, USA
²Department of Physical Sciences, Kingsborough Community College, Brooklyn, New York, USA
³Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA

We report on a suite of microchondrules from three unequilibrated ordinary chondrites (UOCs). Microchondrules, a subset of chondrules that are ubiquitous components of UOCs, commonly occur in fine-grained chondrule rims, although may also occur within matrix. Microchondrules have a variety of textures: cryptocrystalline, microporphyritic, radial, glassy. In some cases, their textures, and in many cases, their compositions, are similar to their larger host chondrules. Bulk compositions for both chondrule populations frequently overlap. The primary material that composes many of the microchondrules has compositions that are pyroxene-normative and is similar to low-Ca-pyroxene phenocrysts from host chondrules; primary material rarely resembles olivine or plagioclase. Some microchondrules are composed of FeO-rich material that has compositions similar to the bulk submicron fine-grained rim material. These microchondrules, however, are not a common compositional type and probably represent secondary FeO-enrichment. Microchondrules may also be porous, suggestive of degasing to form vesicles. Our work shows that the occurrence of microchondrules in chondrule rims is an important constraint that needs to be considered when evaluating chondrule-forming mechanisms. We propose that microchondrules represent melted portions of the chondrule surfaces and/or the melt products of coagulated dust in the immediate vicinity of the larger chondrules. We suggest that, through recycling events, the outer surfaces of chondrules were heated enough to allow microchondrules to bud off as protuberances and become entrained in the surrounding dusty environment as chondrules were accreting fine-grained rims. Microchondrules are thus byproducts of cyclic processing of chondrules in localized environments. Their occurrence in fine-grained rims represents a snapshot of the chondrule-forming environment. We evaluate mechanisms for microchondrule formation and hypothesize a potential link between the emergence of type II chondrules in the early solar system and the microchondrule-bearing fine-grained rims surrounding type I chondrules.

Reference
Bigolski JN, Weisberg MK, Connolly Jr. HC, Ebel DS (2016) Microchondrules in three unequilibrated ordinary chondrites. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12585]
Published by arrangement with John Wiley & Sons

^1,2Alberto Collareta, ¹Massimo D’Orazio, ¹Maurizio Gemelli, ³Andreas Pack,¹Luigi Folco
¹Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy
²Dottorato Regionale in Scienze della Terra Pegaso, Università di Pisa, Pisa, Italy
³Georg-August-Universität Göttingen, Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Göttingen, Germany

The meteorite Mount DeWitt (DEW) 12007 is a polymict regolith breccia mainly consisting of glassy impact-melt breccia particles, gabbroic clasts, feldspathic clasts, impact and volcanic glass beads, basaltic clasts, and mingled breccia clasts embedded in a matrix dominated by fine-grained crystals; vesicular glassy veins and rare agglutinates are also present. Main minerals are plagioclase (typically An>85) and clinopyroxene (pigeonites and augites, sometimes interspersed). The presence of tranquillityite, coupled with the petrophysical data, the O-isotope data (Δ17O = −0.075), and the FeOtot/MnO ratios in olivine (91), pyroxene (65), and bulk rock (77) indicate a lunar origin for DEW 12007. Impactites consist of Al-rich impact-melt splashes and plagioclase-rich meta-melt clasts. The volcanic products belong to the very low titanium (VLT) or low titanium (LT) suites; an unusual subophitic fragment could be cryptomare-related. Gabbroic clasts could represent part of a shallow intrusion within a volcanic complex with prevailing VLT affinity. DEW 12007 has a mingled bulk composition with relatively high incompatible element abundances and shows a high crustal diversity comprising clasts from the Moon’s major terranes and rare lithologies. First-order petrographic and chemical features suggest that DEW 12007 could be launch-paired with other meteorites including Y 793274/981031, QUE 94281, EET 87521/96008, and NWA 4884.

Reference
Collareta A,D’Orazio M,Gemelli M,Pack A,Folco L (2016) High crustal diversity preserved in the lunar meteorite Mount DeWitt 12007 (Victoria Land, Antarctica). Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12597DOI: 10.1111/maps.12597]
Published by arrangement with John Wiley & Sons