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

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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

^1,2Tingting Gu, ^1,2Yingwei Fei, ¹Xiang Wu, ¹Shan Qin
¹Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, Peking University & School of Earth and Space Sciences, Peking University, Beijing 100871, China
²Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C., 20015, U.S.A

Fe-S-P compounds have been observed in many meteorites and could be the important components in planetary cores. Here we investigated the phase stability of Fe3(S,P) solid solutions and synthesized high-quality Fe3(S1−xPx) high-pressure phases in the multi-anvil press. The physical properties of Fe3(S0.5P0.5) were further studied in the diamond-anvil cell by synchrotron X-ray diffraction and emission spectroscopy. The solubility of S in the Fe3(S,P) solid solution increases with increasing pressure. The minimum pressure to synthesize the pure Fe3S and Fe3(S0.13P0.87) is about 21 and 8 GPa, respectively. The observed discontinuity in unit-cell parameters at about 18 GPa is caused by the high-spin to low-spin transition of iron, supported by X-ray emission spectroscopy data. The sulfur solubility in Fe3(S,P) solid solutions could be an excellent pressure indicator if such solid solutions are found in nature.

Reference
Gu T, Fei Y, Wu X, Qin S (2016) Phase stabilities and spin transitions of Fe3(S1−xPx) at high pressure and its implications in meteorites. American Mineralogist 101, 205-210
Link to Article [doi:10.2138/am-2016-5466]
Copyright: The Mineralogical Society of America

¹Stephen E. Haggerty
¹Geology & Environment, Florida International University, Miami 33155, U.S.A.

Spinel is ubiquitous as a rock-forming mineral in terrestrial, lunar, and planetary basalts and closely associated meteoritic equivalents. A major unknown is whether these rocks formed under similar conditions of partial melting of primary or modified mantle, whether redox environments played a role in evolutionary trends, and did mineral crystal chemistry have any influence on elemental partition between solids and liquids? In a novel approach by Papike et al. (2015), spinel is used as an informative, albeit complex indicator of oxygen fugacity, site occupancy of multiple valence elements, and spinel structural types. Planetary basalts may be reduced (IW-3), oxidized (Earth at FMQ), or of intermediate redox state (Mars). Taking an expansive view, the spinel approach holds enormous promise in understanding the magmatic differentiation of asteroids.

Reference
Haggerty SE (2016) Spinel in planetary systems. American Mineralogist 101,5-6
Link to Article[doi:10.2138/am-2016-5554]
Copyright: The Mineralogical Society of America

¹L. Remusat, ^1,2L. Piani, ¹S. Bernard
¹Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), UMR CNRS 7590 – Sorbonne Universités – UPMC – IRD – Muséum National d’Histoire Naturelle, 57 rue Cuvier, Case 52, 75231 Paris Cedex 5, France
²Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan

Carbonaceous and ordinary chondrites (CCs and OCs) contain insoluble organic matter (IOM) with large D-excess compared to other objects in the solar system. The higher the temperature experienced by CCs, the lower the D/H ratio of their IOM. It seems to be the opposite for OCs. Here, we report NanoSIMS H- (and N-) isotopic imaging of IOM of three OCs that experienced thermal metamorphism in the sequence Semarkona, Bishunpur and GRO 95502. In addition, we performed flash heating experiments on the IOM of GRO 95502 at 600 °C and characterized the residues using NanoSIMS, Raman and XANES spectroscopy. The present study shows that, in contrast to IOM of CI, CM and CR, IOM of OCs exhibits very few D-rich (or 15N-rich) hotspots. Furthermore, although the evolution of the molecular structure of OC and CC IOM is similar upon heating, their D/H ratios do not follow the same trend: the D/H of OC IOM drastically increases while the D/H of CC IOM decreases. In contrast to CC IOM, the D-rich component of which does not survive at high temperatures, the present results highlight the thermal recalcitrance of the D-rich component of OC IOM. This suggests that CCs and OCs did not accrete the same organic material, thereby challenging the hypothesis of a common precursor on chondritic parent bodies. The present results support the hypothesis that OC IOM contains an organic component that could originate from the interstellar medium.

Reference
Remusat L, Piani L, Bernard S (2016) Thermal recalcitrance of the organic D-rich component of ordinary chondrites. Earth and Planetary Science Letters 435, 36-44
Link to Article [doi:10.1016/j.epsl.2015.12.009]
Published by arrangement with John Wiley & Sons

¹M. J. Loeffler, ²C. A. Dukes, ³R. Christoffersen,²R. A. Baragiola
¹NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
²University of Virginia, Laboratory for Astrophysics and Surface Physics (LASP), Charlottesville, Virginia, USA
³Jacobs, NASA Johnson Space Center, Houston, Texas, USA

Pulsed-laser irradiation causes the visible-near-infrared spectral slope of olivine (Fo90 and Fo99+) and SiO2 to increase (redden), while the olivine samples darken and the SiO2 samples brighten slightly. XPS analysis shows that irradiation of Fo90 produces metallic Fe. Analytical SEM and TEM measurements confirm that reddening in the Fo90 olivine samples correlates with the production of “nanophase” metallic Fe (npFe0) grains, 20–50 nm in size. The reddening observed in the SiO2 sample is consistent with the formation of SiO or other SiOx species that absorb in the visible. The weak spectral brightening induced by laser irradiation of SiO2 is consistent with a change in surface topography of the sample. The darkening observed in the olivine samples is likely caused by the formation of larger npFe0 particles, such as the 100–400 nm diameter npFe0 identified during our TEM analysis of Fo90 samples. The Fo90 reflectance spectra are qualitatively similar to those in previous experiments suggesting that in all cases formation of npFe0 is causing the spectral alteration. Finally, we find that the accumulation of successive laser pulses cause continued sample darkening in the Vis-NIR, which suggests that repeated surface impacts are an efficient way to darken airless body surfaces.

Reference
Loeffler MJ, Dukes CA, Christoffersen R,Baragiola RA (2016) Space weathering of silicates simulated by successive laser irradiation: In situ reflectance measurements of Fo90, Fo99+, and SiO2. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12581]
Published by arrangement with John Wiley & Sons