¹Jérôme Aléon
¹Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), UMR 7590, Sorbonne Universités, Museum National d’Histoire Naturelle, CNRS, UPMC, IRD, 61 rue Buffon, 75005 Paris, France

A major unanswered question in solar system formation is the origin of the oxygen isotopic dichotomy between the Sun and the planets. Individual Calcium–Aluminum-rich inclusions (CAIs) from CV chondrites exhibit almost the full isotopic range, but how their composition evolved is still unclear, which prevents robust astrochemical conclusions. A key issue is notably the yet unsolved origin of the 16O-rich isotopic composition of pyroxene in type B CAIs. Here, I report an in-situ oxygen isotope study of the archetypal type B CAI USNM-3529-Z from Allende with emphasis on the isotopic composition of pyroxene and its isotopic and petrographic relationships with other major minerals. The O isotopic composition of pyroxene is correlated with indicators of magmatic growth, indicating that the pyroxene evolved from a 16O-poor composition and became progressively enriched in 16O during its crystallization, contrary to the long held assumption that pyroxene was initially 16O-rich. This variation is well explained by isotopic exchange between a 16O-poor partial melt having the isotopic composition of melilite and a 16O-rich gas having the isotopic composition of spinel, during pyroxene crystallization.

The isotopic evolution of 3529-Z is consistent with formation in an initially 16O-rich environment where spinel and gehlenitic melilite crystallized, followed by a 16O-depletion associated with melilite partial melting and recrystallization and finally a return to the initial 16O-rich environment before pyroxene crystallization. This strongly suggests that the environment of CAI formation was globally 16O-rich, with local 16O-depletions systematically associated with high temperature events. The Al/Mg isotopic systematics of 3529-Z further indicates that this suite of isotopic changes occurred in the first 150 000 yr of the solar system, during the main CAI formation period. A new astrophysical setting is proposed, where the 16O-depletion occurs in an optically thin surface layer of the disk and may have originated by evaporation of 16O-poor interstellar dust or non-mass-dependant isotopic fractionation.

Reference
Aléon J (2016) Oxygen isotopes in the early protoplanetary disk inferred from pyroxene in a classical type B CAI. Earth and Planetary Science Letters 440, 62–70.
Link to Article [doi:10.1016/j.epsl.2016.02.007]
Copyright Elsevier

¹Frank Richter, ²Marc Chaussidon, ¹Ruslan Mendybaev, ¹Edwin Kite
¹The University of Chicago, 5734 South Ellis Avenue, Chicago, IL, 60637, U.S.A.
²Institute de Physique du Globe Paris, Sorbonne Paris Cite, UMR CNRS 7154, 1 rue Jussieu, 75005 Paris, France

Lithium concentration and isotopic fractionation profiles across augite grains from two Martian meteorites – MIL 03346 and NWA 817 – were used to determine their thermal history and implications for their geologic setting. The iron-magnesium zoning and associated magnesium isotopic fractionation of olivine grains from NWA 817 were also measured and provide a separate estimate of the cooling rate. The observed correlation of concentration with isotopic fractionation provides the essential evidence that the zoning of these grains was in fact due to diffusion and thus can be used as a measure of their cooling rate. The diffusion rate of lithium in augite depends on the oxygen fugacity, which has to be taken into account when determining a cooling rate based on the lithium zoning. The Fe-Mg exchange in olivine is much less sensitive to oxygen fugacity, but it is significantly anisotropic and for this reason we determined the direction relative to crystallographic axes of the line along which the Fe-Mg zoning was measured. We found that the cooling rate of NWA 817 determined from the lithium zoning in augite grains and that based on the Fe-Mg zoning of olivines are in good agreement at an oxygen fugacity close to that of quartz-fayalite-magnetite oxygen buffer. The cooling rate of MIL 03346 was found to be resolvably faster than that of NWA 817 – of the order of 1°C/hr for the former and of the order of 0.2°C/hr for the latter. An important observation regarding the history of MIL 03346 and NWA 817 is that the lithium and Fe-Mg zoning are only observed where the augite or olivine is in contact with the mesostasis, which implies that they were already about 80% crystallized at the time diffusion began. The augite and olivine core compositions while very homogeneous are not in equilibrium with each other, which we interpret to imply that prior to the rapid cooling there must have been a protracted period of the order of years above the solidus, during which the much faster Fe-Mg exchange in olivine compared to that in augite allowed the olivine to maintain equilibrium with a changing melt composition while the augite was not significantly affected. We suggest two possible geological settings for the origin and evolution of MIL 03346 and NWA 817: (1) A slow cooling stage in a crystallization front in a crustal magma chamber, followed by eruption of melt plus portions of the crystallization front onto the surface where the final fast cooling took place at the bottom of a lava flow or melt pond, and (2) Eruption of a crystal laden melt as a thick long-lived lava flow where the crystals continued to grow as a cumulate and were rapidly cooled when the overlying lava layer was suddenly drained.

Reference
Richter F, Chaussidon M, Mendybaev R, Kite E (2016) Reassessing the Cooling Rate and Geologic Setting of Martian Meteorites MIL 03346 and NWA 817. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.02.020]
Copyright Elsevier

^1,2David Morate, ^1,2Julia de León, ³Mário De Prá, ^1,2Javier Licandro, ^1,4Antonio Cabrera-Lavers, ⁵Humberto Campins, ⁶Noemí Pinilla-Alonso, ⁷Víctor Alí-Lagoa

¹Instituto de Astrofísica de Canarias (IAC), C/vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
²Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
³Observatório Nacional, Coordenação de Astronomia e Astrofísica, 20921-400 Rio de Janeiro, Brazil
⁴GTC Project Office, 38205 La Laguna, Tenerife, Spain
⁵Physics Department, University of Central Florida, PO Box 162385, Orlando, FL 32816-2385, USA
⁶Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, 37996 TN, USA
⁷Laboratoire Lagrange, OCA, Boulevard de l’Observatoire, BP 4229 06304 Nice Cedex 04, France

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Reference
Morate D, de León J, De Prá M, Licandro J, Cabrera-Lavers A, Campins H, Pinilla-Alonso N, Alí-Lagoa V (2016) Compositional study of asteroids in the Erigone collisional family using visible spectroscopy at the 10.4 m GTC. Astronomy & Astrophysics 586, A129
Link to Article [http://dx.doi.org/10.1051/0004-6361/201527453]

The most popular papers on Cosmochemistry Papers in February were:

1-Schwarz WH, Trieloff M, Bollinger K, Gantert N, Fernandes VA, Meyer H-P, Povenmire H, Jessberger EK, Guglielmino M, Koeberl (2016) Coeval ages of Australasian, Central American and Western Canadian tektites reveal multiple impacts 790 ka ago. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.12.037]

2-Ebert S, Bischoff A (2016) Genetic relationship between Na-rich chondrules and Ca,Al-rich inclusions? – Formation of Na-rich chondrules by melting of refractory and volatile precursors in the Solar Nebula. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.01.014]

3-Nguyen AN, Keller LP, Messenger S (2016) MINERALOGY OF PRESOLAR SILICATE AND OXIDE GRAINS OF DIVERSE STELLAR ORIGINS. The Astrophysical Journal 818, 51
Link to Article [http://dx.doi.org/10.3847/0004-637X/818/1/51]

4-Hutzler A et al. (2016) Description of a very dense meteorite collection area in western Atacama: Insight into the long-term composition of the meteorite flux to Earth. Meteoritics & Planetary Sciences (in Press)
Link to Article [DOI: 10.1111/maps.12607]

5-Kawasaki N,Itoh S,Sakamoto N, Yurimoto H (2016) Chronological study of oxygen isotope composition for the solar protoplanetary disk recorded in a fluffy Type A CAI from Vigarano. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.12.031]

^1,2M. Marsset, ²P. Vernazza, ^3,4M. Birlan, ⁵F. DeMeo, ⁵R. P. Binzel, ¹C. Dumas, ¹J. Milli, ^3,4M. Popescu4,3
¹European Southern Observatory (ESO), Alonso de Córdova 3107, 1900 Casilla Vitacura, Santiago, Chile
²Aix Marseille University, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
³IMCCE, Observatoire de Paris, 77 avenue Denfert-Rochereau, 75014 Paris Cedex, France
⁴Astronomical Institute of the Romanian Academy, 5 Cuţitul de Argint, 040557 Bucharest, Romania
⁵Department of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA

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Reference
Marsset M, Vernazza P, Birlan M, DeMeo F, Binzel RP, Dumas C, Milli J, Popescu M (2016) Compositional characterisation of the Themis family. Astronomy & Astrophysics 586 A15
Link to Article [http://dx.doi.org/10.1051/0004-6361/201526962]

¹C. Tubiana, ^1,2C. Snodgrass, ³R. Michelsen, ³H. Haack, ¹H. Böhnhardt, ⁴A. Fitzsimmons, ⁵I. P. Williams
¹Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 33077 Göttingen, Germany
²Planetary and Space Sciences, Department of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
³Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, 1350 Copenhagen, Denmark
⁴Astrophysics Research Centre, Department of Physics and Astronomy, Queen’s University Belfast, Belfast BT7 1NN, UK
⁵School of Physics and Astronomy, Queen Mary, University of London, London E1 4NS, UK

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Reference
Tubiana C, Snodgrass C, Michelsen R, Haack H, Böhnhardt H, Fitzsimmons A, Williams IP (2016) 2P/Encke, the Taurid complex NEOs and the Maribo and Sutter’s Mill meteorites. Astronomy & Astrophysics 584, A97
Link to Article [http://dx.doi.org/10.1051/0004-6361/201425512]

¹M. Hilchenbach (>10)*
¹Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany
*Find the extensive, full author and affiliation list on the publishers website

The COmetary Secondary Ion Mass Analyser instrument on board ESA’s Rosetta mission has collected dust particles in the coma of comet 67P/Churyumov–Gerasimenko. During the early-orbit phase of the Rosetta mission, particles and particle agglomerates have been imaged and analyzed in the inner coma at distances between 100 km and 10 km off the cometary nucleus and at more than 3 AU from the Sun. We identified 585 particles of more than 14 μm in size. The particles are collected at low impact speeds and constitute a sample of the dust particles in the inner coma impacting and fragmenting on the targets. The sizes of the particles range from 14 μm up to sub-millimeter sizes and the differential dust flux size distribution is fitted with a power law exponent of −3.1. After impact, the larger particles tend to stick together, spread out or consist of single or a group of clumps, and the flocculent morphology of the fragmented particles is revealed. The elemental composition of the dust particles is heterogeneous and the particles could contain typical silicates like olivine and pyroxenes, as well as iron sulfides. The sodium to iron elemental ratio is enriched with regard to abundances in CI carbonaceous chondrites by a factor from ~1.5 to ~15. No clear evidence for organic matter has been identified. The composition and morphology of the collected dust particles appear to be similar to that of interplanetary dust particles.

Reference
Hilchenbach M et al. (2016) COMET 67P/CHURYUMOV–GERASIMENKO: CLOSE-UP ON DUST PARTICLE FRAGMENTS. The Astrophysical Journal (Letters) 816, L32
Link to Article [http://dx.doi.org/10.3847/2041-8205/816/2/L32http://dx.doi.org/10.3847/2041-8205/816/2/L32]

^1,2Tuan H. Vu1, Robert Hodyss, ^1,2Mathieu Choukroun, ^1,2Paul V. Johnson
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²NASA Astrobiology Institute

The composition of Europa’s subsurface ocean is a critical determinant of its habitability. However, our current understanding of the ocean composition is limited to its expression on the surface. This work investigates experimentally the composition of mixed sodium–magnesium–sulfate–chloride solutions when frozen to 100 K, simulating conditions that likely occur as ocean fluids are emplaced onto Europa’s surface. Micro-Raman spectroscopy is used to characterize phase composition of the frozen brines at 100 K. Our results show that solutions containing Na+, Cl−, Mg2+, and ${{\mathrm{SO}}_{4}}^{2-}$ preferentially crystallize into Na2SO4 and MgCl2 hydrated minerals upon freezing, even at elevated [Mg2+]/[Na+] ratios. The detection of epsomite (MgSO4•7H2O) on Europa’s surface, if confirmed, may thus imply a relatively sodium-poor ocean composition or a radiolytic process that converts MgCl2 to MgSO4 as suggested by Brown & Hand. The formation of NaCl on the surface, while dependent upon a number of factors such as freezing rate, may indicate an ocean significantly more concentrated in sodium than in magnesium.

Reference
Vu TH, Hodyss R, Choukroun M, Johnson PV (2016) CHEMISTRY OF FROZEN SODIUM–MAGNESIUM–SULFATE–CHLORIDE BRINES: IMPLICATIONS FOR SURFACE EXPRESSION OF EUROPA’S OCEAN COMPOSITION. The Astrophysical Journal (Letters) 816, L26
Link to Article [http://dx.doi.org/10.3847/2041-8205/816/2/L26]

¹S. Xu, ²M. Jura, ³P. Dufour, ²B. Zuckerman
¹European Southern Observatory, Karl-Schwarzschild-Straße 2, D-85748 Garching, Germany
²Department of Physics and Astronomy, University of California, Los Angeles CA 90095-1562, USA
³Institut de Recherche sur les Exoplanètes (iREx), Université de Montréal, Montréal, QC H3C 3J7, Canada

We report high-resolution spectroscopic observations of WD 1145+017—a white dwarf that was recently found to be transitted by multiple asteroid-sized objects within its tidal radius. We discovered numerous circumstellar absorption lines with linewidths of ~300 km s−1 from Mg, Ca, Ti, Cr, Mn, Fe, and Ni, possibly from several gas streams produced by collisions among the actively disintegrating objects. The atmosphere of WD 1145+017 is polluted with 11 heavy elements, including O, Mg, Al, Si, Ca, Ti, V:, Cr, Mn, Fe, and Ni. Evidently, we are witnessing the active disintegration and subsequent accretion of an extrasolar asteroid.

Reference
Xu S, Jura M, Dufour P, Zuckerman B (2016) EVIDENCE FOR GAS FROM A DISINTEGRATING EXTRASOLAR ASTEROID. The Astrophysical journal (Letters), 816 L22
Link to Article [http://dx.doi.org/10.3847/2041-8205/816/2/L22]

¹Laurence E. Nyquist, ²Chi-Yu Shih, ^1,3Francis M. McCubbin, ^3,4Alison R. Santos, ^3,4Charles K. Shearer, ²Zhan X. Peng, ^3,4Paul V. Burger,³Carl B. Agee
¹NASA Johnson Space Center, Mailcode XI, Houston, Texas, USA
²Jacobs, NASA Johnson Space Center, Houston, Texas, USA
³Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, USA
⁴Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA

The bulk matrix domain of the Martian breccia NWA 7034 was examined petrographically and isotopically to better understand the provenance and age of the source material that make up the breccia. Both 147Sm-143Nd and 146Sm-142Nd age results for mineral separates from the bulk matrix portion of breccia NWA 7034 suggest that various lithological components in the breccia probably formed contemporaneously ~4.44 Ga ago. This old age is in excellent agreement with the upper intersection ages (4.35–4.45 Ga) for U-Pb discordia and also concordia defined by zircon and baddeleyite grains in matrix and igneous-textured clasts. Consequently, we confirm an ancient age for the igneous components that make up the NWA 7034 breccia. Substantial disturbance in the Rb-Sr system was detected, and no age significance could be gleaned from our Rb-Sr data. The disturbance to the Rb-Sr system may be due to a thermal event recorded by bulk-rock K-Ar ages of 1.56 Ga and U-Pb ages of phosphates at about 1.35–1.5 Ga, which suggest partial resetting from an unknown thermal event(s), possibly accompanying breccia formation. The NWA 7034 bulk rock is LREE enriched and similar to KREEP-rich lunar rocks, which indicates that the earliest Martian crust was geochemically enriched. This enrichment supports the idea that the crust is one of the enriched geochemical reservoirs on Mars that have been detected in studies of other Martian meteorites.

Reference
Nyquist E, Shih C-Y,McCubbin, FM,Santos AR, Shearer CK, Peng ZX, Burger PV, Agee CB (2016) Rb-Sr and Sm-Nd isotopic and REE studies of igneous components in the bulk matrix domain of Martian breccia Northwest Africa 7034. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12606]
Published by arrangement wit John Wiley & Sons