^1,2Ritesh Kumar Mishra
Current Science 114, 1510-1519 Link to Article [doi: 10.18520/cs/v114/i07/1510-1519]
¹Center for Isotope Cosmochemistry and Geochronology, Astromaterials Research and Exploration Science Division, EISD-XI,
NASA-Johnson Space Center, 2101, NASA Parkway, Houston, TX 77058, USA
²Oak Ridge National Laboratory Associated Universities, Tennessee 37830, Kentucky, USA

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¹E Terán Bobadilla, ²J H Abundis Patiño, ³C Añorve Solano, ³C R Moraila Valenzuela,⁴F Ortega Gutierrez
Astronomy & Geophysics 59, 2.30-2.31 Link to Article [https://doi.org/10.1093/astrogeo/aty084]
¹Facultad de Ciencias Físico Matemáticas, Universidad Autónoma de Sinaloa, Mexico.
²Electrical and Computing Engineering, Technical University of Munich, Germany.
³Facultad de Ciencias de la Tierra y del Espacio, Universidad Autónoma de Sinaloa, Mexico.
⁴Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, Mexico.

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^1,2P. Bonnand, ^1,3A. N. Halliday
Nature Geoscience (in Press) Link to Article [doi:10.1038/s41561-018-0128-2]
¹Department of Earth Sciences, University of Oxford, Oxford, UK
²Laboratoire Magmas et Volcans, Université Clermont Auvergne, CNRS, Clermont-Ferrand, France
³The Earth Institute, Columbia University, New York, NY, USA

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¹Yuki Hibiya et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.04.031]
¹Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
Copyright Elsevier

Northwest Africa (NWA) 6704 is a unique achondrite characterized by a near-chondritic major element composition with a remarkably intact igneous texture. To investigate the origin of this unique achondrite, we have conducted a combined petrologic, chemical, and 187Re–187Os, O, and Ti isotopic study. The meteorite consists of orthopyroxene megacrysts (En55-57Wo3-4Fs40-42; Fe/Mn = 1.4) up to 1.7 cm in length with finer interstices of olivine (Fa50-53; Fe/Mn = 1.1–2.1), chromite (Cr# ∼ 0.94), awaruite, sulfides, plagioclase (Ab92An5Or3) and merrillite. The results of morphology, lattice orientation analysis, and mineral chemistry indicate that orthopyroxene megacrysts were originally hollow dendrites that most likely crystallized under high super-saturation and super-cooling conditions (1–102°C/hr), whereas the other phases crystallized between branches of the dendrites in the order of awaruite, chromite → olivine → merrillite → plagioclase. In spite of the inferred high super-saturation, the remarkably large size of orthopyroxene can be explained as a result of crystallization from a melt containing a limited number of nuclei that are preserved as orthopyroxene megacryst cores having high Mg# or including vermicular olivine. The Re–Os isotope data for bulk and metal fractions yield an isochron age of 4576 ± 250 Ma, consistent with only limited open system behavior of highly siderophile elements (HSE) since formation. The bulk chemical composition is characterized by broadly chondritic absolute abundances and only weakly fractionated chondrite-normalized patterns for HSE and rare earth elements (REE), together with substantial depletion of highly volatile elements relative to chondrites. The HSE and REE characteristics indicate that the parental melt and its protolith had not undergone significant segregation of metals, sulfides, or silicate minerals. These combined results suggest that a chondritic precursor to NWA 6704 was heated well above its liquidus temperature so that highly volatile elements were lost and the generated melt initially contained few nuclei of relict orthopyroxene, but the melting and subsequent crystallization took place on a timescale too short to allow magmatic differentiation. Such rapid melting and crystallization might occur as a result of impact on an undifferentiated asteroid. The O–Ti isotope systematics (Δ17O = −1.052 ± 0.002, 1 SD; ε50Ti = 2.28 ± 0.23, 2 SD) indicate that the NWA 6704 parent body sampled the same isotopic reservoirs in the solar nebula as the carbonaceous chondrite parent bodies. This is consistent with carbonaceous chondrite-like refractory element abundances and oxygen fugacity (FMQ = −2.6) in NWA 6704. Yet, the Si/Mg ratio of NWA 6704 is remarkably higher than those of carbonaceous chondrites, suggesting significant nebular fractionation of forsterite in its provenance.

^1,2C.K. Shearer, ³S. Messenger, ²Z.D. Sharp, ¹P.V. Burger,^3,4 A.N. Nguyen, ³F.M. McCubbin 
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.04.034]
¹Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico 87131
²Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131
³NASA Johnson Space Center, Mailcode XI, 2101 NASA Parkway, Houston, Texas 77058
⁴Jacobs, NASA Johnson Space Center, Houston, Texas 77058
Copyright Elsevier

The δ37Cl from different generations of apatite in martian meteorite Chassigny has a range of ≈10‰ and is almost as great as measurements made on all martian meteorites (≈14‰). This range represents the mixing of distinct Cl isotope reservoirs during the formation of Chassigny: (1) an isotopically light-Cl mantle reservoir (δ37Cl=-4 to -6‰) that exhibits limited variability and (2) an isotopically heavy Cl crustal reservoir (δ37Cl>0) that exhibits significant variability. The mantle component documented in Chassigny melt inclusions that host a solar noble gas composition are derived from pristine, martian mantle. The incompatible element depleted and enriched shergottite sources as defined by radiogenic isotope systematics and trace element concentration ratios have very similar Cl isotopic signatures and suggest that both are derived from the martian mantle. The enrichment of isotopically heavy Cl in the crust resulted from protracted loss of 35Cl to space that started early in the history of Mars. The Cl isotopic signature of the martian mantle is different from the Earth, Moon, and many primitive meteorites (δ37Cl=0), suggesting that these differences represent distinct Cl sources in the solar nebula. The low δ37Cl source represents the primordial Solar System composition from which Mars accreted. The higher δ37Cl values observed for the Earth, Moon, and many chondrites are not primordial, rather they represent the later incorporation of 37Cl-enriched HCl-hydrates into accreting material.

¹Shaolin Li, ^2,3Weibiao Hsu
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13110]
¹Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China
²School of Astronomy and Space Sciences, Nanjing University, Nanjing, China
³Space Science Institute, Macau University of Science and Technology, Macau, China
Published by arrangement with John Wiley & Sons

The disruption of the L chondrite parent body (LCPB) at ~470 Ma is currently the best‐documented catastrophic celestial impact event, based on the large number of L chondritic materials associated with this event. Uranium‐lead (U‐Pb) dating of apatite and its high‐pressure decomposition product, tuite, in the Sixiangkou L6 chondrite provides a temporal link to this event. The U‐Pb system of phosphates adjacent to shock melt veins was altered to varying degrees and the discordance of the U‐Pb system correlates closely with the extent of apatite decomposition. This suggests that the U‐Pb system of apatite could be substantially disturbed by high‐temperature pulse during shock compression from natural impacts, at least on the scale of mineral grains. Although many L chondrites can be temporally related to the catastrophic LCPB impact event, the shock conditions experienced by each individual meteorite vary. This could be due to the different geologic settings of these meteorites on their parent body. The shock pressure and duration derived from most meteorites may only reflect local shock features rather than the impact conditions, although they could provide lower limits to the impact conditions. The Sixiangkou shock duration (~4 s), estimated from high‐pressure transformation kinetics, provides a lower limit to the high‐pressure pulse of the LCPB disruption impact. Combined with available literature data of L chondrites associated with this impact event, our results suggest that the LCPB suffered a catastrophic collision with a large projectile (with a diameter of at least 18–22 km) at a low impact velocity (5–6 km s⁻¹). This is consistent with astronomical estimates based on the dynamical evolution of L chondritic asteroids.

¹Bidong Zhang, ¹Sean R. Shieh, ¹Anthony C. Withers, ¹Audrey Bouvier
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13106]
¹Department of Earth Sciences, Centre for Planetary Science and Exploration, University of Western Ontario, London, Ontario, Canada
Published by arrangement with John Wiley & Sons

We present Raman patterns of enstatite in different classes of enstatite‐rich chondrites and achondrites of various shock levels as previously reported from petrographic observations and X‐ray diffraction analyses. Thin sections or mineral separates of four enstatite chondrites (LaPaz Icefield [LAP] 02225, MacAlpine Hills [MAC] 02837, Pecora Escarpment [PCA] 91020, and Itqiy), three aubrites (Larkman Nunatak [LAR] 04316, Khor Temiki, and Allan Hills [ALH] 84008), and a ureilite (Sayh al Uhaymir [SaU] 559) were examined by laser Raman spectroscopy. We find that the frequencies of fundamental Raman peaks of enstatites from the chondrites and aubrites deviate by ≤2 cm⁻¹ from the values for unshocked enstatite. This small difference implies a negligible effect of shock metamorphism on peak positions. Significant differences (<6 cm⁻¹) for peak positions are found for the pyroxenes of SaU 559 and may be attributed to minor substitution of Fe and Ca for Mg. Linear regressions of peak widths of enstatite chondrites against their established shock stages show a strong positive correlation for each mode (r² > 0.94). From this linear relationship, the 343 and 1014 cm⁻¹ peaks of the aubrites coincide with S4 determined from petrography. For Itqiy, we find S4–5, while the shock levels of SaU 559 exceed the petrologic scheme (S1–6), suggesting that the ureilite might have sustained multiple shock events or have been deformed in a high‐pressure environment. Alternatively, for Itqiy (peak 343 cm⁻¹) and SaU 559 (all peaks) enstatites, minor substitutions of Fe and Ca for Mg may have further broadened their peak widths.

¹S. A. Singerling, ¹A. J. Brearley
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13108]
¹Department of Earth & Planetary Sciences, MSC‐03 2040 1 University of New Mexico, Albuquerque, New Mexico, USA
Published by arrangement with John Wiley & Sons

We have carried out a systematic study involving SEM, EPMA, and TEM analyses to determine the textures and compositions of sulfides and sulfide–metal assemblages in a suite of minimally to weakly altered CM and CR carbonaceous chondrites. We have attempted to constrain the distribution and origin of primary sulfides that formed in the solar nebula, rather than by secondary asteroidal alteration processes. Our study focused primarily on sulfide assemblages associated with chondrules, but also examined some occurrences of sulfides within the matrices of these meteorites. Although sulfides are a minor phase in carbonaceous chondrites, we have determined that primary sulfide grains are actually a major proportion of the sulfide grains in weakly altered CM chondrites and have survived aqueous alteration relatively unscathed. In minimally altered CR chondrites, we have determined that essentially all of the sulfides are of primary origin, confirming the observations of Schrader et al. (2015). The pyrrhotite–pentlandite intergrowth (PPI) grains formed from crystallization of monosulfide solid solution (mss) melts, while sulfide‐rimmed metal (SRM) grains formed from sulfidization of Fe,Ni metal. Micron‐sized metal inclusions in some PPI grains may have formed by co‐crystallization of metal and sulfide from a sulfide melt that experienced S volatilization during the chondrule formation event, or alternatively, may be a remnant of sulfidization of Fe,Ni metal that also occurred during chondrule formation. Sulfur fugacity for SRM grains ranged from −18 to −10 (log units) largely in agreement with predicted solar nebular values. Our observations show that understanding the formation mechanisms of primary sulfide grains provides clues to solar nebular conditions, such as the sulfur fugacity during chondrule formation.

^1,2T. Mohr‐Westheide, ¹A. Greshake, ³R. Wirth, ^1,4,5W. U. Reimold
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13109]
¹Museum für Naturkunde—Leibniz Institute for Evolution and Biodiversity Science, , Berlin, Germany
²Institut für Geologische Wissenschaften, Freie Universität Berlin, , Berlin, Germany
³Helmholtz‐Zentrum Potsdam, Deutsches GeoForschungsZentrum, , Potsdam, Germany
⁴Geochronology Laboratory, University of Brasília, Brasília, Brazil
⁵Humboldt Universität zu Berlin, , Berlin, Germany
Published by arrangement with John Wiley & Sons

The oldest known large bolide impacts onto Earth are represented by approximately 3.47–3.2 Ga old Archean spherule layers of the Barberton Greenstone Belt (BGB) in South Africa and the Pilbara craton in West Australia. These layers were recognized as impact deposits by their excessively high platinum group element (PGE) contents that are indicative of an extraterrestrial component. This was followed by measurements of extraterrestrial Cr isotopic ratios, in some cases. Recently, the extraterrestrial PGE signature in Archean spherule layers from the BGB was localized and positively associated with the presence of submicrometer PGE alloy micronuggets associated with Ni,Cr‐rich spinel. The actual formation of these platinum group mineral (PGM) phases has, however, not yet been resolved. Primary meteoritic particles from the impacting body, the products of impact melting, or condensation from impact vapor plumes have all been proposed as possible genetic process. Resolving this requires detailed microanalytical investigation of the internal microchemical and microstructural compositions, textural characteristics, and crystallographic relationships between the different phases. Here, we report the results of a first transmission electron microscopy (TEM) study of six such PGE microparticles enclosed in Ni‐Cr spinel or occurring in groundmass of Barberton spherule layers from the BARB5 ICDP drill core and from the CT3 exploration core. Results include a variety of chemical and structural PGM compositions that are difficult to explain by a single process, leading to the conclusion that several processes may have been involved in the formation of PGMs in Archean spherule layers from the BGB. There is evidence supporting formation of these PGMs by exsolution from the spinel host phase, precipitation from a melt phase, and condensation from a gas phase (of the impact vapor plume).

¹Brandon Mahan, ^1,2Julien Siebert, ¹Ingrid Blanchard, ¹Stephan Borensztajn, ¹James Badro, ^1,2Frédéric Moynier
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.04.032]
¹Institut de Physique du Globe de Paris, Université Paris Diderot, Sorbonne Paris Cité, CNRS UMR 7154, 1 rue Jussieu, 75238 Paris Cedex 05
²Institut Universitaire de France, Paris, France
Copyright Elsevier

Zinc is a moderately volatile and slightly siderophile element, and therefore provides information into the timing and conditions of volatile element delivery to Earth as well as the redistribution of these elements during planetary differentiation. Specifically, due to its similar volatility with S, it has been assumed that the Earth and its source material maintain the same relative abundances of these elements, and therefore the same S/Zn ratio. In this study, we have conducted Zn metal-silicate partitioning experiments at pressures up to 81 GPa and temperatures up to 4100 K in diamond anvil cells, for two distinct silicate compositions (one pyrolitic, one basaltic) and varying S contents in order to characterize Zn metal-silicate partitioning as a function of these variables. These results have been input into numerous Earth formation models where various parametric controls have been evaluated, namely source material, impactor size and volatile delivery, to determine plausible sets of conditions that can generate present-day bulk silicate Earth (BSE) Zn and S abundances. Modelling results indicate that to arrive at present-day BSE contents for Zn and S, and with core S contents of ∼2 wt% or less, the Earth likely accreted heterogeneously – initially from a volatile-depleted source material chemically akin to the metal and silicate chondrules of CH chondrites, with later delivery of more volatile-rich material.