G. J. Archer^a,b, R. J. Walker^a, J. Tino^a, T. Blackburn^c, T. S. Kruijer^b,d, J. L. Hellmann^b
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.11.012]
^aDepartment of Geology, University of Maryland, College Park, MD 20742, USA
^bInstitut für Planetologie, University of Münster, Münster 48149, Germany
^eEarth and Planetary Sciences, University of California, Santa Cruz, SantacCruz, CA 95064, USA
^dNuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
Copyright Elsevier

The abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Pd), as well as ¹⁸⁷Re-¹⁸⁷Os and ¹⁸²Hf-¹⁸²W isotopic systematics were determined for separated metal, slightly magnetic, and nonmagnetic fractions from seven H4 to H6 ordinary chondrites. The HSE are too abundant in nonmagnetic fractions to reflect metal-silicate equilibration. The disequilibrium was likely a primary feature, as ¹⁸⁷Re-¹⁸⁷Os data indicate only minor open-system behavior of the HSE in the slightly and non-magnetic fractions. ¹⁸²Hf-¹⁸²W data for slightly magnetic and nonmagnetic fractions define precise isochrons for most meteorites that range from 5.2±1.6 Ma to 15.2±1.0 Ma after calcium aluminum inclusion (CAI) formation. By contrast, ¹⁸²W model ages for the metal fractions are typically 2 to 5 Ma older than the slope-derived isochron ages for their respective, slightly magnetic and nonmagnetic fractions, with model ages ranging from 1.4±0.8 Ma to 12.6±0.9 Ma after CAI formation. This indicates that the W present in the silicates and oxides was not fully equilibrated with the metal when diffusive transport among components ceased, consistent with the HSE data. Further, the W isotopic compositions of size-sorted metal fractions from some of the H chondrites also differ, indicating disequilibrium among some metal grains. The chemical/isotopic disequilibrium of siderophile elements among H chondrite components is likely the result of inefficient diffusion of siderophile elements from silicates and oxides to some metal and/or localized equilibration as H chondrites cooled towards their respective Hf-W closure temperatures. The tendency of ¹⁸²Hf-¹⁸²W isochron ages to young from H5 to H6 chondrites may indicate derivation of these meteorites from a slowly cooled, undisturbed, concentrically-zoned parent body, consistent with models that have been commonly invoked for H chondrites. Overlap of isochron ages for H4 and H5 chondrites, by contrast, appear to be more consistent with shallow impact disruption models.

The W isotopic composition of metal from one CR chondrite was examined to compare with H chondrite metals. In contrast to the H chondrites, the CR chondrite metal is characterized by an enrichment in ¹⁸³W that is consistent with nucleosynthetic s-process depletion. Once corrected for the correlative nucleosynthetic effect on ¹⁸²W, the ¹⁸²W model age for this meteorite of 7.0 ± 3.6 Ma is within the range of model ages of most metal fractions from H chondrites. The metal is therefore too young to be a direct nebular condensate, as proposed by some prior studies.

Susann SIEGERT^1,2 and Lutz HECHT^1,2
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13210]
¹Museum f€ur Naturkunde, Leibniz-Institut f€ur Evolutions- und Biodiversitätsforschung, Invalidenstraße 43,10115, Berlin, Germany
²Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstraße 74-100, 12249, Berlin, Germany
Published by arrangement with John Wiley & Sons

Impact melt‐bearing clastic deposits (suevites) are one of the most important records of the impact cratering process. A deeper understanding of their composition and formation is therefore essential. This study focuses on impact melt particles in suevite at Ries, Germany. Textures and chemical evidence indicate that the suevite contains three melt types that originate from different shock levels in the target. The most abundant melt type (“melt type 1”) represents well‐mixed whole‐rock melting of crystalline basement and includes incompletely mixed mafic melt schlieren (“melt type 1 mafic”). Polymineralic melt type 2 comprises mixes between monomineralic melt types 3 and melt type 1. Melt types 2 and 3 are located within melt type 1 as small patches or schlieren but also isolated within the suevite matrix. The main melt type 1 is heterogeneous with respect to trace elements, varying geographically around the crater: in the western sector, it has lower values in trace elements, e.g., Ba, Zr, Th, and Ce, than in the eastern sector. The west–east zoning likely reflects the heterogeneous nature of crystalline basement target rocks with lower trace element contents, e.g., Ba, Zr, Th, and Ce, in the west compared to the east. The chemical zoning pattern of suevite melt type 1 indicates that mixing during ejection and emplacement occurred only on a local (hundreds of meters) scale. The incomplete larger scale mixing indicated by the preservation of these local chemical signatures, and schlieren corroborate the assumption that mixing, ejection, and quenching were very rapid, short‐lived processes.

Tre’Shunda James^1,2 and Renyu Hu^1,3
Astrophysical Journal 867, 17 Link to Article [DOI: 10.3847/1538-4357/aae2bb]
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²Occidental College, Los Angeles, CA 90041, USA
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

Atmospheric chemistry models have shown that molecular oxygen can build up in CO₂-dominated atmospheres on potentially habitable exoplanets without input of life. Existing models typically assume a surface pressure of 1 bar. Here we present model scenarios of CO₂-dominated atmospheres with the surface pressure ranging from 0.1 to 10 bars, while keeping the surface temperature at 288 K. We use a one-dimensional photochemistry model to calculate the abundance of O₂ and other key species, for outgassing rates ranging from a Venus-like volcanic activity up to 20 times Earth-like activity. The model maintains the redox balance of the atmosphere and the ocean, and includes the pressure dependency of outgassing on the surface pressure. Our calculations show that the surface pressure is a controlling parameter in the photochemical stability and oxygen buildup of CO₂-dominated atmospheres. The mixing ratio of O₂ monotonically decreases as the surface pressure increases at very high outgassing rates, whereas it increases as the surface pressure increases at lower-than-Earth outgassing rates. Abiotic O₂ can only build up to the detectable level, defined as 10⁻³ in volume mixing ratio, in 10-bar atmospheres with the Venus-like volcanic activity rate and the reduced outgassing rate of H₂ due to the high surface pressure. Our results support the search for biological activities and habitability via atmospheric O₂ on terrestrial planets in the habitable zone of Sun-like stars.

¹Krivovichev, V.G.,²Charykova, M.V., ^3,4Krivovichev, S.V.
European Journal of Mineralogy 30, 219-230 Link to Article [DOI: 10.1127/ejm/2018/0030-2699]
¹Department of Mineralogy, Institute of Earth Sciences, St. Petersburg State University, University Emb. 7/9, St. Petersburg, 199034, Russian Federation
²Department of Geochemistry, Institute of Earth Sciences, St. Petersburg State University, University Emb. 7/9, St. Petersburg, 199034, Russian Federation
³Department of Crystallography, Institute of Earth Sciences, St. Petersburg State University, St. Petersburg, 199034, Russian Federation

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