¹Sanna Holm-Alwmark,²Auriol S. P. Rae,³Ludovic Ferrière,¹Carl Alwmark,²Gareth S. Collins
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12955]
¹Department of Geology, Lund University, Lund, Sweden
²Department of Earth Science and Engineering, Imperial College London, London, UK
³Natural History Museum, Vienna, Austria
Published by arrangement with John Wiley & Sons

Siljan, central Sweden, is the largest known impact structure in Europe. It was formed at about 380 Ma, in the late Devonian period. The structure has been heavily eroded to a level originally located underneath the crater floor, and to date, important questions about the original size and morphology of Siljan remain unanswered. Here we present the results of a shock barometry study of quartz-bearing surface and drill core samples combined with numerical modeling using iSALE. The investigated 13 bedrock granitoid samples show that the recorded shock pressure decreases with increasing depth from 15 to 20 GPa near the (present) surface, to 10–15 GPa at 600 m depth. A best-fit model that is consistent with observational constraints relating to the present size of the structure, the location of the downfaulted sediments, and the observed surface and vertical shock barometry profiles is presented. The best-fit model results in a final crater (rim-to-rim) diameter of ~65 km. According to our simulations, the original Siljan impact structure would have been a peak-ring crater. Siljan was formed in a mixed target of Paleozoic sedimentary rocks overlaying crystalline basement. Our modeling suggests that, at the time of impact, the sedimentary sequence was approximately 3 km thick. Since then, there has been around 4 km of erosion of the structure.

¹A. A. Maksimova, ¹A. V. Chukin, ¹V. A. Semionkin, ¹M. I. Oshtrakh
Bulletin of the Russian Academy of Sciences: Physics 81, 845-849 Link to Article [DOI https://doi.org/10.3103/S1062873817070188]
¹Institute of Physics and Technology Ural Federal University Yekaterinburg Russia

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

¹M.R.Salvatore, ²T.A.Goudge,³M.S.Bramble, ¹C.S.Edwards,⁴J.L.Bandfield, ⁵E.S.Amador, ³J.F.Mustard, ⁶P.R.Christensen
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.09.019]
¹Department of Physics and Astronomy, Northern Arizona University, NAU Box 6010, Flagstaff, AZ, 86011-6010
²Jackson School of Geosciences, University of Texas at Austin
³Department of Earth, Environmental, and Planetary Sciences, Brown University
⁴Space Science Institute
⁵Department of Earth and Space Sciences, University of Washington
⁶School of Earth and Space Exploration, Arizona State University
Copyright Elsevier

We investigated the area to the northwest of the Isidis impact basin (hereby referred to as “NW Isidis”) using thermal infrared emission datasets to characterize and quantify bulk surface mineralogy throughout this region. This area is home to Jezero crater and the watershed associated with its two deltaic deposits in addition to NE Syrtis and the strong and diverse visible/near-infrared spectral signatures observed in well-exposed stratigraphic sections. The spectral signatures throughout this region show a diversity of primary and secondary surface mineralogies, including olivine, pyroxene, smectite clays, sulfates, and carbonates. While previous thermal infrared investigations have sought to characterize individual mineral groups within this region, none have systematically assessed bulk surface mineralogy and related these observations to visible/near-infrared studies. We utilize an iterative spectral unmixing method to statistically evaluate our linear thermal infrared spectral unmixing models to derive surface mineralogy. All relevant primary and secondary phases identified in visible/near-infrared studies are included in the unmixing models and their modeled spectral contributions are discussed in detail. While the stratigraphy and compositional diversity observed in visible/near-infrared spectra are much better exposed and more diverse than most other regions of Mars, our thermal infrared analyses suggest the dominance of basaltic compositions with less observed variability in the amount and diversity of alteration phases. These results help to constrain the mineralogical context of these previously reported visible/near-infrared spectral identifications. The results are also discussed in the context of future in situ investigations, as the NW Isidis region has long been promoted as a region of paleoenvironmental interest on Mars.

¹Clément Ganino, ¹Guy Libourel
Nature Communications 8, 261 Link to Article [doi:10.1038/s41467-017-00293-1]
¹Université Côte d’Azur, CNRS, OCA, IRD, Géoazur, 250 rue Albert Einstein, Valbonne, 06560, France

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

¹Emmanuel Jacquet,²Yves Marrocchi
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12985]
¹Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, CNRS & Muséum National d’Histoire Naturelle, UMR 7590, Paris, France
²Centre de Recherches Pétrographiques et Géochimiques, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy, France
Publishedby arrangement with John Wiley & Sons

We report combined oxygen isotope and mineral-scale trace element analyses of amoeboid olivine aggregates (AOA) and chondrules in ungrouped carbonaceous chondrite, Northwest Africa 5958. The trace element geochemistry of olivine in AOA, for the first time measured by LA-ICP-MS, is consistent with a condensation origin, although the shallow slope of its rare earth element (REE) pattern is yet to be physically explained. Ferromagnesian silicates in type I chondrules resemble those in other carbonaceous chondrites both geochemically and isotopically, and we find a correlation between 16O enrichment and many incompatible elements in olivine. The variation in incompatible element concentrations may relate to varying amounts of olivine crystallization during a subisothermal stage of chondrule-forming events, the duration of which may be anticorrelated with the local solid/gas ratio if this was the determinant of oxygen isotopic ratios as proposed recently. While aqueous alteration has depleted many chondrule mesostases in REE, some chondrules show recognizable subdued group II-like patterns supporting the idea that the immediate precursors of chondrules were nebular condensates.

¹Tomáš Magna, ²Karel Žák, ³Andreas Pack, ^4,5Frédéric Moynier, ⁴Bérengère Mougel, ³Stefan Peters, ²Roman Skála, ²Šárka Jonášová, ⁶Jiří Mizera, ⁶Zdeněk Řanda
Nature Communications 8, 261, Link to Article [doi:10.1038/s41467-017-00192-5]
¹Czech Geological Survey, Klárov 3, Prague 1, CZ-118 21, Czech Republic
²Institute of Geology of the Czech Academy of Sciences, v.v.i., Rozvojová 269, Prague 6, CZ-165 00, Czech Republic
³Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Universität Göttingen, Goldschmidtstraße 1, Göttingen, D-37077, Germany
⁴Institut de Physique du Globe de Paris, Université Paris Diderot, 1 rue Jussieu, Paris, F-75005, France
⁵Insitut Universitaire de France, Paris, F-75005, France
⁶Nuclear Physics Institute of the Czech Academy of Sciences, v.v.i., Husinec-Řež, CZ-250 68, Czech Republic

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

^1,2Ai-Cheng Zhang, ¹Yi-Fan Bu, ¹Run-Lian Pang, ³Naoya Sakamoto, ^3,4Hisayoshi Yurimoto, ¹Li-Hui Chen, ⁵Jian-Feng Gao, ¹De-Hong Du, ¹Xiao-Lei Wang, ¹Ru-Cheng Wang
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.09.051]
¹State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China
²Lunar and Planetary Science Institute, Nanjing University, Nanjing 210046, China
³Isotope Imaging Laboratory, Creative Research Institution Sousei, Hokkaido University, Sapporo 001-0021, Japan
⁴Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
⁵State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
Copyright Elsevier

Troilite-orthopyroxene intergrowths are present as a common material in the brecciated diogenite Northwest Africa (NWA) 7183. In this study, we report on the petrographic, mineralogical, and rare earth element abundances of the troilite-orthopyroxene intergrowths to constrain their origin and assess their implications for the diverse petrogenesis of diogenites.

Two groups of troilite-orthopyroxene intergrowths with various grain sizes and mineral chemistry have been observed in NWA 7183. One group of intergrowths contains fine-grained (<5 μm) olivine and chromite as inclusions in orthopyroxene (10–20 μm in size). The other group, in which orthopyroxene is more fine-grained (<10 μm in size), is closely associated with coarse irregular olivine grains. The orthopyroxene grains in both groups of troilite-orthopyroxene intergrowths are depleted in Cr, Al, Ti, and Ca compared with diogenitic orthopyroxene. Based on the texture and mineral chemistry, we suggest that the two groups of troilite-orthopyroxene intergrowths formed via reactions between diogenitic olivine and S-rich vapors, probably at different temperatures. The fact that some of the intergrowths are included in diogenitic lithic clasts indicates that the formation of the host diogenite should postdate the formation of the majority of troilite-orthopyroxene intergrowths. This relationship further implies that not all of the diogenites are cumulates that directly crystallized from the Vestan magma ocean. Instead, they probably originated from partial melting and recrystallization of magma ocean cumulates. The replacement of olivine by troilite and orthopyroxene intergrowths can partly explain why the expected olivine-rich lithologies were not detected at the two south pole impact basins on Vesta.

¹Nan Liu,²Andrew Steele,¹Larry R. Nittler,³Rhonda M. Stroud,³Bradley T. De Gregorio,¹Conel M. O’D. Alexander,¹Jianhua Wang
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12954]
¹Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC, 20015, USA
²Geophysical Laboratory, Carnegie Institution for Science, Washington, DC, USA
³Materials Science and Technology Division, Code 6366, US Naval Research Laboratory, Washington, DC, USA
Published by arrangement with John Wiley & Sons

We report the development of a novel method to nondestructively identify presolar silicon carbide (SiC) grains with high initial 26Al/27Al ratios (>0.01) and extreme 13C-enrichments (12C/13C ≤ 10) by backscattered electron-energy dispersive X-ray (EDX) and micro-Raman analyses. Our survey of a large number of presolar SiC demonstrates that (1) ~80% of core-collapse supernova and putative nova SiC can be identified by quantitative EDX and Raman analyses with >70% confidence; (2) ~90% of presolar SiC are predominantly 3C-SiC, as indicated by their Raman transverse optical (TO) peak position and width; (3) presolar 3C-SiC with 12C/13C ≤ 10 show lower Raman TO phonon frequencies compared to mainstream 3C-SiC. The downward shifted phonon frequencies of the 13C-enriched SiC with concomitant peak broadening are a natural consequence of isotope substitution. 13C-enriched SiC can therefore be identified by micro-Raman analysis; (4) larger shifts in the Raman TO peak position and width indicate deviations from the ideal 3C structure, including rare polytypes. Coordinated transmission electron microscopy analysis of one X and one mainstream SiC grain found them to be of 6H and 15R polytypes, respectively; (5) our correlated Raman and NanoSIMS study of mainstream SiC shows that high nitrogen content is a dominant factor in causing mainstream SiC Raman peak broadening without significant peak shifts; and (6) we found that the SiC condensation conditions in different stellar sites are astonishingly similar, except for X grains, which often condensed more rapidly and at higher atmospheric densities and temperatures, resulting in a higher fraction of grains with much downward shifted and broadened Raman TO peaks.

¹Michael D. Higgins, ¹Pierre-Etienne M.C. Martin, ¹Sciences Appliquées
Geochmica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.09.047]
¹Université du Québec à Chicoutimi, Québec G7H 2B1, CANADA
Copyright Elsevier

Abee is an enstatite chondrite breccia dominantly composed of kamacite, enstatite, silica, plagioclase, troilite and niningerite. Clasts are up to 220 mm long and vary in shape from angular to rounded. Some clasts are zoned with kamacite-enriched rims that follow the edge of the clast. Spatial compositional variations were examined in a small block to find out more about the petrological processes that produced this rock, particularly the relationship between the clasts, the matrix and the cores/rims of the zoned clasts. Compositional maps produced using a focussed-beam XRF were segmented into clasts and matrix, and rims and cores where possible. Compositions of most clasts, matrix and rim/cores define a simple, linear trend on simple variation diagrams. If it is assumed that all components were derived from an original homogeneous composition then the variation can be explained either by addition of kamacite or by loss of all other phases. Within this overall compositional variation the kamacite content generally increases as follows: matrix < large homogeneous clasts ≈ zoned clast cores < small homogeneous clasts ≈ zoned clast rims. Production of diversity by addition of kamacite to clasts and rim seems to require a complex history as the source cannot have been the current matrix. It is also difficult to produce the observed chemical variations and zoning by partial melting. However, differentiation by removal of all non-metallic phases may result from repeated impacts: Shock waves would deform kamacite whilst fracturing all other phases. The broken grains would then migrate towards the surface of the clasts where they would spall off into the matrix. This process would also lead to the observed rounding of some clasts. We propose that this shock-differentiation process be called ‘smithing’, as it resembles the ancient process of iron refining.

¹Malcolm J. Rutherford, ¹James W. Head, ¹Alberto E. Saal, ²Erik Hauri, ³Lionel Wilson
American Mineralogist 102, 2045-2053 Link to Article [DOI
https://doi.org/10.2138/am-2017-5994]
¹Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, U.S.A.
²Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015, U.S.A.
³Lancaster Environment Center, Lancaster University, Lancaster LA1 4YQ, U.K.
Copyright: The Mineralogical Society of America

A model for the origin, ascent, and eruption of the lunar A17 orange glass magma has been constructed using petrological constraints from gas solubility experiments and from analyses of the lunar sample 74220 to better determine the nature and origin of this unique explosive eruption. Three stages of the eruption have been identified. Stage 1 of the eruption model extends from ~550 km, the A17 orange glass magma source region based on phase equilibria studies, to 50 km depth in the Moon. Stage 2 extends from ~50 km to 500 m, where a C-O-H-S gas phase formed and grew in volume based on melt inclusion analyses and measurements. The volume of the gas phase at 500 m depth below the surface is calculated to be 7 to 15 vol% of the magma (closed-system) using the minimum and maximum estimates of CO, H2O, and S loss from the melt. In Stage 3, depths shallower than ~450 m, the rising magma exsolved an additional 800–900 ppm H2O and 300 ppm S, increasing the moles in the gas by a factor of 3 to 4. The closed-system gas phase is calculated to reach ~70 vol% at ~130 m depth, enough to fragment the magma and form pyroclastic beads. However, fragmentation (bead formation) is interpreted to have occurred at depths ranging from 600 to 300 m below the lunar surface based on the pressure necessary to explain the C content of the orange glass beads. The gas volume (70%) required to fragment the ascending magma at this depth is a factor of ~5 greater than the volume determined for closed-system degassing of an orange glass magma at 500 m, strongly implying that the gas was produced by open-system degassing as the magma ascended from greater depths.

Formation of the dike carrying the magma up from the ~550 km deep source is considered to occur by a crack propagation mechanism (Wilson and Head 2003, 2017). The rapid dike-propagation process facilitates gas collection by open-system degassing in the upper part of the dike. This is necessary to achieve the gas volumes required for magma fragmentation at 600 m depths, and the magma-ascent velocities to explain the wide areal distribution of the bead deposit. The explosive nature of the picritic orange glass eruption, and the homogeneity of the bead compositions, are consistent with this gas-assisted eruption scenario, as is the evidence of a Fe-metal forming reduction event during Stage 2 followed by a Stage 3 oxidation event in the ascending magma.