^1,2David Baratoux,^3,4Wolf Uwe Reimold
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12678]
¹Géosciences Environnement Toulouse, University of Toulouse, CNRS, IRD, Toulouse, France
²Institut Fondamental d’Afrique Noire, Univesity Cheikh Anta Diop, Dakar, Senegal
³Museum für Naturkunde – Leibniz Institute for Evolution and Biodiversity Research, Berlin, Germany
⁴Humboldt Universität zu Berlin, Berlin, Germany
Published by arrangement with John Wiley & Sons

Shatter cones are a fracture phenomenon that is exclusively associated with shock metamorphism and has also been produced in the laboratory in several shock experiments. The occurrence of shatter cones is the only accepted meso- to macroscopic recognition criterion for impact structures. Shatter cones exhibit a number of geometric characteristics (orientation, apical angles, striation angles, sizes) that can be best described as varied, from case to case. Possible links between geometric properties with impact or crater parameters have remained controversial and the lack of understanding of the mechanism of formation of shatter cones does not offer a physical framework to discuss or understand them. A database of shatter cone occurrences has been produced for this introduction paper to the special issue of Meteoritics and Planetary Science on shatter cones. Distribution of shatter cones with respect to crater size and lithology suggests that shatter cones do not occur in impact craters less than a few kilometers in diameter, with a few, currently questionable exceptions. All pertinent hypotheses of formation are presented and discussed. Several may be discarded in light of the most recent observations. The branching fracture mechanism and the interference models proposed, respectively, by Sagy et al. (2002) and Baratoux and Melosh (2003) require further evaluation. New observations, experiments, or theoretical considerations presented in this special issue promise an important step forward, based on a renewed effort to resolve the enigmatic origin of these important features.

^1,2Maximilian Hasch,^1,3Wolf Uwe Reimold,^1,2Ulli Raschke,¹Patrice Tristan Zaag
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12676]
¹Museum für Naturkunde—Leibniz Institute for Evolution and Biodiversity Science, Berlin, Berlin, Germany
²Geosciences, Freie Universität Berlin, Berlin, Germany
³Humboldt Universität zu Berlin, Berlin, Germany
Published by arrangement with John Wiley & Sons

Shatter cones are the only distinct meso- to macroscopic recognition criterion for impact structures, yet not all is known about their formation. The Keurusselkä impact structure, Finland, is interesting in that it presents a multitude of well-exposed shatter cones in medium- to coarse-grained granitoids. The allegedly 27 km wide Keurusselkä impact structure was formed about 1150 Ma ago in rocks of the Central Finland Granitoid Complex. Special attention was paid in this work to possible relationships between shatter cones and local, as well as regionally occurring, fracture or joint systems. A possible shatter cone find outside the previously suggested edge of the structure could mean that the Keurusselkä impact structure is larger than previously thought. The spacing between joints/fractures from regional joint systems was influenced by the impact, but impact-induced fractures strongly follow the regional joint orientation trends. There is a distinct relationship between shatter cones and joints: shatter cones occur on and against joint surfaces of varied orientations and belonging to the regional orientation trends. Planar fractures (PF) and planar deformation features (PDF) were found in three shatter cone samples from the central-most part of the impact structure, whereas other country rock samples from the same level of exposure but further from the assumed center lack shock deformation features. PDF occurrence is enhanced within 5 mm of shatter cone surfaces, which is interpreted to suggest that shock wave reverberation at preimpact joints could be responsible for this local enhancement of shock deformation. Some shatter cone surfaces are coated with a quasi-opaque material which is also found in conspicuous veinlets that branch off from shatter cone surfaces and resemble pseudotachylitic breccia veins. The vein-filling is composed of two mineral phases, one of which could be identified as a montmorillonitic phyllosilicate. The second phase could not be identified yet. The original composition of the fill could not be determined. Further work is required on this material. Observed joints and fractures were discussed against findings from Barringer impact crater. They show that impact-induced joints in the basement rock do not follow impact-specific orientations (such as radial, conical, or concentric).

¹Rainer Wieler
Chemie der Erde (in Press) Link to Article [doi:10.1016/j.chemer.2016.06.001]
¹ETH Zurich, Department of Earth Sciences, Clausiusstrasse 25, CH-8092 Zurich, Switzerland
Copyright Elsevier

^1,2Sándor Góbi, ^1,2Matthew J. Abplanalp, ^1,2Ralf I. Kaiser
The Astrophysical Journal, Volume 822, 8 Link to Article [http://dx.doi.org/10.3847/0004-637X/822/1/8]
¹Department of Chemistry, University of Hawaii at Mānoa, Honolulu, HI 96822, USA
²W. M. Keck Laboratory in Astrochemistry, University of Hawaii at Mānoa, Honolulu, HI 96822, USA

This work explores the radiolytic decomposition of glycine (H2NCH2COOH) under simulated Martian conditions in the presence of perchlorates (${{\mathrm{ClO}}_{4}}^{-}$), which are abundant oxidizers on the surface of Mars, by energetic electrons at 10, 160, 210, and 260 K, mimicking the radiation exposure of the Martian regolith in the first 5–10 cm depths over about 250 million years. Our experiments present quantitative evidence that the rate constants of the glycine decomposition in the presence of magnesium perchlorate hexahydrate (Mg(ClO4)2 centerdot 6H2O) were a factor of about two higher than that of the pure glycine, suggesting that energetic oxygen atoms (O) released from the ${{\mathrm{ClO}}_{4}}^{-}$ have a significant effect on the decomposition rates and accelerate them by providing a unique oxidizing environment in the radiolyzed samples. Hence, two decay mechanisms exist: radiolysis by the electrons and oxidation by the O atoms. Within the Mars-relevant temperature range covering 160–260 K, the destruction rates are nearly temperature invariant with rates varying as little as 5%. Further, the formation rates of carbon dioxide (CO2) and carbon monoxide (CO) are both accelerated in the presence of ${{\mathrm{ClO}}_{4}}^{-}$ by a factor of three to five, supporting our conclusion of an active oxygen-initiated chemistry. In addition, the degradation rates are significantly higher than the formation rates of CO2 and CO. This suggests that, besides the decarboxylation, alternative degradation pathways such as a polymerization of glycine must exist. Finally, besides CO2 and CO, three alternative products were identified tentatively: methylamine (CH3NH2), methane (CH4), and ammonia (NH3).

¹Stephan Henke, ¹Hans-Peter Gail, ^2,3Mario Trieloff
Astronomy & Astrophysics 589 A41 Link to Article [http://dx.doi.org/10.1051/0004-6361/201527687]
¹Institut für Theoretische Astrophysik, Zentrum für Astronomie, Universität Heidelberg, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
e-mail: gail@uni-heidelberg.de
²Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany
³Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany

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¹A. De Sio et al. (>10)*
Astronomy & Astrophysic 589, A4   Link to Article [http://dx.doi.org/10.1051/0004-6361/201527222]
¹Department of Physic and AstronomyUniversity of Firenze, Largo Enrico Fermi 2, 50125 Firenze, Italy
*Find the extensive, full author and affiliation list on the publishers website

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^1,2S.P.Schwenzer et al. (>10)*
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12668]
¹Department of Environment, Earth and Ecosystems, The Open University, Milton Keynes, UK
²Lunar and Planetary Institute, Houston, Texas, USA
*Find the extensive, full author and affiliation list on the publishers website
Published by arrangement with John Wiley & Sons

We model the fluids involved in the alteration processes recorded in the Sheepbed Member mudstones of Yellowknife Bay (YKB), Gale crater, Mars, as revealed by the Mars Science Laboratory Curiosity rover investigations. We compare the Gale crater waters with fluids modeled for shergottites, nakhlites, and the ancient meteorite ALH 84001, as well as rocks analyzed by the Mars Exploration rovers, and with terrestrial ground and surface waters. The aqueous solution present during sediment alteration associated with phyllosilicate formation at Gale was high in Na, K, and Si; had low Mg, Fe, and Al concentrations—relative to terrestrial groundwaters such as the Deccan Traps and other modeled Mars fluids; and had near neutral to alkaline pH. Ca and S species were present in the 10−3 to 10−2 concentration range. A fluid local to Gale crater strata produced the alteration products observed by Curiosity and subsequent evaporation of this groundwater-type fluid formed impure sulfate- and silica-rich deposits—veins or horizons. In a second, separate stage of alteration, partial dissolution of this sulfate-rich layer in Yellowknife Bay, or beyond, led to the pure sulfate veins observed in YKB. This scenario is analogous to similar processes identified at a terrestrial site in Triassic sediments with gypsum veins of the Mercia Mudstone Group in Watchet Bay, UK.

¹L. Gavilan, ²C. Jäger, ³A. Simionovici, ⁴J. L. Lemaire, ²T. Sabri, ⁵E. Foy, ⁶S. Yagoubi, ⁷T. Henning, ⁸D. Salomon, ⁸G. Martinez-Criado
Astronomy & Astrophysics 587, A144    Link to Article [http://dx.doi.org/10.1051/0004-6361/201527708]
¹Institut d’Astrophysique Spatiale (IAS), CNRS, Univ. Paris Sud, Université Paris-Saclay, 91405 Orsay, France
e-mail: lisseth.gavilan@ias.u-psud.fr
²Laboratory Astrophysics and Cluster Physics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University & Institute of Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany
³Institut des Sciences de la Terre, Observatoire des Sciences de l’Univers de Grenoble, BP 53, 38041 Grenoble, France
⁴Institut des Sciences Moléculaires d’Orsay (ISMO), CNRS, Univ. Paris Sud, Université Paris-Saclay, 91405 Orsay, France
⁵LAPA-IRAMAT, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette, France
⁶LEEL, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette, France
⁷Max-Planck Institute for Astronomy Königstuhl 17, 69117 Heidelberg, Germany
⁸European Synchrotron Radiation Facility, 71 avenue des Martyrs, 38000 Grenoble, France

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¹A. Basu Sarbadhikari,²E. V. S. S. K. Babu,²T. Vijaya Kumar,³H. Chennaoui Aoudjehane
Meteoritics & Planetary Sciences        Link to Article [DOI: 10.1111/maps.12684]
¹Physical Research Laboratory, Ahmedabad, India
²National Geophysical Research Institute (Council of Scientific and Industrial Research), Hyderabad, India
³Hassan II University Casablanca, Faculty of Sciences, GAIA Laboratory, Maârif, Casablanca, Morocco
Published by arrangement with John Wiley & Sons

Tissint, a new unaltered piece of Martian volcanic materials, is the most silica-poor and Mg-Fe-rich igneous rock among the “depleted” olivine-phyric shergottites. Fe-Mg zoning of olivine suggests equilibrium growth (<0.1 °C h−1) in the range of Fo80–56 and olivine overgrowth (Fo55–18) through a process of rapid disequilibrium (~1.0–5.0 °C h−1). The spatially extended (up to 600 μm) flat-top Fe-Mg profiles of olivine indicates that the early-stage cooling rate of Tissint was slower than the other shergottites. The chemically metastable outer rim of olivine (<Fo55) consists of oscillatory phosphorus zoning at the impact-induced melt domains and grew rapidly compared to the early to intermediate-stage crystallization of the Tissint bulk. High-Ca pyroxene to low-Ca pyroxene and high-Ca pyroxene to plagioclase ratios of Tissint are more comparable to the enriched basaltic and enriched olivine-phyric shergottites. Dominance of augite over plagioclase induced augite to control the Ca-buffer in the residual melt suppressing the plagioclase crystallization, which also caused a profound effect on the Al-content in the late-crystallized pyroxenes. Mineral chemical stability, phase-assemblage saturation, and pressure–temperature path of evolution indicates that the parent magma entered the solidus and left the liquidus field at a depth of 40–80 km in the upper mantle. Petrogenesis of Tissint appears to be similar to LAR 06319, an enriched olivine-phyric shergottite, during the early to intermediate stage of crystallization. A severe shock-induced deformation resulted in remelting (10–15 vol%), recrystallization (most Fe-rich phases), and exhumation of Tissint in a time scale of 1–8 yr. Tissint possesses some distinct characteristics, e.g., impact-induced melting and deformation, forming phosphorus-rich recrystallization rims of olivine, and shock-induced melt domains without relative enrichment of LREEs compared to the bulk; and shared characteristics, e.g., modal composition and magmatic evolution with the enriched basaltic shergottites, evidently reflecting unique mantle source in comparison to the clan of the depleted members.

¹Thomas M. Davison, ¹Gareth S. Collins, ²Philip A. Bland
The Astrophysical Journal 821, 68 Link to Article [http://dx.doi.org/10.3847/0004-637X/821/1/68]
¹Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
²Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia

We have developed a method for simulating the mesoscale compaction of early solar system solids in low-velocity impact events using the iSALE shock physics code. Chondrules are represented by non-porous disks, placed within a porous matrix. By simulating impacts into bimodal mixtures over a wide range of parameter space (including the chondrule-to-matrix ratio, the matrix porosity and composition, and the impact velocity), we have shown how each of these parameters influences the shock processing of heterogeneous materials. The temperature after shock processing shows a strong dichotomy: matrix temperatures are elevated much higher than the chondrules, which remain largely cold. Chondrules can protect some matrix from shock compaction, with shadow regions in the lee side of chondrules exhibiting higher porosity that elsewhere in the matrix. Using the results from this mesoscale modeling, we show how the ε − α porous-compaction model parameters depend on initial bulk porosity. We also show that the timescale for the temperature dichotomy to equilibrate is highly dependent on the porosity of the matrix after the shock, and will be on the order of seconds for matrix porosities of less than 0.1, and on the order of tens to hundreds of seconds for matrix porosities of ~0.3–0.5. Finally, we have shown that the composition of the post-shock material is able to match the bulk porosity and chondrule-to-matrix ratios of meteorite groups such as carbonaceous chondrites and unequilibrated ordinary chondrites.