¹J.Wilk, ¹T.Kenkmann
Meteoritics & Planetary Sience (in Press) Link to Article [DOI: 10.1111/maps.12682]
¹Institute of Earth and Environmental Sciences—Geology, Albert-Ludwigs-Universität (ALU) Freiburg, Freiburg, Germany
Published by arrangement with John Wiley & Sons

Shatter cones are the only macroscopic feature considered as evidence for shock metamorphism. Their presence is diagnostic for the discovery and verification of impact structures. The occurrence of shatter cones is heterogeneous throughout the crater record and their geometry can diverge from the typical cone shape. The precise formation mechanism of shatter cones is still not resolved. In this study, we aim at better constraining the boundary conditions of shatter cone formation in impact experiments and test a novel approach to qualitatively and quantitatively describe shatter cone geometries by white light interferometry. We recovered several ejected fragments from MEMIN cratering experiments that show slightly curved, striated surfaces and conical geometries with apices of 36°–52°. These fragments fulfilling the morphological criteria of shatter cones were found in experiments with 20–80 cm sized target cubes of sandstone, quartzite and limestone, but not in highly porous tuff. Targets were impacted by aluminum, steel, and iron meteorite projectiles at velocities of 4.6–7.8 km s−1. The projectile sizes ranged from 2.5–12 mm in diameter and produced experimental peak pressures of up to 86 GPa. In experiments with lower impact velocities shatter cones could not be found. A thorough morphometric analysis of the experimentally generated shatter cones was made with 3D white light interferometry scans at micrometer accuracy. SEM analysis of the surfaces of recovered fragments showed vesicular melt films alternating with smoothly polished surfaces. We hypothesize that the vesicular melt films predominantly form at strain releasing steps and suggest that shatter cones are probably mixed mode fractures.

¹David J. Wilson, ¹Boris T. Gänsicke, ²Jay Farihi,³Detlev Koester
Monthly Notices of the Royal Astronomical Society 459, 3282-3286.
Link to Article [doi: 10.1093/mnras/stw844]
¹Department of Physics, University of Warwick, Coventry CV4 7AL, UK
²University College London, Department of Physics and Astronomy, Gower Street, London WC1E 6BT, UK
³Institut für Theoretische Physik und Astrophysik, University of Kiel, D-24098 Kiel, Germany

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¹J.A. Barrat, ²R.C. Greenwood, ³K. Keil, ⁴M.L. Rouget, ^5,6J.S. Boesenberg, ⁷B. Zanda, ²I.A. Franchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [doi:10.1016/j.gca.2016.07.025]
¹U.B.O.-I.U.E.M., CNRS UMR 66538 (Domaines Océaniques), Place Nicolas Copernic, 29280 Plouzané, France
²Planetary and Space Sciences, Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK76AA,United Kingdom
³Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA
⁴CNRS UMS 3113, I.U.E.M., Place Nicolas Copernic, 29280 Plouzané Cedex, France
⁵Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
⁶Geological Sciences, Brown University, Providence, RI 02912, USA
⁷Muséum National d’Histoire Naturelle, Laboratoire de Minéralogie et de Cosmochimie du Muséum, CNRS UMR7202, 61 rue Buffon, 75005 Paris, France
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We report the abundances of a selected set of “lithophile” trace elements (including lanthanides, actinides and high field strength elements) and high-precision oxygen isotope analyses of a comprehensive suite of aubrites. Two distinct groups of aubrites can be distinguished: a) the main-group aubrites display flat or light-REE depleted REE patterns with variable Eu and Y anomalies; their pyroxenes are light-REE depleted and show marked negative Eu anomalies; b) the Mount Egerton enstatites and the silicate fraction from Larned display distinctive light-REE enrichments, and high Th/Sm ratios; Mount Egerton pyroxenes have much less pronounced negative Eu anomalies than pyroxenes from the main-group aubrites.

Leaching experiments were undertaken to investigate the contribution of sulfides to the whole rock budget of the main-group aubrites. Sulfides contain in most cases at least 50% of the REEs and of the actinides. Among the elements we have analyzed, those displaying the strongest lithophile behaviors are Rb, Ba, Sr and Sc.

The homogeneity of the Δ17O values obtained for main-group aubrite falls [Δ17O = +0.009 ± 0.010 ‰ (2σ)] suggests that they originated from a single parent body whose differentiation involved an early phase of large-scale melting that may have led to the development of a magma ocean. This interpretation is at first glance in agreement with the limited variability of the shapes of the REE patterns of these aubrites. However, the trace element concentrations of their phases cannot be used to discuss this hypothesis, because their igneous trace-element signatures have been modified by subsolidus exchange. Finally, despite similar O isotopic compositions, the marked light-REE enrichments displayed by Mount Egerton and Larned suggest that they are unrelated to the main-group aubrites and probably originated from a distinct parent body.

^1,2,3Steven Goderis, ¹Alan D. Brandon, ⁴Bernhard Mayer, ⁴Munir Humayun
Geochmica et Cosmochimica Acta (in Press) Link to Article [doi:10.1016/j.gca.2016.07.024]
¹Dept. of Earth and Atmospheric Sciences, University of Houston, Science and Research Building 1, Houston, TX 77204, USA
²Earth System Science, Vrije Universiteit Brussel, BE-1050 Brussels, Belgium
³Dept. of Analytical Chemistry, Ghent University, Krijgslaan 281 – S12, BE-9000 Ghent, Belgium
⁴National High Magnetic Field Laboratory and Dept. of Earth, Ocean & Atmospheric Science, Florida State University, Tallahassee, FL 32310, USA
Copyright Elsevier

Northwest Africa (NWA) 7034 and paired stones represent unique samples of martian polymict regolith breccia. Multiple breccia subsamples characterized in this work confirm highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, Pd) contents that are consistently elevated (e.g., Os ∼9.3 to 18.4 ppb) above indigenous martian igneous rocks (mostly < 5 ppb Os), equivalent to ∼3 wt% of admixed CI-type carbonaceous chondritic material, and occur in broadly chondrite-relative proportions. However, a protracted history of impactor component (metal and sulfide) breakdown and redistribution of the associated HSE has masked the original nature of the admixed meteorite signatures. The present-day 187Os/188Os ratios of 0.119 to 0.136 record a wider variation than observed for all major chondrite types. Combined with the measured 187Re/188Os ratios of 0.154 to 0.994, the range in Os isotope ratios indicates redistribution of Re and Os from originally chondritic components early in the history of the regolith commencing at ∼4.4 Ga. Superimposed recent Re mobility reflects exposure and weathering at or near the martian and terrestrial surfaces. Elevated Os concentrations (38.0 and 92.6 ppb Os), superchondritic Os/HSE ratios, and 187Os/188Os of 0.1171 and 0.1197 measured for two subsamples of the breccia suggest the redistribution of impactor material at ∼1.5-1.9 Ga, possibly overlapping with a (partial) resetting event at ∼1.4 Ga recorded by U-Pb isotope systematics in the breccia. Martian alteration of the originally chondritic HSE host phases, to form Os-Ir-rich nuggets and Ni-rich pyrite, implies the influence of potentially impact-driven hydrothermal systems. Multiple generations of impactor component admixture, redistribution, and alteration mark the formation and evolution of the martian regolith clasts and matrix of NWA 7034 and paired meteorites, from the pre-Noachian until impact ejection to Earth.

¹Reto Trappitsch, ²Ingo Leya
The Astrophysical Journal 823, 12 Link to Article [http://dx.doi.org/10.3847/0004-637X/823/1/12]
¹Department of the Geophysical Sciences and Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL 60637, USA
²Space Research and Planetary Sciences, University of Bern, Bern, 3012, Switzerland

Presolar grains are small particles that condensed in the vicinity of dying stars. Some of these grains survived the voyage through the interstellar medium (ISM) and were incorporated into meteorite parent bodies at the formation of the Solar System. An important question is when these stellar processes happened, i.e., how long presolar grains were drifting through the ISM. While conventional radiometric dating of such small grains is very difficult, presolar grains are irradiated with galactic cosmic rays (GCRs) in the ISM, which induce the production of cosmogenic nuclides. This opens the possibility to determine cosmic-ray exposure (CRE) ages, i.e., how long presolar grains were irradiated in the ISM. Here, we present a new model for the production and loss of cosmogenic 3He, 6,7Li, and 21,22Ne in presolar SiC grains. The cosmogenic production rates are calculated using a state-of-the-art nuclear cross-section database and a GCR spectrum in the ISM consistent with recent Voyager data. Our findings are that previously measured 3He and 21Ne CRE ages agree within the (sometimes large) 2σ uncertainties and that the CRE ages for most presolar grains are smaller than the predicted survival times. The obtained results are relatively robust since interferences from implanted low-energy GCRs into the presolar SiC grains and/or from cosmogenic production within the meteoroid can be neglected.

¹Florian J. Zurfluh, ^1,2Beda A. Hofmann, ³Edwin Gnos, ¹Urs Eggenberger,⁴A. J. Timothy Jull
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12690]
¹Institut für Geologie, Universität Bern, Bern, Switzerland
²Naturhistorisches Museum der Burgergemeinde Bern, Bern, Switzerland
³Muséum d’histoire naturelle de la Ville de Genève, Genève 6, Switzerland
⁴Department of Geosciences and NSF-Arizona AMS Laboratory, The University of Arizona, Tucson, Arizona, USA
Published by arrangement with John Wiley & Sons

We have investigated 128 14C-dated ordinary chondrites from Oman for macroscopically visible weathering parameters, for thin section-based weathering degrees, and for chemical weathering parameters as analyzed with handheld X-ray fluorescence. These 128 14C-dated meteorites show an abundance maximum of terrestrial age at 19.9 ka, with a mean of 21.0 ka and a pronounced lack of samples between 0 and 10 ka. The weathering degree is evaluated in thin section using a refined weathering scale based on the current W0 to W6 classification of Wlotzka (1993), with five newly included intermediate steps resulting in a total of nine (formerly six) steps. We find significant correlations between terrestrial ages and several macroscopic weathering parameters. The correlation of various chemical parameters including Sr and Ba with terrestrial age is not very pronounced. The microscopic weathering degree of metal and sulfides with newly added intermediate steps shows the best correlation with 14C terrestrial ages, demonstrating the significance of the newly defined weathering steps. We demonstrate that the observed 14C terrestrial age distribution can be modeled from the abundance of meteorites with different weathering degrees, allowing the evaluation of an age-frequency distribution for the whole meteorite population.

¹Martin R. Lee,²Elias Chatzitheodoridis
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12687]
¹School of Geographical and Earth Sciences, University of Glasgow, Glasgow, UK
²Department of Geological Sciences, School of Mining and Metallurgical Engineering, National Technical University of Athens, Athens, Greece
Published by arrangement with John Wiley & Sons

A scanning and transmission electron microscope study of aluminosilicate glasses within melt inclusions from the Martian meteorite Nakhla shows that they have been replaced by berthierine, an aluminum-iron serpentine mineral. This alteration reaction was mediated by liquid water that gained access to the glasses along fractures within enclosing augite and olivine grains. Water/rock ratios were low, and the aqueous solutions were circumneutral and reducing. They introduced magnesium and iron that were sourced from the dissolution of olivine, and exported alkalis. Berthierine was identified using X-ray microanalysis and electron diffraction. It is restricted in its occurrence to parts of the melt inclusions that were formerly glass, thus showing that under the ambient physico-chemical conditions, the mobility of aluminum and silicon were low. This discovery of serpentine adds to the suite of postmagmatic hydrous silicates in Nakhla that include saponite and opal-A. Such a variety of secondary silicates indicates that during aqueous alteration compositionally distinct microenvironments developed on sub-millimeter length scales. The scarcity of berthierine in Nakhla is consistent with results from orbital remote sensing of the Martian crust showing very low abundances of aluminum-rich phyllosilicates.

^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
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