^1,2,3Wolf Uwe Reimold, ³Natalia Hauser, ⁴Bent T. Hansen, ⁵Matthew Thirlwall, ¹Marie Hoffmann
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.07.040]
¹Museum für Naturkunde – Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstrasse 43, 10115 Berlin, Germany
²Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany
³Institute of Geosciences, Laboratório de Estudos Geocronológicos, Geodinâmicos e Ambientais, Universidade de Brasília, Brasília, DF, CEP 70910-900, Brasil
⁴Department of Isotope Geology, Geoscience Centre, Georg-August Universität, Goldschmidtstraße 3, 37077 Göttingen, Germany
⁵Department of Earth Sciences, Royal Holloway University of London, Egham TW20 0EX, U.K
Copyright Elsevier

Besides impact melt rock, several large terrestrial impact structures, notably the Sudbury (Canada) and Vredefort (South Africa) structures, exhibit considerable occurrences of a second type of impact-generated melt rock, so-called pseudotachylitic breccia (previously often termed “pseudotachylite” – the term today reserved in structural geology for friction melt in shear or fault zones). At the Vredefort Dome, the eroded central uplift of the largest and oldest known terrestrial impact structure, pseudotachylitic breccia is well-exposed, with many massive occurrences of tens of meters width and many hundreds of meters extent. Genesis of these breccias has been discussed variably in terms of melt formation due to friction melting, melting due to decompression after initial shock compression, decompression melting upon formation/collapse of a central uplift, or a combination of these processes. In addition, it was recently suggested that they could have formed by the infiltration of impact melt into the crater floor, coming off a coherent melt sheet and under assimilation of wall rock; even seismic shaking has been invoked. Field evidence for generation of such massive melt bodies by friction on large shear / fault zones is missing. Also, no evidence for the generation of massive pseudotachylitic breccias in rocks of low to moderate shock degree by melting upon pressure release after shock compression has been demonstrated. The efficacy of seismic shaking to achieve sufficient melting as a foundation for massive pseudotachylitic melt generation as typified by the breccias of the Sudbury and Vredefort structures has so far remained entirely speculative. The available petrographic and chemical evidence has, thus, been interpreted to favor either decompression melting (i.e., in situ generation of melt) upon central uplift collapse, or the impact melt infiltration hypothesis. Importantly, all the past clast population and chemical analyses have invariably supported an origin of these breccias from local lithologies only.

Here, the first Rb-Sr, Sm-Nd, and U-Pb isotopic data for Vredefort pseudotachylitic breccias and their host rocks, in comparison to data for Vredefort Granophyre (impact melt rock), are presented. They strongly support that the pseudotachylitic breccias were exclusively formed from local precursor lithologies – in agreement with earlier isotopic results for Sudbury Breccia and chemical results for Vredefort pseudotachylitic breccias. A contribution from a Granophyre-like impact melt component to form Vredefort pseudotachylitic breccia is not indicated. The most likely process for the genesis of voluminous pseudotachylitic breccias in large impact structures remains decompression melting upon formation and collapse of the central uplift, during the modification stage of impact cratering.

¹Michael E. Rutherford, ¹David J. Chapman, ²James G. Derrick, ¹Jack R. W. Patten, ³Philip A. Bland, ⁴Alexander Rack, ²Gareth S. Collins, ¹Daniel E. Eakins
Scientific Reports 7, 45206 Link to Article [doi:10.1038/srep45206]
¹Institute of Shock Physics, Blackett Laboratory, Imperial College London, London SW7 2BW, UK
²Department of Earth Science and Engineering, Imperial College London, London SW7 2BP, UK
³Department of Applied Geology, Curtin University of Technology, Perth, WA 6845, Australia
⁴European Synchrotron Radiation Facility, Structure of Materials, Grenoble, France

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^1,2Zuzana Rodovská, ¹Tomáš Magna, ³Karel Žák, ⁴Chizu Kato, ^4,5,6Paul S. Savage, ⁴Frédéric Moynier, ³Roman Skála, ²Josef Ježek
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12922]
¹Czech Geological Survey, Prague 1, Czech Republic
²Faculty of Science, The Charles University in Prague, Prague 2, Czech Republic
³Institute of Geology, The Czech Academy of Sciences, Prague 6, Czech Republic
⁴Institut de Physique du Globe de Paris, Université Paris Diderot, Paris, France
⁵Department of Earth Sciences, Durham University, Science Labs, Durham, UK
⁶Department of Earth and Environmental Sciences, University of St. Andrews, St. Andrews, Fife, UK
Published by arrangement with John Wiley & Sons

Moldavites are tektites genetically related to the Ries impact structure, located in Central Europe, but the source materials and the processes related to the chemical fractionation of moldavites are not fully constrained. To further understand moldavite genesis, the Cu and Zn abundances and isotope compositions were measured in a suite of tektites from four different substrewn fields (South Bohemia, Moravia, Cheb Basin, Lusatia) and chemically diverse sediments from the surroundings of the Ries impact structure. Moldavites are slightly depleted in Zn (~10–20%) and distinctly depleted in Cu (>90%) relative to supposed sedimentary precursors. Moreover, the moldavites show a wide range in δ66Zn values between 1.7 and 3.7‰ (relative to JMC 3-0749 Lyon) and δ65Cu values between 1.6 and 12.5‰ (relative to NIST SRM 976) and are thus enriched in heavy isotopes relative to their possible parent sedimentary sources (δ66Zn = −0.07 to +0.64‰; δ65Cu = −0.4 to +0.7‰). In particular, the Cheb Basin moldavites show some of the highest δ65Cu values (up to 12.5‰) ever observed in natural samples. The relative magnitude of isotope fractionation for Cu and Zn seen here is opposite to oxygen-poor environments such as the Moon where Zn is significantly more isotopically fractionated than Cu. One possibility is that monovalent Cu diffuses faster than divalent Zn in the reduced melt and diffusion will not affect the extent of Zn isotope fractionation. These observations imply that the capability of forming a redox environment may aid in volatilizing some elements, accompanied by isotope fractionation, during the impact process. The greater extent of elemental depletion, coupled with isotope fractionation of more refractory Cu relative to Zn, may also hinge on the presence of carbonyl species of transition metals and electromagnetic charge, which could exist in the impact-induced high-velocity jet of vapor and melts.

¹L. F. White, ¹J. R. Darling, ²D. E. Moser, ³D. A. Reinhard, ³T. J. Prosa, ¹D. Bullen, ³D. Olson, ³D. J. Larson, ³D. Lawrence, ³I. Martin
Nature Communications 8, 15597 Link to Article [doi:10.1038/ncomms15597]
¹School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth PO1 3QL, UK
²Department of Earth Sciences, University of Western Ontario, London, Canada N6A 5B7
³CAMECA, Madison, Wisconsin 53711, USA

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¹A.A.Maksimova, ¹M.I.Oshtrakh, ²I.Felner, ¹A.V.Chukin, ³M.S.Karabanalov, ¹V.A.Semionkin
Journal of Molecular Structure 1140, 122 Link to Article [https://doi.org/10.1016/j.molstruc.2016.11.042]
¹Institute of Physics and Technology, Ural Federal University, Ekaterinburg, 620002, Russian Federation
²Racah Institute of Physics, The Hebrew University, Jerusalem, Israel
³Institute of Material Science and Metallurgy, Ural Federal University, Ekaterinburg, 620002, Russian Federation

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¹Jean-Pierre Williams, ²Carolyn H. van der Bogert, ³Asmin V. Pathare, ⁴Gregory G. Michael, ⁵Michelle R. Kirchoff, ²Harald Hiesinger
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12924]
¹Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, California, USA
²Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, Germany
³Planetary Science Institute, Tucson, Arizona, USA
⁴Planetary Sciences and Remote Sensing, Institute of Geological Sciences, Freie Universitaet Berlin, Berlin, Germany
⁵Southwest Research Institute, Boulder, Colorado, USA
Published by arrangement with John Wiley & Sons

Determining the ages of young planetary surfaces relies on using populations of small, often sub-km diameter impact craters due to the higher frequency at which they form. Smaller craters however can be less reliable for estimating ages as their size-frequency distribution is more susceptible to alteration with debate as to whether they should be used at all. With the current plethora of meter-scale resolution images acquired of the lunar and Martian surfaces, small craters have been widely used to derive model ages to establish the temporal relation of recent geologic events. In this review paper, we discuss the many factors that make smaller craters particularly challenging to use and should be taken into consideration when crater counts are confined to small crater diameters. Establishing confidence in a model age ultimately requires an understanding of the geologic context of the surface being dated as reliability can vary considerably and limitations of the dating technique should be considered in applying ages to any geologic interpretation.

¹Lily Bossina, ^2,3Nikolaos A.Kazakis, ¹George Kitis, ²Nestor C.Tsirliganis
Applied Radiation and Isotopes 127, 26-34 Link to Article [https://doi.org/10.1016/j.apradiso.2017.05.002]
¹Department of Archaeology, Durham University, UK
²Laboratory of Archaeometry and Physicochemical Measurements, R.C. ‘Athena’, P.O. Box 159, Kimmeria University Campus, 67100 Xanthi, Greece
³Nuclear Physics Laboratory, Physics Department, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece

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¹Evelyn Füri, ¹Etienne Deloulea, ²Reto Trappitsch
Earth and Planetary Science Letters 474, 76-82 Link to Article [https://doi.org/10.1016/j.epsl.2017.05.042]
¹Centre de Recherches Pétrographiques et Géochimiques, CNRS-UL, 15 rue Notre Dame des Pauvres, BP20, 54500 Vandoeuvre-lès-Nancy Cedex, France
²The University of Chicago, Department of Geophysical Sciences and Chicago Center for Cosmochemistry, 5734 South Ellis Avenue, Chicago, IL 60637, USA
Copyright Elsevier

The hydrogen (D/H) isotope ratio is a key tracer for the source of planetary water. However, secondary processes such as solar wind implantation and cosmic ray induced spallation reactions have modified the primordial D/H signature of ‘water’ in all rocks and soils recovered on the Moon. Here, we re-evaluate the production rate of cosmogenic deuterium (D) at the Moon’s surface through ion microprobe analyses of hydrogen isotopes in olivines from eight Apollo 12 and 15 mare basalts. These in situ measurements are complemented by CO2 laser extraction-static mass spectrometry analyses of cosmogenic noble gas nuclides (3He, 21Ne, 38Ar). Cosmic ray exposure (CRE) ages of the mare basalts, derived from their cosmogenic 21Ne content, range from 60 to 422 Ma. These CRE ages are 35% higher, on average, than the published values for the same samples. The amount of D detected in the olivines increases linearly with increasing CRE ages, consistent with a production rate of (2.17±0.11)×10−12 mol(g rock)−1 Ma−1. This value is more than twice as high as previous estimates for the production of D by galactic cosmic rays, indicating that for water-poor lunar samples, i.e., samples with water concentrations ≤50 ppm, corrected D/H ratios have been severely overestimated.

¹Nancy N. Elewa, ¹J. M. Cadogan
Hyperfine Interactions 238, 4 Link to Article [DOI https://doi.org/10.1007/s10751-016-1350-1]
¹School of Physical, Environmental and Mathematical Sciences The University of New South Wales at the Australian Defence Force Academy Canberra Australia

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^1,2Poorna Srinivasan, ^1,3Rhian H. Jones, ¹Adrian J. Brearley
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12921]
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA
²Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, USA
³School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester, UK
Published by arrangement with John Wiley & Sons

We studied textures and compositions of sulfide inclusions in unzoned Fe,Ni metal particles within CBa Gujba, CBa Weatherford, CBb HH 237, and CBb QUE 94411 in order to constrain formation conditions and secondary thermal histories on the CB parent body. Unzoned metal particles in all four chondrites have very similar metal and sulfide compositions. Metal particles contain different types of sulfides, which we categorize as: homogeneous low-Cr sulfides composed of troilite, troilite-containing exsolved daubreelite lamellae, arcuate sulfides that occur along metal grain boundaries, and shock-melted sulfides composed of a mixture of troilite and Fe, Ni metal. Our model for formation proposes that the unzoned metal particles were initially metal droplets that formed from splashing by a partially molten impacting body. Sulfide inclusions later formed as a result of precipitation of excess S from solid metal at low temperatures, either during single stage cooling or during a reheating event by impacts. Sulfides containing exsolution lamellae record temperatures of ≪600 °C, and irregular Fe-FeS intergrowth textures suggest localized shock melting, both of which are indicative of heterogeneous heating by impact processes on the CB parent body. Our study shows that CBa and CBb chondrites formed in a similar environment, and also experienced similar secondary impact processing.