^1,2Thomas S. Kruijer, ¹Thorsten Kleine
Earth and Planetary Science Letters 475, 15-24 Link to Article [https://doi.org/10.1016/j.epsl.2017.07.021]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany
²Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA
Copyright Elsevier

The giant impact model of lunar origin predicts that the Moon mainly consists of impactor material. As a result, the Moon is expected to be isotopically distinct from the Earth, but it is not. To account for this unexpected isotopic similarity of the Earth and Moon, several solutions have been proposed, including (i) post-giant impact Earth–Moon equilibration, (ii) alternative models that make the Moon predominantly out of proto-Earth mantle, and (iii) formation of the Earth and Moon from an isotopically homogeneous disk reservoir. Here we use W isotope systematics of lunar samples to distinguish between these scenarios. We report high-precision 182W data for several low-Ti and high-Ti mare basalts, as well as for Mg-suite sample 77215, and lunar meteorite Kalahari 009, which complement data previously obtained for KREEP-rich samples. In addition, we utilize high-precision Hf isotope and Ta/W ratio measurements to empirically quantify the superimposed effects of secondary neutron capture on measured 182W compositions. Our results demonstrate that there are no resolvable radiogenic 182W variations within the Moon, implying that the Moon differentiated later than 70 Ma after Solar System formation. In addition, we find that samples derived from different lunar sources have indistinguishable 182W excesses, confirming that the Moon is characterized by a small, uniform ∼+26 parts-per-million excess in 182W over the present-day bulk silicate Earth. This 182W excess is most likely caused by disproportional late accretion to the Earth and Moon, and after considering this effect, the pre-late veneer bulk silicate Earth and the Moon have indistinguishable 182W compositions. Mixing calculations demonstrate that this Earth–Moon 182W similarity is an unlikely outcome of the giant impact, which regardless of the amount of impactor material incorporated into the Moon should have generated a significant 182W excess in the Moon. Consequently, our results imply that post-giant impact processes might have modified 182W, leading to the similar 182W compositions of the pre-late veneer Earth’s mantle and the Moon.

¹F.P. Leitzke, ¹R.O.C. Fonseca, ²P. Sprung, ³G. Mallmann, ¹M. Lagos, ¹L.T. Michely, ²C. Münker
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2017.07.009]
¹Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Rheinische Friedrich-Wilhelms-Universität Bonn, 53115 Bonn, Germany
²Institut für Geologie und Mineralogie, Universität zu Köln, 50674 Köln, Germany
³Research School of Earth Sciences, Australian National University, ACT 2601, Canberra, Australia
Copyright Elsevier

We present results of high-temperature olivine-melt, pyroxene-melt and plagioclase-melt partitioning experiments aimed at investigating the redox transition of Mo in silicate systems. Data for a series of other minor and trace elements (Sc, Ba, Sr, Cr, REE, Y, HFSE, U, Th and W) were also acquired to constrain the incorporation of Mo in silicate minerals. All experiments were carried out in vertical tube furnaces at 1 bar and temperatures ranging from ca. 1220 to 1300 °C. Oxygen fugacity was controlled via CO–CO2 gas mixtures and varied systematically from 5.5 log units below to 1.9 log units above the fayalite–magnetite–quartz (FMQ) redox buffer thereby covering the range in oxygen fugacities of terrestrial and lunar basalt genesis. Molybdenum is shown to be volatile at oxygen fugacities above FMQ and that its compatibility in pyroxene and olivine increases three orders of magnitude towards the more reducing conditions covered in this study. The partitioning results show that Mo is dominantly tetravalent at redox conditions below FMQ-4 and dominantly hexavalent at redox conditions above FMQ. Given the differences in oxidation states of the terrestrial (oxidized) and lunar (reduced) mantles, molybdenum will behave significantly differently during basalt genesis in the Earth (i.e. highly incompatible; average View the MathML source) and Moon (i.e. moderately incompatible/compatible; average View the MathML source). Thus, it is expected that Mo will strongly fractionate from W during partial melting in the lunar mantle, given that W is broadly incompatible at FMQ-5. Moreover, the depletion of Mo and the Mo/W range in lunar samples can be reproduced by simply assuming a primitive Earth-like Mo/W for the bulk silicate Moon. Such a lunar composition is in striking agreement with the Moon being derived from the primitive terrestrial mantle after core formation on Earth.

¹Claire-Marie Loudon et al. (>10)*
International journal of Astrobiology (in Press) Link to Article [DOI: https://doi.org/10.1017/S1473550417000234]
¹UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK
*Find the extensive, full author and affiliation list on the publishers website

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¹Thomas S.Kruijer, ¹Thorsten Kleine, ²Lars E.Borg, ¹Gregory A.Brennecka, ³Anthony J.Irving, ¹Addi Bischoff, ⁴Carl B.Agee
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2017.06.047]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Strasse 10, 48149, Münster, Germany
²Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, 7000 East Avenue, CA 94550, USA
³Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA
⁴Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA
Copyright Elsevier

Mars probably accreted within the first 10 million years of Solar System formation and likely underwent magma ocean crystallization and crust formation soon thereafter. To assess the nature and timescales of these large-scale mantle differentiation processes we applied the short-lived 182Hf–182W and 146Sm–142Nd chronometers to a comprehensive suite of martian meteorites, including several shergottites, augite basalt NWA 8159, orthopyroxenite ALH 84001 and polymict breccia NWA 7034. Compared to previous studies the 182W data are significantly more precise and have been obtained for a more diverse suite of martian meteorites, ranging from samples from highly depleted to highly enriched mantle and crustal sources. Our results show that martian meteorites exhibit widespread 182W/184W variations that are broadly correlated with 142Nd/144Nd, implying that silicate differentiation (and not core formation) is the main cause of the observed 182W/184W differences. The combined 182W–142Nd systematics are best explained by magma ocean crystallization on Mars within ∼20–25 million years after Solar System formation, followed by crust formation ∼15 million years later. These ages are indistinguishable from the I–Pu–Xe age for the formation of Mars’ atmosphere, indicating that the major differentiation of Mars into mantle, crust, and atmosphere occurred between 20 and 40 million years after Solar System formation and, hence, earlier than previously inferred based on Sm–Nd chronometry alone.

^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.