¹Lidia Pittarello, ²Gang Ji, ³Akira Yamaguchi, ²Dominique Schryvers, ⁴Vinciane Debaille,¹Philippe Claeys
¹Analytical, Environmental and Geo-Chemistry (AMGC), Earth System Science, Vrije Universiteit Brussel, Brussels, Belgium
²Electron Microscopy for Materials Science (EMAT), University of Antwerp, Antwerp, Belgium
³National Institute of Polar Research, Tachikawa, Tokyo, Japan
⁴Laboratoire G-Time (Géochimie: Tracage isotopique, minéralogique et élémentaire), Université Libre de Bruxelles, Brussels, Belgium

The study of shock metamorphism of olivine might help to constrain impact events in the history of meteorites. Although shock features in olivine are well known, so far, there are processes that are not yet completely understood. In shock veins, olivine clasts with a complex structure, with a ringwoodite rim and a dense network of lamellae of unidentified nature in the core, have been reported in the literature. A highly shocked (S5-6), L6 meteorite, Asuka 09584, which was recently collected in Antarctica by a Belgian–Japanese joint expedition, contains this type of shocked olivine clasts and has been, therefore, selected for detailed investigations of these features by transmission electron microscopy (TEM). Petrographic, geochemical, and crystallographic studies showed that the rim of these shocked clasts consists of an aggregate of nanocrystals of ringwoodite, with lower Mg/Fe ratio than the unshocked olivine. The clast’s core consists of an aggregate of iso-oriented grains of olivine and wadsleyite, with higher Mg/Fe ratio than the unshocked olivine. This aggregate is crosscut by veinlets of nanocrystals of olivine, with extremely low Mg/Fe ratio. The formation of the ringwoodite rim is likely due to solid-state, diffusion-controlled, transformation from olivine under high-temperature conditions. The aggregate of iso-oriented olivine and wadsleyite crystals is interpreted to have formed also by a solid-state process, likely by coherent intracrystalline nucleation. Following the compression, shock release is believed to have caused opening of cracks and fractures in olivine and formation of olivine melt, which has lately crystallized under postshock equilibrium pressure conditions as olivine.

Reference
Pittarello L, Ji G, Yamaguchi A, Schryvers D, Debaille V, Claeys P (2015) From olivine to ringwoodite: a TEM study of a complex process. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12441]

Published by arrangement with John Wiley & Sons

¹Tóth, J. et al. (>10*)
¹Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia
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We provide the circumstances and details of the fireball observation, search expeditions, recovery, strewn field, and physical characteristics of the Košice meteorite that fell in Slovakia on February 28, 2010. The meteorite was only the 15th case of an observed bolide with a recovered mass and subsequent orbit determination. Despite multiple eyewitness reports of the bolide, only three videos from security cameras in Hungary were used for the strewn field determination and orbit computation. Multiple expeditions of professionals and individual searchers found 218 fragments with total weight of 11.3 kg. The strewn field with the size of 5 × 3 km is characterized with respect to the space distribution of the fragments, their mass and size-frequency distribution. This work describes a catalog of 78 fragments, mass, size, volume, fusion crust, names of discoverers, geographic location, and time of discovery, which represents the most complex study of a fresh meteorite fall. From the analytical results, we classified the Košice meteorite as an ordinary H5 chondrite.

Reference
Tóth J et al. (2015) The Košice meteorite fall: Recovery and strewn field. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12447]

Published by arrangement with John Wiley & Sons

¹Rahat Khan, ¹Naoki Shirai, ¹Mitsuru Ebihara
¹Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan

Rare earth elements (REEs), Th, U and P were determined in 15 Rumuruti (R)-type chondrites and the Allende CV chondrite. Repeated analyses of Allende for REEs, Th and U by ICP-MS and P by ICP-AES, and comparisons of these data with literature values ensure high reproducibility (precision) and reliability (accuracy) of acquired data. CI-normalized REE abundances in R chondrites are slightly enriched in heavy REEs with a small, positive Ce anomaly, in contrast to Allende. CI-normalized Pr/Tm and Nd/Yb ratios show a positive correlation, suggesting the heterogeneous mixing of two components (CI-like and refractory-rich materials) during the accretion of the R chondrite parent body. A Ce anomaly, however, was likely homogeneously present in the nebula. A mean Th/U ratio of R chondrites is View the MathML source3.81±0.13(1σ), which is 5.1% higher than the CI ratio. Probably, the Th–U fractionation was inherited from the nebula from which the R chondrite parent body formed. Besides the Th–U fractionation, REEs and Th–U are heterogeneously fractionated in R chondrites, for which parent body processing is assumed to be the cause. A mean P content of R chondrites (1254 μg/g) is higher than for any ordinary chondrite and is close to the EL mean. There appears to be a negative correlation between P and REEs contents in R chondrites. It is probable that REEs were diluted by extraneously supplied, REEs-depleted and P-containing materials (schreibersite or metal). This process must have occurred heterogeneously during accretion so that the heterogeneity of P-containing materials was preserved in the R chondrite parent body and individual R chondrites.

Reference
Khan R, Shirai N, Ebihara M (2015) Chemical characteristic of R chondrites in the light of P, REEs, Th and U abundances. Earth and Planetary Science Letters 422, 18–27
Link to Article [doi:10.1016/j.epsl.2015.04.008]

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¹A.V. Andronikov, , ¹I.E. Andronikova, ¹D.H. Hill
¹Lunar and Planetary Laboratory, University of Arizona, 1415 North 6th Ave, Tucson, AZ, 85705, USA

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Reference
Andronikov AV, Andronikova IE, Hill DH (2015) Impact history of the Chelyabinsk meteorite: Electron microprobe and LA-ICP-MS study of sulfides and metals. Planetary and Space Science (in Press)
Link to Article [doi:10.1016/j.pss.2015.03.028]

¹Yogita Kadlag, ¹Harry Becker
¹Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstrasse 74-100, D-12249 Berlin, Germany

Abundances of highly siderophile elements (HSE: Re, platinum group elements and Au), chalcogens (Te, Se and S), 187Os/188Os and the major and minor elements Mg, Ca, Mn, Fe, Ni and Co were determined in the components of Sahara 97072 (EH3, find) and Kota Kota (EH3, find) in order to understand the element fractionation processes. In a 187Re-187Os isochron diagram, most magnetic components lie close to the 4.56 Ga IIIA iron meteorite isochron, whereas most other components show deviations from the isochron caused by late redistribution of Re, presumably during terrestrial weathering. Metal- and sulfide rich magnetic fractions and metal-sulfide nodules are responsible for the higher 187Os/188Os in bulk rocks of EH chondrites compared to CI chondrites. The HSE and chalcogens are enriched in magnetic fractions relative to slightly magnetic and nonmagnetic fractions and bulk compositions, indicating that Fe-Ni metal is the main host phase of the HSE in enstatite chondrites. HSE abundance patterns indicate mixing of two components, a CI chondrite like end member and an Au-enriched end member. Because of the decoupled variations of Au from those of Pd or the chalcogens, the enrichment of Au in EH metal cannot be due to metal-sulfide-silicate partitioning processes. Metal and sulfide rich nodules may have formed by melting and reaction of pre-existing refractory element rich material with volatile rich gas. A complex condensation and evaporation history is required to account for the depletion of elements having very different volatility than Au in EH chondrites. The depletions of Te relative to HSE, Se and S in bulk EH chondrites are mainly caused by the depletion of Te in metal. S/Se and S/Mn are lower than in CI chondrites in almost all components and predominantly reflect volatility-controlled loss of sulfur. The latter most likely occurred during thermal processing of dust in the solar nebula (e. g., during chondrule formation), followed by the non-systematic loss of S during terrestrial weathering.

Reference
Kadlag Y, Becker H (2015) Fractionation of Highly Siderophile and Chalcogen Elements in Components of EH3 Chondrites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.022]

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^1,2Giulio F.D. Solferino, ^1,3Gregor J. Golabek, ⁴Francis Nimmo, ¹Max W. Schmidt
¹Department of Earth Sciences, ETH Zurich, 8092 Zurich, Switzerland
²School of Biological, Earth and Environmental Sciences, University College Cork, Cork, Ireland
³Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
⁴Department of Earth and Planetary Sciences, University of California Santa Cruz, 95064 Santa Cruz, CA, USA

Despite their relatively simple mineralogical composition (olivine + Fe-Ni metal + FeS +/- pyroxene), the origin of pallasite meteorites remains debated. It has been suggested that catastrophic mixing of olivine fragments with Fe-(Ni)-S followed by various degrees of annealing could explain pallasites bearing solely or prevalently fragmented or rounded olivines. In order to verify this hypothesis, and to quantify the grain growth rate of olivine in a liquid metal matrix, we performed a series of annealing experiments on natural olivine plus synthetic Fe-S mixtures. The best explanation for the observed olivine grain size distributions (GSD) of the experiments are dominant Ostwald ripening for small grains followed by random grain boundary migration for larger grains. Our results indicate that olivine grain growth in molten Fe-S is significantly faster than in solid, sulphur-free metal. We used the experimentally determined grain growth law to model the coarsening of olivine surrounded by Fe-S melt in a 100 to 600 km radius planetesimal. In this model, an impact is responsible for the mixing of olivine and Fe-(Ni)-S. Numerical models suggest that annealing at depths of up to 50 km allow for (i) average grain sizes consistent with the observed rounded olivine in pallasites, (ii) a remnant magnetization of Fe-Ni olivine inclusions as measured in natural pallasites and (iii) for the metallographic cooling rates derived from Fe-Ni in pallasites. This conclusion is valid even if the impact occurs several millions of years after the differentiation of the target body was completed.

Reference
Solferino GFD, Golabek GJ, Nimmo F, Schmidt MW (2015) Fast grain growth of olivine in liquid Fe-S and the formation of pallasites with rounded olivine grains. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.020]

Copyright Elsevier

^1,2M. Schmieder, ¹E. Tohver, ²F. Jourdan, ^1,3S.W. Denyszyn, ⁴P.W. Haines
¹School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, 6009 WA, Australia
²Western Australian Argon Isotope Facility, Department of Applied Geology and John de Laeter Centre for Isotope Research, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
³Centre for Exploration Targeting, University of Western Australia, 35 Stirling Highway, Crawley, 6009 WA, Australia
⁴Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia

This study presents the first optical and scanning electron microscopic characterization and U–Pb SHRIMP dating results for zircon grains separated from the most likely autochthonous impact melt rock in the central domain of the large, ∼40–90 km eroded Ediacaran Acraman impact structure in South Australia. Microtextural characteristics define five zircon subtypes corresponding to different levels of progressive shock metamorphism, from virtually unshocked crystalline zircon grains that exhibit original magmatic zoning in cathodoluminescence images to fully granular zircons that have completely lost their primary zoning pattern and locally contain neocrystallized submicrometer-sized spots of ZrO2 (probably baddeleyite) that pseudomorph pre-impact zircon. The granular zircons correspond to the highest observed level of shock metamorphism and impact-induced recrystallization. ZrO2-bearing granular zircons indicate shock pressures in excess of ∼65–70 GPa, which are considerably higher than previous shock pressure estimates for the Acraman impactites. U–Pb systematics of untreated and chemically abraded melt rock zircons indicate that U–Pb ratios of the Acraman zircons were variably reset during impact. Weakly shocked crystalline grains yield ages on concordia at ∼1.59–1.60 Ga reflecting the magmatic age of the Gawler Range Volcanics. Only the entirely granular zircon population was apparently impact-reset, but based on an Ediacaran age from stratigraphic constraints on the ejecta layer, experienced significant post-impact Pb loss. The microcrystalline nature of granular zircons could have promoted Pb diffusion and α-recoil in post-impact time, as suggested by grain size-dependent diffusion and recoil modeling. A positive correlation of U concentration and shock level suggests that granularization might have preferentially occurred in initially U-rich, probably metamict, zircons. 40Ar/39Ar dating of a melted Yardea Dacite clast from the Acraman melt rock, as well as K-feldspar separated from shocked Yardea Dacite resulted in post-impact alteration plateau ages suggestive of hydrothermal events at ∼500 Ma and ∼450 Ma that selectively affected the impactites that outcrop in the central domain of the Acraman impact structure. Our study demonstrates that the Acraman impact is particularly difficult to date. In the absence of accurate and precise isotopic ages for Acraman, the Ediacaran ejecta-stratigraphic age of ∼635–541 Ma is considered the most reliable age constraint currently available for the timing of the large Acraman impact.

Reference
Schmieder M, Tohver T, Jourdan F, Denyszyn SW, Haines PW (2015) Zircons from the Acraman impact melt rock (South Australia): Shock metamorphism, U–Pb and 40Ar/39Ar systematics, and implications for the isotopic dating of impact Events. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.021]

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^1,2Etienne Médard, ²Max W. Schmidt, ^2,3Markus Wälle, ^4,5Nicole S. Keller, ³Detlef Günther
¹Laboratoire Magmas et Volcans, Université Blaise Pascal – CNRS – IRD, OPGC, 5 rue Kessler, 63038 Clermont Ferrand, France
²Institut für Geochemie und Petrologie, ETH Zürich, Clausiusstrasse 25, CH-8092 Zürich, Switzerland
³Laboratory of Inorganic Chemistry, ETH Zürich, Wolfgang-Pauli-Strasse 10, CH-8093 Zürich, Switzerland
⁴University of Iceland, Institute of Earth Sciences, Sturlugata 7, IS-101 Reykjavik, Iceland
⁵Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

High-pressure, high-temperature experiments have been performed at ∼1.2 GPa and 1360-2100 °C to investigate the partitioning of Pt between a silicate melt and a metallic melt. Our experiments indicate that nanonuggets encountered in previous experiments are experimental artifacts, formed at high temperature by oversaturation caused by high oxygen fugacity during the initial stages of an experiment. Experiments at high-acceleration using a centrifuging piston-cylinder show that nanonuggets can be removed by gravity during the experiment. Formation of nanonuggets can also be avoided by using initially reduced starting materials. The presence of iron is also a key element in reducing the formation of nanonuggets. Our nanonugget-free data are broadly consistent with previous nanonuggets-filtered data, and suggest that Pt partitioning becomes independent of oxygen fugacity below an oxygen fugacity of at least IW+2. Pt is thus possibly dissolved as a neutral species (or even an anionic species) at low fO2, instead of the more common Pt2+ species present at higher fO2. Due to low concentration, the nature of this species cannot be determined, but atomic Pt or Pt- are possible options. Under core-formation conditions, Pt partitioning between metal and silicate is mostly independent of oxygen fugacity, silicate melt composition, and probably also pressure. Partition coefficient during core formation can be expressed by the following equation: View the MathML sourcelogDPtMmetal/silicate=1.0348+14698/T (in weight units). Calculations indicate that the Pt content (and by extension the Highly Siderophile Elements content) of the Earth’s mantle cannot be explained by equilibrium partitioning during core formation, requiring further addition of HSE to the mantle. The mass of this late veneer is approximately 0.4% of the total mass of the Earth (or 0.6% of the mass of the mantle).

Reference
Médard E, Schmidt MW, Wälle M, Kellern NS, Günther D (2015) Platinum partitioning between metal and silicate melts: Core formation, late veneer and the nanonuggets issue. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.019]

Copyright Elsevier

¹N.E.B. Zellner, ²J.W. Delano
¹Department of Physics, Albion College, Albion, MI 49224 USA
²New York Center for Astrobiology, Department of Atmospheric and Environmental Sciences, University at Albany (SUNY), Albany, NY 12222 USA

Lunar impact glasses, which are quenched melts produced during cratering events on the Moon, have the potential to provide not only compositional information about both the local and regional geology of the Moon but also information about the impact flux over time. We present in this paper the results of 73 new 40Ar/39Ar analyses of well-characterized, inclusion-free lunar impact glasses and demonstrate that size, shape, chemical composition, fraction of radiogenic 40Ar retained, and cosmic ray exposure (CRE) ages are important for 40Ar/39Ar investigations of these samples. Specifically, analyses of lunar impact glasses from the Apollo 14, 16, and 17 landing sites indicate that retention of radiogenic 40Ar is a strong function of post-formation thermal history in the lunar regolith, size, and chemical composition. This is because the Ar diffusion coefficient (at a constant temperature) is estimated to decrease by ∼3-4 orders of magnitude with an increasing fraction of non-bridging oxygens, X(NBO), over the compositional range of most lunar impact glasses with compositions from feldspathic to basaltic. Based on these relationships, lunar impact glasses with compositions and sizes sufficient to have retained ∼90% of their radiogenic Ar during 750 Ma of cosmic ray exposure at time-integrated temperatures of up to 290K have been identified and are likely to have yielded reliable 40Ar/39Ar ages of formation. Additionally, ∼50% of the identified impact glass spheres have formation ages of ⩽500 Ma, while ∼75% of the identified lunar impact glass shards and spheres have ages of formation ⩽2000 Ma. Higher thermal stresses in lunar impact glasses quenched from hyperliquidus temperatures are considered the likely cause of poor survival of impact glass spheres, as well as the decreasing frequency of lunar impact glasses in general with increasing age. The observed age-frequency distribution of lunar impact glasses may reflect two processes: (i) diminished preservation due to spontaneous shattering with age; and (ii) preservation of a remnant population of impact glasses from the tail end of the terminal lunar bombardment having 40Ar/39Ar ages up to 3800 Ma. A protocol is described for selecting and analysing lunar impact glasses.

Reference
Zellner NEB, Delano JW (2015) 40Ar/39Ar ages of lunar impact glasses: Relationships among Ar diffusivity, chemical composition, shape, and size. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.013]

Copyright Elsevier

^1,2Marrocchi, Y., ^1,2Avice, G., ^1,2,3Estrade, N.
¹OTELo Department Université de Lorraine Vandoeuvre-lès-Nancy France
²CNRS, CRPG, UMR 7358 Vandoeuvre-lès-Nancy France
³PCIGR, EOS University of British Columbia Vancouver, British Columbia Canada

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Reference
Marrocchi Y, Avice G, Estrade N (2015) Multiple carriers of Q noble gases in primitive meteorites. Geophysical Research Letters (in Press)
Link to Article [DOI: 10.1002/2015GL063198]