M.M.M. Meier^a, B. Schmitz^b, A. Lindskog^a, C. Maden^c, R. Wieler^c

^aLund University, Department of Geology, Sölvegatan 12, SE-22362 Lund, Sweden
^bLund University, Department of Physics, SE-22100 Lund, Sweden
^cETH Zurich, Department of Earth Sciences, CH-8092 Zurich, Switzerland

We measured the He and Ne concentrations of 50 individual extraterrestrial chromite grains recovered from mid-Ordovician (lower Darriwilian) sediments from the Lynna River section near St. Petersburg, Russia. High concentrations of solar wind-like He and Ne found in most grains indicate that they were delivered to Earth as micrometeoritic dust, while their abundance, stratigraphic position and major element composition indicate an origin related to the L chondrite parent body (LCPB) break-up event, 470 Ma ago. Compared to sediment-dispersed extraterrestrial chromite (SEC) grains extracted from coeval sediments at other localities, the grains from Lynna River are both highly concentrated and well preserved. As in previous work, in most grains from Lynna River, high concentrations of solar wind-derived He and Ne impede a clear quantification of cosmic-ray produced He and Ne. However, we have found several SEC grains poor in solar wind Ne, showing a resolvable contribution of cosmogenic ²¹Ne. This makes it possible, for the first time, to determine robust cosmic-ray exposure (CRE) ages in these fossil micrometeorites, on the order of a few hundred-thousand years. These ages are similar to the CRE ages measured in chromite grains from cm-sized fossil meteorites recovered from coeval sediments in Sweden. As the CRE ages are shorter than the orbital decay time of grains of this size by Poynting-Robertson drag, this suggests that the grains were delivered to Earth through direct injection into an orbital resonance. We demonstrate how CRE ages of fossil micrometeorites can be used, in principle, to determine sedimentation rates, and to correlate the sediments at Lynna River with the fossil meteorite-bearing sediment layers in Sweden. In some grains with high concentrations of solar wind Ne, we nevertheless find a well-resolved cosmogenic ²¹Ne signal. These grains must have been exposed for up to several 10 Ma in the regolith layer of the pre-break-up L chondrite parent body. This confirms an earlier suggestion that such regolith grains should be abundant in sediments deposited shortly after the break-up of the LCPB asteroid.

Reference
Meier MMM, Schmitz B, Lindskog A, Maden C and Wieler R (in press) Cosmic-ray exposure ages of fossil micrometeorites from mid-Ordovician sediments at Lynna River, Russia. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.10.026]
Copyright Elsevier

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E. Gnos^1,*, B. A. Hofmann², M. A. Halawani³, Y. Tarabulsi³, M. Hakeem³, M. Al Shanti³, N. D. Greber^2,4, S. Holm⁵, C. Alwmark⁵, R. C. Greenwood⁶, K. Ramseyer⁴

¹Natural History Museum of Geneva, Geneva 6, Switzerland
²Natural History Museum Bern, Bern, Switzerland
³Saudi Geological Survey, Jeddah, Saudi Arabia
⁴Institute of Geological Sciences, University of Bern, Bern, Switzerland
⁵Department of Geology, Lund University, Lund, Sweden
⁶Planetary and Space Sciences, The Open University, Milton Keynes, UK

The very young Wabar craters formed by impact of an iron meteorite and are known to the scientific community since 1933. We describe field observations made during a visit to the Wabar impact site, provide analytical data on the material collected, and combine these data with poorly known information discovered during the recovery of the largest meteorites. During our visit in March 2008, only two craters (Philby-B and 11 m) were visible; Philby-A was completely covered by sand. Mapping of the ejecta field showed that the outcrops are strongly changing over time. Combining information from different visitors with our own and satellite images, we estimate that the large seif dunes over the impact site migrate by approximately 1.0–2.0 m yr⁻¹ southward. Shock lithification took place even at the smallest, 11 m crater, but planar fractures (PFs) and undecorated planar deformation features (PDFs), as well as coesite and stishovite, have only been found in shock-lithified material from the two larger craters. Shock-lithified dune sand material shows perfectly preserved sedimentary structures including cross-bedding and animal burrows as well as postimpact structures such as open fractures perpendicular to the bedding, slickensides, and radiating striation resembling shatter cones. The composition of all impact melt glasses can be explained as mixtures of aeolian sand and iron meteorite. We observed a partial decoupling of Fe and Ni in the black impact glass, probably due to partitioning of Ni into unoxidized metal droplets. The absence of a Ca-enriched component demonstrates that the craters did not penetrate the bedrock below the sand sheet, which has an estimated thickness of 20–30 m.

Reference
Gnos E, Hofmann BA, Halawani MA, Tarabulsi Y, Hakeem M, Al Shanti M, Greber ND, Holm S, Alwmark C, Greenwood RC and Ramseyer K (in press) The Wabar impact craters, Saudi Arabia, revisited. Meteoritics & Planetary Science
[doi:10.1111/maps.12218]
Published by arrangement with John Wiley & Sons

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Indrani Banerjee and Banibrata Mukhopadhyay

Department of Physics, Indian Institute of Science, Bangalore 560 012, India

We investigate nucleosynthesis inside the outflows from gamma-ray burst (GRB) accretion disks formed by the Type II collapsars. In these collapsars, massive stars undergo core collapse to form a proto-neutron star initially, and a mild supernova (SN) explosion is driven. The SN ejecta lack momentum, and subsequently this newly formed neutron star gets transformed to a stellar mass black hole via massive fallback. The hydrodynamics and the nucleosynthesis in these accretion disks have been studied extensively in the past. Several heavy elements are synthesized in the disk, and much of these heavy elements are ejected from the disk via winds and outflows. We study nucleosynthesis in the outflows launched from these disks by using an adiabatic, spherically expanding outflow model, to understand which of these elements thus synthesized in the disk survive in the outflow. While studying this, we find that many new elements like isotopes of titanium, copper, zinc, etc., are present in the outflows. ⁵⁶Ni is abundantly synthesized in most of the cases in the outflow, which implies that the outflows from these disks in a majority of cases will lead to an observable SN explosion. It is mainly present when outflow is considered from the He-rich, ⁵⁶Ni/⁵⁴Fe-rich zones of the disks. However, outflow from the Si-rich zone of the disk remains rich in silicon. Although emission lines of many of these heavy elements have been observed in the X-ray afterglows of several GRBs by Chandra, BeppoSAX, XMM-Newton, etc., Swift seems to have not yet detected these lines.

Reference
Banerjee I and Mukhopadhyay B (2013) Nucleosynthesis in the Outflows Associated with Accretion Disks of Type II Collapsars. The Astrophysical Journal 778:8.
[doi:10.1088/0004-637X/778/1/8]

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Mark H. Thiemens

Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093-0356

Stable isotope ratio variations are regulated by physical and chemical laws. These rules depend on a relation with mass differences between isotopes. New classes of isotope variation effects that deviate from mass dependent laws, termed mass independent isotope effects, were discovered in 1983 and have a wide range of applications in basic chemistry and nature. In this special edition, new applications of these effects to physical chemistry, solar system origin models, terrestrial atmospheric and biogenic evolution, polar paleo climatology, snowball earth geology, and present day atmospheric sciences are presented.

Reference
Thiemens MH (2013) Introduction to Chemistry and Applications in Nature of Mass Independent Isotope Effects Special Feature. PNAS 110:17631-17637.
[doi:10.1073/pnas.1312926110]

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Subrata Chakraborty^a,*, Teresa L. Jackson^a, Musahid Ahmed^b, and Mark H. Thiemens^a

^aDepartment of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093-0356
^bChemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

Select meteoritic classes possess mass-independent sulfur isotopic compositions in sulfide and organic phases. Photochemistry in the solar nebula has been attributed as a source of these anomalies. Hydrogen sulfide (H₂S) is the most abundant gas-phase species in the solar nebula, and hence, photodissociation of H₂S by solar vacuum UV (VUV) photons (especially by Lyman-α radiation) is a relevant process. Because of experimental difficulties associated with accessing VUV radiation, there is a paucity of data and a lack of theoretical basis to test the hypothesis of a photochemical origin of mass-independent sulfur. Here, we present multiisotopic measurements of elemental sulfur produced during the VUV photolysis of H₂S. Mass-independent sulfur isotopic compositions are observed. The observed isotopic fractionation patterns are wavelength-dependent. VUV photodissociation of H₂S takes place through several predissociative channels, and the measured mass-independent fractionation is most likely a manifestation of these processes. Meteorite sulfur data are discussed in light of the present experiments, and suggestions are made to guide future experiments and models.

Reference
Chakraborty S, Jackson TL, Ahmed M and Thiemens MH (2013) Sulfur isotopic fractionation in vacuum UV photodissociation of hydrogen sulfide and its potential relevance to meteorite analysis. PNAS 110:17650-17655.
[doi:10.1073/pnas.1213150110]

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Paula Lindgren^a, Mark C. Price^b, Martin R. Lee^a, Mark J. Burchell^b

^aSchool of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK
^bSchool of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NZ, UK

Calcite twinning in carbonaceous chondrite meteorites can be used to reconstruct the deformation history and the parent body environment during and/or after aqueous alteration, but the shock pressure threshold at which the twins develop is unknown. Accordingly, the aim of this study is to determine the magnitude of shock pressure that is needed to generate calcite twins. This was done by measuring the depths of twinning beneath the resulting craters from experimental impacts in six calcite targets, combined with hydrocode modelling of the peak pressures at the corresponding depths within the targets. Brecciation, fracturing and calcite e-twinning occur below the floors of all the craters and results from the hydrocode modelling show that the twins start to form at shock pressures of ~110 to 480 MPa, which is at least a factor of ten higher than the 10 MPa that is considered to be required to produce calcite twins in low strain rate terrestrial settings. These pressures are equivalent to shock stage S1 as determined by olivine microstructures and consistent with calcite twinning in carbonaceous chondrites being a result of impact gardening in shallow levels of their asteroidal parent bodies.

Reference
Lindgren P, Price MC, Lee MR and Burchell MJ (in press) Constraining the pressure threshold of impact induced calcite twinning: Implications for the deformation history of aqueously altered carbonaceous chondrite parent bodies. Earth and Planetary Science Letters 384:71–80.
[doi:10.1016/j.epsl.2013.10.002]
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Noam R. Izenberg^a, Rachel L. Klima^a, Scott L. Murchie^a, David T. Blewett^a, Gregory M. Holsclaw^b, William E. McClintock^b, Erick Malaret^c, Calogero Mauceri^c, Faith Vilas^d, Ann L. Sprague^e, Jörn Helbert^f, Deborah L. Domingue^d, James W. Head III^g, Timothy A. Goudge^g, Sean C. Solomon^h,i, Charles A. Hibbitts^a, M. Darby Dyar^j

^aPlanetary Exploration Group, The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
^bLaboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA
^cApplied Coherent Technology Corporation, Herndon, VA 20170, USA
^dPlanetary Science Institute, Tucson, AZ 85719, USA
^eLunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
^fInstitute of Planetary Research, Deutsches Zentrum für Luft und Raumfahrt, D-12489 Berlin, Germany
^gDepartment of Geological Sciences, Brown University, Providence, RI 02912, USA
^hDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
ⁱLamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA
^jDepartment of Astronomy, Mount Holyoke College, South Hadley, MA 01075, USA

The MESSENGER spacecraft’s Mercury Atmospheric and Surface Composition Spectrometer (MASCS) obtained more than 1.6 million reflectance spectra of Mercury’s surface from near-ultraviolet to near-infrared wavelengths during the first year of orbital operations. A global analysis of spectra in the wavelength range 300–1450 nm shows little regional variation in absolute reflectance or spectral slopes and a lack of mineralogically diagnostic absorptions. In particular, reflectance spectra show no clear evidence for an absorption band centered near 1 μm that would be associated with the presence of ferrous iron in silicates. There is, however, evidence for an ultraviolet absorption possibly consistent with a very low iron content (2–3 wt% FeO or less) in surface silicates and for the presence of small amounts of metallic iron or other opaque minerals in the form of nano- or micrometer-sized particles. These findings are consistent with MESSENGER X-ray and gamma-ray measurements of Mercury’s surface iron abundance. Although X-ray and gamma-ray observations indicate higher than expected quantities of sulfur on the surface, reflectance spectra show no absorption bands diagnostic of sulfide minerals. Whereas there is strong evidence of water ice in permanently shadowed craters near Mercury’s poles, MASCS spectra provide no evidence for hydroxylated materials near permanently shadowed craters.

Reference
Izenberg NR, Klima RL, Murchie SL, Blewett DT, Holsclaw GM, McClintock WE, Malaret E, Mauceri C, Vilas F, Sprague AL, Helbert J, Domingue DL, Head III JW, Goudge TA, Solomon SC, Hibbitts CA and Dyar MD (in press) The low-iron, reduced surface of Mercury as seen in spectral reflectance by MESSENGER. Icarus
[doi:10.1016/j.icarus.2013.10.023]
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J. M. Paque^1,*, S. R. Sutton², S. B. Simon², J. R. Beckett¹, D. S. Burnett¹, L. Grossman^2,3, H. Yurimoto⁴, S. Itoh⁴, H. C. Connolly Jr.^5,6,7,8

¹Division of Geological and Planetary Sciences, Caltech, Pasadena, California, USA
²Department of Geophysical Sciences, The University of Chicago, Chicago, Illinois, USA
³Enrico Fermi Institute, The University of Chicago, Chicago, Illinois, USA
⁴Department of Natural History Science, Hokkaido University, Sapporo, Hokkaido, Japan
⁵Department of Physical Sciences, Kingsborough Community College of the City University of New York, Brooklyn, New York, USA
⁶Department of Earth and Environmental Sciences, The Graduate Center of the City University of New York, New York, New York, USA
⁷Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
8Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA

Ti valence measurements in MgAl₂O₄ spinel from calcium-aluminum-rich inclusions (CAIs) by X-ray absorption near-edge structure (XANES) spectroscopy show that many spinels have predominantly tetravalent Ti, regardless of host phases. The average spinel in Allende type B1 inclusion TS34 has 87% Ti⁺⁴. Most spinels in fluffy type A (FTA) inclusions also have high Ti valence. In contrast, the rims of some spinels in TS34 and spinel grain cores in two Vigarano type B inclusions have larger amounts of trivalent titanium. Spinels from TS34 have approximately equal amounts of divalent and trivalent vanadium. Based on experiments conducted on CAI-like compositions over a range of redox conditions, both clinopyroxene and spinel should be Ti⁺³-rich if they equilibrated with CAI liquids under near-solar oxygen fugacities. In igneous inclusions, the seeming paradox of high-valence spinels coexisting with low-valence clinopyroxene can be explained either by transient oxidizing conditions accompanying low-pressure evaporation or by equilibration of spinel with relict Ti⁺⁴-rich phases (e.g., perovskite) prior to or during melting. Ion probe analyses of large spinel grains in TS34 show that they are enriched in heavy Mg, with an average Δ²⁵Mg of 4.25 ± 0.028‰, consistent with formation of the spinel from an evaporating liquid. Δ²⁵Mg shows small, but significant, variation, both within individual spinels and between spinel and adjacent melilite hosts. The Δ²⁵Mg data are most simply explained by the low-pressure evaporation model, but this model has difficulty explaining the high Ti⁺⁴ concentrations in spinel.

Reference
Paque JM, Sutton SR, Simon SB, Beckett JR, Burnett DS, Grossman L, Yurimoto H, Itoh S and Connolly HC (in press) XANES and Mg isotopic analyses of spinels in Ca-Al-rich inclusions: Evidence for formation under oxidizing conditions. Meteoritics & Planetary Science
[doi:10.1111/maps.12216]
Published by arrangement with John Wiley & Sons

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Mariana V. Maziviero^1,*, Marcos A. R. Vasconcelos¹, Alvaro P. Crosta¹, Ana M. Góes², Wolf U. Reimold^3,4, Cleyton de C. Carneiro¹

¹Department of Geology and Natural Resources, Institute of Geosciences, University of Campinas, Campinas, São Paulo, Brazil
²Department of Sedimentary and Environmental Geology, Institute of Geosciences, University of São Paulo, São Paulo, Brazil
³Museum für Naturkunde, Leibniz Institute for Research on Evolution and Biodiversity, Berlin, Germany
⁴Humboldt-Universität zu Berlin, Berlin, Germany

Riachão, located at S7°42′/W46°38′ in Maranhão State, northeastern Brazil, is a complex impact structure of about 4.1 km diameter, formed in Pennsylvanian to Permian sedimentary rocks of the Parnaíba Basin sequence. Although its impact origin was already proposed in the 1970s, information on its geology and shock features is still scarce in the literature. We present here the main geomorphological and geological characteristics of the Riachão impact structure obtained by integrated geophysical and remote sensing analysis, as well as geological field work and petrographic analysis. The identified lithostratigraphic units consist of different levels of the Pedra de Fogo Formation and, possibly, the Piauí Formation. Our petrographic analysis confirms the presence of shock-diagnostic planar microdeformation structures in quartz grains of sandstone from the central uplift as evidence for an impact origin of the Riachão structure. The absence of crater-filling impact breccias and melt rocks, shatter cones, as well as the restricted occurrence of microscopic shock effects, suggests that intense and relatively deep erosion has occurred since crater formation.

Reference
Maziviero MV, Vasconcelos MAR, Crosta A., Góes AM, Reimold WU and de C Carneiro C (in press) Geology and impact features of Riachão structure, northern Brazil. Meteoritics & Planetary Science
[doi:10.1111/maps.12213]
Published by arrangement with John Wiley & Sons

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B.C. Johnson^a and H.J. Melosh^a,b

^aDepartment of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907
^bDepartment of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907

We present a model that describes the formation of melt droplets, melt fragments, and accretionary impact lapilli during a hypervelocity impact. Using the iSALE hydrocode, coupled to the ANEOS equation of state for silica, we create high-resolution two-dimensional impact models to track the motion of impact ejecta. We then estimate the size of the ejecta products using simple analytical expressions and information derived from our hydrocode models. Ultimately, our model makes predictions of how the size of the ejecta products depends on impactor size, impact velocity, and ejection velocity. In general, we find that larger impactor sizes result in larger ejecta products and higher ejection velocities result in smaller ejecta product sizes. We find that a 10 km diameter impactor striking at a velocity of 20 km/s creates millimeter scale melt droplets comparable to the melt droplets found in the Chicxulub ejecta curtain layer. Our model also predicts that melt droplets, melt fragments, and accretionary impact lapilli should be found together in well preserved ejecta curtain layers and that all three ejecta products can form even on airless bodies that lack significant volatile content. This prediction agrees with observations of ejecta from the Sudbury and Chicxulub impacts as well as the presence of accretionary impact lapilli in lunar breccia.

Reference
Johnson BC and Melosh HJ (in press) Formation of melt droplets, melt fragments, and accretionary impact lapilli during a hypervelocity impact. Icarus
[doi:10.1016/j.icarus.2013.10.022]
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