Neyda M. Abreu^1,* and Emma S. Bullock²

¹Earth Science Program, Pennsylvania State University—Du Bois Campus, Du Bois, Pennsylvania, USA
²Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia, USA

We have studied the petrologic characteristics of sulfide-metal lodes, polymineralic Fe-Ni nodules, and opaque assemblages in the CR2 chondrite Graves Nunataks (GRA) 06100, one of the most altered CR chondrites. Unlike low petrologic type CR chondrites, alteration of metal appears to have played a central role in the formation of secondary minerals in GRA 06100. Differences in the mineralogy and chemical compositions of materials in GRA 06100 suggest that it experienced higher temperatures than other CR2 chondrites. Mineralogic features indicative of high temperature include: (1) exsolution of Ni-poor and Ni-rich metal from nebular kamacite; (2) formation of sulfides, oxides, and phosphates; (3) changes in the Co/Ni ratios; and (4) carbidization of Fe-Ni metal. The conspicuous absence of pentlandite may indicate that peak temperatures exceeded 600 °C. Opaques appear to have been affected by the action of aqueous fluids that resulted in the formation of abundant oxides, Fe-rich carbonates, including endmember ankerite, and the sulfide-silicate-phosphate scorzalite. We suggest that these materials formed via impact-driven metamorphism. Mineralogic features indicative of impact metamorphism include (1) the presence of sulfide-metal lodes; (2) the abundance of polymineralic opaque assemblages with mosaic-like textures; and (3) the presence of suessite. Initial shock metamorphism probably resulted in replacement of nebular Fe-Ni metal in chondrules and in matrix by Ni-rich, Co-rich Fe metal, Al-Ti-Cr-rich alloys, and Fe sulfides, while subsequent hydrothermal alteration produced accessory oxides, phosphates, and Fe carbonates. An extensive network of sulfide-metal veins permitted effective exchange of siderophile elements from pre-existing metal nodules with adjacent chondrules and matrix, resulting in unusually high Fe contents in these objects.

Reference
Abreu NM and Bullock ES (in press) Opaque assemblages in CR2 Graves Nunataks (GRA) 06100 as indicators of shock-driven hydrothermal alteration in the CR chondrite parent body. Meteoritics & Planetary Science 
[doi:10.1111/maps.12227]
Published by arrangement with John Wiley & Sons

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D. S. Burnett

Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA

The Genesis Discovery mission returned solar matter in the form of the solar wind with the goal of obtaining precise solar isotopic abundances (for the first time) and greatly improved elemental abundances. Measurements of the light noble gases in regime samples demonstrate that isotopes are fractionated in the solar wind relative to the solar photosphere. Theory is required for correction. Measurement of the solar wind O and N isotopes shows that these are very different from any inner solar system materials. The solar O isotopic composition is consistent with photochemical self-shielding. For unknown reasons, the solar N isotopic composition is much lighter than essentially all other known solar system materials, except the atmosphere of Jupiter. Ne depth profiling on Genesis materials has demonstrated that Ne isotopic variations in lunar samples are due to isotopic fractionation during implantation without appealing to higher energy solar particles. Genesis provides a precise measurement of the isotopic differences of Ar between the solar wind and the terrestrial atmosphere. The Genesis isotopic compositions of Kr and Xe agree with data from lunar ilmenite separates, showing that lunar processes have not affected the ilmenite data and that solar wind composition has not changed on 100 Ma time scales. Relative to Genesis solar wind, ArKrXe in Q (the chondrite noble gas carrier) and the terrestrial atmosphere show relatively large light isotope depletions.

Reference
Burnett DS (in press) The Genesis solar wind sample return mission: Past, present, and future. Meteoritics & Planetary Science
[doi:10.1111/maps.12241]
Published by arrangement with John Wiley & Sons

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Yoshihiro Furukawa Taro Samejima, Hiromoto Nakazawa and Takeshi Kakegawa

Department of Earth Science, Graduate School of Science, Tohoku University

Late heavy bombardment (LHB) of extraterrestrial objects supplied carbon with metals to the prebiotic Earth. The early oceans were the major target of these impacts, followed by interactions among the atmosphere, oceanic water, and meteorite constituent materials under high-temperature and high-pressure conditions. Post-impact reactions of these hypervelocity impacts have the potential to produce reduced volatiles and organic compounds, including amino acids. Therefore, understanding the reactions in post-impact plumes is of great importance for the investigation of prebiotic organic compounds. The composition of post-impact plumes has been investigated with thermochemical calculations. However, experimental evidence is still needed to understand the reactions in dynamic systems of post-impact plumes. The present study investigates the effects of reaction temperature and availability of water on products from iron, nickel, graphite, nitrogen, and water in a dynamic gas flow system to investigate reactions in post-impact plumes. Results of this study indicate the formation of CO, H₂, NH₃, and HCN by hypervelocity oceanic impacts of iron-rich extraterrestrial objects. The formation of methane was limited in the present experiments, suggesting that the quenching rate is an influential factor for methane formation in post-impact plumes. Availability of water vapor in the plume was also an influential factor for the formation of reduced volatiles that controlled the CO formation rate from graphite. These results provide experimental evidence for the formation of reduced volatiles in post-impact plumes, which influenced the formation of pre-biotic organic compounds.

Reference
Furukawa Y, Samejima T, Nakazawa H and Kakegawa T (in press) Experimental investigation of reduced volatile formation by high-temperature interactions among meteorite constituent materials, water, and nitrogen. Icarus
[doi:10.1016/j.icarus.2013.11.033]
Copyright Elsevier

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M. Fouchard^a, H. Rickman^b,c, Ch. Froeschlé^d, G.B. Valsecchi^e,f

^aLAL-IMCCE, Université de Lille 1, 1 Impasse de l’Observatoire, F-59000 Lille, France
^bPAS Space Research Center, Bartycka 18A, PL-00-716, Warszawa, Poland
^cDept. of Physics & Astronomy, Uppsala Univ., Box 516, SE-75120 Uppsala, Sweden
^dObservatoire de la Côte d’Azur, UMR Lagrange 7293, Bv. de l’Observatoire, B.P. 4229, F-06304 Nice cedex 4, France
^eIAPS, INAF, via Fosso del Cavaliere 100, I-00133 Roma, Italy
^fIFAC-CNR, Via Madonna del Piano 10, I-50019 Sesto Fiorentino (FI), Italy

We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard et al. 2013b), the planetary perturbations experienced each time the comets penetrate to within 50 AU of the Sun. Each simulation involves an initial sample of four million comets and extends over a maximum of 5 Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. We concentrate on the production of observable comets, reaching for the first time a perihelion within 5 AU of the Sun. We distinguish between four categories, depending on whether the comet jumps across, or creeps through, the Jupiter-Saturn barrier (perihelion distances between 5 and 15 AU), and whether the orbit leading to the observable perihelion is preceded by a major planetary perturbation or not. For reasons explained in the paper, we call the strongly perturbed comets “Kaib-Quinn comets”.
We thus derive a synthetic picture of the Oort spike, from which we draw two main conclusions regarding the full dynamical model. One is that 2/3 of the observable comets are injected with the aid of a planetary perturbation at the previous perihelion passage, and about half of the observable comets are of the Kaib-Quinn type. The other is that the creepers dominate over the jumpers. Due to this fact, the spike peaks at only 31 000AU, and the majority of new comets have semi-major axes less than this value. The creepers show a clear preference for retrograde orbits as a consequence of the need to avoid untimely, planetary ejection before becoming observable. Thus, the new comets should have a 60/40 preference for retrograde against prograde orbits in apparent conflict with observations. However, both these and other results depend on our model assumptions regarding the initial structure of the Oort cloud, which is isotropic in shape and has a relatively steep energy distribution. We also find that they depend on the details of the past history of external perturbations including GMC encounters, and we provide special discussions of those issues.

Reference
Fouchard M, Rickman H, Froeschlé Ch and Valsecchi GB (in press) Planetary perturbations for Oort cloud comets: II. Implications for the origin of observable comets. Icarus
[doi:10.1016/j.icarus.2013.11.032]
Copyright Elsevier

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