M. Darby Dyar¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Department of Astronomy, Mount Holyoke College, South Hadley, Massachusetts 01075, U.S.A.

Although Fe-sulfate minerals occur only rarely on Earth as alteration products of sulfidic basalts or in hydrothermal systems, multiple lines of evidence point to the importance of Fe- (and other) sulfate minerals on the surface of Mars. One such martian data set comes from the MIMOS II Mössbauer spectrometers on the Mars Exploration Rovers, which acquired hundreds of spectra from the martian surface at two locations. Interpretation of those spectra has been limited by the lack of a comprehensive set of laboratory analog spectra of the broad range of naturally occurring sulfate minerals. Accordingly, this study reports Mössbauer data of 98 samples representing 47 different sulfate mineral species, all containing six- or higher-coordinated Fe. The resultant Mössbauer parameters are related to the local polyhedral environment around the Fe cation in each mineral to explain variations in spectral characteristics. Results show that the size of the coordination polyhedron is the best predictor of quadrupole splitting, which increases with both octahedral volume and mean bond length. Species within groups of structurally similar minerals are shown to have comparable spectral peaks that generally fall within small ranges. Although coordination polyhedron geometry is not necessarily unique to any particular mineral species or group, Mössbauer data can be used to help constrain mineral identifications from martian spectra. The number of mineral species is large, but the range of crystal structures and hyperfine parameters may be small, so that in many cases, individual minerals cannot be uniquely fingerprinted. Examples would include quenstedtite, coquimbite, kornelite, and lausenite, which have indistinguishable spectra, as do apjohnite, bilinite, dietrichite, and römerite. Overlap of Mössbauer parameters is a particular complication for identification of Fe³⁺-rich phases because the range of Mössbauer parameters for Fe³⁺ in any coordination number is so small. Previous analyses of martian Mössbauer spectra reported the presence of jarosite (Klingelhöfer et al. 2004; Morris et al. 2004) and an unspecific ferric sulfate (Morris et al. 2008). New data presented here indicate that botryogen, metasideronatrite, and slavikite exhibit Mössbauer spectra similar to those attributed to jarosite at Meridiani Planum. Fibroferrite and rhomboclase have parameters similar to those observed at Arad Samra, and copiapite and parabutlerite could be present at Tyrone Mount Darwin and Berkner Island. Unique mineral identifications are generally not possible from Mössbauer data alone, particularly for paramagnetic phases, although combining Mössbauer results with other data sets enables a greater level of confidence in constraining mineralogy. This study provides a new expansive data set for future interpretation of iron sulfates on Mars.

Reference
Dyar et al. (2013) Mössbauer parameters of iron in sulfate minerals. American Mineralogist 98:1943-1965.
[doi:10.2138/am.2013.4604]
Copyright: The Mineralogical Society of America

Link to Article

Yoram Lithwick

Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA and Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Evanston, IL 60208, USA

When planetesimals begin to grow by coagulation, they first enter an epoch of runaway, during which the biggest bodies grow faster than all the others. The questions of how runaway ends and what comes next have not been answered satisfactorily. We show that runaway is followed by a new stage—the “trans-Hill stage”—that commences when the bodies that dominate viscous stirring (“big bodies”) become trans-Hill, i.e., when their Hill velocity matches the random speed of the small bodies they accrete. Subsequently, the small bodies’ random speed grows in lockstep with the big bodies’ sizes, such that the system remains in the trans-Hill state. Trans-Hill growth is crucial for determining the efficiency of growing big bodies, as well as their growth timescale and size spectrum. Trans-Hill growth has two sub-stages. In the earlier one, which occurs while the stirring bodies remain sufficiently small, the evolution is collisionless, i.e., collisional cooling among all bodies is irrelevant. The efficiency of forming big bodies in this collisionless sub-stage is very low, ~10α  1, where α ~ 0.005(a/AU)^–1 is the ratio between the physical size of a body and its Hill radius. Furthermore, the size spectrum is flat (equal mass per size decade, i.e., q = 4). This collisionless trans-Hill solution explains results from previous coagulation simulations for both the Kuiper Belt and the asteroid belt. The second trans-Hill sub-stage commences once the stirring bodies grow big enough (>α^–1 × the size of the accreted small bodies). After that time, collisional cooling among small bodies controls the evolution. The efficiency of forming big bodies rises and the size spectrum becomes more top heavy. Trans-Hill growth can terminate in one of two ways, depending on the sizes of the small bodies. First, mutual accretion of big bodies can become significant and conglomeration proceeds until half of the total mass is converted into big bodies. This mode of growth may explain the observed size distributions of small bodies in the solar system and is explored in our subsequent work. Second, if the big bodies’ orbits become separated by their Hill radius, oligarchy commences. This mode likely precedes the formation of fully fledged planets.

Reference
Lithwick Y (in press) After Runaway: The Trans-Hill Stage of Planetesimal Growth. The Astrophysical Journal 780:22.
[doi:10.1088/0004-637X/780/1/22]

Link to Article

F. Fröhlich^1,*, G. Poupeau^1,2, A. Badou¹, F. X. Le Bourdonnec², Y. Sacquin³, S. Dubernet², J. M. Bardintzeff^4,5, M. Véran⁶, D. C. Smith⁷, E. Diemer^†

¹Département de Préhistoire, Muséum National d’Histoire Naturelle, UMR CNRS, Paris, France
²Université de Bordeaux 3, CRP2A – UMR CNRS IRAMAT 5060, Pessac, France
³CEA/Saclay, DSM/Irfu, Gif-sur-Yvette Cedex, France
⁴Laboratoire de Pétrographie-Volcanologie/équipe Planétologie, Université Paris-Sud, UMR CNRS IDES 8148, Orsay Cédex, France
⁵Université de Cergy-Pontoise, IUFM, Cergy-Pontoise, France
⁶Département Histoire de la Terre, Muséum National d’Histoire Naturelle, USM 203, Paris, France
⁷Département Histoire de la Terre, Muséum National d’Histoire Naturelle, UMR CNRS 7202, Paris, France
^† Deceased

Results are presented of new geological observations and laboratory analyses on Libyan Desert Glass (LDG), a unique kind of impact glass found in Egypt, probably 28.5–29.4 million years in age. A new LDG occurrence has been discovered some 50 km southward of the main LDG occurrences in the Great Sand Sea. From Fourier transform infrared (FTIR) analysis, the molecular structure of LDG is refined and significant differences are shown between LDG specimens and other pure silica glasses (fulgurite, industrial fused quartz, and amorphous biogenic silica) that are related to differences in their structures. The slight variations observed here for the mean Si-O-Si angle between the different glasses are attributed to their thermal histories. With regard to the other glasses analyzed, the LDG infrared spectral parameters point to a higher ratio of discontinuities and defects in the tetrahedral (SiO₄) network. The quantitative mineralogical constitutions of sandstones and quartzites from the LDG geological setting were analyzed by FTIR. Cretaceous sandstones have a specific composition (about 90 wt% quartz, 10% dickite), clearly different from the Paleozoic ones (about 90 wt% quartz, but ≥7% kaolinite). It is shown that the reddish silts bearing the LDG are constituted mainly of microquartz enriched with dickite, whose particle size distribution is characteristic of fluvio-lacustrine deposits, probably Oligocene to Miocene in age. The target rocks, most probably quartz sand, resulted from the weathering (loss of the cementing microquartz) of the Cretaceous sandstones from the Gilf Khebir Plateau with deposition in a high-energy environment.

Reference
Fröhlich F, Poupeau G, Badou A, Le Bourdonnec FX, Sacquin Y, Dubernet S, Bardintzeff JM, Véran M, Smith DC and Diemer E (in press) Libyan Desert Glass: New field and Fourier transform infrared data. Meteoritics & Planetary Science 
[doi:10.1111/maps.12223]
Published by arrangement with John Wiley & Sons

Link to Article

Agnese Fazio^1,*, Massimo D’Orazio¹, Luigi Folco¹, Jérôme Gattacceca², Corinne Sonzogni²

¹Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy
²CNRS-IRD Aix-Marseille Université, Centre Européen de Recherche et d’Enseignement de Géosciences de l’Environnement (CEREGE), UM34, Aix-en-Provence, France

Northwest Africa (NWA) 6583 is a silicate-bearing iron meteorite with Ni = 18 wt%. The oxygen isotope composition of the silicates (∆′¹⁷O = −0.439 ‰) indicates a genetic link with the IAB-complex. Other chemical, mineralogical, and textural features of NWA 6583 are consistent with classification as a new member of the IAB-complex. However, some unique features, e.g., the low Au content (1.13 μg g⁻¹) and the extremely reducing conditions of formation (approximately −3.5 ∆IW), distinguish NWA 6583 from the known IAB-complex irons and extend the properties of this group of meteorites. The chemical and textural features of NWA 6583 can be ascribed to a genesis by impact melting on a parent body of chondritic composition. This model is also consistent with one of the most recent models for the genesis of the IAB-complex. Northwest Africa 6583 provides a further example of the wide lithological and mineralogical variety that impact melting could produce on the surface of a single asteroid, especially if characterized by an important compositional heterogeneity in space and time like a regolith.

Reference
Fazio A, D’Orazio M, Folco L, Gattacceca J and Sonzogni C (in press) The extremely reduced silicate-bearing iron meteorite Northwest Africa 6583: Implications on the variety of the impact melt rocks of the IAB-complex parent body. Meteoritics & Planetary Science 
[doi:10.1111/maps.12231]
Published by arrangement with John Wiley & Sons

Link to Article

Gabriele Giuli^1,*, Maria Rita Cicconi¹, Sigrid Griet Eeckhout², Christian Koeberl³, Billy P. Glass⁴, Giovanni Pratesi⁵, Mariangela Cestelli-Guidi⁶ and Eleonora Paris¹

¹School of Science and Technology, Geology Division, University of Camerino, Via Gentile III da Varano, 62032, Italy
²European Synchrotron Radiation Facility (ESRF), 6 rue Jules Horowitz, 38043 Grenoble, France
³Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria; and Natural History Museum, Burgring 7, A-1010 Vienna, Austria
⁴Department of Geological Sciences, University of Delaware, Newark, Delaware 19716, U.S.A.
⁵Dipartimento di Scienze della Terra, Università di Firenze, Via G. La Pira 4, 50121, Firenze, Italy
⁶Laboratori Nazionali Frascati, Istituto Nazionale Fisica Nucleare, Via Enrico Fermi, Frascati, Italy

Iron oxidation states and coordination numbers have been determined by micro-X-ray absorption near edge spectroscopy (XANES) on the cores of a large group of microtektites from the Australasian, Ivory Coast, and North American (NA) tektite strewn field. The North American microtektites used in this study have been collected from five sites at different distances from the source crater; most have SiO₂ content between 70 and 80 wt%. Accurate analysis of the pre-edge peak energy position and integrated area allowed determination of Fe³⁺/(Fe²⁺+Fe³⁺) ratios on all samples with an estimated error of ±0.05.
Microtektites from the Australasian and Ivory Coast strewn fields show low values of the Fe³⁺/(Fe²⁺+Fe³⁺) ratios, in fair agreement with tektites from the same strewn field. In contrast, microtektites from the North American strewn fields show a wide range of Fe³⁺/(Fe²⁺+Fe³⁺) ratios from 0.02 to ca. 0.61. Comparison of Fe oxidation state data with chemical composition do not show any relation between Fe³⁺/(Fe²⁺+Fe³⁺) ratios and Na, Ca, or K contents, thus suggesting that the high-Fe oxidation states are not the consequence of sea-water alteration.
The difference between the Fe oxidation state of tektites and microtektites from the North American strewn fields suggests that some factors in the formation of the North American microtektites were different than for the North American tektites and for microtektites in the other strewn fields.
Previous Fe oxidation state data on NA tektites strongly suggest that the wide range in Fe oxidation state we found on NA microtektites is not related to lateral heterogeneity of the target rocks. Despite a correlation between microtektite oxidation state and distance from the source crater, we maintain that Fe oxidation state is not related only to the microtektite droplet flight distance. This is in keeping with the fact that no significant variations in the Fe oxidation state have been found in microtektites from the Australasian strewn field, even for Australasian microtektites recovered in Antarctica. The Fe oxidation state in North American microtektites could be explained by interaction of melt droplets with a H₂O-rich vapor plumes generated during the impact. These data point out that some difference must exist between the thermal histories of microtektites and tektites from the NA strewn field. Moreover, microtektites from the NA strewn field show also distinctively higher oxidation states than those from Ivory Coast or the Australasian strewn fields.

Reference
Giuli G, Cicconi MR, Eeckhout SG, Koeberl C, Glass BP, Pratesi G, Cestelli-Guidi M and Paris E (2013) North American microtektites are more oxidized than tektites. American Mineralogist 98:1930-1937.
[doi:10.2138/am.2013.4505]
Copyright: The Mineralogical Society of America

Link to Article

Gernot Arp*, Claudia Kolepka, Klaus Simon, Volker Karius, Nicole Nolte, Bent T. Hansen

Georg-August-Universität Göttingen, Geowissenschaftliches Zentrum, Göttingen, Germany

The extent of impact-generated hydrothermal activity in the 24 km sized Ries impact structure has been controversially discussed. To date, mineralogical and isotopic investigations point to a restriction of hydrothermal activity to the impact-melt bearing breccias, specifically the crater-fill suevite. Here, we present new petrographic, geochemical, and isotopic data of postimpact carbonate deposits, which indicate a hydrothermal activity more extended than previously assumed. Specifically, carbonates of the Erbisberg, a spring mound located upon the inner crystalline ring of the crater, show travertine facies types not seen in any of the previously investigated sublacustrine soda lake spring mounds of the Ries basin. In particular, the streamer carbonates, which result from the encrustation of microbial filaments in subaerial spring effluents between 60 and 70 °C, are characteristic of a hydrothermal origin. While much of the primary geochemical and isotopic signatures in the mound carbonates have been obliterated by diagenesis, a postimpact calcite vein from brecciated gneiss of the subsurface crater floor revealed a flat rare earth element pattern with a clear positive Eu anomaly, indicating a hydrothermal fluid convection in the crater basement. Finally, the strontium isotope stratigraphic correlation of the travertine mound with the crater basin succession suggests a hydrothermal activity for about 250,000 yr after the impact, which would be much longer than previously assumed.

Reference
Arp G, Kolepka C, Simon K, Karius V, Nolte N and Hansen BT (in press) New evidence for persistent impact-generated hydrothermal activity in the Miocene Ries impact structure, Germany. Meteoritics & Planetary Science 
[doi:10.1111/maps.12235]
Published by arrangement with John Wiley & Sons

Link to Article

Fred J. Ciesla^1,*, Thomas M. Davison², Gareth S. Collins² and David P. O’Brien³

¹Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, 60637, USA
²Impact and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, London, UK
³Planetary Science Institute, Tucson, Arizona, USA

Collisions between planetesimals were common during the first approximately 100 Myr of solar system formation. Such collisions have been suggested to be responsible for thermal processing seen in some meteorites, although previous work has demonstrated that such events could not be responsible for the global thermal evolution of a meteorite parent body. At this early epoch in solar system history, however, meteorite parent bodies would have been heated or retained heat from the decay of short-lived radionuclides, most notably ²⁶Al. The postimpact structure of an impacted body is shown here to be a strong function of the internal temperature structure of the target body. We calculate the temperature–time history of all mass in these impacted bodies, accounting for their heating in an onion-shell–structured body prior to the collision event and then allowing for the postimpact thermal evolution as heat from both radioactivities and the impact is diffused through the resulting planetesimal and radiated to space. The thermal histories of materials in these bodies are compared with what they would be in an unimpacted, onion-shell body. We find that while collisions in the early solar system led to the heating of a target body around the point of impact, a greater amount of mass had its cooling rates accelerated as a result of the flow of heated materials to the surface during the cratering event.

Reference
Ciesla FJ, Davison TM, Collins GS and O’Brien DP (in press) Thermal consequences of impacts in the early solar system. Meteoritics & Planetary Science 
[doi:10.1111/maps.12236]
Published by arrangement with John Wiley & Sons

Link to Article

Alex Meshik^a, Charles Hohenberg^a, Olga Pravdivtseva^a and Donald Burnett^b

^aDepartment of Physics, Washington University, 1 Brookings Drive, Saint Louis, MO 63130, USA
^bGeological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

One of the major goals of the Genesis Mission was to refine our knowledge of the isotopic composition of the heavy noble gases in solar wind and, by inference, the Sun, which represents the initial composition of the solar system. This has now been achieved with permil precision: ³⁶Ar/³⁸Ar = 5.5005 ± 0.0040, ⁸⁶Kr/⁸⁴Kr = .3012 ± .0004, ⁸³Kr/⁸⁴Kr = .2034 ± .0002, ⁸²Kr/⁸⁴Kr = .2054 ± .0002, ⁸⁰Kr/⁸⁴Kr = .0412 ± .0002, ⁷⁸Kr/⁸⁴Kr = .00642 ± .00005, ¹³⁶Xe/¹³²Xe = .3001 ± .0006, ¹³⁴Xe/¹³²Xe = .3691 ± .0007, ¹³¹Xe/¹³²Xe = .8256 ± .0012,¹³⁰Xe/¹³²Xe = .1650 ± .0004, ¹²⁹Xe/¹³²Xe = 1.0405 ± .0010, ¹²⁸Xe/¹³²Xe = .0842 ± .0003, ¹²⁶Xe/¹³²Xe = .00416 ± .00009, and ¹²⁴Xe/¹³²Xe = .00491 ± .00007 (error-weighted averages of all published data). The Kr and Xe ratios measured in the Genesis solar wind collectors generally agree with the less precise values obtained from lunar soils and breccias, which have accumulated solar wind over hundreds of millions of years, suggesting little if any temporal variability of the isotopic composition of solar wind krypton and xenon. The higher precision for the initial composition of the heavy noble gases in the solar system allows (1) to confirm that, exept ¹³⁶Xe and ¹³⁴Xe, the mathematically derived U-Xe is equivalent to Solar Wind Xe and (2) to provide an opportunity for better understanding the relationship between the starting composition and Xe-Q (and Q-Kr), the dominant current “planetary” component, and its host, the mysterious phase-Q.

Reference
Meshik A, Hohenberg C, Pravdivtseva O and Burnett D (in press) Heavy Noble Gases in Solar Wind Delivered by Genesis Mission. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.11.030]
Copyright Elsevier

Link to Article

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. In the companion paper we studied in detail how observable comets are injected from the Oort cloud, when account is taken of the planetary perturbations. In the present paper we concentrate on how the cloud may evolve in the long term and also on the production of decoupled comets, which evolve into semi-major axes less than 1 000AU. Concerning the long-term evolution, we find that the largest stellar perturbations that may statistically be expected during the age of the Solar System induce a large scale migration of comets within the cloud. Thus, comets leave the inner parts, but the losses from the outer parts are even larger, so at the end of our simulations the Oort cloud is more centrally condensed than at the beginning. The decoupled comets, which form a source of centaurs and Halley type comets (roughly in the proportions of 70% and 30%, respectively), are mainly produced by planetary perturbations, Jupiter and Saturn being the most efficient. This effect is dependent on synergies with the Galactic tide and stellar encounters, bringing the perihelia of Oort cloud comets into the planetary region. The star-planet synergy has a large contribution due to the strong encounters that produce major comet showers. However, outside these showers a large majority of decouplings may be attributed to the tide-planet synergy.

Reference
Fouchard M, Rickman H, Froeschlé Ch and Valsecchi GB (in press) Planetary perturbations for Oort cloud comets: III. Evolution of the cloud and production of centaurs and Halley type comets. Icarus
[doi:10.1016/j.icarus.2013.11.034]
Copyright Elsevier

Link to Article

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

Link to Article