• Petrographic comparison of refractory inclusions from different chemical groups of chondrites

      Lin, Y.; Kimura, M.; Miao, B.; Dai, D.; Monoi, A. (The Meteoritical Society, 2006-01-01)
      Twenty-four refractory inclusions (40-230 micrometers, with average of 86 +/- 40 micrometers) were found by X-ray mapping of 18 ordinary chondrites. All inclusions are heavily altered, consisting of finegrained feldspathoids, spinel, and Ca-pyroxene with minor ilmenite. The presence of feldspathoids and lack of melilite are due to alteration that took place under oxidizing conditions as indicated by FeO-ZnO-rich spinel and ilmenite. The pre-altered mineral assemblages are dominated by two types: one rich in melilite, referred to as type A-like, and the other rich in spinel, referred to as spinel-pyroxene inclusions. This study and previous data show similar type and size distributions of refractory inclusions in ordinary and enstatite chondrites. A survey of refractory inclusions was also conducted on Allende and Murchison in order to make unbiased comparison with their counterparts in other chondrites. The predominant inclusions are type A and spinel-pyroxene, with average sizes of 170 +/- 130 micrometers (except for two mm-sized inclusions) in Allende and 150 +/- 100 micrometers in Murchison. The relatively larger sizes are partially due to common conglomerating of smaller nodules in both chondrites. The survey reveals closely similar type and size distributions of refractory inclusions in various chondrites, consistent with our previous data of other carbonaceous chondrites. The petrographic observations suggest that refractory inclusions in various groups of chondrites had primarily formed under similar processes and conditions, and were transported to different chondrite-accreting regions. Heterogeneous abundance and distinct alteration assemblages of refractory inclusions from various chondrites could be contributed to transporting processes and secondary reactions under different conditions.
    • A model for planetesimal meltdown by 26Al and its implications for meteorite parent bodies

      Hevey, Phonsie J.; Sanders, Ian S. (The Meteoritical Society, 2006-01-01)
      The melting of planetesimals heated by 26Al has been modelled using a new finite difference method that incorporates convection. As an example, we consider a planetesimal with a radius of 64 km, which accretes instantaneously at t = 0.75 Myr (after the formation of calcium-aluminum-rich inclusions) from cold (250 K) nebular dust with 50% porosity. At t = 0.9 Myr (T = 700 K), the planetesimal shrinks to a radius of 50 km due to sintering. At t = 1.2 Myr (T = 1425 K), the fully insulated interior, deeper than a few kilometers, starts to melt, and at t = 1.5 Myr (T = 1725 K), with 50% melting, convection starts. By t = 2 Myr, the planetesimal is a globe of molten, convecting slurry inside a thin residual crust. From about t = 2.5 Myr, the crust thickens rapidly as the power of 26Al fades.Planetesimals probably melt in this manner when they accrete before t = 1.3 Myr and are large enough to insulate themselves (R >20 km for accretion at t = 0, rising to >80 km at t = 1.3 Myr). Melting behavior will also be affected by the level of 60Fe in nebular dust, by the extent of devolatilization reactions and basalt segregation during heating, and by gradual accretion.The model suggests that a) the parent bodies of differentiated meteorites had accreted before about t = 1.5 to 2 Myr and before most chondritic parent bodies had formed, and b) that molten planetesimals may be a source for chondrule melt droplets.
    • Conditions in the protoplanetary disk as seen by the type B CAIs

      Richter, Frank M.; Mendybaev, Ruslan A.; Davis, Andrew M. (The Meteoritical Society, 2006-01-01)
      Type B coarse-grained calcium-aluminum-rich inclusions (CAIs) are the oldest known materials to have formed in the solar system and are a unique source of information regarding conditions and processes in the protoplanetary disk around the young sun. Recent experimental results on the crystallization and evaporation of type B-like silicate melts allow us to place the following constraints on the conditions in the protoplanetary disk during the formation of type B CAIs. 1) Once type B CAIs precursors have been condensed from a solar composition gas, they were reheated at 1250-1450 degrees C, as is indicated by their igneous texture. 2) The melilite mantles characteristic of type B1 CAIs could be formed by crystallization of magnesium- and silicon-depleted melt in the outer part of the partially molten droplets. Such depletion can arise when evaporation is fast compared to chemical diffusion in the melt. This requires the pressure of the surrounding solar composition gas to be at least 10^(-4) bars during the initial crystallization of melilite mantle. Type B2 CAIs with uniform distribution of melilite are expected to form at pressures less than 10^(-5) bars. 3) Evaporation calculations are used to place bounds on the thermal history of the type B CAIs. Observed compositional zoning in melilite suggests that the temperatures in the protoplanetary disk where the type B CAIs resided after crystallization could not have exceeded ~1000 degrees C for more than a few tens of thousands of years. A recent calculation of the physical conditions associated with nebular shocks produced transient temperatures and gas pressures very much like what we find is required to melt reasonable CAI precursors and evaporate these sufficiently quickly to make a type B1 CAI.
    • Fabric analysis of Allende matrix using EBSD

      Watt, Lauren E.; Bland, Phil A.; Prior, Dave J.; Russell, Sara S. (The Meteoritical Society, 2006-01-01)
      Fabric analysis of the interstitial matrix material in primitive meteorites offers a novel window on asteroid formation and evolution. Electron backscatter diffraction (EBSD) has allowed fabrics in these fine-grained materials to be visualized in detail for the first time. Our data reveal that Allende, a CV3 chondrite, possesses a uniform, planar, short-axis alignment fabric that is pervasive on a broad scale and is probably the result of deformational shortening related to impact or gravitational compaction. Interference between this matrix fabric and the larger, more rigid components, such as dark inclusions (DIs) and calcium-aluminium-rich inclusions (CAIs), has lead to the development of locally oriented and intensified matrix fabrics. In addition, DIs possess fabrics that are conformable with the broader matrix fabric. These results suggest that DIs were in situ prior to the deformational shortening event responsible for these fabrics, thus providing an argument against dark inclusions being fragments from another lithified part of the asteroid (Kojima and Tomeoka 1996; Fruland et al. 1978). Moreover, both DIs and Allende matrix are highly porous (~25%) (Corrigan et al. 1997). Mobilizing a highly porous DI during impact-induced brecciation without imposing a fabric and incorporating it into a highly porous matrix without significantly compacting these materials is improbable. We favor a model that involves Allende DIs, CAIs, and matrix accreting together and experiencing the same deformation events.
    • In situ identification, pairing, and classification of meteorites from Antarctica through magnetic susceptibility measurements

      Folco, L.; Rochette, P.; Gattacceca, J.; Perchiazzi, N. (The Meteoritical Society, 2006-01-01)
      We report on the effectiveness of using magnetic measurements in the search for meteorites on the Antarctic ice sheet, which is thus far the Earth's most productive terrain. Magnetic susceptibility measurements carried out with a pocket meter (SM30) during the 2003/04 PNRA meteorite collection expedition to northern Victoria Land (Antarctica) proved to be a rapid, sensitive, non-destructive means for the in situ identification, pairing, and classification of meteorites. In blue ice fields characterized by the presence of moraines and glacial drifts (e.g., Miller Butte, Roberts Butte, and Frontier Mountain), magnetic susceptibility measurements allowed discrimination of meteorites from abundant terrestrial stones that look like meteorites thanks to the relatively high magnetic susceptibility of the former with respect to terrestrial rocks. Comparative measurements helped identify 16 paired fragments found at Johannessen Nunataks, thereby reducing unnecessary duplication of laboratory analyses and statistical bias. Following classifications schemes developed by us in this and previous works, magnetic susceptibility measurements also helped classify stony meteorites directly in the field, thereby providing a means for selecting samples with higher research priority. A magnetic gradiometer capable of detecting perturbations in the Earth's magnetic field induced by the presence of meteorites was an efficient tool for locating meteorites buried in snow along the downwind margin of the Frontier Mountain blue ice field. Based on these results, we believe that magnetic sensors should constitute an additional payload for robotic search for meteorites on the Antarctic ice sheet and, by extension, on the surface of Mars where meteorite accumulations are predicted by theoretical works. Lastly, magnetic susceptibility data was successfully used to cross-check the later petrographic classification of the 123 recovered meteorites, allowing the detection of misclassified or peculiar specimens.
    • Stony meteorite characterization by non-destructive measurement of magnetic properties

      Smith, D. L.; Ernst, R. E.; Samson, C.; Herd, R. (The Meteoritical Society, 2006-01-01)
      Four parameters of low-field magnetic susceptibility (bulk value, frequency dependence, degree of anisotropy, and ellipsoid shape) have been determined for 321 stony meteorites from the National Collection of Canada. These parameters provide a basis for rapid, non-destructive, and accurate meteorite classification as each meteorite class tends to have a distinct range of values. Chondrites show a clear trend of increasing bulk susceptibility from LL to L to H to E within the 3.6 to 5.6 log-Chi (in 10^(-9) m^3/kg) range, reflecting increasing Fe-Ni metal and Fe-Ni sulfide content. Achondrite values range in log-Chi from 2.4 to 4.7 and primitive achondrites from 4.2 to 5.7. Frequency dependence is observed, using 19,000 Hz and 825 Hz, with variations in strength among meteorite classes and individual specimen dependence ranging from 1-25.6%. Degrees of anisotropy range from 1 to 53% with both oblate and prolate ellipsoids present. The aubrite class is marked by high degrees of anisotropy, low bulk magnetic susceptibility, and prolate fabric. Camel Donga is set apart from other eucrites, marked by higher bulk susceptibility, degree of anisotropy, and magnitude of oblate ellipsoid shape. The Shergotty, Nakhla, and Chassigny (SNC) meteorites show subclass distinction using frequency dependence and Chassigny is set apart with a relatively strong oblate fabric. The presence of both strong oblate and prolate fabrics among and within meteorite classes of chondritic and achondritic material points to a complex, multi-mechanism origin for anisotropy, more so than previously thought, and likely dominated by impact processes in the later stages of stony parent body formation.
    • Extraterrestrial chromite in Middle Ordovician marine limestone at Kinnekulle, southern Sweden—Traces of a major asteroid breakup event

      Schmitz, Birger; Häggström, Therese (The Meteoritical Society, 2006-01-01)
      The distribution of sediment-dispersed extraterrestrial chromite grains and other Cr-rich spinels (>63 micrometers) has been studied in Middle Ordovician Orthoceratite Limestone from two quarries at Kinnekulle, southern Sweden. In the Thorsberg quarry, an 3.2 m thick sequence of beds previously shown to be rich in fossil meteorites is also rich in sediment-dispersed extraterrestrial chromite grains. Typically, 1-3 grains are found per kilogram of limestone. In the nearby Hällekis quarry, the same beds show similarly high concentrations of extraterrestrial chromite grains, but in samples representing the 9 m downward continuation of the section exposed at this site, only 5 such grains were found in a total of 379 kg of limestone. The extraterrestrial (equilibrated ordinary chondritic) chromite grains can be readily distinguished by a homogeneous and characteristic major element chemistry, including 2.0-3.5 wt% TiO2 and stable V2O3 concentrations close to 0.7 wt%. Terrestrial Cr-rich spinels have a wide compositional range and co-exist with extraterrestrial chromite in some beds. These grains may be derived, for example, from mafic dykes exposed and weathered at the sea floor.Considering lithologic and stratigraphic aspects variations in sedimentation rate cannot explain the dramatic increase in extraterrestrial chromite seen in the upper part of the composite section studied. Instead, the difference may be primarily related to an increase in the ancient flux of extraterrestrial matter to Earth in connection with the disruption of the L chondrite parent body in the asteroid belt at about this time. The coexistence in some beds of high concentrations of chondritic chromite and terrestrial Cr-rich spinels, however, indicates that redistribution of heavy minerals on the sea floor, related to changes in sea level and sea-floor erosion and currents, must also be considered.
    • Density, magnetic susceptibility, and the characterization of ordinary chondrite falls and showers

      Consolmagno, G. J.; Macke, R. J.; Rochette, P.; Britt, D. T.; Gattacceca, J. (The Meteoritical Society, 2006-01-01)
      Bulk and grain densities of 132 ordinary chondrites from the Vatican Observatory collection were measured and compared with their magnetic susceptibility (for the most part using previously measured values; ten new susceptibility measures were taken for this study). Grain density and magnetic susceptibility combined provide a reliable method of classifying un weathered ordinary chondrites. Unlike traditional chemical tests, this method is fast, nondestructive, and characterizes the whole rock, making it especially appropriate for surveying large collections. The system is less viable for finds; extensive weathering of metallic iron in an H chondrite can cause it to plot among L chondrites, while heavily weathered L chondrites plot among the LL group. This system has revealed outlier stones that may be misclassified meteorites or mislabeled samples; in every case where the magnetic susceptibility of a meteorite does not fit its putative classification, the grain density is also found to be in disagreement in a manner consistent with either severe weathering or misidentification. An analysis of stones from five showers shows that, excluding outliers, these samples tend to cluster tightly within their appropriate groups in a plot of grain versus magnetic susceptibility.
    • Stratigraphy and evolution of basalts in Mare Humorum and southeastern Procellarum

      Hackwill, Terence; Guest, John; Spudis, Paul (The Meteoritical Society, 2006-01-01)
      We have studied the mare basalts of Mare Humorum and southeastern Procellarum (30 degrees W-50 degrees W, 0 degrees-40 degrees S). One hundred and nine basaltic units have been identified from differences in their FeO wt% and TiO2 wt% content, and variations in crater densities. Crater counting and reference to isotopically dated Apollo samples have provided an age for 33 major units. Some evidence for three distinct periods of volcanic activity has been found. We found that the large unit in the middle of Mare Humorum is the oldest in the basin. This supports the suggestion that the oldest central unit sank causing the lithosphere to bend and create dykes through which lava flowed to produce the outer units. No evidence of a trend in FeO wt% and TiO2 wt% content against time is found within Mare Humorum. There appears to be no lateral trend of basalts in terms of FeO and TiO2 wt% over the entire area with time. An increase in FeO content with time is found in the 33 major units and there is some evidence for an increase in TiO2 in the same units. A correlation between FeO wt% and TiO2 wt% content is evident when all 109 units are compared. A notable feature of this correlation is a sharp increase in gradient of TiO2 wt% content when the FeO wt% content rises above about 17%.
    • Australasian microtektites and associated impact ejecta in the South China Sea and the Middle Pleistocene supereruption of Toba

      Glass, Billy P.; Koeberl, Christian (The Meteoritical Society, 2006-01-01)
      Australasian microtektites were discovered in Ocean Drilling Program (ODP) Hole 1143A in the central part of the South China Sea. Unmelted ejecta were found associated with the microtektites at this site and with Australasian microtektites in Core SO95-17957-2 and ODP Hole 1144A from the central and northern part of the South China Sea, respectively. A few opaque, irregular, rounded, partly melted particles containing highly fractured mineral inclusions (generally quartz and some K feldspar) and some partially melted mineral grains, in a glassy matrix were also found in the microtektite layer. The unmelted ejecta at all three sites include abundant white, opaque grains consisting of mixtures of quartz, coesite, and stishovite, and abundant rock fragments which also contain coesite and, rarely, stishovite. This is the first time that shock-metamorphosed rock fragments have been found in the Australasian microtektite layer. The rock fragments have major and trace element contents similar to the Australasian microtektites and tektites, except for higher volatile element contents. Assuming that the Australasian tektites and microtektites were formed from the same target material as the rock fragments, the parent material for the Australasian tektites and microtektites appears to have been a fine-grained sedimentary deposit. Hole 1144A has the highest abundance of microtektites (number/cm^2) of any known Australasian microtektite-bearing site and may be closer to the source crater than any previously identified Australasian microtektite-bearing site. A source crater in the vicinity of 22 degrees N and 10 degrees E seems to explain geographic variations in abundance of both the microtektites and the unmelted ejecta the best; however, a region extending NW into southern China and SE into the Gulf of Tonkin explains the geographic variation in abundance of microtektites and unmelted ejecta almost as well. The size of the source crater is estimated to be 43 +/- 9 km based on estimated thickness of the ejecta layer at each site and distance from the proposed source. A volcanic ash layer occurs just above the Australasian microtektite layer, which some authors suggest is from a supereruption of the Toba caldera complex. We estimate that deposition of the ash occurred ~800 ka ago and that it is spread over an area of at least 3.7 x 10^7 km^2.
    • Analysis and survival of amino acids in Martian regolith analogs

      Garry, James R. C.; ten Kate, Inge Loe; Martins, Zita; Nørnberg, Per; Ehrenfreund, Pascale (The Meteoritical Society, 2006-01-01)
      We have investigated the native amino acid composition of two analogs of Martian soil, JSC Mars-1 and Salten Skov. A Mars simulation chamber has been built and used to expose samples of these analogs to temperature and lighting conditions similar to those found at low latitudes on the Martian surface. The effects of the simulated conditions have been examined using high-performance liquid chromatography (HPLC). Exposure to energetic ultraviolet (UV) light in vacuum appears to cause a modest increase in the concentration of certain amino acids within the materials, which is interpreted as resulting from the degradation of microorganisms. The influence of low temperatures shows that the accretion of condensed water on the soils leads to the destruction of amino acids, supporting the idea that reactive chemical processes involving H2O are at work within the Martian soil. We discuss the influence of UV radiation, low temperatures, and gaseous CO2 on the intrinsic amino acid composition of Martian soil analogs and describe, with the help of a simple model, how these studies fit within the framework of life detection on Mars and the practical tasks of choosingand using Martian regolith analogs in planetary research.
    • Monte Carlo simulation of GCR neutron capture production of cosmogenic nuclides in stony meteorites and lunar surface

      Kollr, D.; Michel, R.; Masarik, J. (The Meteoritical Society, 2006-01-01)
      A purely physical model based on a Monte Carlo simulation of galactic cosmic ray (GCR) particle interaction with meteoroids is used to investigate neutron interactions down to thermal energies. Experimental and/or evaluated excitation functions are used to calculate neutron capture production rates as a function of the size of the meteoroid and the depth below its surface. Presented are the depth profiles of cosmogenic radionuclides 36Cl, 41Ca, 60Co, 59Ni, and 129I for meteoroid radii from 10 cm up to 500 cm and a 2-pi irradiation. Effects of bulk chemical composition on n-capture processes are studied and discussed for various chondritic and lunar compositions. The mean GCR particle flux over the last 300 ka was determined from the comparison of simulations with measured 41Ca activities in the Apollo 15 drill core. The determined value significantly differs from that obtained using equivalent models of spallation residue production.
    • The fall and recovery of the Tagish Lake meteorite

      Hildebrand, Alan R.; McCausland, Phil J. A.; Brown, Peter G.; Longstaffe, Fred J.; Russell, Sam D. J.; Tagliaferri, Edward; Wacker, John F.; Mazur, Michael J. (The Meteoritical Society, 2006-01-01)
      The Tagish Lake C2 (ungrouped) carbonaceous chondrite fall of January 18, 2000, delivered 10 kg of one of the most primitive and physically weak meteorites yet studied. In this paper, we report the detailed circumstances of the fall and the recovery of all documented Tagish Lake fragments from a strewnfield at least 16 km long and 3 to 4 km wide. Nearly 1 kg of "pristine" meteorites were collected one week after the fall before new snow covered the strewnfield; the majority of the recovered mass was collected during the spring melt. Ground eyewitnesses and a variety of instrument-recorded observations of the Tagish Lake fireball provide a refined estimate of the fireball trajectory. From its calculated orbit and its similarity to the remotely sensed properties of the D- and P-class asteroids, the Tagish Lake carbonaceous chondrite apparently represents these outer belt asteroids. The cosmogenic nuclide results and modeled production indicate a prefall radius of 2.1-2.4 m (corresponding to 60-90 tons) consistent with the observed fireball energy release. The bulk oxygen-isotope compositions plot just below the terrestrial fractionation line (TFL), following a trend similar to the CM meteorite mixing line. The bulk density of the Tagish Lake material (1.64 +/- 0.02 g/cm^3) is the same, within uncertainty, as the total bulk densities of several C-class and especially D- and P-class asteroids. The high microporosity of Tagish Lake samples (~40%) provides an obvious candidate material for the composition of low bulk density primitive asteroids.
    • UV photolysis of quinoline in interstellar ice analogs

      Elsila, Jamie E.; Hammond, Matthew R.; Bernstein, Max P.; Sandford, Scott A.; Zare, Richard N. (The Meteoritical Society, 2006-01-01)
      The polycyclic aromatic nitrogen heterocycle (PANH) quinoline (C9H7N) was frozen at 20 K in interstellar ice analogs containing either pure water or water mixed with methanol or methane and exposed to ultraviolet (UV) radiation. Upon warming, the photolysis products were analyzed by high-performance liquid chromatography and nanoscale liquid chromatography-electrospray ionization mass spectrometry. A suite of hydroxyquinolines, which were formed by the addition of oxygen atoms to quinoline, was observed as the primary product in all the ices. Quinoline N oxide was not formed, but five hydroxyquinoline isomers were produced with no clear dominance of one isomer. Reduction products, formed by hydrogen atom addition, were also created. Ices created at 20 K with H2O: quinoline ratios of 10:1 to 100:1 showed similar product distributions to those at 122 K, with no apparent temperature or concentration dependence. Increasing the UV dose led to a decrease in overall yield, indicating that quinoline and its products may be photo-destroyed. Methylquinolines were formed upon photolysis of the methanol- and methane-containing ices. In addition, possible methoxyquinolines or quinoline methylene alcohols were formed in the methanolcontaining ice, while methylhydroxyquinolines were created in the methane-containing ice. This work indicates that oxidation of PANHs could occur in icy extraterrestrial environments and suggests that a search for such compounds in carbonaceous meteorites could illuminate the possible link between interstellar ice chemistry and meteoritic organics. Given the importance of oxidized and alkylated PANHs to biochemistry, the formation and delivery of such molecules to the early Earth may have played a role in the origin and evolution of life.
    • Formation of TiC core-graphitic mantle grains from CO gas

      Kimura, Yuki; Nuth, Joseph A.; Ferguson, Frank T. (The Meteoritical Society, 2006-01-01)
      We demonstrate a new formation route for TiC core-graphitic mantle spherules that does not require carbon-atom addition and the very long time scales associated with such growth (Bernatowicz et al. 1996). Carbonaceous materials can be formed from C2H2 and its derivatives, as well as from CO gas. In this paper, we will demonstrate that large-cage-structure carbon particles can be produced from CO gas by the Boudouard reaction. Since the sublimation temperature for such fullerenes is low, the large cages can be deposited onto previously nucleated TiC and produce TiC core-graphitic mantle spherules. New constraints for the formation conditions and the time scale for the formation of TiC core-graphitic mantle spherules are suggested by the results of this study. In particular, TiC core-graphitic mantle grains that are found in primitive meteorites that have never experienced hydration could be mantled by fullerenes or carbon nanotubes rather than by graphite. In situ observations of these grains in primitive anhydrous meteoritic matrix could confirm or refute this prediction and would demonstrate that the graphitic mantle on such grains is a metamorphic feature due to interaction of the presolar fullerenes with water within the meteorite matrix.
    • Sr and Nd analyses of upper Eocene spherules and their implications for target rocks

      Liu, Shaobin; Papanastassiou, D. A.; Ngo, H. H.; Glass, B. P. (The Meteoritical Society, 2006-01-01)
      Upper Eocene impact ejecta has been discovered all over the world. The number of upper Eocene impact layers and the geographic distribution of each layer, based on major chemical composition and biostratigraphic data, are not agreed upon. We have performed four Sr-Nd isotopic analyses of clinopyroxene-bearing spherules (cpx spherules) and three Sr-Nd analyses of microtektites from five Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) sites in the South Atlantic and Indian Oceans. Our data support the hypothesis that there is only one cpx spherule layer in upper Eocene sediments. We also find that the microtektites associated with the cpx spherule layer in the South Atlantic and Indian Oceans are not part of the North American tektite strewn field, but belong to the same event that produced the cpx spherules. The microtektites, together with cpx spherules, are more heterogeneous than microtektites/tektites from other strewn fields. No direct link has been established between the microtektites from this study and possible target rock at the Popigai crater.
    • Polyhedral serpentine grains in CM chondrites

      Zega, Thomas J.; Garvie, Laurence A. J.; Dódony, István; Friedrich, Heiner; Stroud, Rhonda M.; Buseck, Peter R. (The Meteoritical Society, 2006-01-01)
      We used high-resolution transmission electron microscopy (HRTEM), electron tomography, electron energy-loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS) to investigate the structure and composition of polyhedral serpentine grains that occur in the matrices and fine-grained rims of the Murchison, Mighei, and Cold Bokkeveld CM chondrites. The structure of these grains is similar to terrestrial polygonal serpentine, but the data show that some have spherical or subspherical, rather than cylindrical morphologies. We therefore propose that the term polyhedral rather than polygonal be used to describe this material. EDS shows that the polyhedral grains are rich in Mg with up to 8 atom% Fe. EELS indicates that 70% of the Fe occurs as Fe3+. Alteration of cronstedtite on the meteorite parent body under relatively oxidizing conditions is one probable pathway by which the polyhedral material formed. The polyhedral grains are the end-member serpentine in a mineralogic alteration sequence for the CM chondrites.
    • An experimental study on Fischer-Tropsch catalysis: Implications for impact phenomena and nebular chemistry

      Sekine, Yasuhito; Sugita, Seiji; Shido, Takafumi; Yamamoto, Takashi; Iwasawa, Yasuhiro; Kadono, Toshihiko; Matsui, Takafumi (The Meteoritical Society, 2006-01-01)
      Fischer-Tropsch catalysis, by which CO and H2 are converted to CH4 on the surface of transition metals, has been considered to be one of the most important chemical reactions in many planetary processes, such as the formation of the solar and circumplanetary nebulae, the expansion of vapor clouds induced by cometary impacts, and the atmospheric re-entry of vapor condensate due to asteroidal impacts. However, few quantitative experimental studies have been conducted for the catalytic reaction under conditions relevant to these planetary processes. In this study, we conduct Fischer-Tropsch catalytic experiments at low pressures (1.3 10^(4) bar is less than or equal to P which is less than or equal to 5.3 10^(1) bar) over a wide range of H2/CO ratios (0.25-1000) using pure iron, pure nickel, and iron-nickel alloys. We analyze what gas species are produced and measure the CH4 formation rate. Our results indicate that the CH4 formation rate for iron catalysts strongly depends on both pressure and the H2/CO ratio, and that nickel is a more efficient catalyst at lower pressures and lower H2/CO ratios. This difference in catalytic properties between iron and nickel may come from the reaction steps concerning disproportionation of CO, hydrogenation of surface carbon, and the poisoning of the catalyst. These results suggest that nickel is important in the atmospheric re-entry of impact condensate, while iron is efficient in circumplanetary subnebulae. Our results also indicate that previous numerical models of iron catalysis based on experimental data at 1 bar considerably overestimate CH4 formation efficiency at lower pressures, such as the solar nebula and the atmospheric re-entry of impact condensate.
    • The record of Miocene impacts in the Argentine Pampas

      Schultz, Peter H.; Zárate, Marcelo; Hames, Willis E.; Harris, R. Scott; Bunch, T. E.; Koeberl, Christian; Renne, Paul; Wittke, James (The Meteoritical Society, 2006-01-01)
      Argentine Pampean sediments represent a nearly continuous record of deposition since the late Miocene (~10 Ma). Previous studies described five localized concentrations of vesicular impact glasses from the Holocene to late Pliocene. Two more occurrences from the late Miocene are reported here: one near Chasicó (CH) with an 40Ar/39Ar age of 9.24 +/- 0.09 Ma, and the other near Bahía Blanca (BB) with an age of 5.28 +/- 0.04 Ma. In contrast with andesitic and dacitic impact glasses from other localities in the Pampas, the CH and BB glasses are more mafic. They also exhibit higher degrees of melting with relatively few xenoycrysts but extensive quench crystals. In addition to evidence for extreme heating (>1700 degrees C), shock features are observed (e.g., planar deformation features [PDFs] and diaplectic quartz and feldspar) in impact glasses from both deposits. Geochemical analyses reveal unusually high levels of Ba (~7700 ppm) in some samples, which is consistent with an interpretation that these impacts excavated marine sequences known to be at depth. These two new impact glass occurrences raise to seven the number of late Cenozoic impacts for which there is evidence preserved in the Pampean sediments. This seemingly high number of significant impacts over a 10^6 km^2 area in a time span of 10 Myr is consistent with the number of bolides larger than 100 m expected to enter the atmosphere but is contrary to calculated survival rates following atmospheric disruption. The Pampean record suggests, therefore, that either atmospheric entry models need to be reconsidered or that the Earth has received an enhanced flux of impactors during portions of the late Cenozoic. Evidence for the resulting collisions may be best preserved and revealed in rare dissected regions of continuous, low-energy deposition such as the Pampas. Additionally, the rare earth element (REE) concentrations of the target sediments and impact melts associated with the Chasicó event resemble the HNa/K australites of similar age. This suggests the possibility that those enigmatic tektites could have originated as high-angle, distal ejecta from an impact in Argentina, thereby accounting for their rarity and notable chemical and physical differences from other Australasian impact glasses.