• Sahara 03505 sulfide-rich iron meteorite: Evidence for efficient segregation of sulfide-rich metallic melt during high-degree impact melting of an ordinary chondrite

      D'Orazio, M.; Folco, L.; Chaussidon, M.; Rochette, P. (The Meteoritical Society, 2009-01-01)
      The Sahara 03505 meteorite is a 65 g sulfide-rich iron found in an undisclosed locality of the Sahara. It consists of roughly equal volumetric proportion of polycrystalline troilite (crystal size 1.5-7.5 mm) enclosing cellular/dendritic metallic Fe-Ni (width of the dendrite arms, ~100 micrometers). The mineral assemblage is completed by sparse skeletal crystals of chromite, abundant droplets, 5-100 m in size, of anhydrous Fe-, Fe-Na-, and Fe-Mn-Mg-Ca-Na-K-phosphates, tiny crystals of schreibersite, and particles of metallic Cu. The medium- to fine-grained quench texture, and cooling modeling suggest that Sahara 03505 formed through crystallization of a sulfur-rich metallic melt under rapid cooling conditions (1-4 degrees C s^(-1)). The low troilite/metallic Fe-Ni ratio (~0.6 by weight) shows that this liquid was generated at much higher temperatures (1300 degrees C) with respect to the FeS-Fe,Ni cotectic liquids. Based on bulk chemistry and oxygen isotope composition of chromite, we propose that Sahara 03505 formed by extensive impact melting of an ordinary chondrite lithology, followed by the efficient segregation of the immiscible silicate and metallic liquids. The sulfur-rich metallic liquid rapidly cooled either by radiation into space as a small lump, or by conduction to a chondrite country rock as a vein intruded into the walls of an impact crater. Sahara 03505 belongs to a small group of sulfide-rich iron meteorites which are characterized by medium- to fine-grained quench textures and by bulk chemistry that is different from the other iron meteorite groups. We propose here to use the descriptive term sulfide-irons for this meteorite group, by analogy with the stony-irons.
    • Sayh al Uhaymir 094: A new martian meteorite from the Oman desert

      Gnos, E.; Hofmann, B.; Franchi, I. A.; Al-Kathiri, A.; Hauser, M.; Moser, L. (The Meteoritical Society, 2002-01-01)
      Sayh al Uhaymir (SaU) is a 223.3 g, partially crusted, strongly to very strongly shocked melanocratic olivine-porphyric rock of the shergottite group showing a microgabbroic texture. The rock consists of pyroxene (52.0-58.2 vol%)--dominantly prismatic pigeonite (En60-68Fs20-27Wo7-9) associated with minor augite (En46-49Fs15-16Wo28-31)--brown (shock-oxidized) olivine (Fo65-69; 22.1-31%), completely isotropic interstitial plagioclase glass (maskelynite; An50-64Or0.3-0.9; 8.6-13.0%), chromite and titanian magnesian chromite (0.9-1.0%), trace of ilmenite (Ilm80-86), pyrrhotite (Fe92-100; 0.1-0.2%), merrillite (<<0.1%), and pockets (4.8-6.7%) consisting of green basaltic to basaltic andesitic shock glass that is partially devitrified into a brown to black product along boundaries with the primary minerals. The average maximum dimesions of minerals are: olivine (1.5 mm), pyroxene (0.3 mm) and maskelynite (0.3 mm). Primary melt inclusions in olivine and chromite are common and account for 0.1-0.6% of the rock. X-ray tomography revealed that the specimen contains ~0.4 vol% of shock-melt associated vesicles, up to 3 mm in size, which show a preferred orientation. Fluidizatio of the maskelynite, melting and recrystallization of pyroxene, olivine and pyrrhotite indicate shock stage S6. Minor terrestrial weathering resulted in calcite-veining and minor oxidation of sulfides. The meteorite is interpreted as paired with SaU 005/008/051. The modal composition is similar to Dar al Gani 476/489/670/735/876, with the exception that neither mesostasis nor titanomagnetite nor apatite are present and that all phases show little zonation. The restricted mineral composition, predominance of chromite among the oxides, and abundance of olivine indicate affinities to the lherzolitic shergottites.
    • Scanning electron microscopical and cross sectional analysis of extraterrestrial carbonaceous nanoglobules

      Garvie, Laurence A. J.; Baumgardner, Grant; Buseck, Peter R. (The Meteoritical Society, 2008-01-01)
      Carbonaceous nanoglobules are ubiquitous in carbonaceous chondrite (CC) meteorites. The Tagish Lake (C2) meteorite is particularly intriguing in containing an abundance of nanoglobules, with a wider range of forms and sizes than encountered in other CC meteorites. Previous studies by transmission electron microscopy (TEM) have provided a wealth of information on chemistry and structure. In this study, low-voltage scanning electron microscopy (SEM) was used to characterize the globule forms and external structures. The internal structure of the globules was investigated after sectioning by focused ion beam (FIB) milling. The FIB-SEM analysis shows that the globules range from solid to hollow. Some hollow globules show a central open core, with adjoining smaller cores. The FIB with an SEM is a valuable tool for the analysis of extraterrestrial materials, even of sub-micron "soft" carbonaceous particles. The rapid site-specific cross-sectioning capabilities of the FIB allow the preservation of the internal morphology of the nanoglobules, with minimal damage or alteration of the unsectioned areas.
    • Scanning electron microscopy, cathodoluminescence, and Raman spectroscopy of experimentally shock-metamorphosed quartzite

      Gucsik, A.; Koeberl, C.; Brandstätter, F.; Libowitzky, E.; Reimold, W. U. (The Meteoritical Society, 2003-01-01)
      We studied unshocked and experimentally (at 12, 25, and 28 GPa, with 25, 100, 450, and 750 deg degrees C pre-shock temperatures) shock-metamorphosed Hospital Hill quartzite from South Africa using cathodoluminescence (CL) images and spectroscopy and Raman spectroscopy to document systematic pressure or temperature-related effects that could be used in shock barometry. In general, CL images of all samples show CL-bright luminescent patchy areas and bands in otherwise non- luminescent quartz, as well as CL-dark irregular fractures. Fluid inclusions appear dominant in CL images of the 25 GPa sample shocked at 750 degrees C and of the 28 GPa sample shocked at 450 degrees C. Only the optical image of our 28 GPa sample shocked at 25 degrees C exhibits distinct planar deformation features (PDFs). Cathodoluminescence spectra of unshocked and experimentally shocked samples show broad bands in the near-ultraviolet range and the visible light range at all shock stages, indicating the presence of defect centers on, e.g., SiO4 groups. No systematic change in the appearance of the CL images was obvious, but the CL spectra do show changes between the shock stages. The Raman spectra are characteristic for quartz in the unshocked and 12 GPa samples. In the 25 and 28 GPa samples, broad bands indicate the presence of glassy SiO2, while high-pressure polymorphs are not detected. Apparently, some of the CL and Raman spectral properties can be used in shock barometry.
    • Scientific exploration of near-Earth objects via the Orion Crew Exploration Vehicle

      Abell, P. A.; Korsmeyer, D. J.; Landis, R. R.; Jones, T. D.; Adamo, D. R.; Morrison, D. D.; Lemke, L. G.; Gonzales, A. A.; Gershman, R.; Sweetser, T. H.; et al. (The Meteoritical Society, 2009-01-01)
      A study in late 2006 was sponsored by the Advanced Projects Office within NASAs Constellation Program to examine the feasibility of sending the Orion Crew Exploration Vehicle (CEV) to a near-Earth object (NEO). The ideal mission profile would involve two or three astronauts on a 90 to 180 day flight, which would include a 7 to 14 day stay for proximity operations at the target NEO. This mission would be the first human expedition to an interplanetary body beyond the Earth-Moon system and would prove useful for testing technologies required for human missions to Mars and other solar system destinations. Piloted missions to NEOs using the CEV would undoubtedly provide a great deal of technical and engineering data on spacecraft operations for future human space exploration while conducting in-depth scientific investigations of these primitive objects. The main scientific advantage of sending piloted missions to NEOs would be the flexibility of the crew to perform tasks and to adapt to situations in real time. A crewed vehicle would be able to test several different sample collection techniques and target specific areas of interest via extra-vehicular activities (EVAs) more efficiently than robotic spacecraft. Such capabilities greatly enhance the scientific return from these missions to NEOs, destinations vital to understanding the evolution and thermal histories of primitive bodies during the formation of the early solar system. Data collected from these missions would help constrain the suite of materials possibly delivered to the early Earth, and would identify potential source regions from which NEOs originate. In addition, the resulting scientific investigations would refine designs for future extraterrestrial resource extraction and utilization, and assist in the development of hazard mitigation techniques for planetary defense.
    • Search for a meteoritic component in drill cores from the Bosumtwi impact structure, Ghana: Platinum group element contents and osmium isotopic characteristics

      McDonald, I.; Peucker-Ehrenbrink, B.; Coney, L.; Ferrière, L.; Reimold, W. U.; Koeberl, C. (The Meteoritical Society, 2007-01-01)
      An attempt was made to detect a meteoritic component in both crater-fill (fallback) impact breccias and fallout suevites (outside the crater rim) at the Bosumtwi impact structure in Ghana. Thus far, the only clear indication for an extraterrestrial component related to this structure has been the discovery of a meteoritic signature in Ivory Coast tektites, which formed during the Bosumtwi impact event. Earlier work at Bosumtwi indicated unusually high levels of elements that are commonly used for the identification of meteoritic contamination (i.e., siderophile elements, including the platinum group elements [PGE]) in both target rocks and impact breccias from surface exposures around the crater structure, which does not allow unambiguous verification of an extraterrestrial signature. The present work, involving PGE abundance determinations and Os isotope measurements on drill core samples from inside and outside the crater rim, arrives at the same conclusion. Despite the potential of the Os isotope system to detect even small amounts of extraterrestrial contribution, the wide range in PGE concentrations and Os isotope composition observed in the target rocks makes the interpretation of unradiogenic, high-concentration samples as an impact signature ambiguous.
    • Searching for the source regions of martian meteorites using MGS TES: Integrating martian meteorites into the global distribution of igneous materials on Mars

      Hamilton, V. E.; Christensen, P. R.; McSween, H. Y.; Bandfield, J. L. (The Meteoritical Society, 2003-01-01)
      The objective of this study was to identify and map possible source regions for all 5 known martian meteorite lithologies (basalt, lherzolite, clinopyroxenite, orthopyroxenite, and dunite) using data from the Mars Global Surveyor Thermal Emission Spectrometer (MGS TES). We deconvolved the TES data set using laboratory spectra of 6 martian meteorites (Los Angeles, Zagami, ALH A77005, Nakhla, ALH 84001, and Chassigny) as end members, along with atmospheric and surface spectra previously derived from TES data. Global maps (16 pixels/degree) of the distribution of each meteorite end member show that meteorite-like compositions are not present at or above TES detectability limits over most of the planet's dust-free regions. However, we have confidently identified local-scale (100s-1000s km2) concentrations of olivine- and orthopyroxene-bearing materials similar to ALH A77005, Chassigny, and ALH 84001 in Nili Fossae, in and near Ganges Chasma, in the Argyre and Hellas basin rims, and in Eos Chasma. Nakhla-like materials are identified near the detection limit throughout the eastern Valles Marineris region and portions of Syrtis Major. Basaltic shergottites were not detected in any spatially coherent areas at the scale of this study. Martian meteorite-like lithologies represent only a minor portion of the dust-free surface and, thus, are not representative of the bulk composition of the ancient crust. Meteorite-like spectral signatures identified above TES detectability limits in more spatially restricted areas (tens of km) are targets of ongoing analysis.
    • Seasonal variations in the north toroidal sporadic meteor source

      Campbell-Brown, M.; Wiegert, P. (The Meteoritical Society, 2009-01-01)
      Determining the origins of the sporadic meteoroid sources helps determine their current properties. We have analyzed four years of orbital radar data, looking at how the rates, radiants, and orbits of meteoroids in the north toroidal sporadic source change throughout the year. Twelve broad radiant concentrations, separated in either time or radiant location, are identified. Six are broad distributions associated with more focused shower activity, and six are not associated with major showers. Four of the six concentrations not associated with showers have been named Toroidal, Toroidal A, Toroidal B, and Toroidal degrees C, because of their constant location at the north toroidal centre. The other two, which appear close to the north toroidal source and drift toward the helion and antihelion sources respectively, have been named the Helion Arc and the Antihelion Arc. The twelve radiant concentrations generally last for more than ten degrees solar longitude, and those which may have a single parent are likely composed of orbitally evolved material.
    • Secondary alteration of the impactite and mineralization in the basal Tertiary sequence, Yaxcopoil-1, Chicxulub impact crater, Mexico

      Ames, D. E.; Kjarsgaard, I. M.; Pope, K. O.; Dressler, B. O.; Pilkington, M. (The Meteoritical Society, 2004-01-01)
      The 65 Ma Chicxulub impact crater formed in the shallow coastal marine shelf of the Yucatán Platform in Mexico. Impacts into water-rich environments provide heat and geological structures that generate and focus sub-seafloor convective hydrothermal systems. Core from the Yaxcopoil-1 (Yax-1) hole, drilled by the Chicxulub Scientific Drilling Project (CSDP), allowed testing for the presence of an impact-induced hydrothermal system by: a) characterizing the secondary alteration of the 100 m-thick impactite sequence; and b) testing for a chemical input into the lower Tertiary sediments that would reflect aquagene hydrothermal plume deposition. Interaction of the Yax-1 impactites with seawater is evident through redeposition of the suevites (unit 1), secondary alteration mineral assemblages, and the subaqueous depositional environment for the lower Tertiary carbonates immediately overlying the impactites. The least-altered silicate melt composition intersected in Yax-1 is that of a calc-alkaline basaltic andesite with 53.4-56 wt% SiO2 (volatile-free). The primary mineralogy consists of fine microlites of diopside, plagioclase (mainly Ab 47), ternary feldspar (Ab 37 to 77), and trace apatite, titanite,and zircon. The overprinting alteration mineral assemblage is characterized by Mg-saponite, Kmontmorillonite, celadonite, K-feldspar, albite, Fe-oxides, and late Ca and Mg carbonates. Mg and K metasomatism resulted from seawater interaction with the suevitic rocks producing smectite-Kfeldspar assemblages in the absence of any mixed layer clay minerals, illite, or chlorite. Rare pyrite, sphalerite, galena, and chalcopyrite occur near the base of the impactites. These secondary alteration minerals formed by low temperature (0-150 degrees C) oxidation and fixation of alkalis due to the interaction of glass-rich suevite with down-welling seawater in the outer annular trough intersected at Yax-1. The alteration represents a cold, Mg-K-rich seawater recharge zone, possibly recharging higher temperature hydrothermal activity proposed in the central impact basin. Hydrothermal metal input into the Tertiary ocean is shown by elevated Ni, Ag, Au, Bi, and Te concentrations in marcasite and Cd and Ga in sphalerite in the basal 25 m of the Tertiary carbonates in Yax-1. The lower Tertiary trace element signature reflects hydrothermal metal remobilization from a mafic source rock and is indicative of hydrothermal venting of evolved seawater into the Tertiary ocean from an impactgenerated hydrothermal convective system.
    • Sedimentological analysis of resurge deposits at the Lockne and Tvären craters: Clues to flow dynamics

      Ormö, J.; Sturkell, E.; Lindström, M. (The Meteoritical Society, 2007-01-01)
      The Lockne and Tvären craters formed about 455 million years ago in an epicontinental sea where seawater and mainly limestones covered a crystalline basement. The target water depth for Tvären (apparent basement crater diameter D = 2 km) was probably not over 150 m, and for Lockne (D = 7.5 km) recent best-fit numerical simulations suggest the target water depth of 500-700 m. Lockne has crystalline ejecta that partly cover an outer crater (14 km diameter) apparent in the target sediments. Tvären is eroded with only the crater infill preserved. We have line-logged cores through the resurge deposits within the craters in order to analyze the resurge flow. The focus was clast lithology, frequencies, and size sorting. We divide the resurge into resurge proper, with water and debris shooting into the crater and ultimately rising into a central water plume, anti-resurge, with flow outward from the collapsing plume, and oscillating resurge (not covered by the line-logging due to methodological reasons), with decreasing flow in diverse directions. At Lockne, the deposit of the resurge proper is coarse and moderately sorted, whereas the anti-resurge deposit is fining upwards and better sorted. The Tvären crater has a smoothly fining-up section deposited by the resurge proper and may lack anti-resurge deposits. At Lockne, the content of crystalline relative to limestone clasts generally decreases upwards, which is the opposite of Tvären. This may be a consequence of factors such as crater size (i.e., complex versus simple) and the relative target water depth. The mean grain size (i.e., the mean phi value per meter, phi) and standard deviation, i.e., size sorting (sigma) for both craters, can be expressed by the equation sigma = 0.60phi 1.25.
    • Seismic analysis for the Lanzhou fireball

      Dailu, Rong; Yarong, Li (The Meteoritical Society, 2007-01-01)
      In this paper, we convert the Lanzhou fireball's trajectory using seismic data according to the analytical method presented in Pujol et al. (2005, 2006). Taking the same assumptions as Pujol et al., the position of the fireball burst at its terminal has been converted using a relative simple independent method. Both the trajectory and the position of burst are roughly coincident.
    • Selective release of D and 13C from insoluble organic matter of the Murchison meteorite by impact shock

      Mimura, K.; Okamoto, M.; Sugitani, K.; Hashimoto, S. (The Meteoritical Society, 2007-01-01)
      We performed shock-recovery experiments on insoluble organic matter (IOM) purified from the Murchison meteorite, and determined the abundances and isotope ratios of hydrogen and carbon in the shocked IOM sample. We also performed shock experiments on type III kerogen and compared the results of these experiments with the experimental results regarding IOM. The shock selectively released D and 13C from the IOM, while it preferably released H and 12C from the kerogen. The release of these elements from IOM cannot be explained in terms of the isotope effect, whereas their release from kerogen can be explained by this effect. The selective release of heavier isotopes from IOM would be due to its structure, in which D and 13C-enriched parts are present as an inhomogeneity and are weakly attached to the main network. Shock gave rise to a high release of D even at a lower degree of dehydrogenation compared with the stepwise heating of IOM. This effective release of D is probably an inherent result of shock, in which a dynamic high pressure and high-temperature condition prevails. Thus, shock would effectively control the hydrogen isotope behavior of extraterrestrial organic matter during the evolution of the solar nebula.
    • Shape, metal abundance, chemistry, and origin of chondrules in the Renazzo (CR) chondrite

      Ebel, D. S.; Weisberg, M. K.; Hertz, J.; Campbell, A. J. (The Meteoritical Society, 2008-01-01)
      We used synchrotron X-ray microtomography to image in 3-dimensions (3D) eight whole chondrules in a ~1 cm^3 piece of the Renazzo (CR) chondrite at ~17 micrometers per volume element (voxel) edge. We report the first volumetric (3D) measurement of metal/silicate ratios in chondrules and quantify indices of chondrule sphericity. Volumetric metal abundances in whole chondrules range from 1 to 37 volume % in 8 measured chondrules and by inspection in tomography data. We show that metal abundances and metal grain locations in individual chondrules cannot be reliably obtained from single random 2D sections. Samples were physically cut to intersect representative chondrules multiple times and to verify 3D data. Detailed 2D chemical analysis combined with 3D data yield highly variable whole-chondrule Mg/Si ratios with a supra-chondritic mean value, yet the chemically diverse, independently formed chondrules are mutually complementary in preserving chondritic (solar) Fe/Si ratios in the aggregate CR chondrite. These results are consistent with localized chondrule formation and rapid accretion resulting in chondrule + matrix aggregates (meteorite parent bodies) that preserve the bulk chondritic composition of source regions.
    • Shergottites Dhofar 019, SaU 005, Shergotty, and Zagami: 40Ar-39Ar chronology and trapped Martian atmospheric and interior argon

      Korochantseva, E. V.; Trieloff, M.; Buikin, A. I.; Hopp, J. (The Meteoritical Society, 2009-01-01)
      We report a high-resolution 40Ar-39Ar study of mineral separates and whole-rock samples of olivine-phyric (Dhofar 019, Sayh al Uhaymir [SaU] 005) and basaltic (Shergotty, Zagami) shergottites. Excess argon is present in all samples. The highest (40Ar/36Ar)trapped ratios are found for argon in pyroxene melt inclusions (~1500), maskelynite (~1200), impact glass (~1800) of Shergotty and impact glass of SaU 005 (~1200). A high (40Ar/36Ar)trapped componentusually uniquely ascribed to Martian atmosphere--can also originate from the Martian interior, indicating a heterogeneous Martian mantle composition. As additional explanation of variable high (40Ar/ 36Ar)trapped ratios in shocked shergottites, we suggest argon implantation from a "transient atmosphere" during impact induced degassing. The best 40Ar-39Ar age estimate for Dhofar 019 is 642 +/- 72 Ma (maskelynite). SaU 005 samples are between 700-900 Ma old. Relatively high 40Ar-39Ar ages of melt inclusions within Dhofar 019 (1086 +/- 252 Ma) and SaU 005 olivine (885 +/- 66 Ma) could date entrapment of a magmatic liquid during early olivine crystallization, or reflect unrecognized excess 40Ar components. The youngest 40Ar-39Ar age of Shergotty separates (maskelynite) is ~370 Ma, that of Zagami is ~200 Ma. The 40Ar-39Ar chronology of Dhofar 019 and SaU 005 indicate <1 Ga ages. Apparent ages uncorrected for trapped (e.g., Martian atmosphere, mantle) argon components approach 4.5 Ga, but are not caused by inherited 40Ar, because excess 40Ar is supported by 36Artrapped. Young ages obtained by 40Ar-39Ar and other chronometers argue for primary rather than secondary events. The cosmic ray exposure ages calculated from cosmogenic argon are 15.7 +/- 0.7 Ma (Dhofar 019), 1.0-1.6 Ma (SaU 005), 2.1-2.5 Ma (Shergotty) and 2.2-3.0 Ma (Zagami).
    • Shişr 043 (IIIAB medium octahedrite): The first iron meteorite from the Oman desert

      Al-Kathiri, A.; Hofmann, B. A.; Gnos, E.; Eugster, O.; Welten, K. C.; Krähenbühl, U. (The Meteoritical Society, 2006-01-01)
      The iron meteorite Shişr 043 is a single mass of 8267 g found in the south Oman desert 42 km NE of the Shişr village. It is the first iron identified among the >1400 individual meteorites reported from Oman. The meteorite is a slightly elongated mass showing only minor rusting, a partially smooth and partially rough surface with octahedral cleavage, and a partially preserved metallic fusion crust typically 0.75 mm thick. The undeformed Widmanstätten pattern with a mean kamacite bandwidth of 1.0 +/- 0.1 mm (n = 97) indicates structural classification as a medium octahedrite. From the bulk composition, Ni = 8.06 wt%, Ga = 18.8 ppm, Ge = 37.25 ppm, and Ir = 3.92 ppm, the meteorite is classified as IIIAB, the most common group of iron meteorites. The cosmic-ray exposure (CRE) age based on 3He, 21Ne, 38Ar concentrations and 10Be-21Ne, 26Al-21Ne, and 36Cl-36Ar ratios is 290 +/- 20 Ma. This age falls within the range observed for type IIIAB iron meteorites, but does not coincide with the main cluster. The cosmogenic noble gas and radionuclide data indicate that Shişr 043 had a relatively small preatmospheric mass. The low degree of weathering is consistent with a young terrestrial age of <10,000 years based on the saturated 41Ca concentration. Shişr 043 is not paired with any of the other eight known iron meteorites from the Arabian Peninsula.
    • Shock and thermal history of Martian meteorite Allan Hills 84001 from transmission electron microscopy

      Barber, David J.; Scott, Edward R. D. (The Meteoritical Society, 2006-01-01)
      Microstructures in the Allan Hills 84001 meteorite were studied using optical and electron microscopy, putting emphasis on shock effects, which are widespread. Some orthopyroxene exhibits only (100) slip, but more typical grains suffered extensive slip, microfracturing, and frequently contain (100) clino-inversion lamellae. In fracture zones, shock deformation of orthopyroxene has produced all three effects in profusion, together with intergranular pockets of orthopyroxene glass and intragranular glass lamellae, which were apparently created by shearing on low index planes, usually (100) or {110}. Both types of plane are loci that pseudo-planar fractures tend to follow. Thus, the glass lamellae, which have not been observed in other meteorites, probably formed by frictional heating during the sliding of microscale corrugated surfaces, one over another, leading to local melting. We infer that the orthopyroxene glass and the fracture zones both formed from shear stresses created by strong shock. Ubiquitous undeformed micrometer and submicrometer euhedral chromites in orthopyroxene and plagioclase glasses and carbonate probably crystallized after shock heating and fracture zone formation. Nanocrystals of eskolaite (Cr2O3) coating silica glass grains are probably also a result of shock-induced thermal decomposition of chromite. Iron sulfides (pyrite and pyrrhotite were identified) tended to be associated with plagioclase glass. A carbonate disk showing no evidence for shock deformation had a substructure of elongated, slightly misoriented subcells in the exterior; interior regions had more eqiaxed subcells. Both microstructures probably formed during growth, but the conditions are undetermined. Chemical composition varied on a micron scale, but the rim of the disk was more ferroan; oxide precipitates and voids were widely distributed as in fracture-filling carbonates. If the fracture zones and opx glass are the result of strong shock, as we deduce, it is very unlikely that pores could have filled by carbonate long after the fracture zones formed. We infer that the carbonate, like the phosphate, olivine, pyrrhotite, eskolaite, and many euhedral, submicrometer chromites, crystallized during the final stages of the impact that created the fracture zones and glasses with compositions of plagioclase, silica, and orthopyroxene.
    • Shock melting of ordinary chondrite powders and implications for asteroidal regoliths

      Hörz, F.; Cintala, M. J.; See, T. H.; Le, L. (The Meteoritical Society, 2005-01-01)
      A series of 59 impacts in the laboratory reduced a coherent 460 g piece of the L6 ordinary chondrite ALH 85017 to a coarse-grained regolith. We then subjected the 125-250 micrometer fines from this sample to reverberation shock stresses of 14.567 GPa in order to delineate the melting behavior of porous, unconsolidated, chondritic asteroid surfaces during meteorite impact. The initial pore space (40-50%) was completely closed at 14.5 GPa and a dense aggregate of interlocking grains resulted. Grain-boundary melting commenced at <27 GPa and ~50% of the total charge was molten at 67 GPa; this stress corresponds to typical asteroid impacts at ~5 km/sec. Melting of the entire sample most likely mandates >80 GPa, which is associated with impact velocities >8 km/sec. The Fe-Ni and troilite clasts of the original meteorite melted with particular ease, forming immiscible melts that are finely disseminated throughout the silicate glass. These metal droplets are highly variable in size, extending to <100 nm and most likely to superparamagnetic domains; such opaques are also observed in the natural melt veins of ordinary chondrites. It follows that melting and dissemination of pre-existing, Fe-rich phases may substantially affect the optical properties of asteroidal surfaces. It seems unnecessary to invoke reduction of Fe2+ (or Fe3+) by sputtering or impact-processes--in analogy to the lunar surface--to produce space weathering effects on S-type asteroids. We note that HED meteorites contain ample FeO (comparable to that in lunar basalts) for reduction processes to take place, yet their probable parent object(s), Vesta and its collisional fragments, display substantially unweathered surfaces. Howardites, eucrites, and diogenites (HEDs), however, contain little native metal (typically <0.5%), in contrast to ordinary chondrites (commonly 10-15%) and their S-type parent objects. These considerations suggest that the modal content of native metal and sulfides is more important for space weathering on asteroids than total FeO.
    • Shock melts in QUE 94411, Hammadah al Hamra 237, and Bencubbin: Remains of the missing matrix?

      Meibom, A.; Righter, K.; Chabot, N.; Dehn, G.; Antignano, A.; McCoy, T. J.; Krot, A. N.; Zolensky, M. E.; Petaev, M. I.; Keil, K. (The Meteoritical Society, 2005-01-01)
      We have studied the CB carbonaceous chondrites Queen Alexandra Range (QUE) 94411, Hammadah al Hamra (HH) 237, and Bencubbin with an emphasis on the petrographical and mineralogical effects of the shock processing that these meteorite assemblages have undergone. Ironnickel metal and chondrule silicates are the main components in these meteorites. These hightemperature components are held together by shock melts consisting of droplets of dendritically intergrown Fe,Ni-metal/sulfide embedded in silicate glass, which is substantially more FeO-rich (30 40 wt%) than the chondrule silicates (FeO <5 wt%). Fine-grained matrix material, which is a major component in most other chondrite classes, is extremely scarce in QUE 94411 and HH 237, and has not been observed in Bencubbin. This material occurs as rare, hydrated matrix lumps with major and minor element abundances roughly similar to the ferrous silicate shock melts (and CI). We infer that hydrated, fine-grained material, compositionally similar to these matrix lumps, was originally present between the Fe,Ni-metal grains and chondrules, but was preferentially shock melted. Other shockrelated features in QUE 94411, HH 237, and Bencubbin include an alignment and occasionally strong plastic deformation of metal and chondrule fragments. The existence of chemically zoned and metastable Fe,Ni-metal condensates in direct contact with shock melts indicates that the shock did not substantially increase the average temperature of the rock. Because porphyritic olivine-pyroxene chondrules are absent in QUE 94411, HH 237, and Bencubbin, it is difficult to determine the precise shock stage of these meteorites, but the shock was probably relatively light (S2-S3), consistent with a bulk temperature increase of the assemblages of less than ~300 degrees C. The apparently similar shock processing of Bencubbin, Weatherford, Gujba (CBa) and QUE 94411/HH 237 (CBb) supports the idea of a common asteroidal parent body for these meteorites.
    • Shock metamorphism of quartz at the submarine Mjølnir impact crater, Barents Sea

      Sandbakken, P. T.; Langenhorst, F.; Dypvik, H. (The Meteoritical Society, 2005-01-01)
      Shock metamorphosed quartz grains have been discovered in a drill core from the central peak of the Late Jurassic, marine Mjølnir structure; this finding further corroborates the impact origin of Mjølnir. The intersected strata represent the Upper Jurassic Hekkingen Formation and underlying Jurassic and Upper Triassic formations. The appearance, orientation, and origin of shock features in quartz grains and their stratigraphic distribution within the core units have been studied by optical and transmission electron microscopy. The quartz grains contain planar fractures (PFs), planar deformation features (PDFs), and mechanical Brazil twins. The formation of PFs is the predominant shock effect and is attributed to the large impedance differences between the water-rich pores and constituent minerals in target sediments. This situation may have strengthened tensional/extensional and shear movements during shock compression and decompression. The combination of various shock effects indicates possible shock pressures between 5 and at least 20 GPa for three core units with a total thickness of 86 m (from 74.00 m to 171.09 m core depth). Crater-fill material from the lower part of the core typically shows the least pressures, whereas the uppermost part of the allochthonous crater deposits displays the highest pressures. The orientations of PFs in studied quartz grains seem to become more diverse as the pressure rises from predominantly (0001) PFs to a combination of (0001), {0011}, and {0022} orientations. However, the lack of experimental data on porous sedimentary rocks does not allow us to further constrain the shock conditions on the basis of PF orientations.
    • Shock re-equilibration of fluid inclusions in crystalline basement rocks from the Ries crater, Germany

      Elwood Madden, Megan E.; Kring, David A.; Bodnar, Robert J. (The Meteoritical Society, 2006-01-01)
      This study examines the effects of shock metamorphism on fluid inclusions in crystalline basement target rocks from the Ries crater, Germany. The occurrence of two-phase fluid inclusions decreases from shock stage 0 to shock stage 1, while single-phase inclusions increase, likely as a result of re-equilibration. In shock stages 2 and 3, both two-phase and single-phase inclusions decrease with increasing shock stage, indicating that fluid inclusion vesicles are destroyed due to plastic deformation and phase changes in the host minerals. However, quartz clasts entrained in shock stage 4 melts contain both single-phase and two-phase inclusions, demonstrating the rapid quenching of the melt and the heterogeneous nature of impact deformation. Inclusions in naturally shocked polycrystalline samples survive at higher shock pressures than those in single crystal shock experiments. However, fluid inclusions in both experimental and natural samples follow a similar trend in re-equilibration at low to moderate shock pressures leading to destruction of inclusion vesicles in higher shock stages. This suggests that shock processing may lead to the destruction of fluid inclusions in many planetary materials and likely contributed to shock devolatilization of early planetesimals.