• Ries and Chicxulub: Impact craters on Earth provide insights for Martian ejecta blankets

      Kenkmann, T.; Schönian, F. (The Meteoritical Society, 2006-01-01)
      Terrestrial impact structures provide field evidence for cratering processes on planetary bodies that have an atmosphere and volatiles in the target rocks. Here we discuss two examples that may yield implications for Martian craters: 1. Recent field analysis of the Ries crater has revealed the existence of subhorizontal shear planes (detachments) in the periphery of the crater beneath the ejecta blanket at 0.9-1.8 crater radii distance. Their formation and associated radial outward shearing was caused by weak spallation and subsequent dragging during deposition of the ejecta curtain. Both processes are enhanced in rheologically layered targets and in the presence of fluids. Detachment faulting may also occur in the periphery of Martian impacts and could be responsible for the formation of lobe-parallel ridges and furrows in the inner layer of double-layer and multiple-layer ejecta craters. 2. The ejecta blanket of the Chicxulub crater was identified on the southeastern Yucatán Peninsula at distances of 3.0-5.0 crater radii from the impact center. Abundance of glide planes within the ejecta and particle abrasion both rise with crater distance, which implies a ground-hugging, erosive, and cohesive secondary ejecta flow. Systematic measurement of motion indicators revealed that the flow was deviated by a preexisting karst relief. In analogy with Martian fluidized ejecta blankets, it is suggested that the large runout was related to subsurface volatiles and the presence of basal glide planes, and was influenced by eroded bedrock lithologies. It is proposed that ramparts may result from enhanced shear localization and a stacking of ejecta material along internal glide planes at decreasing flow rates when the flow begins to freeze below a certain yield stress.
    • Role of water in the formation of the Late Cretaceous Wetumpka impact structure, inner Gulf Coastal Plain of Alabama, USA

      King, David T.; Ormö, Jens; Petruny, Lucille W.; Neathery, Thornton L. (The Meteoritical Society, 2006-01-01)
      The effect of shallow marine water (30-100 m deep) in the late excavation and early modification stages of a marine-target crater 5 km in diameter, as exemplified by the Late Cretaceous Wetumpka impact structure in Alabama, USA, is manifest in the early collapse of a weak part of the rim. Excavation flow and connate marine water are interpreted to be factors in this collapse. This partial rim collapse catastrophically emplaced an upper-structure-filling unit of broken and redistributed sedimentary target formations, which presently mantles the deeper fallback breccia deposits within the structure. Furthermore, rim collapse flow facilitated the formation of a structurally modified, extrastructure terrain, which is located outside and adjacent to the collapsed rim segment. This extrastructure terrain appears to be the product of extensive slumping of poorly consolidated target sedimentary formations.
    • 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 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.
    • Shock, post-shock annealing, and post-annealing shock in ureilites

      Rubin, Alan E. (The Meteoritical Society, 2006-01-01)
      The thermal and shock histories of ureilites can be divided into four periods: 1) formation, 2) initial shock, 3) post-shock annealing, and 4) post-annealing shock. Period 1 occurred 4.55 Ga ago when ureilites formed by melting chondritic material. Impact events during period 2 caused silicate darkening, undulose to mosaic extinction in olivines, and the formation of diamond, lonsdaleite, and chaoite from indigenous carbonaceous material. Alkali-rich fine-grained silicates may have been introduced by impact injection into ureilites during this period. About 57% of the ureilites were unchanged after period 2. During period 3 events, impact-induced annealing caused previously mosaicized olivine grains to become aggregates of small unstrained crystals. Some ureilites experienced reduction as FeO at the edges of olivine grains reacted with degrees C from the matrix. Annealing may also be responsible for coarsening of graphite in a few ureilites, forming euhedral-appearing, idioblastic crystals. Orthopyroxene in Meteorite Hills (MET) 78008 may have formed from pigeonite by annealing during this period. The Rb-Sr internal isochron age of ~4.0 Ga for MET 78008 probably dates the annealing event. At this late date, impacts are the only viable heat source. About 36% of ureilites experienced period 3 events, but remained unchanged afterwards. During period 4, ~7% of the ureilites were shocked again, as is evident in the polymict breccia, Elephant Moraine (EET) 83309. This rock contains annealed mosaicized olivine aggregates composed of small individual olivine crystals that exhibit undulose extinction.Ureilites may have formed by impact-melting chondritic material on a primitive body with heterogeneous O isotopes. Plagioclase was preferentially lost from the system due to its low impedance to shock compression. Brief melting and rapid burial minimized the escape of planetary-type noble gases from the ureilitic melts. Incomplete separation of metal from silicates during impact melting left ureilites with relatively high concentrations of trace siderophile elements.
    • Shock-induced melting, recrystallization, and exsolution in plagioclase from the Martian lherzolitic shergottite GRV 99027

      Wang, Deqiang; Chen, Ming (The Meteoritical Society, 2006-01-01)
      Plagioclase in the Martian lherzolitic shergottite Grove Mountains (GRV) 99027 was shocked, melted, and recrystallized. The recrystallized plagioclase contains lamellae of pyroxene, olivine, and minor ilmenite (<1 micrometers wide). Both the pyroxene and the olivine inclusions enclosed in plagioclase and grains neighboring the plagioclase were partially melted into plagioclase melt pools. The formation of these lamellar inclusions in plagioclase is attributed to exsolution from recrystallizing melt. Distinct from other Martian meteorites, GRV 99027 contains no maskelynite but does contain recrystallized plagioclase. This shows that the meteorite experienced a slower cooling than maskelynite-bearing meteorites. We suggest that the parent rock of GRV 99027 could have been embedded in hot rocks, which facilitated a more protracted cooling history.
    • Shock-metamorphosed zircon in terrestrial impact craters

      Wittmann, A.; Kenkmann, T.; Schmitt, R. T.; Stöffler, D. (The Meteoritical Society, 2006-01-01)
      To ascertain the progressive stages of shock metamorphism of zircon, samples from three well-studied impact craters were analyzed by optical microscopy, scanning electron microscopy (SEM), and Raman spectroscopy in thin section and grain separates. These samples are comprised of well-preserved, rapidly quenched impactites from the Ries crater, Germany, strongly annealed impactites from the Popigai crater, Siberia, and altered, variably quenched impactites from the Chicxulub crater, Mexico. The natural samples were compared with samples of experimentally shock-metamorphosed zircon. Below 20 GPa, zircon exhibits no distinct shock features. Above 20 GPa, optically resolvable planar microstructures occur together with the high-pressure polymorph reidite, which was only retained in the Ries samples. Decomposition of zircon to ZrO2 only occurs in shock stage IV melt fragments that were rapidly quenched. This is not only a result of post-shock temperatures in excess of ~1700 degrees C but could also be shock pressure-induced, which is indicated by possible relics of a high-pressure polymorph of ZrO2. However, ZrO2 was found to revert to zircon with a granular texture during devitrification of impact melts. Other granular textures represent recrystallized amorphous ZrSiO4 and reidite that reverted to zircon. This requires annealing temperatures >1100 degrees C. A systematic study of zircons from a continuous impactite sequence of the Chicxulub impact structure yields implications for the post-shock temperature history of suevite-like rocks until cooling below ~600 degees C.
    • SIMS studies of Allende projectiles fired into Stardust-type aluminum foils at 6 km/sec

      Hoppe, Peter; Stadermann, Frank J.; Stephan, Thomas; Floss, Christine; Leitner, Jan; Marhas, Kuljeet K.; Hörz, Friedrich (The Meteoritical Society, 2006-01-01)
      We have explored the feasibility of degrees C, N, and O isotopic measurements by NanoSIMS and of elemental abundance determinations by time-of-flight secondary ion mass spectrometry (TOF-SIMS) on residues of Allende projectiles that impacted Stardust-type aluminum foils in the laboratory at 6 km/sec. These investigations are part of a consortium study aimed at providing the foundation for the characterization of matter associated with microcraters that were produced during the encounter of the Stardust space probe with comet 81P/Wild-2. Eleven experimental impact craters were studied by NanoSIMS and eighteen by TOF-SIMS. Crater sizes were between 3 and 190 micrometers. The NanoSIMS measurements have shown that the crater morphology has only a minor effect on spatial resolution and on instrumental mass fractionation. The achievable spatial resolution is always better than 200 nm, and degrees C and O isotopic ratios can be measured with a precision of several percent at a scale of several 100 nm, which is the typical size of presolar grains. This clearly demonstrates that presolar matter, provided it survives the impact into the aluminum foil partly intact, is recognizable even if embedded in material of solar system origin. TOF-SIMS studies are restricted to materials from the crater rim. The element ratios of the major rock-forming elements in the Allende projectiles are well-characterized by the TOF-SIMS measurements, indicating that fractionation of those elements during impact can be expected to be negligible. This permits chemical information on the type of impactor material to be obtained. For any more detailed assignments to specific chondrite groups, however, information on the abundances of the light elements, especially degrees C, is crucial. This information could not be obtained in the present study due to unavoidable contamination during impact experiments.
    • Spatial heterogeneity of 26Al/27Al and stable oxygen isotopes in the solar nebula

      Boss, A. P. (The Meteoritical Society, 2006-01-01)
      The degree of isotopic spatial heterogeneity in the solar nebula has long been a puzzle, with different isotopic systems implying either large-scale initial spatial homogeneity (e.g., 26Al chronometry) or a significant amount of preserved heterogeneity (e.g., ratios of the three stable oxygen isotopes, 16O, 17O, and 18O). We show here that in a marginally gravitationally unstable (MGU) solar nebula, the efficiency of large-scale mixing and transport is sufficient to spatially homogenize an initially highly spatially heterogeneous nebula to dispersions of ~10% about the mean value of 26Al/27Al on time scales of thousands of years. A similar dispersion would be expected for 17O/16O and 18O/16O ratios produced by ultraviolet photolysis of self-shielded molecular CO gas at the surface of the outer solar nebula. In addition to preserving a chronological interpretation of initial 26Al/27Al ratios and the self-shielding explanation for the oxygen isotope ratios, these solar nebula models offer a self-consistent environment for achieving large-scale mixing and transport of thermally annealed dust grains, shock-wave processing of chondrules and refractory inclusions, and giant planet formation.
    • Spectral properties of angrites

      Burbine, T. H.; McCoy, T. J.; Hinrichs, J. L.; Lucey, P. G. (The Meteoritical Society, 2006-01-01)
      Angrites are generally believed to be fragments of a basaltic asteroid that differentiated under relatively oxidizing conditions. Almost all angrites (e.g., D'Orbigny, Lewis Cliff [LEW] 86010, and Sahara 99555) are composed predominately of anorthite, Al-Ti diopside-hedenbergite, and Ca-rich olivine, except for the type specimen, Angra dos Reis, which is composed almost entirely of Al-Ti diopside-hedenbergite. D'Orbigny, LEW 86010, and Sahara 99555 also have spectral properties very different from Angra dos Reis. These newly measured angrites all have broad absorption features centered near 1 m with very weak to absent absorption bands at ~2 micrometers, which is characteristic of some clinopyroxenes. The spectrum of Angra dos Reis has the characteristic 1 and 2 micrometers features due to pyroxene. One asteroid, 3819 Robinson, has similar spectral properties to the newly measured angrites in the visible wavelength region, but does not appear to spectrally match these angrites in the near-infrared.
    • 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.
    • Stellar nucleosynthetic contribution of extinct short-lived nuclei in the early solar system and the associated isotopic effects

      Sahijpal, S.; Soni, P. (The Meteoritical Society, 2006-01-01)
      A wide range of stellar nucleosynthetic sources has been analyzed to derive their contributions of short-lived and stable nuclei to the presolar cloud. This detailed study is required to infer the most plausible source(s) of short-lived nuclei through a critical comparison among the various stellar sources that include AGB stars, novae, supernovae II, Ia, and Wolf-Rayet stars that evolved to supernovae Ib/c. In order to produce the canonical value of 26Al/27Al in the early solar system, almost all stellar sources except low-mass AGB stars imply large isotopic anomalies in Ca-Al-rich inclusions (CAIs). This is contrary to the observed isotopic compositions of CAIs. The discrepancy could impose stringent constraints on the formation and thermal evolution of CAIs from different chondrites. Among the various stellar scenarios, the injection of short-lived nuclei into the previously formed solar protoplanetary disc by a massive star of an ad hoc chosen high-injection mass cut is a possible scenario. There is a possibility of the contribution of short-lived nuclides by a 1.5-3 Mʘ AGB star as it implies the smallest shift in stable isotopes. A low-mass AGB star of relatively low metallicity would be even a better source of short-lived nuclei. However, this scenario would require independent gravitational collapse of the presolar cloud coupled with ambipolar diffusion of magnetic flux. Alternatively, numerous scenarios can be postulated that involve distant (greater than or equal to 10 pc) massive stars can contribute 60Fe to the presolar cloud and can trigger its gravitational collapse. These scenarios would require production of 26Al and 41Ca by irradiation in the early solar system. Significant production of 26Al and 60Fe can be explained if massive, rotating Wolf-Rayet stars that evolved to supernovae Ib/c were involved.
    • 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.
    • Stratigraphy and composition of lava flows in Mare Nubium and Mare Cognitum

      Bugiolacchi, Roberto; Spudis, Paul D.; Guest, John E. (The Meteoritical Society, 2006-01-01)
      Three major periods of basaltic activity characterize the infill of the basins. Each of these periods was itself punctuated by discrete phases of widespread magma eruptions: three during both the Late Imbrian Epoch and the early Eratosthenian Period and then two in the late Eratosthenian Period. We found the youngest lavas off the eastern border of the Fra Mauro peninsula and, mantling a much larger area, over most of the central western Nubium basin.Our results place the Nubium/Cognitum basalts in the low-Ti category (1-5 wt% TiO2).The data indicate that the majority (~90%) of the mare terrain has iron content between 18 and 22 wt%. In particular, FeO contents tend to concentrate toward two compositional poles, each of ~20 wt%, and a much smaller one of ~15 wt%. These values are typical of nearside lunar maria.To complement our compositional data, we present a census of craters larger than 500 m using Orbiter IV images. The result was a crater count average with frequency 5.6 x 10^(-2)km^(-2), translating into an inferred mean age of 3300 Ma for the exposed lava flows.By combining lava chemistry with age, we find a possible correlation between the ages of the most prominent flow units and their estimated titanium content, with younger basalts becoming progressively Ti-richer with time (from 2-3 to 4-5 wt% TiO2).
    • 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%.
    • Surface electronic states of meteoritic nanodiamonds

      Garvie, Laurence A. J.; Buseck, Peter R. (The Meteoritical Society, 2006-01-01)
      The C K edge of Orgueil nanodiamonds (C-delta diamonds) was acquired by electron energy-loss spectroscopy (EELS), with an energy resolution of 300 meV. The spectra show peaks at 282.5, 284.7, and 286.4 eV, which occur in the band gap below the main diamond edge and are absent from the bulk diamond spectrum. These peaks are attributed to transitions from degrees C 1s surface core levels to unoccupied surface states, and arise from single and pi-bonded dangling bonds and C-H bonds. A shoulder to the main absorption edge at 287.8 eV may correspond to hydrocarbon adsorbates. These results can be used to further our understanding of C-delta diamond structure and may reveal the presence of a fullerene-like surface. The unique surface electronic states of the C-delta diamond surfaces are expected to affect their optical properties, which are dependent on features such as extent of H coverage, particle size, and surface structure.
    • Survival of organic phases in porous IDPs during atmospheric entry: A pulse-heating study

      Matrajt, G.; Brownlee, D.; Sadilek, M.; Kruse, L. (The Meteoritical Society, 2006-01-01)
      In this study, we have performed pulse-heating experiments at different temperatures for three organic molecules (a polycyclic aromatic hydrocarbon [PAH], a ketone, and an amino acid) (absorbed into microporous aluminum oxide (Al2O3) in order to imitate the heating of the organic molecules in interplanetary dust particles (IDPs) and micrometeorites (MMs) during atmospheric entry and to investigate their survival. We have shown that modest amounts (a few percent) of these organic molecules survive pulse-heating at temperatures in the 700 to 900 degrees C range. This suggests that the porosity in IDPs and MMs, combined with a sublimable phase (organic material, water), produces an ablative cooling effect, which permits the survival of organic molecules that would otherwise be lost either by thermal degradation or evaporation during atmospheric entry.
    • Synchrotron-based infrared microspectroscopy as a useful tool to study hydration states of meteorite constituents

      Moroz, L. V.; Schmidt, M.; Schade, U.; Hiroi, T.; Ivanova, M. A. (The Meteoritical Society, 2006-01-01)
      We present the results of the infrared (IR) microscopic study of the anomalous carbonaceous chondrites Dhofar (Dho) 225 and Dhofar 735 in comparison to typical CM2 chondrites Cold Bokkeveld, Murray, and Mighei. The Fourier transform infrared (FTIR) 2.5-14 micrometers reflectance measurements were performed on conventional polished sections using an infrared microscope with a synchrotron radiation source. We demonstrate that the synchrotron-based IR microspectroscopy is a useful, nondestructive tool for studying hydration states of meteorite constituents in situ. Our results show that the matrices of Dho 225 and Dho 735 are dehydrated compared to the matrices of typical CM2 chondrites. The spectra of the Dho 225 and Dho 735 matrices lack the 2.7-2.8 micrometers absorption feature present in the spectra of Cold Bokkeveld, Murray, and Mighei. Spectral signatures caused by Si-O vibrations in fine-grained, Fe-rich olivines dominate the 10 micrometers spectral region in the spectra of Dho 225 and Dho 735 matrices, while the spectra of normal CM2 chondrites are dominated by spectral signatures due to Si-O vibrations in phyllosilicates. We did not detect any hydrated phases in the spectra of Dho 225 and Dho 735 polished sections. In addition, the near-infrared reflectance spectra of Dho 225 and Dho 735 bulk powders show spectral similarities to the Antarctic metamorphosed carbonaceous chondrites. We confirm the results of previous mineralogical, chemical, and isotopic studies indicating that the two meteorites from Oman are the first non-Antarctic metamorphosed carbonaceous chondrites.
    • TEM studies and the shock history of a "mysterite" inclusion from the Krymka LL chondrite

      Weber, I.; Semenenko, V. P.; Stephan, T.; Jessberger, E. K. (The Meteoritical Society, 2006-01-01)
      The microstructure and composition of the matrix of one carbonaceous inclusion (K1) in the Krymka LL3.1 chondrite were studied using transmission electron microscopy (TEM). K1 has previously shown an enigmatic nature and similarities with volatile-rich, fine-grained, dark inclusions of Krymka called "mysterite."In the present study, four minerals were identified by TEM. Olivine, pyroxene, and pyrrhotite typically have grain sizes of one micrometer; graphite occurs as flakes of a similar size. Olivine shows a moderately high dislocation density most probably caused by shock. Pyroxene shows coexisting ortho- and clinoenstatite lamellae that probably originated from shear stress after a shock event or from the rapid cooling of the protoenstatite stability field. However, we demonstrate that in this case, a shock trigger is more likely. Pyrrhotite in the studied sample occurs as a 4C monoclinic superstructure. The graphite flakes in the fragment are well crystallized, as can be seen by discrete spots in the diffraction pattern. In graphite, the degree of crystallization increases with the metamorphic grade. Based on the microstructure of this mineral we conclude that after a first moderate shock event, the residual temperature between 300 degrees C and 500 degrees C led to thermal metamorphism. A second shock event, possibly at excavation from the parent body, is responsible for the shock features observed in olivine, pyroxene, and graphite.