• Three-dimensional structures and elemental distributions of Stardust impact tracks using synchrotron microtomography and X-ray fluorescence analysis

      Tsuchiyama, A.; Nakamura, T.; Okazaki, T.; Uesugi, K.; Nakano, T.; Sakamoto, K.; Akaki, T.; Iida, Y.; Kadono, T.; Jogo, K.; et al. (The Meteoritical Society, 2009-01-01)
      Three-dimensional structures and elemental abundances of four impact tracks in silica aerogel keystones of Stardust samples from comet 81P/Wild 2 (bulbous track 67 and carrot-type tracks 46, 47, and 68) were examined non-destructively by synchrotron radiation-based microtomography and X-ray fluorescence analysis. Track features, such as lengths, volumes and width as a function of track depth, were obtained quantitatively by tomography. A bulbous portion was present near the track entrance even in carrot-type tracks. Each impact of a cometary dust particle results in the particle disaggregated into small pieces that were widely distributed on the track walls as well as at its terminal. Fe, S, Ca, Ni, and eight minor elements are concentrated in the bulbous portion of track 68 as well as in terminal grains. It was confirmed that bulbous portions and thin tracks were formed by disaggregation of very fine fragile materials and relatively coarse crystalline particles, respectively. The almost constant ratio of whole Fe mass to track volume indicates that the track volume is almost proportional to the impact kinetic energy. The size of the original impactor was estimated from the absolute Fe mass by assuming its Fe content (CI) and bulk density. Relations between the track sizes normalized by the impactor size and impact conditions are roughly consistent with those of previous hypervelocity impact experiments.
    • Book Review: Isotope Geology, Claude J. Allègre

      Jull, A. J. T. (The Meteoritical Society, 2009-01-01)
    • A combined ToF-SIMS and EMP/SEM study of a three-phase symplectite in the Los Angeles basaltic shergottite

      Rost, D.; Stephan, T.; Greshake, A.; Fritz, J.; Weber, I.; Jessberger, E. K.; Stöffler, D. (The Meteoritical Society, 2009-01-01)
      A rare three-phase symplectite consisting of Ca-rich pyroxene, Fe-rich olivine, and a silica phase is frequently found rimming pyroxene in the Martian meteorite Los Angeles. This assemblage is usually interpreted as the breakdown product of metastable pyroxferroite, a very rare pyroxenoid mineral itself. However, its origin is not entirely understood, mainly because the extremely small average size of the constituent phases represents a challenge for precise high-resolution analysis. In addition to electron microbeam methods, the present study uses time-of-flight secondary ion mass spectrometry (ToF-SIMS) to overcome the limits of spatial resolution and to comprehensively study this mineral assemblage. The prevailing breakdown hypothesis is supported by the following results: (1) The three symplectite phases are very homogenous in composition from 100 micrometers down to the micrometer scale. (2) The silica phase could be shown to be almost pure SiO2. (3) The symplectite bulk composition is consistent with pyroxferroite. Sub micrometer sized Ti-oxide grains are found within the symplectite (but not within the Ca-rich pyroxene) and probably represent a minor breakdown phase in addition to the three main phases.
    • Estimate of the magnetic field of Mars based on the magnetic characteristics of the Yamato 000593 nakhlite

      Funaki, M.; Hoffmann, V.; Imae, N. (The Meteoritical Society, 2009-01-01)
      Yamato 000593, a nakhlite, was analyzed in terms of its magnetic record and magnetomineralogy. The natural remanent magnetization (NRM: 3.55-6.07 x 10^(-5) Am^2/kg) was thermally demagnetized at ~320 degrees degrees C, and it was unstable against alternating field demagnetization. Based on analyses of thermomagnetic curves, the temperature dependence of hysteresis parameters, and microscopic observations, the magnetic minerals mainly consist of magnetite (0.68 wt% of the sample, including ~5% Fe2TiO4) of less than 100 m in size, associated with minor amounts of monoclinic pyrrhotite (<0.069 wt% of the sample) and goethite. Thermal demagnetization of NRM at ~330 degrees C is explained due to an offset of magnetization of antipodal NRM components of magnetite, whereas it is not due to a pyrrhotite Curie point. Large magnetite grains show exsolution texture with ilmenite laths, and are cut by silicate (including goethite) veins that formed along cracks. Numerous single-domain (SD) and pseudo-single-domain (PSD) magnetite grains are scattered in the mesostasis and adjacent olivine grains. Moderate coercive forces of HC = 6.8 mT and HRC = 31.1 mT suggest that Yamato 000593 is fundamentally able to carry a stable NRM; however, NRM was found to be unstable. Accordingly, the meteorite was possibly crystallized at 1.3 Ga under an extremely weak or absent magnetic field, or was demagnetized by impact shock at 12 Ma (ejection age) on Mars. This finding differs from the results of previous paleomagnetic studies of SNC (shergottites, nakhlites, chassignites, and orthopyroxenite) Martian meteorites. The significant dipole magnetic field resulting from the molten metallic core was probably absent during the Amazonian Epoch (after 1.8 Ga) on Mars.
    • Identification of the Bloody Creek structure, a possible impact crater in southwestern Nova Scotia, Canada

      Spooner, I.; Stevens, G.; Morrow, J.; Pufahl, P.; Grieve, R.; Raeside, R.; Pilon, J.; Stanley, C.; Barr, S.; McMullin, D. (The Meteoritical Society, 2009-01-01)
      An approximately 0.4 km diameter elliptical structure formed in Devonian granite in southwestern Nova Scotia, herein named the Bloody Creek structure (BCS), is identified as a possible impact crater. Evidence for an impact origin is based on integrated geomorphic, geophysical, and petrographic data. A near-continuous geomorphic rim and a 10 m deep crater that is infilled with lacustrine sediments and peat define the BCS. Ground penetrating radar shows that the crater has a depressed inner floor that is sharply ringed by a 1 m high buried scarp. Heterogeneous material under the floor, interpreted as deposits from collapse of the transient cavity walls, is overlain by stratified and faulted lacustrine and wetland sediments. Alteration features found only in rim rocks include common grain comminution, polymict lithic microbreccias, kink-banded feldspar and biotite, single and multiple sets of closely spaced planar microstructures (PMs) in quartz and feldspar, and quartz mosaicism, rare reduced mineral birefringence, and chlorite showing plastic deformation and flow microtextures. Based on their form and crystallographic orientations, the quartz PMs consist of planar deformation features that document shock-metamorphic pressures less than or equal to 25 GPa. The age of the BCS is not determined. The low diameter-to-depth ratio of the crater, coupled with anomalously high shock-metamorphic pressures recorded at its exposed rim, may be a result of significant post-impact erosion. Alternatively, impact onto glacier ice during the waning stages of Wisconsinian deglaciation (about 12 ka BP) may have resulted in dissipation of much impact energy into the ice, resulting in the present morphology of the BCS.
    • Stuart H. Perry's contributions to meteorite collection and research, 1927-1957

      Plotkin, H.; Clarke, R. S. (The Meteoritical Society, 2009-01-01)
      Stuart H. Perry (1874-1957), an influential Michigan newspaper editor and publisher and a vice president of the Associated Press, developed a passionate interest in collecting and studying meteorites in the 1920s and 1930s. Firmly believing that meteorites belong in great museums where they can be properly investigated, he generously donated his meteorites to various museums after he finished his own study of them. He had a sincere interest in the National Collection of Meteorites, and donated 192 specimens--mostly irons--to the U.S. National Museum; these constituted some of the most important meteorites in its collection, and moved iron meteorites to center stage, a position still occupied. By applying current metallographic methods to the study of iron meteorites, Perry directed scientists to a powerful new research tool, which led to major advances in our understanding of meteoritic irons and helped give rise to a new field within planetary sciences. His groundbreaking monograph The metallography of meteoric iron served as a standard reference collection of metallographic photomicrographs of iron meteorites for more than 30 years. It remained an insightful and useful work on the structure of meteoritic iron until improved binary and ternary phase diagrams in the Fe-Ni(-P) system allowed a more detailed treatment of the formation of iron meteorites. Perry received many honors for his work, and held office in the Meteoritical Society, serving as a councilor from 1941-1950, and as a vice president from 1950-1957.
    • Collisional modification of the acapulcoite/lodranite parent body revealed by the iodine-xenon system in lodranites

      Crowther, S. A.; Whitby, J. A.; Busfield, A.; Holland, G.; Busemann, H.; Gilmour, J. D. (The Meteoritical Society, 2009-01-01)
      The I-Xe system of three lodranites has been investigated. Samples of Gibson yielded no isochrons, and late model ages are attributed to late addition of iodine. Two metal and one silicate separate from the transitional lodranite Graves Nunataks (GRA) 95209 gave ages that are consistent with each other and with the literature I-Xe age of Acapulco feldspar. These yield a mean closure age 4.19 +/- 0.53 Ma after the Shallowater enstatite reference age (4562.3 +/- 0.4 Ma). Such identical I-Xe ages from distinct phases imply that the parent material underwent a period of rapid cooling, the absolute age of this event being 4558.1 +/- 0.7 Ma. Such rapid cooling indicates an increase in the rate at which heat could be conducted away, requiring a significant modification of the parent body. We suggest the parent body was modified by an impact at or close to the time recorded by the I-Xe system. An age of 10.4 +/- 2.3 Ma after Shallowater has been determined for one whole-rock sample of Lewis Cliff (LEW) 88280. Since the release pattern is similar to that of GRA 95209 this hints that the larger grain size of this sample may reflect slower cooling due to deeper post impact burial.
    • What metal-troilite textures can tell us about post-impact metamorphism in chondrite meteorites

      Tomkins, A. G. (The Meteoritical Society, 2009-01-01)
      Metal-troilite textures are examined in metamorphosed and impact-affected ordinary chondrites to examine the response of these phases to rapid changes in temperature. Complexly intergrown metal-troilite textures are shown to form in response to three different impact-related processes. (1) During impacts, immiscible melt emulsions form in response to spatially focused heating. (2) Immediately after impact events, re-equilibration of heterogeneously distributed heat promotes metamorphism adjacent to zones of maximum impact heating. Where temperatures exceed ~850 degrees degrees C, this post-impact metamorphism results in melting of conjoined metal-troilite grains in chondrites that were previously equilibrated through radiogenic metamorphism. When the resulting Fe-Ni-S melt domains crystallize, a finely intergrown mixture of troilite and metal forms, which can be zoned with kamacite-rich margins and taenite-rich cores. (3) At lower temperatures, post-impact metamorphism can also cause liberation of sulfur from troilite, which migrates into adjacent Fe-Ni metal, allowing formation of troilite and occasionally copper within the metal during cooling. Because impact events cause heating within a small volume, post-impact metamorphism is a short duration event (days to years) compared with radiogenic metamorphism (>10^6 years). The fast kinetics of metal-sulfide reactions allows widespread textural changes in conjoined metal-troilite grains during post-impact metamorphism, whereas the slow rate of silicate reactions causes these to be either unaffected or only partially annealed, except in the largest impact events. Utilizing this knowledge, information can be gleaned as to whether a given meteorite has suffered a post-impact thermal overprint, and some constraints can be placed on the temperatures reached and duration of heating.
    • Chemical identification of comet 81P/Wild 2 dust after interacting with molten silica aerogel

      Rietmeijer, F. J. M. (The Meteoritical Society, 2009-01-01)
      Flight aerogel in Stardust allocation C2092,2,80,47,6 contains percent level concentrations of Na, Mg, Al, S, Cl, K, Ca, Cr, Mn, Fe, and Ni that have a distinctive Fe- and CI-normalized distribution pattern, which is similar to this pattern for ppb level chemical impurities in pristine aerogel. The elements in this aerogel background were assimilated in non-vesicular and vesicular glass with the numerous nanometer Fe-Ni-S compound inclusions. After correction for the background values, the chemical data show that this piece of comet Wild 2 dust was probably an aggregate of small (<500 nm) amorphous ferromagnesiosilica grains with many tiny Fe,Ni-sulfide inclusions plus small Ca-poor pyroxene grains. This distinctive Fe- and CI-normalized element distribution pattern is found in several Stardust allocations. It appears to be a common feature in glasses of quenched aerogel melts but its exact nature is yet to be established.
    • Formation age of the lunar crater Giordano Bruno

      Morota, T.; Haruyama, J.; Miyamoto, H.; Honda, C.; Ohtake, M.; Yokota, Y.; Matsunaga, T.; Hirata, N.; Demura, H.; Takeda, H.; et al. (The Meteoritical Society, 2009-01-01)
      Using the Terrain Camera onboard the Japanese lunar explorer, SELENE (Kaguya), we obtained new high-resolution images of the 22-kilometer-diameter lunar crater Giordano Bruno. Based on crater size-frequency measurements of small craters (<200 m in diameter) superposed on its continuous ejecta, the formation age of Giordano Bruno is estimated to be 1 to 10 Ma. This is constructive evidence against the crater's medieval age formation hypothesis.
    • Shocked quartz grains in the polymict breccia of the Granby structure, Sweden—Verification of an impact

      Alwmark, C. (The Meteoritical Society, 2009-01-01)
      The Middle Ordovician Granby structure in Sweden is generally considered the result of an asteroidal or cometary collision with Earth, although no hard evidence, i.e., shock metamorphic features or traces of the impactor, have been presented to date. In this study, drill core samples of a sedimentary breccia from the Granby structure have been investigated for microscopic shock metamorphic evidence in an attempt to verify the impact genesis of the structure. The finding of multiple sets of decorated planar deformation features (PDFs) in quartz grains in these samples provides unambiguous evidence that the structure is impact derived. Furthermore, the orientation of the PDFs, e.g., omega {1013}, pi {1012} and r, z {1011}, is characteristic for impact deformation. The fact that a majority of the PDFs are decorated implies a water-bearing target. The shocked quartz grains can be divided into two groups; rounded grains found in the breccia matrix likely originated from mature sandstone, and angular grains in fragments from crystalline target rocks. The absence of melt particles provides an estimated maximum shock pressure for the sedimentary derived quartz of 15-20 GPa and the frequency distribution of PDF orientations in the bedrock quartz implies pressures of the order of 10 GPa.
    • Atmospheric erosion induced by oblique impacts

      Shuvalov, V. (The Meteoritical Society, 2009-01-01)
      Previous investigations of impact-induced atmospheric erosion considered vertical impacts only. Numerical simulations of oblique impacts presented in this paper show that the loss of air strongly depends on trajectory inclination and it increases as the impact angle decreases. The results of numerical simulations over the wide range of impact parameters (projectile sizes from 1 to 30 km, impact velocities from 15 to 70 km/s, escape velocities from 5 to 11.2 km/s, projectile densities from 1 to 3.3 g/cm3, normal atmospheric densities varying by three orders of magnitude) can be approximated by simple analytical formulae.