• Chondrule thermal history from unequilibrated H chondrites: A transmission and analytical electron microscopy study

      Ferraris, C.; Folco, L.; Mellini, M (The Meteoritical Society, 2002-01-01)
      Sixteen texturally different (porphyritic, barred, radial, cryptocrystalline) FeO-rich chondrules from the unequilibrated ordinary chondrites Brownfield, Frontier Mountain (FRO) 90003 and FRO 90032 were characterized by optical and scanning electron microscopy and then thoroughly studied by transmission and analytical electron microscopy. Nanotextural and nanochemical data indicate similar thermal evolution for chondrules of the same textural groups; minor, yet meaningful differences occur among the different groups. Olivine is the earliest phase formed and crystallizes between 1500 and 1400 degrees C. Protoenstatite crystallizes at temperatures higher than 1350-1200 C; it later inverts to clinoenstatite in the 1250-1200 degrees C range. Enstatite is surrounded by pigeonitic or (less frequently) augitic rims; the minimal crystallization temperature for the rims is 1000 degrees C; high pigeonite later inverts to low pigeonite, between 935 and 845 degrees C. The outer pigeonitic or augitic rims are constantly exsolved, producing sigmoidal augite or enstatite precipitates; sigmoidal precipitates record exsolution temperatures between 1000 and 640 degrees C. Cooling rate (determined using the speedometer based upon ortho-clinoenstatite intergrowth) was in the order of 50-3000 degrees C/h at the clinoenstatite-orthoenstatite transition temperature (close to 1250-1200 degrees C), but decreased to 5-10 degrees C/h or slower at the exsolution temperature (between 1000 and 650 degrees C), thus revealing nonlinear cooling paths. Nanoscale observations indicate that the individual chondrules formed and cooled separately from 1500 down to at least 650 degrees C. Accretion into chondritic parent body occurred at temperatures lower than 650 degrees C.
    • Combined noble gas and trace element measurements on individual stratospheric interplanetary dust particles

      Kehm, K.; Flynn, G. J.; Sutton, S. R.; Hohenberg, C. M (The Meteoritical Society, 2002-01-01)
      The trace element compositions and noble gas contents of 32 individual interplanetary dust particles (IDPs) collected in the Earth's stratosphere were measured. Trace element compositions are generally similar to CI meteorites, with occasional depletions in Zn/Fe with respect to CI. Noble gases were detected in all but one of the IDPs. Noble gas elemental compositions are consistent with the presence of fractionated solar wind. A rough correlation between surface-normalized He abundances and Zn/Fe ratios is observed; Zn-poor particles generally have lower He contents than the other IDPs. This suggests that both elements were lost by frictional heating during atmospheric entry and confirms the view that Zn can serve as an entry-heating indicator in IDPs.
    • Discovery of an asteroid and quasi-satellite in an Earth-like horseshoe orbit

      Connors, M.; Chodas, P.; Mikkola, S.; Wiegert, P.; Weillet, C.; Innanen, K. (The Meteoritical Society, 2002-01-01)
      The newly discovered asteroid 2002 AA29 moves in a very Earth-like orbit that relative to Earth has a unique horseshoe shape and allows transitions to a quasi-satellite state. This is the first body known to be in a simple heliocentric horseshoe orbit, moving along its parent planet's orbit. It is similarly also the first true co-orbital object of Earth, since other asteroids in 1:1 resonance with Earth have orbits very dissimilar from that of our planet. When a quasi-satellite, it remains within 0.2 AU of the Earth for several decades. 2002 AA29 is the first asteroid known to exhibit this behavior. 2002 AA29 introduces an important new class of objects offering potential targets for space missions and clues to asteroid orbit transfer evolution.
    • Ejection ages from krypton-81-krypton-83 dating and pre-atmospheric sizes of martian meteorites

      Eugster, O.; Busemann, H.; Lorenzetti, S.; Terribilini, D. (The Meteoritical Society, 2002-01-01)
      Cosmic-ray exposure (CRE) ages and Mars ejection times were calculated from the radionuclide 81Kr and stable Kr isotopes for seven martian meteorites. The following 81Kr-Kr CRE ages were obtained: Los Angeles = 3.35 +/- 0.70 Ma; Queen Alexandra Range 94201 = 2.22 +/- 0.35 MA; Shergotty = 3.05 +/- 0.50 Ma; Zagami = 2.98 +/- 0.30 Ma; Nakhla = 10.8 +/- 0.8 Ma; Chassigny = 10.6 +/- 2.0 Ma; and Allan Hills 84001 = 15.4 +/- 5.0 Ma. Comparison of these age with previously obtained CRE ages from the stable noble gas nuclei 3He, 21Ne, and 38Ar shows excellent agreement. This indcates that the method for the production rate calculation for the stable nuclei is reliable. in all martian meteorites we observe effects induced by secondary cosmic-ray produced epithermal neutrons. Epithermal neutron fluxes, phi(n) (30-300 cV), are calculated based on the reaction 79Br(n, gamma x Beta)80Kr. We show that the neutron capture effects were induced in free space during Mars-Earth transfer of the meteorids and that they are not due to a pre-exposure on Mars before ejection of the meteoritic material. Neutron fluxes and slowing down densities experienced by the meteoroids are calculated and pre-atmospheric sizes are estimated. We obtain minimum radii in the range of 22-25 cm and minimum masses of 150-220 kg. These results are in good agreement with the mean sizes reported for model calculations using current semiempirical data.
    • Gas-melt interactions and their bearing on chondrule formation

      Tissandier, T.; Libourel, G.; Robert, F. (The Meteoritical Society, 2002-01-01)
      Interactions between nebular gas and molten silictes or oxides coul have played a major role in the formation and differentiation of the first solids formed in the solar system. In order to simulate such interactions, we set up a new experimental device in which isothermal condensation experiments have been conducted. Partially molten chondrule-like samples have been exposed to high SiO(g) partial pressures, for intervals between 80 and 300 s and at temperatures ranging from 1600 to 1750 K. Results show that silica entering from the gas phase could be responsible for several textural and mineralogical featues observed in natural chondrules. For instance, these experiments reproduce not only the mineralogical zonation of porphyritic olivine-pyroxene chondrules with the peripheral location of pyroxenes, but also olivine resorption textures and the common poikilitical enclosure of olivines in pyroxenes. In the light of these similarities, we advocate that gas-melt interactions through condensation are viable mechanisms for chondrule formation and hence may place severe constrains on the history of these primitive objects. In the nebula, high SiO(g) partial pressures could have been established by the volatization of regions with high dust/gas ratio. A possible scenario for this stochastic thermal activity is the intense activity of the protosun in its young stellar object phase.
    • Impact-induced microbial endolithic habitats

      Cockell, C. S.; Lee, P.; Osinski, G.; Horneck, G.; Broady, P. (The Meteoritical Society, 2002-01-01)
      Asteroid and comet impacts on Earth are commonly viewed as agents of ecosystem destruction, be it on local or global scales. However, for some microbial communities, impacts may represent an opportunity for habitat formation as some substrates are rendered more suitable for colonization when processed by impacts. We describe how heavily shocked gneissic crystalline basement rocks exposed at the Haughton impact structure, Devon Island, Nunavut, Arctic Canada, are hosts to endolithic photosynthetic microorganisms in significantly greater abundance than lesser-shocked or unshocked gneisses. Two factors contribute to this enhancement: (a) increased porosity due to impact fracturing and differential mineral vaporization, and (b) increased translucence due to the selective vaporization of opaque mineral phases. Using biological ultraviolet radiation dosimetry, and by measuring the concentrations of photoprotective compounds, we demonstrate that a covering of 0.8 mm of shocked gneiss can provide substantial protection from ultraviolet radiation, reducing the inactivation of Bacillus subtilis spores by 2 orders of magnitude. The colonisation of the shocked habitat represents a potential mechanism for pioneer microorganisms to invade an impact structure in the earliest stages of post-impact primary succession. The communities are analogous to the endolithic communities associated with sedimentary rocks in Antarctica, but because they occur in shocked crystalline rocks, they illustrate a mechanism for the creation of microbial habitats on planetary surfaces that do not have exposed sedimentary units. This might have been the case on early Earth. The data have implications for the microhabitats in which biological signatures might be sought on Mars.
    • Microscopic search for the carrier phase Q of the trapped planetary noble gases in Allende, Leoville, and Vigarano

      Vis, R. D.; Mrowiec, A.; Kooyman, P. J.; Matsubara, K.; Heymann, D. (The Meteoritical Society, 2002-01-01)
      High-resolution transmission electron microscopy micrographs of acid-resistant residues of the Allende, Leoville, and Vigarano meteorites show a great variety of carbon structures: curved and frequently twisted and intertwined graphene sheets, abundant carbon black-like particles, and hollow "sacs". It is suggested that perhaps all of these are carriers for the planetary Q-noble gases in these meteorites. Most of these materials are pyrocarbons that probably formed by the pyrolysis of hydrocarbons either in a gas phase, or on hot surfaces of minerals. An attempt was made to analyze for argon with particle-induced x-ray emission in 143 spots of grains of floating and suspended matter from freeze-dry cycles of an Allende bulk sample in water, and floating "black balls" from sonication in water of samples from the Allende meteorite. The chemical compositions of these particles were obtained, but x-ray signals at the wavelength of argon were obtained on only a few spots.
    • Properties of chondrules in EL3 chondrites, comparison with EH3 chondrites, and the implications for the formation of enstatite chondrites

      Schneider, D. M.; Symes, S. J. K.; Benoit, P. H.; Sears, D. W. G. (The Meteoritical Society, 2002-01-01)
      The study of chondrules provides information about processes occuring in the early solar system. In order to ascertain to what extent these processes played a role in determining the properties of the enstatite chondrites, the physical and chemical properties of chondrules from three EL3 chondrites and three EH3 chondrites have been examined by optical, cathodoluminescence (CL), and electron microprobe techniques. Properties examined include size, texture, CL, and composition of both individual phases and bulk chondrules. The textures, distribution of textures, and composition of silicates of the EL3 chondrules resemble those of EH3 chondrules. However, the chondrules from the two classes differ in that (1) the size distribution of the EL chondrules is skewed to larger values than EH chondrules, (2), the enstatite in EL chondrules displays varying shades of red CL due to the presence of fine-grained sulfides and meetal in the silicates, and (3) the mesostatis of EH chondrules is enriched in Na relative to that of EL chondrules. The similarities between the chondrules of the two classes suggest similar precursor materials, while the differences suggest that there was not a single reservoir of meteoritic chondrules, but that their origin was fairly local. The differences in the size distribution of chondrules in EH and EL chondrites may be explained by aerodynamic and gravitational sorting during accumulation of the meteoritic material, while differences in CL and mesostasis properties may reflect differences in formation conditions and cooling rate following chondrule formation. We argue that our observations are consistent with the formation of enstatite chondrites in a thick dynamic regolith on their parent body.
    • Size-frequency distributions of chondrules and chondrule fragments in LL3 chondrites: Implications for parent-body fragmentation of chondrules

      Nelson, V. E.; Rubin, A. E. (The Meteoritical Society, 2002-01-01)
      We measured the sizes and textural types of 719 intact chondrules and 1322 chondrule fragments in thin sections of Semarkona (LL3.0), Bishunpur (LL3.1), Krymka (LL3.1), Piancaldoli (LL3.4) and Lewis Cliff 88175 (LL3.8). The mean apparent diameter of chondrules in these LL3 chondrites is 0.80 phi units or 570 micrometers, much smaller than the previous rough estimate of ~900 micrometers. Chondrule fragments in the five LL3 chondrites have a mean apparent cross-section of 1.60 phi units or 330 micrometers. The smallest fragments are isolated olivine and pyroxene grains; these are probably phenocrysts liberated from disrupted porphyritic chondrules. All five LL3 chondrites have fragment/chondrule number ratios exceeding unity, suggesting that substantial numbers of the chondrules in these rocks were shattered. Most fragmentation probably occurred on the parent asteroid. Porphyritic chondrules (porphyritic olivine + porphyritic pyroxene + porphyritic olivine-pyroxene) are more readily broken than droplet chondrules (barred olivine + radial pyroxene + cryptocrystalline). The porphyritic fragment/chondrule number ratio (2.0) appreciably exceeds that of droplet-textured objects (0.9). Intact droplet chondrules have a larger mean size than intact porphyritic chondrules, implying that large porphyritic chondrules are fragmented preferentially. This is consistent with the relatively low percentage of porphyritic chondrules within the set of the largest chondrules (57%) compared to that within the set of the smallest chondrules (81%). Differences in mean size among chondrule textural types may be due mainly to parent-body chondrule-fragmentation events and not to chondrule-formation processes in the solar nebula.
    • The first known natural occurrence of calcium monoaluminate, in a calcium-aluminum-rich inclusion from the CH chondrite Northwest Africa 470

      Ivanova, M. A.; Petaev, M. I.; MacPherson, G. J.; Nazarov, M. A.; Taylor, L. A.; Wood, J. A. (The Meteoritical Society, 2002-01-01)
      Natural calcium monoaluminate, CaA12O4, has been found in a grossite-rich calcium-aluminum-rich inclusion (CAI) from the CH chondrite Northwest Africa 470. The calcium monoaluminate occurs as colorless ~10 micrometer subhedral grains intergrown with grossite, perovskite, and melilite. Nebular condensation is the most likely origin for the precursor matrials of this CAI, but calculations suggest that dust/gas ratios substantially enhanced over solar are required to stabilize CaA12O4.
    • Yamato 792947,793408 and 82038: The most primitive H chondrites, with abundant refractory inclusions

      Kimura, M.; Hiyagon, H.; Palme, H.; Spettel, B.; Wolf, D.; Clayton, R. N.; Mayeda, T. K.; Sato, T.; Suzuki, A.; Kojima, H. (The Meteoritical Society, 2002-01-01)
      In this paper we report petrological and chemical data of the unusual chondritic meteorites Yamato (Y)-792947, Y-93408 and Y-82038. The three meteorites are very similar in texture and chemical composition, suggesting that they are pieces of a single fall. The whole-rock oxygen isotopes and the chemical compositions are indicative ofH chondrites. In addition, the mineralogy, and the abundances of chondrule types, opaque minerals and matrices suggest that these meteorites are H3 chondrites. They were hardly affected by thermal and shock metamorphism. The degree of weathering is very low. We conclude that these are the most primitive H chondrites, H3.2-3.4 (SI), known to date. On the other hand, these chondrites contain extraordinarily high amounts of refractory inclusions, intermediate between those of ordinary and carbonaceous chondrites. The distribution of the inclusions may have been highly heterogeneous in the primitive solar nebula. The mineralogy, chemistry and oxygen isotopic compositions of inclusions studied here are similar to those in CO and E chondrites.