• A comparative study of opaque phases in Qingzhen (EH3) and MacAlpine Hills 88136 (EL3): Representatives of EH and EL parent bodies

      Lin, Y.; Goresy, A. El (The Meteoritical Society, 2002-01-01)
      Opaque minerals in the Qingzhen (EH3) and MacAlpine Hills (MAC) 88136 (EL3) enstatite chondrites were studied and compared with other EH and EL chondrites. All opaque minerals usually occur in multi-sulfide-metal clasts and nodules in the matrix between chondrules (El Goresy et al., 1988). The higher abundance of opaque minerals, the occurrence of niningerite and various alkali-sulfides (e.g., caswellsilverite, phases A and B, djerfisherite) are diagnostic criteria for EH chondrites, while alabandite is characteristic for EL chondrites. In addition, EH chondrites are characterized by enrichments of Si in both kamacite and perryite, and alkali elements in sphalerite and chalcopyrite. The Mn contents of daubreelite and sphalerite are lower in EH than in EL chondrites. These are consistent with lower oxygen fugacity and higher H2S fugacity of EH than EL chondrites. In contrast, the discovery of sphalerite and Zn-rich daubreelite in MAC 88136 indicates that their absence in EL6 chondrites is probably related to thermal metamorphism in the parent body. Schreibersite microspherules are commonly enclosed in most sulfides in Qingzhen, but are absent in MAC 88136. They were once molten, and probably predated all sulfide host phases. The petrographic setting and chemical compositions of the sulfide hosts of the schreibersite microspherules in EH3 chondrites are consistent with formation by condensation. The earliest sulfide condensates oldhamite and niningerite occupy the interiors of the clasts and nodules, whereas the rims consist of troilite and djerfisherite. In addition, in Qingzhen, some other troilite, djerfisherite and sphalerite assemblages coexist with perryite. They were produced by sulfurization of metallic Fe-Ni in the nebula. In MAC 88136, sulfurization of Si-bearing Fe-Ni metal is less pronounced, and it produced troilite, schreibersite and less abundant perryite. Two kinds of normal zoning and a reverse zoning trends of niningerite, and both normal and reverse zoning of sphalerite were found in clasts and nodules in Qingzhen. The coexistence of normal and reverse zoning profiles in niningerite grains in the same meteorite strongly suggests that they formed before accretion in the parent body, because an asteroidal metamorphic or an impact event in the parent body would have erased these contrasting profiles and destroyed the textural settings. In contrast, alabandite in MAC 88136 shows only normal zoning, with the FeS content decreasing to 9.3 mol% toward troilite, indicating very slow cooling at low temperature
    • A hibonite-corundum inclusion from Murchison: A first-generation condensate from the solar nebula

      Simon, S. B.; Davis, A. M.; Grossman, L.; McKeegan, K. D. (The Meteoritical Society, 2002-01-01)
      Through freeze-thaw disaggregation of the Murchison (CM) carbonaceous chondrite, we have recovered a ~90 x 75 micrometers refractory inclusion that consists of corundum and hibonite with minor perovskite. Corundum occurs as small (~10 micrometers), rounded grains enclosed in hibonite laths (~10 micrometers wide and 30-40 micrometers long) throughout the inclusion. Perovskite predominantly occurs near the edge of the inclusion. The crystallization sequence inferred petrographically--corundum followed by hibonite followed by perovskite--is that predicted for the first phases to form by equilibrium condensation from a solar gas for Ptot is less than or equal to 5 x 10^(-3) atm. In addition, the texture of the inclusion, with angular voids between subhedral hibonite laths and plates, is also consistent with formation of the inclusion by condensation. Hibonite has heavy rare earth element (REE) abundances of ~40 x CI chondrites, light REE abundances ~20 x CI chondrites, and negative Eu anomalies. The chondrite-normalized abundance patterns, especially one for a hibonite-perovskite spot, are quite similar to the patterns of calculated solid/gas partition coefficients for hibonite and perovskite at 10^(-3) atm and are not consistent with formation of the inclusion by closed-system fractional crystallization. In contrast with the features that are consistent with a condensation origin, there are problems with any model for the formation of this inclusion that includes a molten stage, relic grains, or volatilization. If thermodynamic models of equilibrium condensation are correct, then this inclusion formed at pressures <5 x 10^(-3) atm, possibly with enrichments (<1000x) in CI dust relative to gas at low pressures (below 10^(-4) atm). Both hibonite and corundum have delta-17O is approximately equal to delta-18O which is approximately equal to -50 ppm, indicating formation from an 16O-rich source. The inclusion does not contain radiogenic 26Mg and apparently did not contain live 26Al when it formed. If the short-lived radionuclides were formed in a supernova and injected into the early solar nebula, models of this process suggest that 26Al-free refractory inclusions such as this one formed within the first ~6 x 10^5 years of nebular collapse.
    • An improved shadow measurement technique for constraining the morphometry of simple impact craters

      Chappelow, J. E.; Sharpton, V. L. (The Meteoritical Society, 2002-01-01)
      The lengths of the shadows cast within simple, bowl-shaped impact craters have been used to constrain their depths on a variety of planetary bodies. This technique, however, only yields the "true" crater depth if the shadow transects the crater center where the floor is deepest. In the past, attempts have been made to circumvent this limitation by choosing only craters where the shadow tip lies very near the crater center; but this approach may introduce serious artifacts that adversely affect the slope of the regressed depth vs. diameter data and its variance. Here we introduce an improved method for deriving depth information from shadow measurements that considers three basic shape variations of simple craters: paraboloidal, conical, and flat-floored. We show that the shape of the cast shadow can be used to constrain crater shape and we derive improved equations for finding the depths of these simple craters.
    • Book Review: Astrobiology, Monica Grady

      Brewer, J. (The Meteoritical Society, 2002-01-01)
    • Book Review: Moon Lander, Thomas J. Kelly

      Warren, P. H. (The Meteoritical Society, 2002-01-01)
    • Book Review: Noble Gas Geochemistry, Minoru Ozima, Frank A. Podosek

      Wieler, R. (The Meteoritical Society, 2002-01-01)
    • Book Review: Solar System Dynamics, C. D. Murray, S. F. Dermott

      Scheeres, D. J. (The Meteoritical Society, 2002-01-01)
    • From the Editors: About noble gases in E chondrites

      Busemann, H. (The Meteoritical Society, 2002-01-01)
    • From the Editors: Opaque minerals in primitive stony meteorites

      Lauretta, D. S. (The Meteoritical Society, 2002-01-01)
    • Itawa Bhopji (L3-5) chondrite regolith breccia: Fall, classification, and cosmogenic records

      Bhandari, N.; Murty, S. V. S.; Shukla, P. N.; Shukla, A. D.; Mahajan, R. R.; Sarin, M. M.; Srinivasan, G.; Suthar, K. M.; Sisodia, M. S.; Jha, S.; et al. (The Meteoritical Society, 2002-01-01)
      A stony meteorite fell at Itawa Bhopji, Rajasthan, India on 2000 May 30. This is the fifth recorded fall in a small area of Rajasthan during the past decade. The meteorite is an ordinary chondrite with light clasts in a dark matrix, consisting of a mixture of equilibrated (mainly type 5) and unequilibrated components. Olivine is Fa24-26 and pyroxene Fs20-22 but, within the unequilibrated components, olivine (Fa5-29) and low calcium pyroxene (Fs5-37) are highly variable. Based on petrographic studies and chemical analyses, it is classified as L(3-5) regolith breccia. Studies of various cosmogenic records, including several gamma-emitting radionuclides varying in half-life from 5.6 day 52Mn to 0.73 Ma 26Al, tracks and rare gases have been carried out. The exposure age of the meteorite is estimated from cosmogenic components of rare gases to be 19.6 Ma. The track density varies by a factor of ~3 (from 4 to 12 x 10^6/cm^2) within the meteorite, indicating a preatmospheric body of ~9 cm radius (corresponding to a meteoroid mass of ~11 kg) and small ablation (1.5 to 3.6 cm). Trapped components in various rare gases are high and the solar component is present in the dark portion of the meteorite. Large excess of neutron-produced 82Kr and 128Xe in both the light and the dark lithology but very low 60Co, indicating low neutron fluxes received by the meteoroid in the interplanetary space, are clear signatures of an additional irradiation on the parent body.
    • Noble gases in enstatite chondrites II: The trapped component

      Patzer, A.; Schultz, L. (The Meteoritical Society, 2002-01-01)
      The trapped noble gas record of 57 enstatite chondrites (E chondrites) has been investigated. Basically, two different gas patterns have been identified dependent on the petrologic type. All E chondrites of type 4 to 6 show a mixture of trapped common chondritic rare gases (Q) and a subsolar component (range of elemental ratios for E4-6 chondrites: 36Ar/132Xe = 582 +/- 270 and 36Ar/84Kr = 242 +/- 88). E3 chondrites usually contain Q gases, but also a composition with lower 36Ar/132Xe and 36Ar/84Kr ratios, which we call sub-Q (36Ar/132Xe = 37.0 +/- 18.0 and 36Ar/84Kr = 41.7 +/- 18.1). The presence of either the subsolar or the sub-Q signature in particular petrologic types cannot be readily explained by parent body metamorphism as postulated for ordinary chondrites. We therefore present a different model that can explain the bimodal distribution and composition of trapped heavy noble gases in E chondrites. Trapped solar noble gases have been observed only in some E3 chondrites. About 30% of each group, EH3 and EL3 chondrites, amounting to 9% of all analyzed E chondrites show the solar signature. Notably, only one of those meteorites has been explicitly described as a regolith breccia.
    • Petrographic studies of the impact melts from Meteor Crater, Arizona, USA

      Hörz, F.; Mittlefehldt, D. W.; See, T. H.; Galindo, C. (The Meteoritical Society, 2002-01-01)
      We investigated the ballistically dispersed melts from Meteor Crater, Arizona, USA to determine the stratigraphic extent of its melt zone from the compositional relationship of melts and target rocks. Most melt particles are crystallized, hydrated, and oxidized; pristine glasses are rare. Hydration and oxidation occurred at ambient temperatures long after the impact. The preserved glasses are generally clear and texturally homogeneous, but unlike typical impact melts, they have unusually heterogeneous compositions, both within individual particles and from sample to sample. For example, the average SiO2 for individual particles ranges from 43 to 65%. The projectile content is unusually high and it is distributed bimodally, with specific samples containing either 5-10% or 20-30% FeO. These compositional heterogeneities most likely reflect the high carbonate content of the target rocks and the release of copious CO2 that dispersed the melts, thereby terminating melt flow and mixing. The high projectile content and the CO2 depleted residue of purely sedimentary rocks produced mafic melts that crystallized fine-grained olivine and pyroxene. The melts fall into three compositional groups reflecting variable proportions of the major target formations, Moenkopi, Kaibab, and Coconino. Least-square mixing calculations revealed one group to contain 55% Moenkopi, 40% quartz-rich, upper Kaibab, and 5% meteorite, suggesting a source depth of <30 m from the pre-impact surface. The other two melt groups have higher contents of meteorite (15-20%) and Kaibab (50-70%) and contain more SiO2 than average Kaibab. The additional quartz may have been derived from Coconino or the upper Kaibab, implying melt depths >90 m or <30 m, respectively. Additional studies, especially hydrocode calculations, are needed to better understand the source depth of these melts and their exceptionally high projectile content.
    • Petrology and chemistry of the basaltic shergottite North West Africa 480

      Barrat, J. A.; Gillet, Ph.; Sautter, V.; Jambon, A.; Javoy, M.; Göpel, C.; Lesourd, M.; Keller, F.; Petit, E. (The Meteoritical Society, 2002-01-01)
      North West Africa (NWA) 480 is a new martian meteorite of 28 g found in the Moroccan Sahara in November 2000. It consists mainly of large gray pyroxene crystals (the largest grains are up to 5 mm in length) and plagioclase converted to maskelynite. Excluding the melt pocket areas, modal analyses indicate the following mineral proportions: 72 vol% pyroxenes extensively zoned, 25% maskelynite, 1% phosphates (merrillite and chlorapatite), 1% opaque oxides (ilmenite, ulvospinel and chromite) and sulfides, and 1% others such as silica and fayalite. The compositional trend of NWA 480 pyroxenes is similar to that of Queen Alexandra Range (QUE) 94201 but in NWA 480 the pyroxene cores are more Mg-rich (En77-En65). Maskelynites display a limited zoning (An42-50Ab54-48Or2-4). Our observations suggest that NWA 480 formed from a melt with a low nuclei density at a slow cooling rate. The texture was achieved via a single-stage cooling where pyroxenes grew continuously. A similar model was previously proposed for QUE 94201 by McSween et al. (1996). NWA 480 is an Al-poor ferroan basaltic rock and resembles Zagami or Shergotty for major elements and compatible trace element abundances. The bulk rock analysis for oxygen isotopes yields Delta-17O = +0.42 ppm, a value in agreement at the high margin, with those measured on other shergottites (Clayton and Mayeda, 1996; Romanek et al., 1998; Franchi et al., 1999). Its CI-normalized rare earth element pattern is similar to those of peridotitic shergottites such as Allan Hills (ALH)A77005, suggesting that these shergottites shared a similar parent liquid, or at least the same mantle source.
    • Strontium and neodymium isotopic study of Libyan Desert Glass: Inherited Pan-African age signatures and new evidence for target material

      Schaaf, P.; Müller-Sohnius, D. M. (The Meteoritical Society, 2002-01-01)
      Libyan Desert Glass (LDG) is an impact-related, natural glass of still unknown target material. We have determined Rb-Sr and Sm-Nd isotopic ratios from seven LDG samples and five associated sandstones from the LDG strewn field in the Great Sand Sea, western Egypt. Planar deformation features were recently detected in quartz from these sandstones. 87Sr/86Sr ratios and e-Nd values for LDG range between 0.71219 and 0.71344, and between -16.6 and -17.8, respectively, and hence are distinct from the less radiogenic 87Sr/86Sr ratios of 0.70910-0.71053 and e-Nd values from -6.9 to -9.6 for the local sandstones from the LDG strewn field. Previously published isotopic ratios from the Libyan BP and Oasis crater sandstones are generally incompatible with our LDG values. LDG formation undoubtedly occurred at 29 Ma, but neither the Rb-Sr nor the Sm-Nd isotopic system were rehomogenised during the impact event, as we can deduce from Pan-African ages of ~540 Ma determined from the regression lines from a total of 14 LDG samples from this work and the literature. Together with similar Sr and Nd isotopic values for LDG and granitoid rocks from northeast Africa west of the Nile, these findings point to a sandy matrix target material for the LDG derived from a Precambrian crystalline basement, ruling out the Cretaceous sandstones of the former "Nubian Group" as possible precursors for LDG.