• Asteroid 3628 Božněmcová: Covered with angrite-like basalts?

      Cloutis, E. A.; Binzel, R. P.; Burbine, T. H.; Gaffey, M. J.; McCoy, T. J. (The Meteoritical Society, 2006-01-01)
      A detailed analysis of the reflectance spectrum of asteroid 3628 Božněmcová, previously identified as a possible ordinary chondrite parent body, indicates that its surface consists of an assemblage dominated by clinopyroxene and plagioclase feldspar. The clinopyroxene is Fe2+-bearing (likely in the range Fs~10-20), with >90% of the Fe2+ being present in the M1 crystallographic site (spectral type A). The clinopyroxene:plagioclase feldspar ratio is between ~2 and 3 (~55-75% clinopyroxene, ~20-33% plagioclase feldspar). If olivine is present, the clinopyroxene:olivine ratio is >~3 (<20% olivine). The derived mineralogy of Božněmcov is most similar, but not identical, to the known angrite meteorites. The data suggest that Božněmcov formed by melting and differentiation of an oxidized chondritic precursor and probably represents an unsampled angrite-like body.
    • Carbon isotopic composition of acetic acid generated by hydrous pyrolysis of macromolecular organic matter from the Murchison meteorite

      Oba, Y.; Naraoka, H. (The Meteoritical Society, 2006-01-01)
      Low molecular weight monocarboxylic acids, including acetic acid, are some of the most abundant organic compounds in carbonaceous chondrites. So far, the 13C- and D-enriched signature of water-extractable carboxylic acids has implied an interstellar contribution to their origin. However, it also has been proposed that monocarboxylic acids could be formed by aqueous reaction on the meteorite parent body. In this study, we conducted hydrous pyrolysis of macromolecular organic matter purified from the Murchison meteorite (CM2) to examine the generation of monocarboxylic acids with their stable carbon isotope measurement. During hydrous pyrolysis of macromolecular organic matter at 270-330 degrees C, monocarboxylic acids with carbon numbers ranging from 2 (C2) to 5 (C5) were detected, acetic acid (CH3COOH; C2) being the most abundant. The concentration of the generated acetic acid increased with increasing reaction temperature; up to 0.48 mmol acetic acid/g macromolecular organic matter at 330 degrees C. This result indicates that the Murchison macromolecule has a potential to generate at least ~0.4 mg acetic acid/g meteorite, which is about four times higher than the amount of water-extractable acetic acid reported from Murchison. The carbon isotopic composition of acetic acid generated by hydrous pyrolysis of macromolecular organic matter is ~-27 ppm (versus PDB), which is much more depleted in 13C than the water-extractable acetic acid reported from Murchison. Intramolecular carbon isotope distribution shows that methyl (CH3-)-C is more enriched in 13C relative to carboxyl (-COOH)-C, indicating a kinetic process for this formation. Although the experimental condition of this study (i.e., 270-330 degrees C for 72 h) may not simulate a reaction condition on parent bodies of carbonaceous chondrite, it may be possible to generate monocarboxylic acids at lower temperatures for a longer period of time.
    • Chronostratigraphy, composition, and origin of Ni-rich spinel from the Late Eocene Fuente Caldera section in Spain: One impact or more?

      Robin, E.; Molina, E. (The Meteoritical Society, 2006-01-01)
      Here we report on the stratigraphic distribution and chemical composition of Ni-rich spinel, a specific mineral tracer of meteorite impacts, in the Fuente Caldera section in Spain. A major peak in spinel abundance is observed in a biostratigraphic interval defined by the last occurrence of the planktic foraminifera Porticulasphaera semiinvoluta and the first occurrence of the planktic foraminifera Turborotalia cunialensis. Two other peaks of lower abundances are observed higher up in the same biostratigraphic interval, but geochemical considerations suggest that they likely originate from redeposition by turbiditic currents. Biostratigraphic correlations with the global stratotype section and point for the Eocene/Oligocene boundary of Massignano in Italy give an age of 35.4 +/- 0.2 Ma (1-sigma) for the major peak. This age is indistinguishable from the age of the impact horizon at Massignano (35.5 +/- 0.2 Ma) and within the age uncertainties for the Popigai (35.7 +/- 0.2 Ma) and Chesapeake Bay (35.5 +/- 0.5 Ma) craters. The Fuente Caldera spinel, as the Massignano spinel, is assumed to be a relic mineral of microkrystites, which are believed to derive from a unique source related to the Popigai impact crater. The morphologies and Cr compositions of the Fuente Caldera and Massignano spinel crystals are markedly different, however: the Fuente Caldera spinel occurs mostly as octahedral and skeletal crystals with 85% of the grains belonging to the Cr-rich magnetite series and 15% to the Fe-rich chromite series, whereas the Massignano spinel occurs mostly as dendritic crystals with 90% of the grains belonging to the Cr-poor magnetite series. It is unlikely that these differences are the result of post-depositional alteration processes because the compositions of the crystals, as well as their morphologies, are in general very similar to those reported for primary spinel crystals, i.e., spinel crystals present in meteorite fusion crust or synthetized from meteoritic material. In addition, spinel crystals have quite homogeneous compositions except for a few grains (<10%) showing Cr zonations, but these are assigned to primary crystallization processes. One possible explanation that is consistent with a single impact event producing spatial variations in spinel compositions and morphologies is that microkrystites are locally generated by the ablation in the atmosphere of impact debris. An alternative explanation is that Fuente Caldera and Massignano microkrystites derive from two closely spaced impact events, which however requires another, so-far unknown source crater for microkrystites.
    • Frontier Mountain 93001: A coarse-grained, enstatite-augite-oligoclase-rich, igneous rock from the acapulcoite-lodranite parent asteroid

      Folco, L.; D'Orazio, M.; Burroni, A. (The Meteoritical Society, 2006-01-01)
      The Frontier Mountain (FRO) 93001 meteorite is a 4.86 g fragment of an unshocked, medium-to coarse-grained rock from the acapulcoite-lodranite (AL) parent body. It consists of anhedral orthoenstatite (FS13.3 +/- .04 WO 3.1 +/- 0.2), augite (FS6.1 +/- 0.7 WO42.3 +/- 0.9; Cr2O3 = 1.54 +/- 0.03), and oligoclase (Ab80.5 +/- 3.3 Or 3.1 +/- 0.6) up to >1 cm in size enclosing polycrystalline aggregates of fine-grained olivine (average grain size: 460 +/- 210 micrometers) showing granoblastic textures, often associated with Fe,Ni metal, troilite, chromite (cr# = 0.91 +/- 0.03; fe# = 0.62 +/ 0.04), schreibersite, and phosphates. Such aggregates appear to have been corroded by a melt. They are interpreted as lodranitic xenoliths. After the igneous (the term "igneous" is used here strictly to describe rocks or minerals that solidified from molten material) lithology intruding an acapulcoite host in Lewis Cliff (LEW) 86220, FRO 93001 is the second-known silicate-rich melt from the AL parent asteroid. Despite some similarities, the silicate igneous component of FRO 930011 (i.e., the pyroxene-plagioclase mineral assemblage) differs in being coarser-grained and containing abundant enstatite. Melting-crystallization modeling suggests that FRO 93001 formed through high-degree partial melting (greater than or equal to 35 wt%; namely, greater than or equal to 15 wt% silicate melting and ~20 wt% metal melting) of an acapulcoite source rock, or its chondritic precursor, at temperatures greater than or equal to 1200 degrees C, under reducing conditions. The resulting magnesium-rich silicate melt then underwent equilibrium crystallization; prior to complete crystallization at ~1040 degrees C, it incorporated lodranitic xenoliths. FRO 93001 is the highest-temperature melt from the AL parent-body so far available in laboratory. The fact that FRO 93001 could form by partial melting and crystallization under equilibrium conditions, coupled with the lack of quench-textures and evidence for shock deformation in xenoliths, suggests that FRO 93001 is a magmatic rock produced by endogenic heating rather than impact melting.
    • Laboratory simulation of terrestrial meteorite weathering using the Bensour (LL6) ordinary chondrite

      Lee, M. R.; Smith, C. L.; Gordon, S. H.; Hodson, M. E. (The Meteoritical Society, 2006-01-01)
      Laboratory dissolution experiments using the LL6 ordinary chondrite Bensour demonstrate that meteoritic minerals readily react with distilled water at low temperatures, liberating ions into solution and forming reaction products. Three experiments were performed, all for 68 days and at atmospheric fO2 but using a range of water/rock ratios and different temperatures. Experiments 1 and 2 were batch experiments and undertaken at room temperature, whereas in experiment 3, condensed boiling water was dripped onto meteorite subsamples within a Soxhlet extractor. Solutions from experiment 1 were chemically analyzed at the end of the experiment, whereas aliquots were extracted from experiments 2 and 3 for analysis at regular intervals. In all three experiments, a very significant proportion of the Na, Cl, and K within the Bensour subsamples entered solution, demonstrating that chlorapatite and feldspar were especially susceptible to dissolution. Concentrations of Mg, Al, Si, Ca, and Fe in solution were strongly affected by the precipitation of reaction products and Mg and Ca may also have been removed by sorption. Calculations predict saturation of experimental solutions with respect to Al hydroxides, Fe oxides, and Fe (oxy)hydroxides, which would have frequently been accompanied by hydrous aluminosilicates. Some reaction products were identified and include silica, a Mg-rich silicate, Fe oxides, and Fe (oxy)hydroxides. The implications of these results are that even very short periods of subaerial exposure of ordinary chondrites will lead to dissolution of primary minerals and crystallization of weathering products that are likely to include aluminosilicates and silicates, Mg-Ca carbonates, and sulfates in addition to the ubiquitous Fe oxides and (oxy)hydroxides.
    • Microbial diversity on the Tatahouine meteorite

      Benzerara, K.; Chapon, V.; Moreira, D.; López-García, P. (The Meteoritical Society, 2006-01-01)
      Biological processes can alter the chemistry and mineralogy of meteorites in a very short time, even in cold or hot deserts. It is thus important to assess the diversity of microorganisms that colonize meteorites in order to better understand their physiological capabilities. Microscopy observations of Tatahouine meteorite fragments that were exposed for 70 years in the Sahara desert showed that they were colonized by morphologically diverse biomorphs. A molecular diversity study based on 16S rRNA gene amplification of DNA supported the conclusion that a huge taxonomic diversity of prokaryotes colonized the Tatahouine meteorite in less than 70 years in the Tatahouine sand. Eleven different bacterial divisions were evidenced, among which Cytophaga-Flexibacter- Bacteroides (CFB), Cyanobacteria, and Alpha-Proteobacteria were dominantly represented. Crenarcheota were also detected. Most of the Tatahouine meteorite phylotypes were related to sequences identified in the surrounding Tatahouine more generally to sequences detected in soils. Some of them, in particular many of the archaeal phylotypes, were detected in arid regions in association with desert varnish. The results suggest that the diversity of the clone library generated from the meteorite fraction was reduced compared with that of the Tatahouine sand clone library, which can be explained as the result of partial colonization of the meteorite and/or a specific selection of colonizing bacteria by the substrate. We discuss the possibility that several groups detected in this study may play a prominent role in the various alteration processes detected at the surface of the Tatahouine meteorite.
    • Noble gas space exposure ages of individual interplanetary dust particles

      Kehm, K.; Flynn, G. J.; Sutton, S. R.; Hohenberg, C. M. (The Meteoritical Society, 2006-01-01)
      The He, Ne, and Ar compositions of 32 individual interplanetary dust particles (IDPs) were measured using low-blank laser probe gas extraction. These measurements reveal definitive evidence of space exposure. The Ne and Ar isotopic compositions in the IDPs are primarily a mixture between solar wind (SW) and an isotopically heavier component dubbed fractionated solar (FS), which could be implantation-fractionated solar wind or a distinct component of the solar corpuscular radiation previously identified as solar energetic particles (SEP). Space exposure ages based on the Ar content of individual IDPs are estimated for a subset of the grains that appear to have escaped significant volatile losses during atmosphere entry. Although model-dependent, most of the particles in this subset have ages that are roughly consistent with origin in the asteroid belt. A short (<1000 years) space exposure age is inferred for one particle, which is suggestive of cometary origin. Among the subset of grains that show some evidence for relatively high atmospheric entry heating, two possess elevated 21Ne/22Ne ratios generated by extended exposure to solar and galactic cosmic rays. The inferred cosmic ray exposure ages of these particles exceeds 107 years, which tends to rule out origin in the asteroid belt. A favorable possibility is that these 21Ne‐rich IDPs previously resided on a relatively stable regolith of an Edgeworth‐Kuiper belt or Oort cloud body and were introduced into the inner solar system by cometary activity. These results demonstrate the utility of noble gas measurements in constraining models for the origins of interplanetary dust particles.
    • Polygonal impact craters in the Argyre region, Mars: Evidence for influence of target structure on the final crater morphology

      Öhman, T.; Aittola, M.; Kostama, V.-P.; Hyvärinen, M.; Raitala, J. (The Meteoritical Society, 2006-01-01)
      Impact craters that in plan view are distinctly polygonal rather than circular or elliptical are common on Mars and other planets (hman et al. 2005). Their actual formation mechanism, however, is somewhat debatable. We studied the polygonal craters of different degradational stages in the region of the Argyre impact basin, Mars. The results show that in the same areas, heavily degraded, moderately degraded, and fresh polygonal craters display statistically similar strike distributions of the straight rim segments. The fact that the strike distributions are not dependent on lighting conditions was verified by using two data sets (Viking and MOC-WA) having different illumination geometries but similar resolutions. In addition, there are no significant differences in the amount of polygonality of craters in different degradational stages. These results clearly imply that large-scale polygonality is not caused by degradation, but originates from the cratering process itself, concurring with the findings regarding lunar craters by Eppler et al. (1983). The straight rims of polygonal craters apparently reflect areal fracture patterns that prevail for a geologically long time.
    • 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.
    • 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.