• Density, magnetic susceptibility, and the characterization of ordinary chondrite falls and showers

      Consolmagno, G. J.; Macke, R. J.; Rochette, P.; Britt, D. T.; Gattacceca, J. (The Meteoritical Society, 2006-01-01)
      Bulk and grain densities of 132 ordinary chondrites from the Vatican Observatory collection were measured and compared with their magnetic susceptibility (for the most part using previously measured values; ten new susceptibility measures were taken for this study). Grain density and magnetic susceptibility combined provide a reliable method of classifying un weathered ordinary chondrites. Unlike traditional chemical tests, this method is fast, nondestructive, and characterizes the whole rock, making it especially appropriate for surveying large collections. The system is less viable for finds; extensive weathering of metallic iron in an H chondrite can cause it to plot among L chondrites, while heavily weathered L chondrites plot among the LL group. This system has revealed outlier stones that may be misclassified meteorites or mislabeled samples; in every case where the magnetic susceptibility of a meteorite does not fit its putative classification, the grain density is also found to be in disagreement in a manner consistent with either severe weathering or misidentification. An analysis of stones from five showers shows that, excluding outliers, these samples tend to cluster tightly within their appropriate groups in a plot of grain versus magnetic susceptibility.
    • Desert Meteorites Workshop: Abstracts

      The Meteoritical Society, 2006-01-01
    • Determination of the Fe oxidation state of the Chassigny kaersutite: A microXANES spectroscopic study

      Monkawa, Akira; Mikouchi, Takashi; Koizumi, Eisuke; Sugiyama, Kazumasa; Miyamoto, Masamichi (The Meteoritical Society, 2006-01-01)
      In order to elucidate the formation of low-H kaersutites in Martian meteorites, the Fe3+/Sigma-Fe ratio of Chassigny kaersutites in magmatic inclusions was directly determined by the synchrotron microXANES analysis. XANES analysis for standard kaersutites with known Fe3+/Sigma-Fe ratios shows a linear relationship between centroid energy positions of XANES pre-edge spectra and the Fe3+/Sigma-Fe ratio. Based on the linear relationship, the Fe3+/Sigma-Fe ratio of Chassigny kaersutites is estimated to be about 0.05. The low Fe3+/Sigma-Fe ratio clearly suggests that low-H kaersutites in Chassigny are not likely to be formed by the oxidation-dehydrogenation reactions. The low-H content of the Chassigny kaersutites is mainly due to the presence of a Ti oxy-component.
    • Differentiation and evolution of the IVA meteorite parent body: Clues from pyroxene geochemistry in the Steinbach stony-iron meteorite

      Ruzicka, A.; Hutson, M. (The Meteoritical Society, 2006-01-01)
      We analyzed the Steinbach IVA stony-iron meteorite using scanning electron microscopy (SEM), electron microprobe analysis (EMPA), laser ablation inductively-coupled-plasma mass spectroscopy (LA-ICP-MS), and modeling techniques. Different and sometimes adjacent low-Ca pyroxene grains have distinct compositions and evidently crystallized at different stages in a chemically evolving system prior to the solidification of metal and troilite. Early crystallizing pyroxene shows evidence for disequilibrium and formation under conditions of rapid cooling, producing clinobronzite and type 1 pyroxene rich in troilite and other inclusions. Subsequently, type 2 pyroxene crystallized over an extensive fractionation interval. Steinbach probably formed as a cumulate produced by extensive crystal fractionation (~60-70% fractional crystallization) from a high-temperature (~1450-1490 degrees C) silicate-metallic magma. The inferred composition of the precursor magma is best modeled as having formed by greater than or equal to 30-50% silicate partial melting of a chondritic protolith. If this protolith was similar to an LL chondrite (as implied by O-isotopic data), then olivine must have separated from the partial melt, and a substantial amount (~53-56%) of FeO must have been reduced in the silicate magma. A model of simultaneous endogenic heating and collisional disruption appears best able to explain the data for Steinbach and other IVA meteorites. Impact disruption occurred while the parent body was substantially molten, causing liquids to separate from solids and oxygen-bearing gas to vent to space, leading to a molten metal-rich body that was smaller than the original parent body and that solidified from the outside in. This model can simultaneously explain the characteristics of both stony-iron and iron IVA meteorites, including the apparent correlation between metal composition and metallographic cooling rate observed for metal.
    • Effect of volatiles and target lithology on the generation and emplacement of impact crater fill and ejecta deposits on Mars

      Osinski, Gordon R. (The Meteoritical Society, 2006-01-01)
      Impact cratering is an important geological process on Mars and the nature of Martian impact craters may provide important information as to the volatile content of the Martian crust. Terrestrial impact structures currently provide the only ground-truth data as to the role of volatiles and an atmosphere on the impact-cratering process. Recent advancements, based on studies of several well-preserved terrestrial craters, have been made regarding the role and effect of volatiles on the impact-cratering process. Combined field and laboratory studies reveal that impact melting is much more common in volatile-rich targets than previously thought, so impact-melt rocks, melt-bearing breccias, and glasses should be common on Mars. Consideration of the terrestrial impact-cratering record suggests that it is the presence or absence of subsurface volatiles and not the presence of an atmosphere that largely controls ejecta emplacement on Mars. Furthermore, recent studies at the Haughton and Ries impact structures reveal that there are two discrete episodes of ejecta deposition during the formation of complex impact craters that provide a mechanism for generating multiple layers of ejecta. It is apparent that the relative abundance of volatiles in the near-surface region outside a transient cavity and in the target rocks within the transient cavity play a key role in controlling the amount of fluidization of Martian ejecta deposits. This study shows the value of using terrestrial analogues, in addition to observational data from robotic orbiters and landers, laboratory experiments, and numerical modeling to explore the Martian impact-cratering record.
    • Effects of experimental aqueous alteration on the abundances of argon-rich noble gases in the Ningqiang carbonaceous chondrite

      Yamamoto, Yukio; Okazaki, Ryuji; Nakamura, Tomoki (The Meteoritical Society, 2006-01-01)
      Ar-rich noble gases, the so-called "subsolar" noble gases, are a major component of heavy primordial noble gases in unequilibrated ordinary chondrites and some classes of anhydrous carbonaceous chondrites, whereas they are almost absent in hydrous carbonaceous chondrites that suffered extensive aqueous alteration. To understand the effects of aqueous alteration on the abundance of Ar-rich noble gases, we performed an aqueous alteration experiments on the Ningqiang type 3 carbonaceous chondrite that consists entirely of anhydrous minerals and contains Ar-rich noble gases. Powdered samples and deionized neutral water were kept at 200 degrees C for 10 and 20 days, respectively. Mineralogical analyses show that, during the 10-day alteration, serpentine and hematite formed at the expense of olivine, low-Ca pyroxene, and sulfide. Noble gas analyses show that the 10-day alteration of natural Ningqiang removed 79% of the primordial 36Ar, 68% of the 84Kr, and 60% of the 132Xe, but only 45% of the 4He and 53% of the primordial 20Ne. Calculated elemental ratios of the noble gases removed during the 10-day alteration are in the range of those of Ar-rich noble gases. These results indicate that Ar-rich noble gases are located in materials that are very susceptible to aqueous alteration. In contrast, heavy primordial noble gases remaining in the altered samples are close to Q gas in elemental and isotope compositions. This indicates that phase Q is much more resistant to aqueous alteration than the host phases of Ar-rich noble gases. In the 20-day sample, the mineralogical and noble gas signatures are basically similar to those of the 10-day sample, indicating that the loss of Ar-rich noble gases was completed within the 10-day alteration. Our results suggest that almost all of the Ar-rich noble gases were lost from primitive asteroids during early, low-temperature aqueous alteration.
    • Erratum

      Bhandari, N. (The Meteoritical Society, 2006-01-01)
    • Erratum

      The Meteoritical Society, 2006-01-01
    • Erratum

      The Meteoritical Society, 2006-01-01
      Shock waves-Phenomenology, experimental, and numerial simulationi
    • Erratum

      The Meteoritical Society, 2006-01-01
    • Establishing the link between the Chesapeake Bay impact structure and the North American tektite strewn field: The Sr-Nd isotopic evidence

      Deutsch, Alexander; Koeberl, Christian (The Meteoritical Society, 2006-01-01)
      The Chesapeake Bay impact structure, which is about 35 Ma old, has previously been proposed as the possible source crater of the North American tektites (NAT). Here we report major and trace element data as well as the first Sr-Nd isotope data for drill core and outcrop samples of target lithologies, crater fill breccias, and post-impact sediments of the Chesapeake Bay impact structure. The unconsolidated sediments, Cretaceous to middle Eocene in age, have epsilon-Srt = 35.7 Ma of +54 to +272, and epsilon-Ndt = 35.7 Ma ranging from -6.5 to -10.8; one sample from the granitic basement with a TNdCHUR model age of 1.36 Ga yielded an epsilon-Srt = 35.7 Ma of +188 and an epsilon-Ndt = 35.7 Ma of -5.7. The Exmore breccia (crater fill) can be explained as a mix of the measured target sediments and the granite, plus an as-yet undetermined component. The post-impact sediments of the Chickahominy formation have slightly higher TNdCHUR model ages of about 1.55 Ga, indicating a contribution of some older materials. Newly analyzed bediasites have the following isotope parameters: +104 to +119 (epsilon-Srt = 35.7 Ma), -5.7 (epsilon-Ndt = 35.7 Ma), 0.47 Ga (TSrUR), and 1.15 Ga (TNdCHUR), which is in excellent agreement with previously published data for samples of the NAT strewn field. Target rocks with highly radiogenic Sr isotopic composition, as required for explaining the isotopic characteristics of Deep Sea Drilling Project (DSDP) site 612 tektites, were not among the analyzed sample suite. Based on the new isotope data, we exclude any relation between the NA tektites and the Popigai impact crater, although they have identical ages within 2 errors. The Chesapeake Bay structure, however, is now clearly constrained as the source crater for the North American tektites, although the present data set obviously does not include all target lithologies that have contributed to the composition of the tektites.
    • Estimating shock pressures based on high-pressure minerals in shock-induced melt veins of L chondrites

      Xie, Z.; Sharp, T. G.; De Carli, P. S. (The Meteoritical Society, 2006-01-01)
      Here we report the transmission electron microscopy (TEM) observations of the mineral assemblages and textures in shock-induced melt veins from seven L chondrites of shock stages ranging from S3 to S6. The mineral assemblages combined with phase equilibrium data are used to constrain the crystallization pressures, which can be used to constrain shock pressure in some cases. Thick melt veins in the TenhamL6 chondrite contain majorite and magnesiowstite in the center, and ringwoodite, akimotoite, vitrified silicate-perovskite, and majorite in the edge of the vein, indicating crystallization pressure of ~25 GPa. However, very thin melt veins (5-30 micrometers wide) in Tenham contain glass, olivine, clinopyroxene, and ringwoodite, suggesting crystallization during transient low-pressure excursions as the shock pressure equilibrated to a continuum level. Melt veins of Umbarger include ringwoodite, akimotoite, and clinopyroxene in the vein matrix, and Fe2SiO4-spinel and stishovite in SiO2-FeO-rich melt, indicating a crystallization pressure of ~18 GPa. The silicate melt veins in Roy contain majorite plus ringwoodite, indicating pressure of ~20 GPa. Melt veins of Ramsdorf and Nakhon Pathon contain olivine and clinoenstatite, indicating pressure of less than 15 GPa. Melt veins of Kunashak and La Lande include albite and olivine, indicating crystallization at less than 2.5 GPa. Based upon the assemblages observed, crystallization of shock veins can occur before, during, or after pressure release. When the assemblage consists of high-pressure minerals and that assemblage is constant across a larger melt vein or pocket, the crystallization pressure represents the equilibrium shock pressure.
    • Experimental petrology of the basaltic shergottite Yamato-980459: Implications for the thermal structure of the Martian mantle

      Mussel White, Donald S.; Dalton, Heather A.; Kiefer, Walter S.; Treiman, Allan H. (The Meteoritical Society, 2006-01-01)
      The Martian meteorite Yamato (Y-) 980459 is an olivine-phyric shergottite. It has a very primitive character and may be a primary melt of the Martian mantle. We have conducted crystallization experiments on a synthetic Y-980459 composition at Martian upper mantle conditions in order to test the primary mantle melt hypothesis. Results of these experiments indicate that the cores of the olivine megacrysts in Y-980459 are in equilibrium with a melt of bulk rock composition, suggesting that these megacrysts are in fact phenocrysts that grew from a magma of the bulk rock composition. Multiple saturation of the melt with olivine and a low-calcium pyroxene occurs at approximately 12 +/- 0.5 kbar and 1540 +/- 10 degrees C, suggesting that the meteorite represents a primary melt that separated from its mantle source at a depth of ~100 km. Several lines of evidence suggest that the Y-980459 source underwent extensive melting prior to and/or during the magmatic event that produced the Y-980459 parent magma. When factored into convective models of the Martian interior, the high temperature indicated for the upper Martian mantle and possibly high melt fraction for the Y-980459 magmatic event suggests a significantly higher temperature at the core-mantle boundary than previously estimated.
    • Extent of chondrule melting: Evaluation of experimental textures, nominal grain size, and convolution index

      Nettles, Jeffrey W.; Lofgren, Gary E.; Carlson, William D.; McSween, Harry Y. (The Meteoritical Society, 2006-01-01)
      Dynamic crystallization experiments on the ordinary chondrite Queen Alexandra Range (QUE) 97008 document textural features that occur in partially melted chondrules with changes in the degree of partial melting and cooling rate. We carried out a matrix of experiments, at peak temperatures of 1250, 1350, 1370, and 1450 degrees C, and cooling rates of 1000, 100, and 10 degrees C/h, and quenched. All experimentally produced textures closely resemble textures of porphyritic chondrules. Because peak temperatures were well below the liquidi for typical chondrule compositions, the textural similarities support an incomplete melting origin for most porphyritic chondrules. Our experiments can be used to determine the extent of melting of natural chondrules by comparing textural relationships among the experimental results with those of natural chondrules. We used our experiments along with X-ray computerized tomography scans of a Semarkona chondrule to evaluate two other methods that have been used previously to quantify the degree of melting: nominal grain size and convolution index. Proper applications of these methods can result in valid assessments of a chondrule's degree of melting, but only if accompanied by careful interpretation, as chondrule textures are controlled by more than just the extent of melting. Such measurements of single aspects of chondrule textures might be coupled with qualitative analysis of other textural aspects to accurately determine degree of melting.
    • Extraterrestrial chromite in Middle Ordovician marine limestone at Kinnekulle, southern Sweden—Traces of a major asteroid breakup event

      Schmitz, Birger; Häggström, Therese (The Meteoritical Society, 2006-01-01)
      The distribution of sediment-dispersed extraterrestrial chromite grains and other Cr-rich spinels (>63 micrometers) has been studied in Middle Ordovician Orthoceratite Limestone from two quarries at Kinnekulle, southern Sweden. In the Thorsberg quarry, an 3.2 m thick sequence of beds previously shown to be rich in fossil meteorites is also rich in sediment-dispersed extraterrestrial chromite grains. Typically, 1-3 grains are found per kilogram of limestone. In the nearby Hällekis quarry, the same beds show similarly high concentrations of extraterrestrial chromite grains, but in samples representing the 9 m downward continuation of the section exposed at this site, only 5 such grains were found in a total of 379 kg of limestone. The extraterrestrial (equilibrated ordinary chondritic) chromite grains can be readily distinguished by a homogeneous and characteristic major element chemistry, including 2.0-3.5 wt% TiO2 and stable V2O3 concentrations close to 0.7 wt%. Terrestrial Cr-rich spinels have a wide compositional range and co-exist with extraterrestrial chromite in some beds. These grains may be derived, for example, from mafic dykes exposed and weathered at the sea floor.Considering lithologic and stratigraphic aspects variations in sedimentation rate cannot explain the dramatic increase in extraterrestrial chromite seen in the upper part of the composite section studied. Instead, the difference may be primarily related to an increase in the ancient flux of extraterrestrial matter to Earth in connection with the disruption of the L chondrite parent body in the asteroid belt at about this time. The coexistence in some beds of high concentrations of chondritic chromite and terrestrial Cr-rich spinels, however, indicates that redistribution of heavy minerals on the sea floor, related to changes in sea level and sea-floor erosion and currents, must also be considered.
    • Fabric analysis of Allende matrix using EBSD

      Watt, Lauren E.; Bland, Phil A.; Prior, Dave J.; Russell, Sara S. (The Meteoritical Society, 2006-01-01)
      Fabric analysis of the interstitial matrix material in primitive meteorites offers a novel window on asteroid formation and evolution. Electron backscatter diffraction (EBSD) has allowed fabrics in these fine-grained materials to be visualized in detail for the first time. Our data reveal that Allende, a CV3 chondrite, possesses a uniform, planar, short-axis alignment fabric that is pervasive on a broad scale and is probably the result of deformational shortening related to impact or gravitational compaction. Interference between this matrix fabric and the larger, more rigid components, such as dark inclusions (DIs) and calcium-aluminium-rich inclusions (CAIs), has lead to the development of locally oriented and intensified matrix fabrics. In addition, DIs possess fabrics that are conformable with the broader matrix fabric. These results suggest that DIs were in situ prior to the deformational shortening event responsible for these fabrics, thus providing an argument against dark inclusions being fragments from another lithified part of the asteroid (Kojima and Tomeoka 1996; Fruland et al. 1978). Moreover, both DIs and Allende matrix are highly porous (~25%) (Corrigan et al. 1997). Mobilizing a highly porous DI during impact-induced brecciation without imposing a fabric and incorporating it into a highly porous matrix without significantly compacting these materials is improbable. We favor a model that involves Allende DIs, CAIs, and matrix accreting together and experiencing the same deformation events.
    • Focused ion beam recovery of hypervelocity impact residue in experimental craters on metallic foils

      Graham, G. A.; Teslich, N.; Dai, Z. R.; Bradley, J. P.; Kearsley, A. T.; Hörz, F. (The Meteoritical Society, 2006-01-01)
      The Stardust sample return capsule returned to Earth in January 2006 with primitive debris collected from Comet 81P/Wild-2 during the fly-by encounter in 2004. In addition to the cometary particles embedded in low-density silica aerogel, there are microcraters preserved in the aluminum foils (1100 series; 100 micrometers thick) that are wrapped around the sample tray assembly. Soda lime spheres (~49 micrometers in diameter) have been accelerated with a light gas gun into flight-grade aluminum foils at 6.35 km s^(-1) to simulate the capture of cometary debris. The experimental craters have been analyzed using scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDX) to locate and characterize remants of the projectile material remaining within the craters. In addition, ion beam-induced secondary electron imaging has proven particularly useful in identifying areas within the craters that contain residue material. Finally, high-precision focused ion beam (FIB) milling has been used to isolate and then extract an individual melt residue droplet from the interior wall of an impact. This has enabled further detailed elemental characterization that is free from the background contamination of the aluminum foil substrate. The ability to recover "pure" melt residues using FIB will significantly extend the interpretations of the residue chemistry preserved in the aluminum foils returned by Stardust.
    • Formation of the binary near-Earth object 1996 FG3: Can binary NEOs be the source of short-CRE meteorites?

      Morbidelli, A.; Levison, H. F.; Bottke, W. F. (The Meteoritical Society, 2006-01-01)
      1996 FG3 is a binary near-Earth object (NEO) that was likely formed during a tidal disruption event. Our results indicate that the formation of this binary object was unlikely to have occurred when the progenitor had a encounter velocity with the Earth significantly smaller than its current value (10.7 km/s); The formation of the binary object on an orbit similar to the present one is possible, and the survival of the satellite constrains this to have happened less than 1.6 Ma ago. However, the binary object could also have been formed when the progenitor's encounter velocity with Earth was >12 km/s, and in this case we cannot constrain its formation age. Our results indicate that tidal disruptions occurring among NEOs with low velocity encounters with Earth are unlikely to produce long-lasting NEO binaries. Thus, tidal disruption may not be able to completely re-supply the observed population. This would imply that a significant fraction of the observed NEO binaries evolved out of the main asteroid belt. Overall, our results suggest to us that the CM2 meteorites having cosmic ray exposure (CRE) ages of ~200,000 yr were likely liberated by the tidal disruption of a primitive NEO with a relative velocity with the Earth significantly smaller than that of 1996 FG3. We propose a list of such objects, although as far as we know, none of the candidates is a binary for the reasons described above.
    • Formation of TiC core-graphitic mantle grains from CO gas

      Kimura, Yuki; Nuth, Joseph A.; Ferguson, Frank T. (The Meteoritical Society, 2006-01-01)
      We demonstrate a new formation route for TiC core-graphitic mantle spherules that does not require carbon-atom addition and the very long time scales associated with such growth (Bernatowicz et al. 1996). Carbonaceous materials can be formed from C2H2 and its derivatives, as well as from CO gas. In this paper, we will demonstrate that large-cage-structure carbon particles can be produced from CO gas by the Boudouard reaction. Since the sublimation temperature for such fullerenes is low, the large cages can be deposited onto previously nucleated TiC and produce TiC core-graphitic mantle spherules. New constraints for the formation conditions and the time scale for the formation of TiC core-graphitic mantle spherules are suggested by the results of this study. In particular, TiC core-graphitic mantle grains that are found in primitive meteorites that have never experienced hydration could be mantled by fullerenes or carbon nanotubes rather than by graphite. In situ observations of these grains in primitive anhydrous meteoritic matrix could confirm or refute this prediction and would demonstrate that the graphitic mantle on such grains is a metamorphic feature due to interaction of the presolar fullerenes with water within the meteorite matrix.
    • Fracture-related intracrystalline transformation of olivine to ringwoodite in the shocked Sixiangkou meteorite

      Chen, Ming; Li, Hui; El Goresy, Ahmed; Liu, Jing; Xie, Xiande (The Meteoritical Society, 2006-01-01)
      Magnesium-iron olivine in the Sixiangkou L6 chondrite contains abundant fractures induced by plastic deformation during shock metamorphism. This study reports the discovery of lamellar ringwoodite that incoherently nucleated and grew along planar and irregular fractures in olivine. Magnesium-iron interdiffusion took place between olivine matrix and crystallizing ringwoodite at high pressures and high temperatures, which resulted in higher FeO content in ringwoodite lamellae than in olivine. This suggests that a quasi-hydrostatic high pressure lasting for several minutes should have been produced in the shock veins of the meteorite. The intracrystalline transformation of olivine to ringwoodite also has implications for phase transitions in subducting lithospheric slabs because planar and irregular fractures are commonly produced in olivine that suffered plastic deformation.