• Meteorite finds from southern Tunisia

      Laridhi Ouazaa, N.; Perchiazzi, N.; Kassaa, S.; Zeoli, A.; Ghanmi, M.; Folco, L. (The Meteoritical Society, 2009-01-01)
      We report on the meteorite search campaign of April 2008, conducted by a joint Tunisian- Italian scientific expedition in southern Tunisia (Dahar region). Nine likely unpaired meteorites(seven H-class and two L-class chondrites) totalling ~1.3 kg were recovered by exploring an approximately 45 km^2 area, therefore demonstrating that southern Tunisia is a suitable terrain for systematic searches for meteorites.
    • Petrography, mineralogy, and trace element geochemistry of lunar meteorite Dhofar 1180

      Zhang, A.; Hsu, W. (The Meteoritical Society, 2009-01-01)
      Here we report the petrography, mineralogy, and trace element geochemistry of the Dhofar 1180 lunar meteorite. Dhofar 1180 is predominantly composed of fine-grained matrix with abundant mineral fragments and a few lithic and glassy clasts. Lithic clasts show a variety of textures including cataclastic, gabbroic, granulitic, ophitic/subophitic, and microporphyritic. Both feldspathic and mafic lithic clasts are present. Most feldspathic lithic clasts have a strong affinity to ferroan anorthositic suite rocks and one to magnesian suite rocks. Mafic lithic clasts are moderately to extremely Fe-rich. The Ti/[Ti+Cr]-Fe/[Fe+Mg] compositional trend of pyroxenes in mafic lithic clasts is consistent with that of low-Ti mare basalts. Glasses display a wide chemical variation from mafic to feldspathic. Some glasses are very similar to those from Apollo 16 soils. KREEP components are essentially absent in Dhofar 1180. One glassy clast is rich in K, REE and P, but its Mg/[Mg+Fe] is very low (0.25). It is probably a last-stage differentiation product of mare basalt. Molar Fe/Mn ratios of both olivine and pyroxene are essentially consistent with a lunar origin. Dhofar 1180 has a LREE-enriched (La 18 x CI, Sm 14 x CI) pattern with a small positive Eu anomaly (Eu x 15 CI). Th concentration is 0.7 ppm in Dhofar 1180. Petrography, mineralogy, and trace element geochemistry of Dhofar 1180 are different from those of other lunar meteorites, indicating that Dhofar 1180 represents a unique mingled lunar breccia derived from an area on the lunar nearside but far away from the center of the Imbrium Basin.
    • 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.
    • 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.
    • 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.
    • Book Review: Isotope Geology, Claude J. Allègre

      Jull, A. J. T. (The Meteoritical Society, 2009-01-01)
    • Cometary glycine detected in samples returned by Stardust

      Elsila, J. E.; Glavin, D. P.; Dworkin, J. P. (The Meteoritical Society, 2009-01-01)
      Our previous analysis of cometary samples returned to Earth by NASAs Stardust spacecraft showed several amines and amino acids, but the origin of these compounds could not be firmly established. Here, we present the stable carbon isotopic ratios of glycine and -aminon- caproic acid (EACA), the two most abundant amino acids identified in Stardust-returned foil samples measured by gas chromatographymass spectrometry coupled with isotope ratio mass spectrometry. The delta-13C value for glycine of +29 +/- 6 ppm strongly suggests an extraterrestrial origin for glycine, while the delta-13C value for EACA of -25 +/- 2 ppm indicates terrestrial contamination by Nylon-6 during curation. This represents the first detection of a cometary amino acid.
    • Along-track compositional and textural variation in extensively melted grains returned from comet 81P/Wild 2 by the Stardust mission: Implications for capture-melting process

      Velbel, M. A.; Harvey, R. P. (The Meteoritical Society, 2009-01-01)
      Five amorphous (extensively melted) grains from Stardust aerogel capture Track 35 were examined by transmission electron microscopy (TEM); two from the bulb, two from near the bulbstylus transition, and one from near the terminal particle. Melted grains consist largely of a texturally and compositionally heterogeneous emulsion of immiscible metal/sulfide beads nanometers to tens of nanometers in diameter in a silica-rich vesicular glass. Most metal/sulfide beads are spherical, but textures of non-spherical beads indicate that some solidified as large drops during stretching and breaking while in translational and rotational motion, and others solidified from lenses of immiscible liquid at the silicate-melt/vesicle (vapor) interface. Melted grains appear to become richer in Fe relative to Mg, and depleted in S relative to Fe and Ni with increasing penetration distance along the aerogel capture track. Fe/S ratios are near unity in grains from the bulb of Track 35, consistent with the dominance of Fe-monosulfide minerals inferred by previous research on Stardust materials. Near stoichiometric Fe/S in melted grains from the bulb suggests that Fe-sulfides in the bulb were dispersed and melted during formation of the bulb but did not lose S. Along-track increases in Fe/S in melted grains from the bulb through the bulb-stylus transition and continuing into the stylus indicate that S initially present as iron monosulfide may have been progressively partially volatilized and lost from the melted grains with greater penetration of the grains deeper into the aerogel during capture-melting of comet dust. Extensively melted grains from the bulbs of aerogel capture tracks may preserve better primary compositional information with less capture-related modification than grains from farther along the same capture tracks.
    • A cometary aggregate interplanetary dust particle as an analog for comet Wild 2 grain chemistry preserved in silica-rich Stardust glass

      Rietmeijer, F. (The Meteoritical Society, 2009-01-01)
      Many of the nanometer-scale grains from comet 81P/Wild 2 did not survive hypervelocity capture. Instead, they melted and interacted with silica melt derived from the aerogel used by the Stardust mission. Their petrological properties were completely modified, but their bulk chemistry was preserved in the chemical signatures of mostly vesicular Si-rich glass with its typical Fe-Ni-S compound inclusions. Chondritic aggregate IDP L2011A9 that experienced atmospheric pre-entry thermal modification was selected as an analog to investigate these Wild 2 chemical signatures. The chemical, petrologic, and mineralogical properties of the individual constituents in this aggregate IDP are presented and used to match the chemical signatures of these Wild 2 grains. Mixing of comet material and pure silica, which is used in a diagram that recognizes this mixing behavior, is used to constrain the probable petrologic and minerals that caused the Wild 2 signatures. The Wild 2 nanometer-scale grain signatures in Si-rich glass allocations from three different deceleration tracks resembled mixtures of ultrafine-grained principal components and dense agglomerate-like material, Mg-rich silicates (<500 nm) and Fe,Ni sulfides (<100 nm), and Si-rich amorphous material. Dust resembling the mixed matrix of common chondritic aggregate IDPs was present in Jupiter-family comet Wild 2.
    • Connection between micrometeorites and Wild 2 particles: From Antarctic snow to cometary ices

      Dobricǎ, E.; Engrand, C.; Duprat, J.; Gounelle, M.; Leroux, H.; Quirico, E.; Rouzaud, J.-N. (The Meteoritical Society, 2009-01-01)
      We discuss the relationship between large cosmic dust that represents the main source of extraterrestrial matter presently accreted by the Earth and samples from comet 81P/Wild 2 returned by the Stardust mission in January 2006. Prior examinations of the Stardust samples have shown that Wild 2 cometary dust particles contain a large diversity of components, formed at various heliocentric distances. These analyses suggest large-scale radial mixing mechanism(s) in the early solar nebula and the existence of a continuum between primitive asteroidal and cometary matter. The recent collection of CONCORDIA Antarctic micrometeorites recovered from ultra-clean snow close to Dome degrees C provides the most unbiased collection of large cosmic dust available for analyses in the laboratory. Many similarities can be found between Antarctic micrometeorites and Wild 2 samples, in terms of chemical, mineralogical, and isotopic compositions, and in the structure and composition of their carbonaceous matter. Cosmic dust in the form of CONCORDIA Antarctic micrometeorites and primitive IDPs are preferred samples to study the asteroid-comet continuum.
    • Analytical SuperSTEM for extraterrestrial materials research

      Bradley, J. P.; Dai, Z. R. (The Meteoritical Society, 2009-01-01)
      Electron-beam studies of extraterrestrial materials with significantly improved spatial resolution, energy resolution, and sensitivity are enabled using a 300 keV SuperSTEM scanning transmission electron microscope(STEM) with a monochromator and two spherical aberration correctors. The improved technical capabilities enable analyses previously not possible. Mineral structures can be directly imaged and analyzed with single-atomic-column resolution, liquids, and implanted gases can be detected, and UV-VIS optical properties can be measured. Detection limits for minor/trace elements in thin (<100 nm thick) specimens are improved such that quantitative measurements of some extend to the sub-500 ppm level. Electron energy-loss spectroscopy (EELS) can be carried out with 0.10-0.20 eV energy resolution and atomic-scale spatial resolution such that variations in oxidation state from one atomic column to another can be detected. Petrographic mapping is extended down to the atomic scale using energy dispersive X-ray spectroscopy (EDS) and energy-filtered transmission electron microscopy (EFTEM) imaging. Technical capabilities and examples of the applications of SuperSTEM to extraterrestrial materials are presented, including the UV spectral properties and organic carbon K-edge fine structure of carbonaceous matter in interplanetary dust particles (IDPs), X-ray elemental maps showing the nanometer-scale distribution of carbon within GEMS (glass with embedded metal and sulfides), the first detection and quantification of trace Ti in GEMS using EDS, and detection of molecular H2O in vesicles and implanted H2 and He in irradiated mineral and glass grains.
    • Kosmochloric Ca-rich pyroxenes and FeO-rich olivines (Kool grains) and associated phases in Stardust tracks and chondritic porous interplanetary dust particles: Possible precursors to FeO-rich type II chondrules in ordinary chondrites

      Joswiak, D. J.; Brownlee, D. E.; Matrajt, G.; Westphal, A. J.; Snead, C. J. (The Meteoritical Society, 2009-01-01)
      Terminal particles and mineral fragments from comet 81P/Wild 2 were studied in 16 aerogel tracks by transmission and secondary electron microscopy. In eight tracks clinopyroxenes with correlated Na2O and Cr2O3 contents as high as 6.0 wt% and 13.0 wt%, respectively, were found. Kosmochloric (Ko) clinopyroxenes were also observed in 4 chondritic interplanetary dust particles (IDPs). The Ko clinopyroxenes were often associated with FeO-rich olivine +/- Cr-rich spinel +/- aluminosilicate glass or albitic feldspar, assemblages referred to as Kool grains (Ko = kosmochloric Ca-rich pyroxene, ol = olivine). Fine-grained (submicron) Kool fragments have textures suggestive of crystallization from melts while coarse-grained (>1 micrometer) Kool fragments are often glass-free and may have formed by thermal metamorphism in the nebula. Average major and minor element distributions between clinopyroxenes and coexisting FeO-rich olivines are consistent with these phases forming at or near equilibrium. In glass-bearing fine-grained Kool fragments, high concentrations of Na in the clinopyroxenes are inconsistent with existing experimentally determined partition coefficients at equilibrium. We speculate that the availability of Cr in the melt increased the clinopyroxene Na partition coefficient via a coupled substitution thereby enhancing this phase with the kosmochlor component. The high temperature minerals, fine-grain sizes, bulk compositions and common occurrence in the SD tracks and IDPs support the idea that Kool grains could have been precursors to type II chondrules in ordinary chondrites. These grains, however, have not been observed in these meteorites suggesting that they were destroyed during chondrule formation and recycling or were not present in the nebula at the time and location where meteoritic chondrules formed.
    • Rietveld analysis of X-ray powder diffraction patterns as a potential tool for the identification of impact-deformed carbonate rocks

      Huson, S. A.; Foit, F. F.; Watkinson, A. J.; Pope, M. C. (The Meteoritical Society, 2009-01-01)
      Previous X-ray powder diffraction (XRD) studies revealed that shock deformed carbonates and quartz have broader XRD patterns than those of unshocked samples. Entire XRD patterns, single peak profiles and Rietveld refined parameters of carbonate samples from the Sierra Madera impact crater, west Texas, unshocked equivalent samples from 95 miles north of the crater and the Mission Canyon Formation of southwest Montana and western Wyoming were used to evaluate the use of X-ray powder diffraction as a potential tool for distinguishing impact deformed rocks from unshocked and tectonically deformed rocks. At Sierra Madera dolostone and limestone samples were collected from the crater rim (lower shock intensity) and the central uplift (higher shock intensity). Unshocked equivalent dolostone samples were collected from well cores drilled outside of the impact crater. Carbonate rocks of the Mission Canyon Formation were sampled along a transect across the tectonic front of the Sevier and Laramide orogenic belts. Whereas calcite subjected to significant shock intensities at the Sierra Madera impact crater can be differentiated from tectonically deformed calcite from the Mission Canyon Formation using Rietveld refined peak profiles, weakly shocked calcite from the crater rim appears to be indistinguishable from the tectonically deformed calcite. In contrast, Rietveld analysis readily distinguishes shocked Sierra Madera dolomite from unshocked equivalent dolostone samples from outside the crater and tectonically deformed Mission Canyon Formation dolomite.
    • Nebular mixing constrained by the Stardust samples

      Ogliore, R. C.; Westphal, A. J.; Gainsforth, Z.; Butterworth, A. L.; Fakra, S. C.; Marcus, M. A. (The Meteoritical Society, 2009-01-01)
      Using X-ray microprobe analysis of samples from comet Wild 2 returned by the Stardust mission, we determine that the crystalline Fe-bearing silicate fraction in this Jupiter-family comet is greater than 0.5. Assuming this mixture is a composite of crystalline inner solar system material and amorphous cold molecular cloud material, we deduce that more than half of Wild 2 has been processed in the inner solar system. Several models exist that explain the presence of crystalline materials in comets. We explore some of these models in light of our results.
    • A TEM study of four particles extracted from the Stardust track 80

      Stodolna, J.; Jacob, D.; Leroux, H. (The Meteoritical Society, 2009-01-01)
      Four particles extracted from track 80 at different penetration depths have been studied by analytical transmission electron microscopy (ATEM). Regardless of their positions within the track, the samples present a comparable microstructure made of a silica rich glassy matrix embedding a large number of small Fe-Ni-S inclusions and vesicles. This microstructure is typical of strongly thermally modified particles that were heated and melted during the hypervelocity impact into the aerogel. X-ray intensity maps show that the particles were made of Mg-rich silicates (typically 200 nm in diameter) cemented by a fine-grained matrix enriched in iron sulfide. Bulk compositions of the four particles suggest that the captured dust particle was an aggregate of grains with various iron sulfide fraction and that no extending chemical mixing in the bulb occurred during the deceleration. The bulk S/Fe ratios of the four samples are close to CI and far from the chondritic meteorites from the asteroidal belt, suggesting that the studied particles are compatible with chondritic-porous interplanetary dust particles or with material coming from a large heliocentric distance for escaping the S depletion.
    • Organic matter from comet 81P/Wild 2, IDPs, and carbonaceous meteorites; similarities and differences

      Wirick, S.; Flynn, G. J.; Keller, L. P.; Nakamura-Messenger, K.; Peltzer, C.; Jacobsen, C.; Sandford, S. A.; Zolensky, M. E. (The Meteoritical Society, 2009-01-01)
      During preliminary examination of 81P/Wild 2 particles collected by the NASA Stardust spacecraft, we analyzed seven, sulfur embedded and ultramicrotomed particles extracted from five different tracks. Sections were analyzed using a scanning transmission X-ray microscope (SXTM) and carbon X-ray absorption near edge structure (XANES) spectra were collected. We compared the carbon XANES spectra of these Wild 2 samples with a database of spectra on thirty-four interplanetary dust particles(IDPs) and with several meteorites. Two of the particles analyzed are iron sulfides and there is evidence that an aliphatic compound associated with these particles can survive high temperatures. An iron sulfide from an IDP demonstrates the same phenomenon. Another, mostly carbon free containing particle radiation damaged, something we have not observed in any IDPs we have analyzed or any indigenous organic matter from the carbonaceous meteorites, Tagish Lake, Orgueil, Bells and Murchison. The carbonaceous material associated with this particle showed no mass loss during the initial analysis but chemically changed over a period of two months. The carbon XANES spectra of the other four particles varied more than spectra from IDPs and indigenous organic matter from meteorites. Comparison of the carbon XANES spectra from these particles with 1. the carbon XANES spectra from thirty-four IDPs (<15 micron in size) and 2. the carbon XANES spectra from carbonaceous material from the Tagish Lake, Orgueil, Bells, and Murchison meteorites show that 81P/Wild 2 carbon XANES spectra are more similar to IDP carbon XANES spectra then to the carbon XANES spectra of meteorites.
    • The comparative behavior of apatite-zircon U-Pb systems in Apollo 14 breccias: Implications for the thermal history of the Fra Mauro Formation

      Nemchin, A. A.; Pidgeon, R. T.; Healy, D.; Grange, M. L.; Whitehouse, M. J.; Vaughan, J. (The Meteoritical Society, 2009-01-01)
      We report secondary ion mass spectrometry (SIMS) U-Pb analyses of zircon and apatite from four breccia samples from the Apollo 14 landing site. The zircon and apatite grains occur as cogenetic minerals in lithic clasts in two of the breccias and as unrelated mineral clasts in the matrices of the other two. SIMS U-Pb analyses show that the ages of zircon grains range from 4023 +/- 24 Ma to 4342 +/- 5 Ma, whereas all apatite grains define an isochron corresponding to an age of 3926 +/- 3 Ma. The disparity in the ages of cogenetic apatite and zircon demonstrates that the apatite U-Pb systems have been completely reset at 3926 +/- 3 Ma, whereas the U-Pb system of zircon has not been noticeably disturbed at this time. The apatite U-Pb age is slightly older than the ages determined by other methods on Apollo 14 materials highlighting need to reconcile decay constants used for the U-Pb, Ar-Ar and Rb-Sr systems. We interpret the apatite age as a time of formation of the Fra Mauro Formation. If the interpretation of this Formation as an Imbrium ejecta is correct, apatite also determines the timing of Imbrium impact. The contrast in the Pb loss behavior of apatite and zircon places constraints on the temperature history of the Apollo 14 breccias and we estimate, from the experimentally determined Pb diffusion constants and an approximation of the original depth of the excavated samples in the Fra Mauro Formation, that the breccias experienced an initial temperature of about 1300-1100 degrees degrees C, but cooled within the first five to ten years.
    • Nuclear field shift effect as a possible cause of Te isotopic anomalies in the early solar system—An alternative explanation of Fehr et al. (2006 and 2009)

      Moynier, F.; Fujii, T.; Albarède, F. (The Meteoritical Society, 2009-01-01)
      We explore the possibility that Te isotopic anomalies measured in Ca-Al-rich inclusions (Fehr et al. 2009) and in leachates of carbonaceous chondrites (Fehr et al. 2006) may be due to mass-independent effects controlled by nuclear field shift rather than to nucleosynthetic processes. Fehr et al.s spectrum of mass-independent anomalies of Te isotopes shows a smooth correlation with mass number and nuclear charge distribution. Ratios of even to odd isotopes, as the 125Te/126Te ratio used by these authors for normalization are particularly prone to nuclear field shift effects. We show that the alternative normalization of isotopic ratios to 130Te/126Te strongly reduces the trend of isotopic fractionation with mass number, leaving only 125Te as truly anomalous. For both normalizations (125Te/126Te and 130Te/126Te), Fehr et al.s results fit the theory of Bigeleisen (1996), which suggests that the nuclear field shift effect can potentially account for the observed Te isotope abundances, as an alternative to nucleosynthetic processes. We propose that these mass-independent effects may be acquired during accretion of sulfides from the solar nebula.
    • Incompletely compacted equilibrated ordinary chondrites

      Sasso, M. R.; Macke, R. J.; Boesenberg, J. S.; Britt, D. T.; Rivers, M. L.; Ebel, D. S.; Friedrich, J. M. (The Meteoritical Society, 2009-01-01)
      We document the size distributions and locations of voids present within five highly porous equilibrated ordinary chondrites using high-resolution synchrotron X-ray microtomography (CT) and helium pycnometry. We found total porosities ranging from ~10 to 20% within these chondrites, and with CT we show that up to 64% of the void space is located within intergranular voids within the rock. Given the low (S1-S2) shock stages of the samples and the large voids between mineral grains, we conclude that these samples experienced unusually low amounts of compaction and shock loading throughout their entire post accretionary history. With Fe metal and FeS metal abundances and grain size distributions, we show that these chondrites formed naturally with greater than average porosities prior to parent body metamorphism. These materials were not fluffed on their parent body by impact-related regolith gardening or events caused by seismic vibrations. Samples of all three chemical types of ordinary chondrites (LL, L, H) are represented in this study and we conclude that incomplete compaction is common within the asteroid belt.
    • Interpreting the I-Xe system in individual silicate grains from Toluca IAB

      Pravdivtseva, O. V.; Meshik, A. P.; Hohenberg, C. M.; Petaev, M. (The Meteoritical Society, 2009-01-01)
      Detailed isotopic and mineralogical studies of silicate inclusions separated from a troilite nodule of the Toluca IAB iron meteorite reveal the presence of radiogenic 129Xe in chlorapatite, plagioclase, perryite, and pyroxene grains. Subsequent I-Xe studies of 32 neutron-irradiated pyroxene grains indicate that high-Mg and low-Mg pyroxenes have distinctive I-Xe signatures. The I-Xe system in high-Mg pyroxenes closed at 4560.5 +/- 2.4 Ma, probably reflecting exsolution of silicates from the melt, while the low-Mg pyroxenes closed at 4552.0 +/- 3.7 Ma, 8.5 Ma later, providing a means for determining the cooling rate at the time of exsolution. If the host Toluca graphite-troiliterich inclusion formed after the breakup and reassembly of the IAB parent body as has been suggested, the I-Xe ages of the high-Mg pyroxenes separated from this inclusions indicate that this catastrophic impact occurred not later than 4560.5 Ma, 6.7 Ma after formation of CAIs. The cooling rate at the time of silicates exsolution in Toluca is 14.5 +/- 10.0 degrees C/Ma.