• Cosmogenic nuclides in stony meteorites revisited

      Leya, I.; Masarik, J. (The Meteoritical Society, 2009-01-01)
      We present new model calculations for depth and size dependent cosmogenic production rates in ordinary and carbonaceous chondrites by galactic cosmic rays. This model, essentially that of Leya et al. (2000a), folds together particle spectra and cross sections for the relevant nuclear reactions, but has been significantly improved due to major improvements in the neutron cross section database and better Monte Carlo modeling of the primary and the secondary particle spectra. The data presented here replace (and extend) the results of our earlier model predictions. Here we give for ordinary and carbonaceous chondrites elemental production rates for the cosmogenic radionuclides 10Be, 14C, 26Al, 36Cl, 41Ca, 53Mn, 60Fe, and 129I as well as for the noble gas isotopes 3He, 4He, 20Ne, 21Ne, 22Ne, 36Ar, and 38Ar. Using the new data and expressing size and depth scales to the unit [g/cm2], we are able to demonstrate that the matrix effect for both chondrite types is negligible for all target product combinations, except for those which are dominated by thermal or very low energy neutron reactions. Based on the new model predictions, we present a variety of elemental and isotopic production rate ratios allowing for a reliable determination of preatmospheric sizes, shielding depths, cosmic-ray exposure ages, and diffusive losses.
    • Elemental and isotope behavior of macromolecular organic matter from CM chondrites during hydrous pyrolysis

      Oba, Y.; Naraoka, H. (The Meteoritical Society, 2009-01-01)
      A new insight into carbon and hydrogen isotope variations of insoluble organic matter (IOM) is provided from seven CM chondrites, including Murchison and six Antarctic meteorites (Y-791198, Y-793321, A-881280, A-881334, A-881458 and B-7904) as well as Murchison IOM residues after hydrous pyrolysis at 270-330 degrees C for 72 h. Isotopic compositions of bulk carbon (delta-13Cbulk) and hydrogen (delta-D) of the seven IOMs vary widely, ranging from 15.1 to 7.6 and +133 to +986 per mil, respectively. Intramolecular carboxyl carbon (delta-13CCOOH) is more enriched in 13C by 7.5 -11 per mil than bulk carbon. After hydrous pyrolysis of Murchison IOM at 330 degrees degrees C, H/C ratio, delta-13Cbulk, delta-13CCOOH, and delta-D values decrease by up to 0.31, 3.5 per mil, 5.5 per mil, and 961 per mil, respectively. The O/C ratio increases from 0.22 to 0.46 at 270 degrees C and to 0.25 at 300 degrees degrees C, and decreases to 0.10 at 330 degrees C. delta-13Cbulk-delta-D cross plot of Murchison IOM and its pyrolysis residues shows an isotopic sequence. Of the six Antarctic IOMs, A-881280, A-881458, Y-791198 and B-7904 lie on or near the isotopic sequence depending on the degree of hydrous and/or thermal alteration, while A-881334 and Y-793321 consist of another distinct isotope group. A delta-13Cbulk-delta-13CCOOH cross-plot of IOMs, including Murchison pyrolysis residues, has a positive correlation between them, implying that the oxidation process to produce carboxyls is similar among all IOMs. These isotope distributions reflect various degree of alteration on the meteorite parent bodies and/or difference in original isotopic compositions before the parent body processes.
    • Geochemical identification of impactor for Lonar crater, India

      Misra, S.; Newsom, H. E.; Shyam Prasad, M. Shyam; Geissman, J. W.; Dube, A.; Sengupta, D. (The Meteoritical Society, 2009-01-01)
      The only well-known terrestrial analogue of impact craters in basaltic crusts of the rocky planets is the Lonar crater, India. For the first time, evidence of the impactor that formed the crater has been identified within the impact spherules, which are ~0.3 to 1 mm in size and of different aerodynamic shapes including spheres, teardrops, cylinders, dumbbells and spindles. They were found in ejecta on the rim of the crater. The spherules have high magnetic susceptibility (from 0.31 to 0.02 SI-mass) and natural remanent magnetization (NRM) intensity. Both NRM and saturation isothermal remanent magnetization (SIRM) intensity are ~2 Am2/kg. Demagnetization response by the NRM suggests a complicated history of remanence acquisition. The spherules show schlieren structure described by chains of tiny dendritic and octahedral-shaped magnetite crystals indicating their quenching from liquid droplets. Microprobe analyses show that, relative to the target basalt compositions, the spherules have relatively high average Fe2O3 (by ~1.5 wt%), MgO (~1 wt%), Mn (~200 ppm), Cr (~200 ppm), Co (~50 ppm), Ni (~1000 ppm) and Zn (~70 ppm), and low Na2O (~1 wt%) and P2O5 (~0.2 wt%). Very high Ni contents, up to 14 times the average content of Lonar basalt, require the presence of a meteoritic component in these spherules. We interpret the high Ni, Cr, and Co abundances in these spherules to indicate that the impactor of the Lonar crater was a chondrite, which is present in abundances of 12 to 20 percent by weight in these impact spherules. Relatively high Zn yet low Na2O and P2O5 contents of these spherules indicate exchange of volatiles between the quenching spherule droplets and the impact plume.
    • Lunar dust and lunar simulant activation and monitoring

      Wallace, W. T.; Taylor, L. A.; Liu, Y.; Cooper, B. L.; McKay, D. S.; Chen, B.; Jeevarajan, A. S. (The Meteoritical Society, 2009-01-01)
      NASA plans to resume human exploration of the Moon in the next decade. One of the pressing concerns is the effect that lunar dust (the fraction of the lunar regolith <20 micrometers in diameter) will have on systems, both human and mechanical, due to the fact that various problems were caused by dust during the Apollo missions. The loss of vacuum integrity in the lunar sample containers during the Apollo era ensured that the present lunar samples are not in the same condition as they were on the Moon; they have been passivated by oxygen and water vapor. To mitigate the harmful effects of lunar dust on humans, methods of "reactivating" the dust must be developed for experimentation, and, ideally, it should be possible to monitor the level of activity to determine methods of deactivating the dust in future lunar habitats. Here we present results demonstrating that simple grinding, as a simple analog to micrometeorite crushing, is capable of substantially activating lunar dust and lunar simulant, and it is possible to determine the level of chemical activity by monitoring the ability of the dust to produce hydroxyl radicals in aqueous solution. Comparisons between ground samples of lunar dust, lunar simulant, and quartz reveal that ground lunar dust is capable of producing over three times the amount of hydroxyl radicals as lunar simulant and an order of magnitude more than ground quartz.
    • 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.
    • Multiple fluvial reworking of impact ejecta--A case study from the Ries crater, southern Germany

      Buchner, E.; Schmieder, M. (The Meteoritical Society, 2009-01-01)
      Impact ejecta eroded and transported by gravity flows, tsunamis, or glaciers have been reported from a number of impact structures on Earth. Impact ejecta reworked by fluvial processes, however, are sparsely mentioned in the literature. This suggests that shocked mineral grains and impact glasses are unstable when eroded and transported in a fluvial system. As a case study, we here present a report of impact ejecta affected by multiple fluvial reworking including rounded quartz grains with planar deformation features and diaplectic quartz and feldspar glass in pebbles of fluvial sandstones from the "Monheimer Hhensande" ~10 km east of the Ries crater in southern Germany.
    • Tellurium isotope compositions of calcium-aluminum-rich inclusions

      Fehr, M. A.; Rehkämper, M.; Halliday, A. N.; Hattendorf, B.; Günther, D. (The Meteoritical Society, 2009-01-01)
      A method for the precise and accurate determination of the tellurium (Te) isotope compositions of calcium-aluminum-rich inclusions (CAIs) has been developed. The technique utilizes multiple-collector inductively coupled plasma-mass spectrometry (MC-ICPMS) with either Faraday detectors or a dual ion-counting system. The external reproducibility (2-sigma) for 126Te/125Te was ~15 per mil and ~2 per mil when 3 pg and 65 pg of Te were analyzed with the electron multipliers. Measurements performed on 200 pg of Te using Faraday detectors and time-resolved software displayed an external reproducibility of ~8 per mil for 126Te/124Te, whereas 3 ng Te could be measured to a precision of about 0.6 per mil. Analyses of five CAIs from the Allende chondrite yielded Te concentrations that range from 12 to 537 ppb and the inclusions are therefore depleted in Te relative to bulk Allende by factors of about 2 to 86. The Sn/Te ratios of the CAIs are also fractionated compared to bulk Allende (which displays 124Sn/128Te is approximately equal to 0.1) with 124Sn/128Te ratios of about 0.1 to 2.5. The Te isotope measurements for these refractory inclusions yielded no 126Te excesses from the decay of the short-lived radionuclide 126Sn (tau-1/2 = 234,500 years) and the most precise analysis provided a epsilon-126Te value of 1 +/- 6 (epsilon-126Te = 126Te/ 124Te normalized to 122Te/124Te = 0.53594 and reported relative to the JMC Te standard). Minor differences in the Te isotope composition of the CAIs relative to the terrestrial standard and bulk Allende hint at the presence of small deficits in r-process Te isotopes or excess of s-process Te, but these nucleosynthetic anomalies are barely resolvable given the analytical uncertainties. Hence, it is also conceivable that these effects reflect small unresolved analytical artifacts.
    • The Carancas meteorite impact crater, Peru: Geologic surveying and modeling of crater formation and atmospheric passage

      Kenkmann, T.; Artemieva, N. A.; Wünnemann, K.; Poelchau, M. H.; Elbeshausen, D.; Núñez del Prado, H. (The Meteoritical Society, 2009-01-01)
      The recent Carancas meteorite impact event caused a worldwide sensation. An H4-5 chondrite struck the Earth south of Lake Titicaca in Peru on September 15, 2007, and formed a crater 14.2 m across. It is the smallest, youngest, and one of two eye-witnessed impact crater events on Earth. The impact violated the hitherto existing view that stony meteorites below a size of 100 m undergo major disruption and deceleration during their passage through the atmosphere and are not capable of producing craters. Fragmentation occurs if the strength of the meteoroid is less than the aerodynamic stresses that occur in flight. The small fragments that result from a breakup rain down at terminal velocity and are not capable of producing impact craters. The Carancas cratering event, however, demonstrates that meter-sized stony meteoroids indeed can survive the atmospheric passage under specific circumstances. We present results of a detailed geologic survey of the crater and its ejecta. To constrain the possible range of impact parameters we carried out numerical models of crater formation with the iSALE hydrocode in two and three dimensions. Depending on the strength properties of the target, the impact energies range between approximately 100-1000 MJ (0.024-0.24 t TNT). By modeling the atmospheric traverse we demonstrate that low cosmic velocities (12- 1-4 kms^(-1)) and shallow entry angles (<20 degrees) are prerequisites to keep aerodynamic stresses low (<10 MPa) and thus to prevent fragmentation of stony meteoroids with standard strength properties. This scenario results in a strong meteoroid deceleration, a deflection of the trajectory to a steeper impact angle (40-60 degrees), and an impact velocity of 350-600 ms^(-1), which is insufficient to produce a shock wave and significant shock effects in target minerals. Aerodynamic and crater modeling are consistent with field data and our microscopic inspection. However, these data are in conflict with trajectories inferred from the analysis of infrasound signals.
    • The use of Auger spectroscopy for the in situ elemental characterization of sub-micrometer presolar grains

      Stadermann, F. J.; Floss, C.; Bose, M.; Lea, A. S. (The Meteoritical Society, 2009-01-01)
      Presolar grains are small samples of stardust that can be found at low abundances in some of the most unaltered types of extraterrestrial materials. While earlier laboratory studies of stardust mainly focused on grain types that can be extracted from bulk meteorites by acid dissolution techniques, such as silicon carbide and graphite, recent analyses of presolar silicates rely on isotope imaging searches for locating these grains in situ. Since presolar silicates are generally less than a micrometer in diameter and represent at best only a few hundred ppm of their host materials (e.g., primitive meteorites or interplanetary dust particles), locating and studying these particles can be analytically challenging. Recently, we began using scanning Auger spectroscopy for the in situ elemental characterization of presolar silicate grains as a complement to NanoSIMS isotopic studies for obtaining spatially matched compositional data. Auger spectroscopy is a well-established analytical technique for elemental characterizations in the material sciences, but has not been widely used in geological applications. We discuss the application of this technique to sub-micrometer sized silicate grains and address practical issues such as sample preparation, measurement settings, spatial resolution, data processing, and elemental quantification.
    • Trace element chemistry of Cumulus Ridge 04071 pallasite with implications for main group pallasites

      Danielson, L. R.; Righter, K.; Humayun, M. (The Meteoritical Society, 2009-01-01)
      Pallasites have long been thought to represent samples from the metallic core–silicate mantle boundary of a small asteroid-sized body, with as many as ten different parent bodies recognized recently. This report focuses on the description, classification, and petrogenetic history of pallasite Cumulus Ridge (CMS) 04071 using electron microscopy and laser ablation ICP-MS. Most olivines are angular in CMS 04071, but there are some minor occurrences of small rounded olivines, such as in the Eagle Station pallasite. Olivine, chromite, and metal compositions indicate that CMS 04071 can be classified as a Main Group pallasite. The kamacite/taenite partition coefficients (D) for highly siderophile elements (HSE) are all close to 1, but comparison with previous studies on iron meteorites and pallasites shows that variation of some D values is controlled by the Ni content oftaenite. D(HSE)metal/sulfide for Re, Cu, and Cr all are <1, indicating chalcophile behavior for these three elements, in agreement with experimental Dmetal/sulfide. D(HSE)metal/olivine are variable, which isperhaps due to small metallic inclusions in the olivine that are present to variable extents in different pallasites. All of these data, together with results from previous studies, indicate that the CMS pallasites were likely formed at the core-mantle boundary of a small asteroid, but not necessarily related to the core that produced the IIIAB irons. In addition, they share a similar volatile element depletion to HEDs that is distinct from other bodies such as Earth, Mars, Angrite Parent Body, and the parent body of the brachinites.