• Weathering features in shocked quartz from the Ries impact crater, Germany

      Leroux, H. (The Meteoritical Society, 2005-01-01)
      Shocked quartz from the ejecta of the Ries impact structure has been investigated by analytical transmission electron microscopy (ATEM). Quartz grains display numerous planar fractures (PFs) and planar deformation features (PDFs). Both are partly or fully replaced by a mineral of the kaolinite group (likely halloysite). Its formation involves fluid circulation into the dense fracture networks, dissolution and removal of the amorphous phase initially present in PDFs, and finally, precipitation and crystallization of the kaolinite group mineral from solutions resulting from the chemical alteration of adjacent minerals (feldspars and biotite). Kaolinite group minerals are typical of hydrothermal alteration at low temperature, in humid climate, and under moderately acid conditions and, thus, this alteration may not be directly related to the impact event itself. However, the weathering features were strongly enhanced by the shock-generated microstructure, in particular by fractures that provided pathways for fluid circulation.
    • Weathering of meteorites from Oman: Correlation of chemical and mineralogical weathering proxies with 14C terrestrial ages and the influence of soil chemistry

      Al-Kathiri, A.; Hofmann, B. A.; Jull, A. J. T.; Gnos, E. (The Meteoritical Society, 2005-01-01)
      Fifty-four fragments of ordinary chondrites from 50 finds representing all searched areas in central Oman and all weathering stages were selected to compare the physical, chemical, and mineralogical effect of terrestrial weathering with 14C terrestrial ages. 14C ages range from 2.0 to >49 kyr with a median value of 17.9 kyr. The peak of the age range, which is between 10-20 kyr, falls in an arid climate period. A comparison of the chemical composition of Omani chondrites with literature data for unweathered H and L chondrites demonstrates a strong enrichment in Sr and Ba, and depletion in S during weathering. Water contents in H chondrites increase with terrestrial age, whereas L chondrites show a rapid initial increase followed by nearly constant water content. Correlating Sr, Ba, and H2O with age indicates two absorption trends: i) an initial alteration within the first 20 kyr dominated by H2O uptake, mainly reflecting Fe-Ni metal alteration, and ii) a second Baand Sr-dominated stage correlated with slower and less systematic weathering of troilite that starts after H2O reaches ~2 wt%. Sulfur released from troilite partly combines with Ba and Sr to form sulfate minerals. Other parameters correlated with 14C age are degree of weathering, color of powdered meteorites, and the Ni/Fe ratio. Chemical analyses of 145 soils show a high degree of homogeneity over the entire interior Oman Desert, indicating large-scale mixing by wind. Soil samples collected from beneath meteorite finds typically are enriched in Ni and Co, confirming mobilization from the meteorites. High Cr and Ni concentrations in reference soil samples, which decrease from NE to SW, are due to detrital material from ultramafic rocks of the Oman Mountains.
    • Weston: Revised position for a historically significant meteorite fall

      Robson, M.; Pagliaro, F. (The Meteoritical Society, 2009-01-01)
      Weston is the first well-documented meteorite fall in the New World. The fall occurred on December 14, 1807. The Weston event began the study of meteoritics in the United States in the decade that science accepted that stones do, in fact, fall from the sky. It is unfortunate that much of the literature regarding this historically significant fall is erroneous. This paper will deal with the geographic position of the fall site. One of us (Robson 2007) proposed a new set of coordinates for Weston that was accepted by the Meteoritical Society. At the societys 70th annual meeting, new Weston coordinates were suggested; with the caveat that research was ongoing. However, it was also stated in the presentation that it was unlikely that either coordinate would change by as much as a minute. Further research indicates a final revision is required. Our revised mean fall position of the seven documented fragments of Weston is: 41 degrees 16' N, 73 degrees 16' W (WGS 84 coordinates, to the nearest minute). A quirk of history is a main factor in the derivation of faulty positions for Weston. The historically changing positions given for the fall are explored. Our methodology is discussed and the newly discovered Weston manuscripts, maps, and communications of Yales foremost meteoric astronomer, Professor H. A. Newton, support our findings.
    • What metal-troilite textures can tell us about post-impact metamorphism in chondrite meteorites

      Tomkins, A. G. (The Meteoritical Society, 2009-01-01)
      Metal-troilite textures are examined in metamorphosed and impact-affected ordinary chondrites to examine the response of these phases to rapid changes in temperature. Complexly intergrown metal-troilite textures are shown to form in response to three different impact-related processes. (1) During impacts, immiscible melt emulsions form in response to spatially focused heating. (2) Immediately after impact events, re-equilibration of heterogeneously distributed heat promotes metamorphism adjacent to zones of maximum impact heating. Where temperatures exceed ~850 degrees degrees C, this post-impact metamorphism results in melting of conjoined metal-troilite grains in chondrites that were previously equilibrated through radiogenic metamorphism. When the resulting Fe-Ni-S melt domains crystallize, a finely intergrown mixture of troilite and metal forms, which can be zoned with kamacite-rich margins and taenite-rich cores. (3) At lower temperatures, post-impact metamorphism can also cause liberation of sulfur from troilite, which migrates into adjacent Fe-Ni metal, allowing formation of troilite and occasionally copper within the metal during cooling. Because impact events cause heating within a small volume, post-impact metamorphism is a short duration event (days to years) compared with radiogenic metamorphism (>10^6 years). The fast kinetics of metal-sulfide reactions allows widespread textural changes in conjoined metal-troilite grains during post-impact metamorphism, whereas the slow rate of silicate reactions causes these to be either unaffected or only partially annealed, except in the largest impact events. Utilizing this knowledge, information can be gleaned as to whether a given meteorite has suffered a post-impact thermal overprint, and some constraints can be placed on the temperatures reached and duration of heating.
    • William Lee Quaide, 1927-2004

      French, B. (The Meteoritical Society, 2005-01-01)
    • Woodleigh impact structure, Australia: Shock petrography and geochemical studies

      Reimold, W. U.; Koeberl, C.; Hough, R. M.; McDonald, I.; Bevan, A.; Amare, K.; French, B. M. (The Meteoritical Society, 2003-01-01)
      The large, complex Woodleigh structure in the Carnarvon basin of Western Australia has recently been added to the terrestrial impact crater record. Many aspects of this structure are, however, still uncertain. This work provides a detailed petrographic assessment of a suite of representative drill core samples from the borehole Woodleigh 1 that penetrated uplifted basement rocks of the central part of this structure. Fundamental rock and mineral deformation data and high-precision chemical data, including results of PGE and oxygen isotopic analysis, are presented. The sampled interval displays likely impact-produced macrodeformation in the form of fracturing and breccia veining at the microscopic scale. Contrary to earlier reports that these breccias represent pseudotachylite (friction melt) or even shock/shear-produced pseudotachylitic melt breccia cannot be confirmed due to pervasive post-impact alteration. Abundant planar deformation features (PDFs) in quartz, in addition to diaplectic glass and partial isotropization, are the main shock deformation effects observed, confirming that Woodleigh is of impact origin. Over the investigated depth interval, the statistics of quartz grains with a variable number of sets of PDFs does not change significantly, and the patterns of crystallographic orientations of PDFs in randomly selected quartz grains does not indicate a change in absolute shock pressure with depth either. The value of oxygen isotopes for the recognition of meteoritic contamination, as proposed by earlier Woodleigh workers, is critically assessed. Neither INA nor PGE analyses of our samples support the presence of a meteoritic component within this basement section, as had been claimed in earlier work.
    • Workshop on Chondrites and the Protoplanetary Disk Kaua'i, Hawai'i, 2004

      Liffman, Kurt (The Meteoritical Society, 2006-01-01)
    • Workshop summary on physical and chemical properties of potential Earth impactors

      Huebner, W. F.; Greenberg, J. M. (The Meteoritical Society, 2002-01-01)
      From 2001 June 17 to 25, we held the first international workshop in Erice, Italy, dedicated to the determination of geological and geophysical properties of near-Earth objects (NEOs). The goal was to develop a roadmap for determining the physical chemical properties of NEOs in the coming decades to meet the scientific requirements for development of Earth collision avoidance technology. We identified many properties that are desired, but four measurements are needed most critically for any potentially hazardous NEO: (1) its mass, (2) its mass distribution, (3) its material strengths, and (4) its internal structure. Global (whole-body) properties, such as material strengths and internal structure, can be determined best from the analyses of permeating waves: artificially initiated seismology and multifrequency reflection and transmission radio tomography. Seismology provides the best geophysical (material strengths) data of NEOs composed of consolidated materials while radio tomography provides the best geological data (e.g., the state of fracture) of electrically nonconducting media. Thus, teh two methods are complimentary: seismology is most suitable for stony and metallic asteroids, while radio tomography is most appropriate for comet nuclei and carbonaceous asteroids. The three main conclusions are (1) remote sensing for physical characterization should be increased, (2) several dedicated NEO missions should be prepared for geophysical and geological investigations, and (3) that it is prudent to develop and prove the technology to make geophysical measurements on NEOs now.
    • Yamato 792947,793408 and 82038: The most primitive H chondrites, with abundant refractory inclusions

      Kimura, M.; Hiyagon, H.; Palme, H.; Spettel, B.; Wolf, D.; Clayton, R. N.; Mayeda, T. K.; Sato, T.; Suzuki, A.; Kojima, H. (The Meteoritical Society, 2002-01-01)
      In this paper we report petrological and chemical data of the unusual chondritic meteorites Yamato (Y)-792947, Y-93408 and Y-82038. The three meteorites are very similar in texture and chemical composition, suggesting that they are pieces of a single fall. The whole-rock oxygen isotopes and the chemical compositions are indicative ofH chondrites. In addition, the mineralogy, and the abundances of chondrule types, opaque minerals and matrices suggest that these meteorites are H3 chondrites. They were hardly affected by thermal and shock metamorphism. The degree of weathering is very low. We conclude that these are the most primitive H chondrites, H3.2-3.4 (SI), known to date. On the other hand, these chondrites contain extraordinarily high amounts of refractory inclusions, intermediate between those of ordinary and carbonaceous chondrites. The distribution of the inclusions may have been highly heterogeneous in the primitive solar nebula. The mineralogy, chemistry and oxygen isotopic compositions of inclusions studied here are similar to those in CO and E chondrites.
    • Yamato 86029: Aqueously altered and thermally metamorphosed CI-like chondrite with unusual textures

      Tonui, E. K.; Zolensky, M. E.; Lipschutz, M. E.; Wang, M.-S.; Nakamura, T. (The Meteoritical Society, 2003-01-01)
      We describe the petrologic and trace element characteristics of the Yamato 86029 (Y-86029) meteorite. Y-86029 is a breccia consisting of a variety of clasts, and abundant secondary minerals including coarse- and fine-grained phyllosilicates, Fe-Ni sulfides, carbonates, and magnetite. There are no chondrules, but a few anhydrous olivine-rich grains are present within a very fine-grained phyllosilicate-rich matrix. Analyses of 14 thermally mobile trace elements suggest that Y-86029 experienced moderate, open-system thermal metamorphism. Comparison with data for other heated carbonaceous chondrites suggests metamorphic temperatures of 500-600 degrees C for Y-86029. This is apparent petrographically, in partial dehydration of phyllosilicates to incompletely re-crystallized olivine. This transformation appears to proceed through `intermediate' highly-disordered `poorly crystalline' phases consisting of newly formed olivine and residual desiccated phyllosilicate and their mixtures. Periclase is also present as a possible heating product of Mg-rich carbonate precursors. Y-86029 shows unusual textures rarely encountered in carbonaceous chondrites. The periclase occurs as unusually large Fe-rich clasts (300-500 micrometers). Fine-grained carbonates with uniform texture are also present as small (10-15 micrometers in diameter), rounded to sub-rounded `shells' of ankerite/siderite enclosing magnetite. These carbonates appear to have formed by low temperature aqueous alteration at specific thermal decomposition temperatures consistent with thermodynamic models of carbonate formation. The fine and uniform texture suggests crystallization from a fluid circulating in interconnected spaces throughout entire growth. One isolated aggregate in Y-86029 also consists of a mosaic of polycrystalline olivine aggregates and sulfide blebs typical of shock-induced melt re- crystallization. Except for these unusual textures, the isotopic, petrologic and chemical characteristics of Y- 86029 are quite similar to those of Y-82162, the only other heated CI-like chondrite known. They were probably derived from similar asteroids rather than one asteroid, and hence may not necessarily be paired.
    • Yellow impact glass from the K/T boundary at Beloc (Haiti): XANES determination of the Fe oxidation state and implications for formation conditions

      Giuli, Gabriele; Eeckhout, Sigrid Griet; Koeberl, Christian; Pratesi, Giovanni; Paris, Eleonora (The Meteoritical Society, 2008-01-01)
      We determined the iron oxidation state and coordination number in five samples of yellow impact glass from the Cretaceous-Tertiary (K/T) boundary section at Beloc, Haiti, which formed as the result of impact melting during the Chicxulub impact event. The samples were analyzed by Fe K-edge XANES spectroscopy and the results were compared with published data on eight black impact glasses and one high Si-K impact spherule from the same impact layer. The pre-edge peak of our high-resolution XANES spectra displays evident variations indicative of significant changes in the Fe oxidation state, spanning a wide range from about 75 to 100 mole% Fe3+. Yellow K/T glasses have significantly higher Fe3+/(Fe2+ + Fe3+) ratios compared to black K/T impact glasses (from 20 to 75 mole% Fe3+) and high Si-K glass (20 mole% Fe3+). In particular, all the pre-edge peak data on these three types of impact glasses plot between two mixing lines joining a point calculated as the mean of a group of tektites studied so far (consisting of [4]Fe2+ and [5]Fe2+) to [4]Fe3+ and [5]Fe3+, respectively. Thus, the XANES spectra of the yellow K/T glasses can be interpreted as a mixture of [4]Fe2+, [5]Fe2+, [4]Fe3+, and [5]Fe3+. Our observations can be explained by a very large range of oxygen fugacity conditions during melt formation. Furthermore, there is a clear positive relationship between the Fe3+/(Fe2+ + Fe3+) ratio and the Ca content of these glasses, suggesting that the Fe oxidation state was influenced by the relative contribution of Ca-sulfate-and Ca-carbonate-bearing sedimentary rocks at the impact site.
    • Zoned chondrules in Semarkona: Evidence for high- and low-temperature processing

      Grossman, Jeffrey N.; Alexander, Conel M. O'D.; Wang, Jianhua; Brearley, Adrian J. (The Meteoritical Society, 2002-01-01)
      At least 15% of the low-FeO chondrules in Semarkona (LL3.0) have mesostases that are concentrically zoned in Na, with enrichments near the outer margins. We have studied zoned chondrules using electron microprobe methods (x-ray mapping plus quantitative analysis), ion microprobe analysis for trace elements and hydrogen isotopes, cathodoluminescence imaging, and transmission electron microscopy in order to determine what these objects can tell us about the environment in which chondrules formed and evolved. Mesostases in these chondrules are strongly zoned in all moderately volatile elements and H (interpreted as water). Calcium is depleted in areas of volatile enrichment. Titanium and Cr generally decrease toward the chondrule surfaces, whereas Al and Si may either increase or decrease, generally in opposite directions to one another; Mn follows Na in some chondrules but not in others; Fe and Mg are unzoned. D/H ratios increase in the water-rich areas of zoned chondrules. Mesostasis shows cathodoluminescence zoning in most zoned chondrules, with the brightest yellow color near the outside. Mesostasis in zoned chondrules appears to be glassy, with no evidence for devitrification. Systematic variations in zoning patterns among pyroxene- and olivine-rich chondrules may indicate that fractionation of low- and high-Ca pyroxene played some role in Ti, Cr, Mn, Si, Al, and some Ca zoning. But direct condensation of elements into hot chondrules, secondary melting of late condensates into the outer portions of chondrules, and subsolidus diffusion of elements into warm chondrules cannot account for the sub-parallel zoning profiles of many elements, the presence of H2O, or elemental abundance patterns. Zoning of moderately volatile elements and Ca may have been produced by hydration of chondrule glass without devitrification during aqueous alteration on the parent asteroid. This could have induced structural changes in the glass allowing rapid diffusion and exchange of elements between altered glass and surrounding matrix and rim material. Calcium was mainly lost during this process, and other nonvolatile elements may have been mobile as well. Some unzoned, low-FeO chondrules appear to have fully altered mesostasis.
    • Zoning patterns of Fe and V in spinel from a type B Ca-Al-rich inclusion: Constraints on subsolidus thermal history

      Paque, J. M.; Burnett, D. S.; Beckett, J. R. (The Meteoritical Society, 2007-01-01)
      We obtained two-dimensional concentration maps for the minor elements Fe and V in 21 spinel crystals in the Allende type B1 inclusion TS-34 with a 4-5 micrometer resolution. Locally high concentrations of Fe occur along at least one edge of the spinels and decrease toward the center of the grains. Enrichment in V can also occur along edges or at corners. In general, there is no overall correlation of the Fe and V distributions, but in local regions of two grains, the V and Fe distributions are correlated, strongly suggesting a local source for both elements. In these two grains, opaque assemblages are present that appear to locally control the V distributions. This, coupled with previous work, suggests that prior to alteration, TS-34 contained V-rich metal. Oxidation of this metal during alteration can account for the edge/corner V enrichments, but provide only minor FeO contributions, explaining the overall lack of correlation between Fe and V. Most of the FeO appears to have been externally introduced along spinel boundaries during alteration. These alteration phases served as sources for diffusion of FeO into spinel. FeO distributions in spinel lead to a mean attenuation length of ~8 micrometer and, using literature diffusion coefficients in isothermal and exponential cooling approximations for peak temperatures in the range 600-700 degrees C, this leads to a time scale for calciumaluminum- rich inclusion (CAI) alteration in the range of decades to centuries.