• Petrographic, X-ray diffraction, and electron spin resonance analysis of deformed calcite: Meteor Crater, Arizona

      Burt, J. B.; Pope, M. C.; Watkinson, A. J. (The Meteoritical Society, 2005-01-01)
      Calcite crystals within the Kaibab limestone in Meteor Crater, Arizona, are examined to understand how calcite is deformed during a meteorite impact. The Kaibab limestone is a silty finegrained and fossiliferous dolomudstone and the calcite crystals occur as replaced evaporite nodules with impact-induced twinning. Twinning in the calcite is variable with deformational regimes based on abundances of crystals with twins and twin densities within crystals. The twins are similar to those that are seen in low tectonically deformed regimes. Low levels of shock are inferred from minor peak broadening of the X-ray diffraction patterns (XRD) of the calcite crystals. In addition, electron spin resonance (ESR) spectroscopy data also indicates low shock levels (<3.0 GPa). Quantitative shock pressures and correlation between the XRD and ESR results are poor due to the inferred low shock levels in conjunction with the analytical error associated in calculation of the shock pressures.
    • Regolith history of lunar meteorites

      Lorenzetti, S.; Busemann, H.; Eugster, O. (The Meteoritical Society, 2005-01-01)
      The regolith evolution of the lunar meteorites Dhofar (Dho) 081, Northwest Africa (NWA) 032, NWA 482, NWA 773, Sayh al Uhaymir (SaU) 169, and Yamato (Y-) 981031 was investigated by measuring the light noble gases He, Ne, and Ar. The presence of trapped solar neon in Dho 081, NWA 773, and Y-981031 indicates an exposure at the lunar surface. A neon three-isotope diagram for lunar meteorites yields an average solar 20Ne/22Ne ratio of 12.48 +/- 0.07 representing a mixture of solar energetic particles neon at a ratio of 11.2 and solar wind neon at a ratio of 13.8. Based on the production rate ratio of 21Ne and 38Ar, the shielding depth in the lunar regolith of NWA 032, NWA 482, SaU 169, and Y-981031 was obtained. The shielding depth of these samples was between 10.5 g/cm2 and >500 g/cm^2. Based on spallogenic Kr and Xe, the shielding depth of Dho 081 was estimated to be most likely between 120 and 180 g/cm^2. Assuming a mean density of the lunar regolith of 1.8 g/cm^3, 10.5 g/cm^2 corresponds to a depth of 5.8 cm and 500 g/cm^2 to 280 cm below the lunar surface. The range of regolith residence time observed in this study is 100 Ma up to 2070 Ma.
    • The search for fullerenes in rocks from the Ries impact crater

      Frank, O.; Jehlička, J.; Hamplová, V.; Svatoš, A. (The Meteoritical Society, 2005-01-01)
      Since their discovery, fullerenes have been reported from various geological environments. One group of these findings has been related to bolide impacts, e.g., the Sudbury crater and the K-T and P-T boundaries. Impact rocks of the Ries crater, Germany, including samples of suevites, metamorphosed crystalline clasts, and glass bombs, have been collected in the Otting, Altebürg, and Seelbronn quarries. No fullerenes in concentrations above 1 ppb have been found in analyzed samples. Laser desorption time-of-flight mass spectrometry (LD-TOF-MS) confirmed the absence of fullerenes in the analyzed samples. These results support the concept that the Ries impactor was a stony meteorite.
    • The formation of the Widmanstätten structure in meteorites

      Yang, J.; Goldstein, J. I. (The Meteoritical Society, 2005-01-01)
      We have evaluated various mechanisms proposed for the formation of the Widmanstätten pattern in iron meteorites and propose a new mechanism for low P meteoritic metal. These mechanisms can also be used to explain how the metallic microstructures developed in chondrites and stony-iron meteorites. The Widmanstätten pattern in high P iron meteorites forms when meteorites enter the three-phase field alpha + gamma + Ph via cooling from the gamma + Ph field. The Widmanstätten pattern in low P iron meteorites forms either at a temperature below the (alpha + gamma)/(alpha + gamma + Ph) boundary or by the decomposition of martensite below the martensite start temperature. The reaction gamma --> alpha + gamma, which is normally assumed to control the formation of the Widmanstätten pattern, is not applicable to the metal in meteorites. The formation of the Widmanstätten pattern in the vast majority of low P iron meteorites (which belong to chemical groups IAB-IIICD, IIIAB, and IVA) is controlled by mechanisms involving the formation of martensite alpha2. We propose that the Widmanstätten structure in these meteorites forms by the reaction gamma --> alpha2 + gamma --> alpha + gamma, in which alpha2 decomposes to the equilibrium alpha and gamma phases during the cooling process. To determine the cooling rate of an individual iron meteorite, the appropriate formation mechanism for the Widmanstätten pattern must first be established. Depending on the Ni and P content of the meteorite, the kamacite nucleation temperature can be determined from either the (gamma + Ph)(alpha + gamma + Ph) boundary, the (alpha + gamma)/(alpha + gamma + Ph) boundary, or the Ms temperature. With the introduction of these three mechanisms and the specific phase boundaries and the temperatures where transformations occur, it is no longer necessary to invoke arbitrary amounts of under-cooling in the calculation of the cooling rate. We conclude that martensite decomposition via the reactions gamma --> alpha 2 --> alpha + gamma and gamma --> alpha2 + gamma --> alpha + gamma are responsible for the formation of plessite in irons and the metal phases of mesosiderites, chondrites, and pallasites. The hexahedrites (low P members of chemical group IIAB) formed by the massive transformation through the reaction gamma --> alpha-m --> alpha at relatively high temperature in the two-phase alpha + gamma region of the Fe-Ni-P phase diagram near the alpha/(alpha + gamma) phase boundary.
    • Portales Valley: Petrology of a metallic-melt meteorite breccia

      Ruzicka, A.; Killgore, M.; Mittlefehldt, D. W.; Fries, M. D. (The Meteoritical Society, 2005-01-01)
      Portales Valley (PV) is an unusual metal-veined meteorite that has been classified as an H6 chondrite. It has been regarded either as an annealed impact melt breccia, as a primitive achondrite, or as a meteorite with affinities to silicated iron meteorites. We studied the petrology of PV using a variety of geochemical-mineralogical techniques. Our results suggest that PV is the first welldocumented metallic-melt meteorite breccia. Mineral-chemical and other data suggest that the protolith to PV was an H chondrite. The composition of FeNi metal in PV is somewhat fractionated compared to H chondrites and varies between coarse vein and silicate-rich portions. It is best modeled as having formed by partial melting at temperatures of ~940-1150 degrees C, with incomplete separation of solid from liquid metal. Solid metal concentrated in the coarse vein areas and S-bearing liquid metal concentrated in the silicate-rich areas, possibly as a result of a surface energy effect. Both carbon and phosphorus must have been scavenged from large volumes and concentrated in metallic liquid. Graphite nodules formed by crystallization from this liquid, whereas phosphate formed by reaction between P-bearing metal and clinopyroxene components, depleting clinopyroxene throughout much of the meteorite and growing coarse phosphate at metal-silicate interfaces. Some phosphate probably crystallized from P-bearing liquids, but most probably formed by solid-state reaction at ~975-725 degrees C. Phosphate-forming and FeO-reduction reactions were widespread in PV and entailed a change in the mineralogy of the stony portion on a large scale. Portales Valley experienced protracted annealing from supersolidus to subsolidus temperatures, probably by cooling at depth within its parent body, but the main differences between PV and H chondrites arose because maximum temperatures were higher in PV. A combination of a relatively weak shock event and elevated pre-shock temperatures probably produced the vein-and-breccia texture, with endogenic heating being the main heat source for melting, and with stress waves from an impact event being an essential trigger for mobilizing metal. Portales Valley is best classified as an H7 metallic-melt breccia of shock stage S1. The meteorite is transitional between more primitive (chondritic) and evolved (achondrite, iron) meteorite types and offers clues as to how differentiation could have occurred in some asteroidal bodies.
    • Studies and characterizations of the Al Zarnkh meteorite

      Gismelseed, A. M.; Bashir, S.; Worthing, M. A.; Yousif, A. A.; Elzain, M. E.; Al Rawas, A. D.; Widatallah, H. M. (The Meteoritical Society, 2005-01-01)
      A newly fallen Sudanese meteorite named Al Zarnkh was investigated using room and liquid nitrogen temperature Mössbauer measurements, X-ray diffraction (XRD), and electron probe microanalysis (EPMA) in conjunction with energy dispersive X-ray microscopy. The Mössbauer spectra exhibited strong paramagnetic doublets with magnetic sextets. The doublets are assigned to olivine and pyroxene, while the magnetic sextets are assigned to troilite and kamacite. Based on microprobe analyses and textural studies, olivine is the most abundant phase and occurs as fine to medium grained laths both in the groundmass and in barred olivine chondrules. Both orthopyroxenes and clinopyroxenes are present and these tend to be granular. Plagioclase is an abundant interstitial groundmass phase. Chromites were detected in some groundmass olivine and are highly chromiumand iron-rich with no Fe3+ detected. The kamacite contains small amounts of Co. The mole fraction of the Fe end-member of olivine (fayalite) and orthopyroxene (ferrosilite) are found to be about 28% and 23%, respectively. These values are compared with that obtained from two chondritic meteorites. Based on these results, the studied meteorite is classified as an ordinary LL5 chondrite.
    • Chondrules in Antarctic micrometeorites

      Genge, M. J.; Gileski, A.; Grady, M. M. (The Meteoritical Society, 2005-01-01)
      Previous studies of unmelted micrometeorites (>50 micrometers) recovered from Antarctic ice have concluded that chondrules, which are a major component of chondritic meteorites, are extremely rare among micrometeorites. We report the discovery of eight micrometeorites containing chondritic igneous objects, which strongly suggests that at least a portion of coarse-grained crystalline micrometeorites represent chondrule fragments. Six of the particles are identified as composite micrometeorites that contain chondritic igneous objects and fine-grained matrix. These particles suggest that at least some coarse-grained micrometeorites (cgMMs) may be derived from the same parent bodies as fine-grained micrometeorites. The new evidence indicates that, contrary to previous suggestions, the parent bodies of micrometeorites broadly resemble the parent asteroids of chondrulebearing carbonaceous chondrites.
    • A meteorite impact crater field in eastern Bavaria? A preliminary report

      Fehr, K. T.; Pohl, J.; Mayer, W.; Hochleitner, R.; Fassbinder, J.; Geiss, E.; Kerscher, H. (The Meteoritical Society, 2005-01-01)
      Numerous circular depressions north of Burghausen in eastern Bavaria, with diameters ranging from meters to tens of meters in size and dispersed over an area of at least 11 7 km, are suspected to have an extraterrestrial origin since they resemble other small meteorite impact craters. The depressions are bowl-shaped, have high circularity and a characteristic rim. Most of them were formed in unconsolidated glacial gravels and pebbles intermixed with fine-grained sand and clay. Magnetic investigations reveal weak anomalies with amplitudes of less than +/- 10 nanoTesla (nT). In some cases, the origins of the anomalies are suspected to be due to human activity within the structures. So far, no traces of meteoritic material have been detected. An evident archaeological or local geological explanation for the origin of the craters does not exist. A World War I and II explosive origin can be excluded since trees with ages exceeding 100 years can be found in some craters. One crater was described in 1909. Carbon-14 dating of charcoal found in one crater yielded an age of 1790 +/- 60 years. Hence, a formation by meteorite impacts that occurred in Celtic or early medieval times should be considered. A systematic archaeological excavation of some structures and an intensified search for traces of meteoritic material are planned.
    • Shock-melted material in the Krymka LL3.1 chondrite: Behavior of the opaque minerals

      Semenenko, V. P.; Perron, C. (The Meteoritical Society, 2005-01-01)
      Six large millimeter- to centimeter-size regions of one specimen of the Krymka LL3.1 ordinary chondrite show evidence of having been completely or nearly completely shock-melted in situ, a phenomenon rarely observed in primitive chondrites. The shock pressure, nominally in the range of 75-90 GPa, could only have been 30-35 GPa in a porous material like fine-grained matrix. The melted regions have an igneous texture and their silicates are zoned and unequilibrated. Large metal-troilite intergrowths formed in these regions. The metal has a nickel content corresponding to martensite and the troilite contains up to 4.2 wt% nickel. Melting must have been very short and cooling very fast (>100 degrees C/h at high temperature). The metal contains up to 0.7 wt% phosphorus. Abundant chromite crystals and sodium-iron phosphate glass globules are found in troilite. The differences in composition between the opaque phases found in the melted regions and those generally observed in unmetamorphosed chondrules are assigned to melting under closed system conditions. Surprisingly high Co concentrations (up to 13 wt%) were found in some metal grains in or at the periphery of melted regions. They likely resulted from sulfurization of metal by sulfur vapor produced during the shock. After solidification, at least one other shock led to mechanical effects in the melted regions.
    • Internal structure of type I deep-sea spherules by X-ray computed microtomography

      Feng, H.; Jones, K. W.; Tomov, S.; Stewart, B.; Herzog, G. F.; Schnabel, C.; Brownlee, D. E. (The Meteoritical Society, 2005-01-01)
      The internal structures of type I spherules (melted micrometeorites rich in iron) have been investigated using synchrotron-based computed microtomography. Variations from sphericity are smallthe average ratio of the largest to the smallest semimajor axis is 1.07 +/- 0.06. The X-ray tomographs reveal interior cavities, four spherules with metal cores with diameters ranging from 57 to 143 micrometers and, in two spherules, high attenuation features thought to be nuggets rich in platinumgroup elements. Bulk densities range from 4.2 to 5.9 g/cm3 and average grain densities from 4.5 to 6.5 (g/cm3) with uncertainties of 10-15%. The average grain densities are those expected for materials containing mostly oxides of iron and nickel. The tomographic density measurements indicate an average void space of 5 +8/-5%. The void spaces may be contraction features or the skeletons of bubbles that formed in the molten precursors during atmospheric passage.
    • Ar-Ar and I-Xe ages and the thermal history of IAB meteorites

      Bogard, D. D.; Garrison, D. H.; Takeda, H. (The Meteoritical Society, 2005-01-01)
      Studies of several samples of the large Caddo County IAB iron meteorite reveal andesitic material enriched in Si, Na, Al, and Ca, which is essentially unique among meteorites. This material is believed to have formed from a chondritic source by partial melting and to have further segregated by grain coarsening. Such an origin implies extended metamorphism of the IAB parent body. New 39Ar-40Ar ages for silicate from three different Caddo samples are consistent with a common age of 4.50-4.51 Gyr. Less well-defined Ar-Ar degassing ages for inclusions from two other IABs, EET (Elephant Moraine) 83333 and Udei Station, are ~4.32 Gyr, whereas the age for Campo del Cielo varies considerably over about 3.23-4.56 Gyr. New 129I-129Xe ages for Caddo County and EET 83333 are 4557.9 +/- 0.1 Myr and 4557-4560 Myr, respectively, relative to an age of 4562.3 Myr for Shallowater. Considering all reported Ar-Ar degassing ages for IABs and related winonaites, the range is ~4.32-4.53 Gyr, but several IABs give similar Ar ages of 4.50-4.52 Gyr. We interpret these older Ar ages to represent cooling after the time of last significant metamorphism on the parent body and the younger ages to represent later 40Ar diffusion loss. The older Ar-Ar ages for IABs are similar to Sm-Nd and Rb-Sr isochron ages reported in the literature for Caddo County. Considering the possibility that IAB parent body formation was followed by impact disruption, reassembly, and metamorphism (e.g., Benedix et al. 2000), the Ar-Ar ages and IAB cooling rates deduced from Ni concentration profiles in IAB metal (Herpfer et al. 1994) are consistent if the time of the postassembly metamorphism was as late as about 4.53 Gyr ago. However, I-Xe ages reported for some IABs define much older ages of about 4558-4566 Myr, which cannot easily be reconciled with the much younger Ar-Ar and Sm-Nd ages. An explanation for the difference in radiometric ages of IABs may reside in combinations of the following: a) I-Xe ages have very high closure temperatures and were not reset during metamorphism about 4.53 Gyr ago; b) a bias exists in the 40K decay constants which makes these Ar-Ar ages approximately 30 Myr too young; degrees C) the reported Sm-Nd and Rb-Sr ages for Caddo are in error by amounts equal to or exceeding their reported 2-sigma uncertainties; and d) about 30 Myr after the initial heating that produced differentiation of Caddo silicate and mixing of silicate and metal, a mild metamorphism of the IAB parent body reset the Ar-Ar ages.
    • Spectral reflectance of Martian meteorites: Spectral signatures as a template for locating source region on Mars

      McFadden, L. A.; Cline, T. P. (The Meteoritical Society, 2005-01-01)
      We report the spectral reflectance of Martian meteorites from 0.3-2.6 microns for the purpose of cataloguing spectra and the association of their spectral properties with mineralogy and petrology. We fit the spectra to a series of overlapping, modified Gaussian absorptions using least squares fitting. The results are validated against established relationships between photon interactions with mineral chemistry and the band parameters. These resultant band parameters can be used to constrain interpretations of Martian reflectance spectra in the search for the source region of meteorites from Mars. The limitations of the fitting method are discussed.