• 40Ar-39Ar ages of H-chondrite impact melt breccias

      Swindle, T. D.; Isachsen, C. E.; Weirich, J. R.; Kring, D. A. (The Meteoritical Society, 2009-01-01)
      40Ar-39Ar analyses of a total of 26 samples from eight shock-darkened impact melt breccias of H-chondrite affinity (Gao-Guenie, LAP 02240, LAP 03922, LAP 031125, LAP 031173, LAP 031308, NWA 2058, and Ourique) are reported. These appear to record impacts ranging in time from 303 +/- 56 Ma (Gao-Guenie) to 4360 +/- 120 Ma (Ourique) ago. Three record impacts 300-400 Ma ago, while two others record impacts 3900-4000 Ma ago. Combining these with other impact ages from H chondrites in the literature, it appears that H chondrites record impacts in the first 100 Ma of solar system history, during the era of the lunar cataclysm and shortly thereafter (3500-4000 Ma ago), one or more impacts ~300 Ma ago, and perhaps an impact ~500 Ma ago (near the time of the L chondrite parent body disruption). Records of impacts on the H chondrite parent body are rare or absent between the era of planetary accretion and the lunar cataclysm (4400-4050 Ma), during the long stretch between heavy bombardment and recent breakup events (3500-1000 Ma), or at the time of final breakup into meteorite-sized bodies (<50 Ma).
    • A meteorite crater on Earth formed on September 15, 2007: The Carancas hypervelocity impact

      Tancredi, G.; Ishitsuka, J.; Schultz, P. H.; Harris, R. S.; Brown, P.; ReVelle, D. O.; Antier, K.; Le Pichon, A.; Rosales, D.; Vidal, E.; et al. (The Meteoritical Society, 2009-01-01)
      On September 15, 2007, a bright fireball was observed and a big explosion was heard by many inhabitants near the southern shore of Lake Titicaca. In the community of Carancas (Peru), a 13.5 m crater and several fragments of a stony meteorite were found close to the site of the impact. The Carancas event is the first impact crater whose formation was directly observed by several witnesses as well as the first unambiguous seismic recording of a crater-forming meteorite impact on Earth. We present several lines of evidence that suggest that the Carancas crater was a hypervelocity impact. An event like this should have not occurred according to the accepted picture of stony meteoroids ablating in the Earths atmosphere, therefore it challenges our present models of entry dynamics. We discuss alternatives to explain this particular event. This emphasizes the weakness in the pervasive use of average parameters (such as tensile strength, fragmentation behavior and ablation behavior) in current modeling efforts. This underscores the need to examine a full range of possible values for these parameters when drawing general conclusions from models about impact processes.
    • Constraints on the cooling history of the H-chondrite parent body from phosphate and chondrule Pb-isotopic dates from Estacado

      Blinova, A.; Amelin, Y.; Samson, C. (The Meteoritical Society, 2007-01-01)
      To constrain the metamorphic history of the H-chondrite parent body, we dated phosphates and chondrules from four H6 chondritic meteorites using U-Pb systematics. Reconnaissance analyses revealed that only Estacado had a sufficiently high 206Pb/204Pb ratio suitable for our purposes. The Pb- Pb isochron date for Estacado phosphates is measured to be 4492 +/- 15 Ma. The internal residue second leachate isochron for Estacado chondrules yielded the chondrule date of 4546 +/- 18 Ma. An alternative age estimate for Estacado chondrules of 4527.6 +/- 6.3 Ma is obtained from an isochron including two chondrules, two magnetically separated fractions, and four bulk chondrite analyses. This isochron date might represent the age of termination of Pb diffusion from the chondrules to the matrix. From these dates and previously established closure temperatures for Pb diffusion in phosphates and chondrules, we estimate an average cooling rate for Estacado between 5.5 +/- 3.2 Myr/degrees C and 8.3 +/- 5.0 Myr/degrees C. Using previously published results for Ste. Marguerite (H4) and Richardton (H5), our data reveal that the cooling rates of H chondrites decrease markedly with increasing metamorphic grade, in agreement with the predictions of the onion-shell asteroid model. Several issues, however, need to be addressed before confirming this model for the H-chondrite parent body: the discrepancies between peak metamorphic temperatures established by various mineral thermometers need to be resolved, diffusion and other mechanisms of element migration in polycrystalline solids must be better understood, and dating techniques should be further improved.
    • Heavily-hydrated lithic clasts in CH chondrites and the related, metal-rich chondrites Queen Alexandra Range 94411 and Hammadah al Hamra 237

      Greshake, A.; Krot, A. N.; Meibom, A.; Weisberg, M. K.; Zolensky, M. E.; Keil, K. (The Meteoritical Society, 2002-01-01)
      Fine-grained, heavily-hydrated lithic clasts in the metal-rich (CB) chondrites Queen Alexandra Range (QUE) 94411 and Hammadah al Hamra 237 and CH chondrites, such as Patuxent Range (PAT) 91546 and Allan Hills (ALH) 85085, are mineralogically similar suggesting genetic relationship between these meteorites. These clasts contain no anhydrous silicates and consist of framboidal and platelet magnetite, prismatic sulfides (pentlandite and pyrrhotite), and Fe-Mn-Mg-bearing Ca-carbonates set in a phyllosilicate-rich matrix. Two types of phyllosilicates were identified: serpentine, with basal spacing of ~0.73 nm, and saponite, with basal spacings of about 1.1-1.2 nm. Chondrules and FeNi-metal grains in CB and CH chondrites are believed to have formed at high temperature (>1300 K) by condensation in a solar nebula region that experienced complete vaporization. The absence of aqueous alteration of chondrules and metal grains in CB and CH chondrites indicates that the clasts experienced hydration in an asteroidal setting prior to incorporation into the CH and CB parent bodies. The hydrated clasts were either incorporated during regolith gardening or accreted together with chondrules and FeNi-metal grains after these high-temperature components had been transported from their hot formation region to a much colder region of the solar nebula.
    • Partial melting of H6 ordinary chondrite Kernouve: Constraints on the effects of reducing conditions on oxidized compositions

      Ford, Rena L.; Benedix, Gretchen K.; McCoy, Timothy J.; Rushmer, Tracy (The Meteoritical Society, 2008-01-01)
      Partial melting experiments at temperatures of 950-1300 degrees C were conducted on the H6 chondrite Kernouv under reducing conditions using CO-CO2 gas mixing and graphite-buffered sealed silica tubes to examine the effect of reducing conditions during melting of starting materials that are more oxidized relative to the oxygen fugacity conditions of the experiments. The experiments produced a range of mineralogical and compositional changes. Olivine exhibits significant reduction to compositions of Fa25 at temperatures of 1300 degrees C. In contrast, orthopyroxene exhibits only slight reduction until the highest temperatures. Chromite is sometimes consumed by intruding sulfides, and displays increasingly magnesian compositions ranging as low as Fe/Fe + Mg of 0.1 at a constant Cr/Cr + Al ratio. The compositional changes with increasing temperature reflect a complex set of reactions, including oxidation-reduction. One application of these experiments address whether primitive achondrites could have formed from ordinary chondrite-like precursors by partial melting under reducing conditions. While changes observed in olivine and troilite compositions might support such an idea, differences in oxygen isotopic composition, Cr/Cr + Al in chromite, orthopyroxene compositions, and thermodynamic evidence against reduction during melting of primitive achondrites (Benedix et al. 2005) firmly refute such an idea.
    • Reclassification and thermal history of Trenzano chondrite

      Fioretti, A. M.; Domeneghetti, M. C.; Molin, G.; Cámara, F.; Alvaro, M.; Agostini, L. (The Meteoritical Society, 2007-01-01)
      We present a new single-crystal X-ray diffraction (XRD) study performed on a suite of six orthopyroxene grains from the low-shocked H6 Trenzano meteorite. The quenched intracrystalline Fe^(2+)-Mg ordering state in orthopyroxene preserves the memory of the cooling rate near closure temperature Tc, thus yielding useful constraints on the last thermal event undergone by the host rock. The orthopyroxene Tc of 522 +/- 13 degrees C, calculated using a new calibration equation obtained by Stimpfl (2005b), is higher than in previously published H chondrite data. The orthopyroxene cooling rate at this Tc is about 100 degrees C/kyr. This fast rate is inconsistent with the much slower cooling rate expected for H6 in the onion shell structural and thermal model of chondrite parent bodies. A petrographic study carried out at the same time indicated that the Trenzano meteorite is an H5 chondrite and not an H6 chondrite, as it is officially classified. Furthermore, the two-pyroxene equilibrium temperature of Trenzano (824 +- 24 degrees C), calculated with QUILF95, is similar to the two-pyroxene temperature of 750-840 degrees C obtained for the Carcote (H5) chondrite (Kleinschrot and Okrusch 1999).
    • The Asco meteorite (1805): New petrographic description, chemical data, and classification

      Gattacceca, J.; Bourot-Denise, M.; Brandstaetter, F.; Folco, L.; Rochette, P. (The Meteoritical Society, 2007-01-01)
      We present magnetic measurements, chemical analyses, and petrographic observations of the poorly studied Asco historical meteorite fall (1805). These new data indicate that this meteorite has been previously misclassified as an L6 ordinary chondrite. Asco is reclassified as an H6 ordinary chondrite with shock stage S3. An interesting feature of this meteorite is the presence of chromiteplagioclase assemblages with variable textures.
    • The Cali meteorite fall: A new H/L ordinary chondrite

      Trigo-Rodríguez, J. M.; Llorca, J.; Rubin, A. E.; Grossman, J. N.; Sears, D. W. G.; Naranjo, M.; Bretzius, S.; Tapia, M.; Guarín Sepúlveda, M. H. (The Meteoritical Society, 2009-01-01)
      The fall of the Cali meteorite took place on 6 July 2007 at 16 h 32 +/- 1 min local time (21 h 32 +/- 1 min UTC). A daylight fireball was witnessed by hundreds of people in the Cauca Valley in Colombia from which 10 meteorite samples with a total mass of 478 g were recovered near 3 degrees 24.3'N, 76 degrees 30.6'W. The fireball trajectory and radiant have been reconstructed with moderate accuracy. From the computed radiant and from considering various plausible velocities, we obtained a range of orbital solutions that suggest that the Cali progenitor meteoroid probably originated in the main asteroid belt. Based on petrography, mineral chemistry, magnetic susceptibility, thermoluminescence, and bulk chemistry, the Cali meteorite is classified as an H/L4 ordinary chondrite breccia.
    • The fall of Hoima, an H6 chondrite from Uganda

      Greshake, A.; Reimold, W. U.; Tuhumwire, J. T.; Baguma, Z.; De Villiers, M. E. (The Meteoritical Society, 2006-01-01)
      The Hoima meteorite fell on March 30, 2003, in the Hoima district near Butema, Uganda. According to its mineralogy, texture, and mineral chemical characteristics, Hoima is classified as a brecciated H6 ordinary chondrite of shock stage S2 and weathering grade W0. After the meteorites Maziba, Soroti, Awere, and Mbale, Hoima represents the fifth meteorite recorded from Uganda.
    • The mineralogy of the Yaringie Hill meteorite—A new H5 chondrite from South Australia

      Tappert, R.; Foden, J.; Pring, A. (The Meteoritical Society, 2009-01-01)
      The Yaringie Hill meteorite is a new H5 ordinary chondrite found in the Gawler Ranges, South Australia. The meteorite, which shows only minor signs of terrestrial weathering, is predominantly composed of olivine (Fa17.2), orthopyroxene (Fs15.1Wo1.1), and three distinct phases of nickeliferous iron metal (kamacite, taenite, tetrataenite). Other minerals include troilite, plagioclase (Ab81An16Or3), clinopyroxene (En52Wo42Fs6), chlorapatite, merrillite, ilmenite, and native copper. Three types of spinel with distinctive textures (coarse, skeletal aggregates, rounded aggregates) and with compositions close to the join MgAl2O4-FeCr2O4 are also present. Chondrules within the Yaringie Hill meteorite, which often have poorly defined boundaries, are placed in a recrystallized matrix. Shock indicators suggest that the meteorite experienced only weak shock metamorphism (S3).
    • Traces of an H chondrite in the impact-melt rocks from the Lappajärvi impact structure, Finland

      Tagle, R.; Öhman, T.; Schmitt, R. T.; Erzinger, J.; Claeys, Ph. (The Meteoritical Society, 2007-01-01)
      Here we present the results of a geochemical study of the projectile component in impact melt rocks from the Lappajrvi impact structure, Finland. Main- and trace-element analyses, including platinum group elements (PGEs), were carried out on twenty impact-melt rock samples from different locations and on two shocked granite fragments. The results clearly illustrate that all the impact melt rocks are contaminated with an extraterrestrial component. An identification of the projectile type was performed by determining the projectile elemental ratios and comparing the corresponding element ratios in chondrites. The projectile elemental ratios suggest an H chondrite as the most likely projectile type for the Lappajrvi impact structure. The PGE composition of the highly diluted projectile component (~0.05 and 0.7 wt% in the impact-melt rocks) is similar to the recent meteorite population of H chondrites reaching Earth. The relative abundance of ordinary chondrites, including H, L, and LL chondrites, as projectiles at terrestrial impact structures is most likely related to the position of their parent bodies relative to the main resonance positions. This relative abundance of ordinary chondrites suggests a strong bias of the impactor population toward inner Main Belt objects.