• A study of ordinary chondrites by Mossbauer spectroscopy with high-velocity resolution

      Oshtrakh, M. I.; Petrova, E. V.; Grokhovsky, I.; Semionkin, V. A. (The Meteoritical Society, 2008-01-01)
      An improvement in the velocity resolution and quality of Mössbauer spectra has been applied to a group of ordinary chondrites. This improvement permitted us to carry out a more detailed study of the iron bearing phases in these samples than has previously been possible. Mössbauer spectra of 11 ordinary chondrites of L and H chemical groups were measured using 4096 channels and presented for further analysis in 1024 channels. Subspectra of the metal grains of several chondrites demonstrated the presence of at least two magnetic sextets related to the main Fe(Ni, Co) phases. Moreover, Mössbauer study of extracted metal grains from Tsarev L5 revealed three sextets and one singlet spectral components related to various alpha-Fe(Ni, Co), alpha'-Fe(Ni, Co), alpha2-Fe(Ni, Co), and gamma-Fe(Ni, Co) phases. Each subspectrum of olivine and pyroxene in Mössbauer spectra of ordinary chondrites was fitted by superposition of two quadrupole doublets related to M1 and M2 sites in minerals for the first time. An analysis of relative areas and Mössbauer hyperfine parameters was performed and some differences for L and H chondrites as well as for M1 and M2 sites were observed. Mössbauer parameters of troilite and oxidized iron were analyzed. In contrast to a previous study with 512-channel spectra, the presence of oxidized iron was found in all chondrites.
    • Analyses of the chondritic meteorite Orvinio (H6): Insight into the origins and evolution of shocked H chondrite material

      Grier, J. A.; Kring, D. A.; Swindle, T. D.; Rivkin, A. S.; Cohen, B. A.; Britt, D. T. (The Meteoritical Society, 2004-01-01)
      We have studied the petrography, reflectance spectra, and Ar-Ar systematics of the Orivinio meteorite. Orvinio is an H chondrite not an L chondrite as sometimes reported. The material in the meteorite was involved in several impact events. One impact event produced large swaths of impact melt from H chondrite material surrounding relict clasts of chondrule-bearing material. Not only were portions of a bulk H chondrite planestesimal melted during the impact event, but shock redistribution of metal and sulfide phases in the meteorite dramatically altered its reflectance spectra. Both the melt and relict clasts are darker than unshocked H chondrite material, bearing spectral similarities to some C-class asteroids. Such shock metamorphism, which lowers the albedo of an object without increasing its spectral slope, may partially explain some of the variation among S-class asteroids and some of the trends seen on asteroid 433 Eros. Noble gases record the evidence of at least two, and perhaps three, impact events in the meteorite and its predecessor rocks. The most significant evidence is for an event that occurred 600 Ma ago or less, perhaps ~325 Ma ago or less. There is also a signature of 4.2 Ga in the Ar-Ar systematics, which could either reflect complete degassing of the rock at that time or partial degassing of even the most retentive sites in the more recent event.
    • Thuathe, a new H4/5 chondrite from Lesotho: History of the fall, petrography, and geochemistry

      Reimold, W. U.; Buchanan, P. C.; Ambrose, D.; Koeberl, C.; Franchi, I.; Lalkhan, C.; Schultz, L.; Franke, L.; Heusser, G. (The Meteoritical Society, 2004-01-01)
      On July 21, 2002, a meteorite fall occurred over the Thuathe plateau of western Lesotho. The well-defined strewn field covers an area of 1.9 x 7.4 km. Many of the recovered specimens display a brecciated texture with leucocratic, angular to subrounded clasts in a somewhat darker groundmass. Mineralogical and chemical data, as well as oxygen isotopic analysis, indicate that Thuathe is an H4/ 5, S2/3 meteorite, with local H3 or H6 character. A number of anomalous features include somewhat high Co contents of kamacite and taenite relative to normal H-group chondrites. Oxygen isotopic data plot at the edge of the normal H chondrite data field. Variable contents of metallic mineral phases and troilite result in a heterogeneous bulk composition (e.g., with regard to Si, Fe, and Mg), resulting in a spread of major element ratios that is not consistent with previously accepted H-group composition. Trace element abundances are generally consistent with H chondritic composition, and Kr and Xe isotopic data agree with an H4 classification for this meteorite. Noble gas analysis gave U, Th-4He gas retention and K-Ar ages typical for H chondrites; no major thermal event affected this material since ~3.7 Ga. The exposure age for Thuathe is 5 Ma, somewhat lower than for other H chondrites. Cosmogenic nuclide analysis indicates a pre-atmospheric radius of this meteorite between 35 and 40 cm. In the absence of evidence for solar gases, we classify Thuathe as a fragmental breccia. Numerous narrow, black veins cut across samples of Thuathe and are the result of a brittle deformation event that also caused local melting, especially in portions rich in sulfide. The formation of these veinlets is not the result of locally enhanced shock pressures (i.e., of shock melting) but rather of shearing under brittle conditions with local, friction-related temperature excursions causing melting mostly of Fe-sulfide and FeNi-metal but also, locally, of silicate minerals. Frictional temperature excursions must have attained values in excess of 1500 degrees C to permit complete melting of forsteritic olivine.