• Clast fabric examination of impact-generated breccias, borehole LB-07A, Bosumtwi, Ghana

      Morris, W. A.; Ugalde, H.; Clark, C.; Miles, B. (The Meteoritical Society, 2007-01-01)
      An impact event always creates a cloud of ejecta generated through excavation of the target. Subsequent in-filling of the void by crater-fill deposits provides a record of post-impact processes. Full-core digital photographic scans of core segments from borehole LB-07 in the Bosumtwi impact crater provide a complete record of the in-fill process. The shape, orientation, and size of clasts within the impact breccia were measured using a best-fit ellipsoid approach. Clast size and variance, together with clast orientation data, suggest the impact breccias at Bosumtwi can be divided into a simple two-fold subdivision that loosely agrees with the lithological zonation of a lower monomict breccia overlain by a polymict breccia. The lower unit is characterized by a uniform and finer-grained clast size together with a uniform flat-lying clast orientation. The boundary between the two zones is defined by a sharp increase in clast size. The upper zone shows an average increase in clast size with decreasing depth, but full grain size spectrum together with increased grain size deviation suggest that this is a result of mixing between two populations with different grain size distribution. The main population of clasts shows an incremental decrease of clast size with decreasing depth. The upper zone also contains weakly defined shallowly dipping clast fabrics, which may be suggestive of horizontal transport or deposition onto an inclined surface.
    • Fabric analysis of Allende matrix using EBSD

      Watt, Lauren E.; Bland, Phil A.; Prior, Dave J.; Russell, Sara S. (The Meteoritical Society, 2006-01-01)
      Fabric analysis of the interstitial matrix material in primitive meteorites offers a novel window on asteroid formation and evolution. Electron backscatter diffraction (EBSD) has allowed fabrics in these fine-grained materials to be visualized in detail for the first time. Our data reveal that Allende, a CV3 chondrite, possesses a uniform, planar, short-axis alignment fabric that is pervasive on a broad scale and is probably the result of deformational shortening related to impact or gravitational compaction. Interference between this matrix fabric and the larger, more rigid components, such as dark inclusions (DIs) and calcium-aluminium-rich inclusions (CAIs), has lead to the development of locally oriented and intensified matrix fabrics. In addition, DIs possess fabrics that are conformable with the broader matrix fabric. These results suggest that DIs were in situ prior to the deformational shortening event responsible for these fabrics, thus providing an argument against dark inclusions being fragments from another lithified part of the asteroid (Kojima and Tomeoka 1996; Fruland et al. 1978). Moreover, both DIs and Allende matrix are highly porous (~25%) (Corrigan et al. 1997). Mobilizing a highly porous DI during impact-induced brecciation without imposing a fabric and incorporating it into a highly porous matrix without significantly compacting these materials is improbable. We favor a model that involves Allende DIs, CAIs, and matrix accreting together and experiencing the same deformation events.
    • Lithological and structural characteristics of the Lake Bosumtwi impact crater, Ghana: Interpretation of acoustic televiewer images

      Hunze, S.; Wonik, T. (The Meteoritical Society, 2007-01-01)
      Bosumtwi is a very well-preserved 1.07 Myr old, complex terrestrial impact crater locatedin south-central Ghana, West Africa. The impact structure has a diameter of about 10.5 km and wasformed in 2.1–2.2 Gyr Precambrian metasedimentary and metavolcanic rocks. Drilling and logging was carried out during the Lake Bosumtwi Drilling Project (BCDP) which was supported by the International Continental Scientific Drilling Program (ICDP). One of the aims of this project is to achieve detailed information on the subsurface structure and crater fill of one of the best preserved large young impact structures. We interpreted the wireline logs and televiewer images. The physical properties including shallow resistivity, p-wave velocity, magnetic susceptibility, and borehole diameter of the breccia differ significantly from those of the meta-graywackes and slate/phyllites. Fractures observed in the televiewer images are interpreted to determine their characteristic structural features. The fracture dip angles are steep (50–70 degrees) and the two main dip directions are southeast and southwest. Most fractures observed in the borehole are open. The indicated main stress direction is north-south.
    • Stony meteorite characterization by non-destructive measurement of magnetic properties

      Smith, D. L.; Ernst, R. E.; Samson, C.; Herd, R. (The Meteoritical Society, 2006-01-01)
      Four parameters of low-field magnetic susceptibility (bulk value, frequency dependence, degree of anisotropy, and ellipsoid shape) have been determined for 321 stony meteorites from the National Collection of Canada. These parameters provide a basis for rapid, non-destructive, and accurate meteorite classification as each meteorite class tends to have a distinct range of values. Chondrites show a clear trend of increasing bulk susceptibility from LL to L to H to E within the 3.6 to 5.6 log-Chi (in 10^(-9) m^3/kg) range, reflecting increasing Fe-Ni metal and Fe-Ni sulfide content. Achondrite values range in log-Chi from 2.4 to 4.7 and primitive achondrites from 4.2 to 5.7. Frequency dependence is observed, using 19,000 Hz and 825 Hz, with variations in strength among meteorite classes and individual specimen dependence ranging from 1-25.6%. Degrees of anisotropy range from 1 to 53% with both oblate and prolate ellipsoids present. The aubrite class is marked by high degrees of anisotropy, low bulk magnetic susceptibility, and prolate fabric. Camel Donga is set apart from other eucrites, marked by higher bulk susceptibility, degree of anisotropy, and magnitude of oblate ellipsoid shape. The Shergotty, Nakhla, and Chassigny (SNC) meteorites show subclass distinction using frequency dependence and Chassigny is set apart with a relatively strong oblate fabric. The presence of both strong oblate and prolate fabrics among and within meteorite classes of chondritic and achondritic material points to a complex, multi-mechanism origin for anisotropy, more so than previously thought, and likely dominated by impact processes in the later stages of stony parent body formation.