Computer Science
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.489l&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 489-490
Computer Science
Chondrites, Crystal Lattices, Impact Damage, Metamorphism (Geology), Transmission Electron Microscopy, Crystal Dislocations, Electron Diffraction, Fusion (Melting), Mechanical Twinning, Pressure Effects
Scientific paper
Among the ordinary chondrites, the Tenham meteorite (L6) is an instructive example of strong shock metamorphism. It is randomly pervaded by pseudotachylite-like shock veins that contain ringwoodite and majorite, the high-pressure phases of olivine and pyroxene, respectively. According to the revised shock nomenclature of ordinary chondrites, this shock signature is characteristic for shock stage S6 with pressures in excess of 50-55 GPa. In contrast to previous transmission electron microscope (TEM) studies, concentrating on the formation of the high-pressure phases in shock veins, our goal was to characterize the shock defects in olivine, pyroxene, and plagioclase occurring in the bulk of Tenham, i.e., outside shock veins. In olivine, shock has caused the formation of irregular and planar fractures as well as the activation of numerous dislocations. Planar fractures can be either filled with alteration products of Fe-Ni alloy. By far, diopside displays the greatest diversity of shock-induced defects; these are mechanical twins, numerous dislocations, and planar deformation features (PDFs). Diffuse scattering rings in electron diffraction patterns reveal that they consist of amorphous material. In analogy to quartz, it is reasonable to call them PDFs, too. The predominant shock effect in plagioclase is the formation of PDFs. Up to three crossing sets of such parallel, amorphous lamellae have been observed. It is concluded that almost all lattice defects observed in the silicate phases of Tenham are compatible with an impact origin. Some of the defects have been calibrated by shock experiments and, hence, give clues to the shock pressure.
Doukhan Jean Claude
Joreau Pascal
Langenhorst Falko
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