Physics
Scientific paper
Oct 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996m%26ps...31..767l&link_type=abstract
Meteoritics, vol. 31, pages 767-776.
Physics
8
Scientific paper
A large number of ordinary chondrites contain micron-sized particles of metal and/or troilite dispersed in their silicate grains. Such metallic phases are responsible for the so-called darkening of the silicate grains and might be either precipitates, which formed during reduction of the silicates, or inclusions injected as a melt during a shock event. We have investigated these tiny foreign phases by analytical transmission electron microscopy in three unweathered, metamorphosed ordinary chondrites (Saint Siverin, LL6, Tsarev, L6 and Kernouvi, H6). We also looked for remnant shock indices. Our TEM observations suggest the following sequence of events in the three meteorites. First, a number of relatively strong shock events occurred on the parent body/bodies producing an Fe-FeS melt which was injected into silicate grains along a dense network of open fractures. Most of these shock defects were subsequently erased by high-temperature (700-900 C) thermal metamorphism. Some remnants of the shock events are the observed trails of tiny metal and/or sulfide inclusions which formed as a result of fracture healing. Chemical homogenization of the silicates and limited oxidation of the metallic blebs also occurred during this high temperature annealing event, resulting in Ni-rich inclusions. This effect was especially pronounced in the L and LL-chondrites studied. During subsequent cooling of the body/bodies, inclusions of chromite and phosphate precipitated, nucleating preferentially on lattice defects (dislocations, sub-grain boundaries) and on the metal and sulfide inclusions. A later shock event of moderate intensity, probably corresponding to the separation of the meteorite from its parent body, produced new shock features in the silicate grains of the Saint Siverin meteorite including mechanical twins in diopside and straight free screw dislocations in olivine.
Doukhan Jean Claude
Guyot Frangois
Leroux Hugues
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