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An analytical electron microscopy (AEM) investigation of opaque inclusions in some type 6 ordinary chondrites
Author(s) -
LEROUX Hugues,
DOUKHAN JeanClaude,
GUYOT François
Publication year - 1996
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.1996.tb02111.x
Subject(s) - chondrite , shock metamorphism , troilite , parent body , silicate , meteorite , ordinary chondrite , mineralogy , kamacite , geology , olivine , metal , materials science , chemistry , astrobiology , metallurgy , physics , organic chemistry
— A large number of ordinary chondrites contains 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 Séverin, LL6, Tsarev, L6 and Kernouvé, 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 that 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 that 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, subgrain 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 Séverin meteorite, including mechanical twins in diopside and straight free screw dislocations in olivine.