TEM Examination of Shock Veins in Ordinary Chondrites

Details TEM Examination of Shock Veins in Ordinary Chondrites TEM Examination of Shock Veins in Ordinary Chondrites

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.537l&link_type=abstract

Meteoritics, vol. 30, no. 5, page 537

Computer Science

2

Chondrites, L, Majorite, Metamorphism, Shock, Meteorites, Acfer 040, Coorara, Johnson City, Ringwoodite, Tenham, Veins

Scientific paper

Introduction: As part of a research project on shock metamorphism of chondrites [1] we studied shock veins in different L-chondrites to understand their formation conditions. These thin shock veins (>20 micrometers thickness) have been found in the meteorites L5-6-Acfer 040, L6-Coorara, L6-Johnson City and L6-Tenham and were studied in detail by analytical transmission electron microscopy (ATEM). Texture. Shock veins consist of a fine-grained opaque matrix with embedded large fragments of the host chondrite. High-pressure polymorphs of olivine (ringwoodite and wadsleyite) and enstatite (majorite), which are clear indicators of strong shock metamorphism [1], are present in both matrix and fragments. In the matrix, majorite and, to a minor extent, ringwoodite coexist with globules of Fe-Ni alloy and troilite (< 30 micrometers). In fragments, ringwoodite and majorite occur as polycrystalline aggregates of tiny crystals (< 2 - 3 micrometers). Lattice defects. Ringwoodite shows generally a high density of stacking faults parallel to 110 planes. These stacking faults are interpreted as growth defects due to rapid cooling [2]. Majorite is defect-free. Chemistry. Ringwoodite in fragments is chemically less homogeneous and tends to a higher FeO-content than olivine in the host chondrite [3, 4]. A tendency to chemical heterogeneity has also been detected for majorite in fragments. The composition of the majorite in the matrix is distinctly heterogeneous and differs systematically from that of the enstatite, because it shows a distinct enrichment in Al, Mg, Ca, Na and a significant depletion in Si, Mn and especially in Fe, which is up to 50% lower than in the primary enstatite. Discussion. Chemical heterogeneity and small grain sizes of high-pressure phases as well as presence of Fe-Ni alloy and troilite in the form of globules indicates that shock veins represent quenched melts. The high-pressure phases are interpreted as crystallization products of these melts. The localized temperature excursions result probably from friction of the meteoritic material during the shock event. Phases with the lowest melting points (troilite, Fe-Ni alloy, plagioclase, orthopyroxene and diopside) are assumed to melt preferentially and to produce the melt matrix. This interpretation explains the dominance of majorite grains in the matrix and their special chemical composition, which reflects mixing of enstatite, diopside, and plagioclase. The silicate matrix melt, which is immiscible with the metal melt, exsolves the troilite and metal globules. References. [1] Stoffler D. et al. (1991) GCA, 55, 3845-3867. [2] Langenhorst et al. (1995) GCA, in press. [3] Lingemann C. M. and Stoffler D. (1994) Meteoritics, 29, 491-492. [4] Lingemann C. M. and Stoffler D. (1995) LPS XXVI, 851-852.

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