Computer Science
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.491l&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 491-492
Computer Science
3
Chondrites, Meteoritic Composition, Meteoritic Microstructures, Mineralogy, Phase Transformations, Pressure Effects, Scanning Electron Microscopy, Temperature Effects, Transmission Electron Microscopy
Scientific paper
Since the discovery of ringwoodite in the meteorite Tenham, further studies confirmed that this mineral occurs preferentially in melt veins and melt pockets of highly shocked L chondrules. Except for some special Transmission Electron Microscopy (TEM) studies a comprehensive study of this high-pressure phase of olivine in the general scope of shock metamorphism of chondrites is missing. Eighteen ringwoodite-bearing L6(S6) chondrites were studied, some of them in more detail: L6(S6)-Catherwood, L6(S6)-Coorara, L6(S6)-Johnson City, L6(S6)-Pampa del Infierno, L6(S6)-Tenham, and L5-6(S6)-Acfer 040. They were analyzed by the optical microscope, Scanning Electron Microscopy (SEM), microprobe, and TEM. Ringwoodite is formed in chondrites at locations where localized melting occurs by shock waves exceeding 50 GPa. The localized shock-pressure and temperature concentrations are obviously favorable for the kinetics of the olivine-spinel transition. Ringwoodite crystallizes most probably not during the short peak pressure pulse but rather during the more extended phase of pressure release, where the temperature is still extremely high, presumably near the solidus of chondrites. The observed chemical variation of individual ringwoodite grains and the indication that wadsleyite coexists with ringwoodite, could be explained as follows. On release from a high-pressure-shock state, olivine transforms to an assemblage of the Beta phase (wadsleyite) and the gamma phase (ringwoodite) in the pressure range between about 20 and 13 GPa. In the two-phase region, where the Beta and gamma phases coexist under equilibrium conditions, the two phases would have compositions in the range Fa(13-25) and Fa(25-31) respectively if the primary olivine composition is 24-25 mol% Fa. This range is very near to the observed chemical variation within polycrystalline ringwoodite grains Fa(19-31).
Lingemann C. M.
Stäffler Dieter
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