Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.242k&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 242
Other
2
Scientific paper
Introduction. LEW88516, a shergottite find from Antarctica, shows evidence for extensive shock metamorphism, including: the development of shock melt in common pockets and pervasive veins, the formation of maskelynite, and strong mosaicism in olivine and pyroxene (Lindstrom et al., 1992). Here we report more detailed studies of shock features in LEW88516 using electron microscopy. Our optical and scanning electron microscope (SEM) observations are from three petrographic thin sections. For the transmission electron microscope (TEM) studies, regions of interest were extracted from one of the polished thin sections, mounted on support grids, and ion thinned. Preliminary TEM observations are from two regions; in the vicinity of a large ~4-mm melt pocket and an area including maskelynite and opaque veinlets. Results. Olivine (OL) shows a variety of microstructures. Far from the melt pockets, OL (Fo(sub)67) is merely deformed, with patches or streaks of light-brown (to clear) pleochroism, strong mosaicism, and multiple sets of planar elements (typically lamellar features parallel to {130} and {001}) similar to those in other strongly shocked meteorites (e.g. Ashworth, 1985). Nearer to the melt pockets is a recrystallized zone, in which the original OL has broken down into many small micrometer-sized, compositionally homogeneous subgrains. The individual subgrains are relatively free of defects and share planar equilibrium grain boundaries. OL in the recrystallized zone lacks pleochroism. Immediately adjacent to the melt pockets is a thin, partially melted zone, ~100 micrometers wide, of submicrometer, compositionally heterogeneous OL subgrains. Backscattered SEM images of the partially melted zone indicate that the OL subgrains are unequilibrated, with grains of composition Fo(sub)67 coexisting with more magnesian subgrains with compositions similar to the OL dendrites in melt pockets. Traces of the partial melts are preserved as glass at grain boundaries. Similar partial melts occur in OL grains in the shergottite ALHA77005. Finally, much of the shock melt in LEW88516 contains defect-free, coarse-grained OL dendrites. Pyroxenes also shows deformed, recrystallized, and partially melted zones. Far from the melt pockets, pigeonite is deformed to yield strong mosaicism and planar features parallel to (001). Recrystallized pigeonite is extensively twinned, and consists of monoclinic and orthorhombic polytypes intergrown on a fine scale. This intergrowth of polytypes causes strong streaking along a* in selected-area electron diffraction (SAED) patterns. Augite far from the melt pockets shows mosaicism and planar features parallel to (001). Far from the melt pockets, plagioclase has been completely transformed to maskelynite, yet retains its euhedral grain morphology. SAED patterns of maskelynite show only diffuse scattering typical of amorphous materials. Olivine and augite in contact with maskelynite show only mosaicism and planar deformation features; recrystallization textures are absent. Adjacent to the melt pockets, however, the maskelynite shows flow lines and fine vesicles and coexists with recrystallized and partially melted OL and pyroxene. Minor phases in LEW88516 also show evidence of shock. Whitlockite contains numerous defects and shows mosaicism. Ilmenites show deformation twinning along (101) similar to that in shocked ilmenites from lunar samples (Sclar et al., 1973). The sulfide phase in the opaque shock veins is pyrrhottite; SAED patterns indicate a mixture of the 1c and 4c polytypes. Discussion and Conclusions. Shock effects vary in intensity on the cm-scale, but overall LEW88516 is highly shocked. Olivine in particular shows a range of microstructures from partial melting and recrystallization to mosaicism and development of planar deformation features. Applying the shock classification scheme of Stoffler et al. (1991), LEW88516 has been shocked to stage S6 (very strongly shocked) as evidenced by the abundant melt pockets, recrystallized olivine, and maskelynite. The overall shock effects are similar to those in ALHA 77005, making these two meteorites the most heavily shocked shergottites. References Ashworth, J. R. (1985) EPSL 73, 17; Lindstrom, M. M. et al. (1992) Lunar Planet. Sci. (abstract) 23, 783; Sclar, C. B. et al. (1973) Proc. Lunar Planet. Sci. 4th, 841; Stoffler, D. et al. (1991) GCA 55, 3845.
Keller Lindsay P.
Treiman Allan H.
Wentworth Sue J.
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