Shock Deformation Texture of Olivine Crystals of the Eeta79001 Shergottite

All the shergottite meteorites display intense shock metamorphisms. Especially, plagioclase shows a characteristic shock feature: formation of diaplectic glass (maskelynite) or melting (Duke, 1968; McSween and Stoffler, 1980; McSween and Jarosewich, 1983). By studying the shergottite meteorites, we can get a better understanding of the nature of shock metamorphism of olivine, in comparison with that of coexisting plagioclase which is the most sensitive and well-characterized pressure indicator, in natural impact events. Olivine occurs in two shergottites, ALHA 77005 and EET A 7900 I. Previously we reported shock deformation microstructures of olivine crystals in the ALHA77005 shergottite (Mori and Takeda, 1983). Transmission electron microscopic (TEM) study was conducted to elucidate the shock deformation microstructure of olivine in the EETA 79001 shergottite. One olivine megacryst was separated from the chip of EETA79001. The olivine crystal was examined on an X-ray precession camera to determine the crystallographic orientation and the diffraction asterism. Sample preparation for TeM study was performed using a conventional ion-thinning technique after X-ray study. Optically, the olivine shows irregular fractures and undulatory extinction. TEM observation reveals deformation microstructures characteristic of shock (i.e. dislocation, crack) are very common in the olivine. Long straight [001] screw dislocations are predominant. Some dislocations form dipole configurations. Ashworth and Barber (1975) observed similar dislocation substructures of olivine in the Olivenza and Hedjaz chondrites. Dislocation densities are ca. 108 1010 cm' in EET A 7900 I . Zones of very high dislocation density occur in association w;th microcracks. In some cases, small amounts of amorphous zone are observed along the microcracks. Such an amorphous zone may represent a diaplectic glass produced by shock compression (Jeanloz et al., 1979). Shock-induced thermal effect is apparently negligible in EETA79001, judging from the small degree of dislocation recovery and the preservation of the diaplectic glass. McSween and Jarosewich (1983) estimated peak shock pressures of 35-45 GPa experienced by EET A 7900 I. Their estimate is based on the following diagnostic observations: mosaic ism and granulation in olivine, formation of maskelynite, and production of melt pockets and veins. As far as our observation on the deformation microstructures of olivine is concerned, lower shock pressures (ca. 20-30 GPa) are inferred than those of Mcsween and Jarosewich (1983). This may reflect a heterogeneous distribution of shock deformation or a refractory nature of olivine for shock deformation.