Deformation of olivine in stony meteorites

The deformational characteristics of olivine and, to a lesser extent, pyroxene in thirty-six stony meteorites were evaluated, by petrographic means, by making comparisons with olivines deformed in dynamic experiments and static experiments and by natural terrestrial deformations. Shock-wave compression experiments in the peak pressure range 0.15 to 1.0 Mb were performed on dunite, harzburgite, and single crystals of fayalite and augite. Very distinctive textural changes take place with increasing shock pressure: (1) well-developed fracturing and local undulatory extinction at 0.15–0.2 Mb, (2) a pervasive mosaic structure at 0.25 to > 0.5 Mb, and (3) abundant recrystallization at 1.0 Mb, with all gradations between. These results have been used as the basis for classification of the degree of shock deformation of the silicates. Deformational textures in the twenty-five chondrites studied fit into this classification very well, and the degree of shock deformation so determined is in excellent agreement with that deduced by others on the basis of gas-retention ages, Na volatility in feldspar and maskelynite, macroscopic textures, and Laue photographs. Most of the textures and structural features of the achondrites, especially the ureilites, and mesosiderites cannot be described in this scheme, but the data are few. Plastic deformation in olivine of the chondrites, as evidenced by irregular kink bands and undulatory extinction, has taken place predominantly on the system T = {110}, (100), t = [001], the low temperature and/or high strain-rate systems in static experiments. Regular, sharp kink bands and undulatory extinction in the Novo Urei achondrite and Pinnaroo and Estherville mesosiderites have formed by slip on the systems T = {110}, t = [001] and T = {0kl}, t = [100]. The latter system predominates in terrestrially deformed olivines and in static experiments of high temperatures and/or low strain rates. All the deformational features in silicates of the chondrites may be explained by shock deformation due to pre-terrestrial collisions. The interpretation for achondrites and stony irons is, however, ambiguous. With the exception of recrystallization of olivine in Goalpara, the deformational textures, appearance of extinction bands, and the responsible slip systems in achondrites and stony irons are more nearly akin to those in statically deformed olivine. Perhaps the achondrites and stony irons were at substantial static pressures and temperatures on break-up of the parent body.