Shock effects of the Leoville CV carbonaceous chondrite: a transmission electron microscope study

Leoville is a CV carbonaceous chondrite with a strong preferred orientation of the elongated chondrules and inclusions. The texture probably resulted from deformation. Olivine grains in most chondrules show undulatory extinction and fine planar fractures. Transmission electron microscope observations reveal that most olivine grains in the matrix exhibit high densities of micro-cracks and dislocations with Burgers vector b = [001]; the densities of the dislocations are comparable to those in chondrule olivines. Olivine in the matrix also forms aggregates in places, which comprise extremely small, rounded to sub-rounded grains and glassy material, closely resembling the recrystallized olivine commonly seen in shocked ordinary chondrites. Enstatite in the matrix has numerous dislocations and lamellae that are caused by (100) stacking faults. These micro-textures are characteristic of deformation at high strain rates and are very similar to those in shocked ordinary chondrites and experimentally shocked materials. Therefore, this suggests that the chondrules and matrix in the Leoville experienced shock, probably after accretion to the meteorite parent body. Comparison of the textures to those in experimentally shocked materials suggests that the shock pressures experienced by Leoville were in the range 5-20 Gpa. We believe that multiple impacts by such relatively mild shock pressures compacted the Leoville meteorite and caused the deformation of chondrules and the foliation.