Impact-induced textural changes of CV carbonaceous chondrites: Experimental reproduction

Shock-recovery experiments were carried out on the Allende CV carbonaceous chondrite using a single-stage propellant gun. Allende samples were impacted at room temperature with peak shock pressures of 11 and 21 GPa. Another samples were impacted twice from the same direction at room temperature with first/second shock pressures of 11/11 GPa and 21/21 GPa to study shock effects by multiple impacts. The other samples were preheated and impacted at 300 degrees C and 11 GPa, 300 degrees C and 21 GPa, and 600 degrees C and 21 GPa to study shock effects at high temperature. In all recovered samples, chondrules were flattened perpendicular to the direction of shock wave propagation and showed alignment. The whole shapes and microtextures of flattened chondrules are very similar to those observed in naturally shocked CV chondrites, The degree of chondrule flattening increases with shock intensity, initial temperature, and the number of impacts. Silicate minerals in chondrules shocked at high temperatures were blackened due to injection of Fe, Ni, and S-rich melt into fine fractures. The matrix was compressed to varying degrees by impacts. Matrices that were impacted at high temperatures melted partially to form veins filled by a mixture of silicate fragments and Fe, Ni, and S-rich melt. A detailed comparison of textures between experimentally shocked Allende and naturally shocked CV chondrites indicates that the Allende sample impacted twice at 21 GPa and room temperature is very similar to the Leoville and Efremovka CV chondrites that have experienced the highest level of impacts among carbonaceous chondrites of petrologic type 3. The Allende sample impacted once at 21 GPa and room temperature is similar to the Gronsnaja CV chondrite. The Allende samples impacted at 300 and 600 degrees C differ in texture from any naturally shocked CV chondrite, These results suggest that CV carbonaceous chondrites found so far on the Earth have undergone mild impacts on cold meteorite parent bodies. (C) 2000 Academic Press.