High strain-rate deformation as an impact process: Ordinary chondrite and carbonate-silicate frictional melting experiments and their comparison with naturally deformed materials.

摘要

The goal of this study was to investigate the effects and importance of high strain-rate deformation during the impact process. Two different materials were chosen for frictional melting experiments: (1) ordinary chondrites and (2) carbonate-silicate rocks. High-speed friction experiments were performed on ordinary chondrites El Hammami (H5, S2) and Sahara 97001 (L6, S3) using an axial friction-welding apparatus. Each sample was subjected to a strain rate of 103 to 104 s-1 . The shear zones generated during the experiment are composed of silicate glass and mineral fragments intermingled with dispersed submicron-size FeNi and FeS blebs. Fracturing of mineral grains and formation of metallic veins occur beyond the shear zone. The experimental shear zones are similar to certain vein systems in naturally deformed ordinary chondrites. The experiments show that shock deformation is not required for the formation of veins and darkening in ordinary chondrites. Updated shock classification of "black" ordinary chondrites, thought to be heavily shocked, reveals that all "black" ordinary chondrites are not heavily shocked. Three types of "black" ordinary chondrites were observed: gas-rich ordinary chondrites, veined ordinary chondrites and ordinary chondrites with a homogenous distribution of mm-scale metal grains. The identification of frictional melts unrelated to shock deformation in ordinary chondrites requires the absence of high pressure polymorphs and annealing of shock features.; The response of carbonates to impact processes has primarily been studied via shock experiments. Although the most recent shock experiments with carbonates indicate that they are highly resistant to shock devolatilization, impact experiments suggest that high strain-rate deformation is a key factor in the degassing of carbonate targets, particularly for oblique impacts. Experimental high strain-rate deformation (102 to 103 s -1 for 1 second) resulted in the formation of an opaque white carbonate-silicate material. Ca-Mg silicates and oxides were produced within the shear zone and adjacent veins. Pervasive vesicles and excess MgO and CaO in silicate-free portions of the shear zone suggest the loss of at least 5 wt% CO2. The experimental results indicate that carbonate targets are susceptible to degassing caused by high strain-rate deformation, which may occur during oblique impacts or impact-directed flow.