Chelyabinsk meteorite: Shock metamorphism, black veins and impact melt dikes, and the Hugoniot

The Chelyabinsk meteorite (which fell on February 15, 2013) is a LL5 chondrite of shock stage S4, whose fragments are classified into light and dark lithologies. According to the intensity of their shock metamorphism, light lithology fragments are subdivided into two groups, which were affected by peak pressures within the ranges of 20–25 and 25–30 GPa, respectively. The material of the dark lithology was shocked at 25–30 GPa but was then annealed, which resulted in a decrease in the discernible degree of shock metamorphism. Black veins cutting across both the light and the dark lithologies and impact melt dikes in the dark lithology were produced by friction melting along boundaries of blocks that had been generated by fragmentation in a shock wave. The impact melt of the dikes is slightly enriched in Si, Al, Ca, Na, and K and has an oxygen isotopic composition similar to the chondrite matrix. It is thought that black vein melt started to crystallize in a rarefaction wave. Melt in the dikes and the central portions of the black veins crystallized after total pressure release. Heating of material hosting the melt dikes resulted in its blackening and annealing of its shock metamorphic features. The Hugoniot obtained for the Chelyabinsk meteorite was utilized to calculate the post-shock and shock temperatures within a broad pressure range. According to these evaluations, the meteorite was heated for 65–135 degrees during the impact event. The melting of the LL chondrite started at a load of approximately 100 GPa because of the high “equilibrium” post-shock temperatures, and a pressure of 140 GPa resulted in the complete melting of the material.