Thermal and deformation history of the Shergotty meteorite deduced from clinopyroxene microstructure

Abstract

Methods of transmission electron microscopy (TEM), including imaging, electron diffraction, and energy dispersive X-ray microanalysis, have been used to study the microstructure of clinopyroxenes from the Shergotty meteorite. The aim was to contribute to the knowledge of the thermal and deformational history of this meteorite which stems probably from the planet Mars. Previous studies by several groups have shown that the Shergotty minerals had been deformed and transformed by shock waves. TEM reveals a complex microstructural pattern, consisting of “001” exsolution lamellae of augite in a pigeonite matrix or vice versa, growth twins, antiphase domains, mechanical twins, stacking faults, shear fractures, areas of low crystalline order, and bending. It was proven that augite, but not pigeonite, is twinned on (001). Twinning of clinopyroxenes on (001) is characteristic of shock deformation. In addition, polysynthetic mechanical twins and stacking faults on (100) in pigeonite and augite were observed. Comparison with our earlier studies of exsolution microstructures in clinopyroxenes from the basaltic Whin Sill, Northern England, suggests that the cooling rate of the Shergotty clinopyroxenes was on the order of 2 · 10 −3 K/h between 1100-800°C, probably lower at the high temperature end as indicated by thicker exsolution lamellae in Shergotty clinopyroxenes. The deformation twins and shear fractures were clearly formed after the exsolution. The deformation features among themselves display a sequence of succession. However, this sequence can be explained by a single shock event. Therefore, it is concluded that the shock metamorphism of Shergotty is connected with the impact event ejecting the meteorite into space.