Shock physics mesoscale modeling of shock stage 5 and 6 in ordinary and enstatite chondrites

Shock-darkening, the melting of metals and iron sulfides into a network of veins within silicate grains, altering reflectance spectra of meteorites, was previously studied using shock physics mesoscale modeling. Melting of iron sulfides embedded in olivine was observed at pressures of 40-50 GPa. This pressure range is at the transition between shock stage 5 (C-S5) and 6 (C-S6) of the shock metamorphism classification in ordinary and enstatite chondrites. To better characterize C-S5 and C-S6 with a mesoscale modeling approach and assess post-shock heating and melting, we used multi-phase (i.e. olivine/enstatite, troilite, iron, pores, and plagioclase) meshes with realistic configurations of grains. We carried out a systematic study of shock compression in ordinary and enstatite chondrites at pressures between 30 and 70 GPa. To setup mesoscale sample meshes with realistic silicate, metal, iron sulfide, and open pore shapes, we converted backscattered electron microscope images of three chondrites. The resolved macroporosity in meshes was 3-6%. Transition from shock C-S5 to C-S6 was observed through (1) the melting of troilite above 40 GPa with melt fractions of similar to 0.7-0.9 at 70 GPa, (2) the melting of olivine and iron above 50 GPa with melt fraction of similar to 0.001 and 0.012, respectively, at 70 GPa, and (3) the melting of plagioclase above 30 GPa (melt fraction of 1, at 55 GPa). Post-shock temperatures varied from similar to 540 K at 30 GPa to similar to 1300 K at 70 GPa. We also constructed models with increased porosity up to 15% porosity, producing higher post-shock temperatures (similar to 800 K increase) and melt fractions (similar to 0.12 increase) in olivine. Additionally we constructed a pre-heated model to observe post-shock heating and melting during thermal metamorphism. This model presented similar results (melting) at pressures 10-15 GPa lower compared to the room temperature models. Finally, we demonstrated dependence of post-shock heating and melting on the orientation of open cracks relative to the shock wave front. In conclusion, the modeled melting and post-shock heating of individual phases were mostly consistent with the current shock classification scheme (Stoffler et al., 1991, 2018).