Shock-induced melting textures and high-pressure polymorphs in 178 ordinary chondrites of all chemical groups and petrologic types were investigated. The shock-induced melting modes were classified into three types, namely pocket, line, and network. The type of shock-induced melting depends on the petrologic type. The width of the shock-induced melt increased with increasing the petrologic type number. The approximate estimated shock-pressure ranges recorded in and around the shock-induced melts of the H-group ordinary chondrites based on the identified high-pressure polymorphs were as follows: H3, less than 2 GPa; H4-H6, 2-6 GPa. For ordinary chondrites of the L/LL group, the values were as follows: L/LL3, 2-6 GPa; L/LL4, 2-14 GPa; L5: 14-20 GPa; LL5, 2-14 GPa; L6, 17-23 GPa; and LL6, 14-18 GPa. After adopting the estimated shock pressures into the onion shell-structured parent body model, the shock pressure on the surface was much lower than in the interior. One possibility is that the apparent lower shock pressure on the surface is due to spallation during the impact. Considering the features of the high-pressure polymorphs, the major disruption history of the parent bodies is different in each chemical group, although the L/LL chondrite parent bodies may have a similar major disruption history.

We investigated the back-transformation mechanisms of ringwoodite and majorite occurring in a shock-melt vein (SMV) of the Yamato 75267 H6 ordinary chondrite during atmospheric entry heating. Ringwoodite and majorite in the shock melt near the fusion crust have back-transformed into olivine and enstatite, respectively. Ringwoodite (Fa~1 8) occurs in the SMV as a fine-grained polycrystalline assemblage. Approaching the fusion crust, fine-grained polycrystalline olivine becomes dominant instead of ringwoodite. The back-transformation from ringwoodite to olivine proceeds by incoherent nucleation and by an interface-controlled growth mechanism: nucleation occurs on the grain boundaries of ringwoodite, and subsequently olivine grains grow. Majorite (Fs 16-17 En 82-83 Wo 1) occurs in the SMV as a fine-grained polycrystalline assemblage. Approaching the fusion crust, the majorite grains become vitrified. Approaching the fusion crust even more, clino/ orthoenstatite grains occur in the vitrified majorite. The back-transformation from majorite to enstatite is initiated by the vitrification, and growth continues by the subsequent nucleation in the vitrified majorite.