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~18) 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 (Fs16–17En82–83Wo1) 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.
ASJC Scopus subject areas
- Space and Planetary Science