Coherent and subsequent incoherent ringwoodite growth in olivine of shocked L6 chondrites

Pervasive replacement of original olivine grains by ringwoodite adjacent to a shock-melt vein of Yamato 791384L6 chondrite was observed with a FEG-SEM. In these olivines, we observed three distinct olivine-ringwoodite textures arranged in a spatially successive arrangement from the wall of the shock-melt vein into the olivine grains: (1) polycrystalline ringwoodite, (2) oriented several sets of ringwoodite lamellae and (3) oriented single lamellae. TEM images show that the polycrystalline and oriented several sets of lamellae parts consist of polycrystalline ringwoodite, indicating that phase transformation from olivine to ringwoodite is controlled by incoherent growth mechanism. On the other hand, the set of oriented single lamellae consists of thin ringwoodite platelets (<~10nm in thickness) depicting, as observed here for the first time in nature, coherent crystallographic orientation: (100)_Ol//{111}_Rgt. Previous TEM studies reported that only the incoherent growth mechanism was active during the transformation in the shocked chondrite. However, our TEM studies revealed evidence for formation of the coherent intracrystalline lamella growth mechanism then followed by incoherent growth in the shocked chondrite in nature, thus allowing to constrain a robust time scale of the shock and at the origin of the veins.