Ringwoodite lamellae in olivine: Clues to olivine-ringwoodite phase transition mechanisms in shocked meteorites and subducting slabs

The first natural occurrence of ringwoodite lamellae was found in the olivine grains inside and in areas adjacent to the shock veins of a chondritic meteorite, and these lamellae show distinct growth mechanism. Inside the veins where pressure and temperature were higher than elsewhere, ringwoodite lamellae formed parallel to the [101] planes of olivine, whereas outside they lie parallel to the (100) plane of olivine. The lamellae replaced the host olivine from a few percent to complete. Formation of these lamellae relates to a diffusion-controlled growth of ringwoodite along shear-induced planar defects in olivine. The planar defects and ringwoodite lamellae parallel to the [101] planes of olivine should have been produced in higher shear stress and temperature region than that parallel to the (100) plane of olivine. This study suggests that the time duration of high pressure and temperature for the growth of ringwoodite lamellae might have lasted at least for several seconds, and that an intracrystalline transformation mechanism of ringwoodite in olivine could favorably operate in the subducting lithospheric slabs in the deep Earth.