Ringwoodite rim around olivine core in shock-induced melt-vein of GRV 022321 chondrite: Transformation kinetics of olivine to ringwoodite
Abstract Here we report the natural occurrence of the ringwoodite rims around olivine cores in shock-induced melt veins of the Antarctic chondrite GRV 022321. Electron microprobe analysis (EMPA), Raman spectroscopy, Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) were used to examine the sample to better elucidate the mechanisms of transformation of the olivine to ringwoodite and Fe-Mg partitioning in olivine under the shock. The GRV 022321 is an L6 chondrite with a network of black veins enclosing abundant olivine host-rock fragments. Some of the enclosed fragments ranging from 5 µm to 100 µm in size have bright rims up to 20 µm wide, and a dark core under reflected light and backscatter electron imaging. Raman spectroscopy reveals that rims are made of ringwoodite, and cores are predominantly olivine. EMPA data show the ringwoodites in rims are richer in Fe (Fa46) than the olivine cores (Fa10-Fa23). The olivine cores have variable contrast in BSE images with the heterogeneities in fayalite content (Fa10 to Fa23) and a branching network of low-Fa olivine. FIB-TEM observations reveal that the ringwoodite rims are polycrystalline with crystallite sizes from 200 nm to 800 nm, while the olivine cores are also polycrystalline, but with smaller crystallites from 100 nm to 200 nm. Based on observation, we conclude that the original Fa23 olivine transformed to Fa10 olivine and Fa46 ringwoodite by a solid-state diffusion-controlled growth mechanism during shock, and the branching network of low-Fa olivine acted as long-range(up to 10µm)high-diffusion pathways for grain-boundary Fe-Mg interdiffusion through highly deformed nano-crystalline olivine to accommodate the diffusion-controlled growth of ringwoodite.