16O enrichments in aluminum-rich chondrules from ordinary chondrites

Black melt veins in the Martian basaltic meteorite Zagami were investigated using analytical transmission electron microscopy, with the goal to constrain the mechanisms and PTt conditions that result in the formation of high-pressure phases therein. Our study reveals the existence of a number of high-pressure minerals: stishovite, K- and Na,Ca-rich hollandites, akimotoite (silicate ilmenite), as well as amorphous grains of silicate perovskite composition. Crystals of a ferro-magnesian silicate titanite with a low Ca content have been identified for the first time. The veins formed by rapid shear melting and solidified in extremely short times. As a consequence, the vein melt could not homogeneously mix and it partially preserves the chemical heterogeneity of precursor minerals, accounting for the large diversity of high-pressure minerals. Most of the tiny phases directly crystallized from the melt under high pressure at very high temperatures. Silicate perovskite probably amorphized upon decompression and evolved, at its grain boundaries, by solid-state diffusion toward the lower pressure assemblage wüstite+stishovite. Not only do these observations help to understand the formation of high-pressure phases in shock events, but they also provide clues to the mineralogical complexity that can be expected in planetary mantles.