Anatomy of black veins in Zagami: clues to the formation of high-pressure phases

Black melt veins in the Martian basaltic meteorite Zagami were investigated using analytical transmission electron microscopy, with the goal to constr ain the mechanisms and PTt conditions that result in the formation of high- pressure phases therein. Our study reveals the existence of a number of hig h-pressure minerals: stishovite, K- and Na,Ca-rich hollandites, akimotoite (silicate ilmenite), as well as amorphous grains of silicate perovskite com position. Crystals of a ferro-magnesian silicate titanite with a low Ca con tent have been identified for the first time. The veins formed by rapid she ar melting and solidified in extremely short times. As a consequence, the v ein melt could not homogeneously mix and it partially preserves the chemica l heterogeneity of precursor minerals, accounting for the large diversity o f high-pressure minerals. Most of the tiny phases directly crystallized fro m the melt under high pressure at very high temperatures. Silicate perovski te probably amorphized upon decompression and evolved, at its grain boundar ies, by solid-state diffusion toward the lower pressure assemblage wustitestishovite. Not only do these observations help to understand the formation of high-pressure phases in shock events, but they also provide clues to th e mineralogical complexity that can be expected in planetary mantles. (C) 2 000 Elsevier Science B.V. All rights reserved.