High-pressure polymorphs of magnesian orthopyroxene from a shock vein in the Yamato-000047 lherzolitic shergottite

Abstract

Abstract– We found a simple thin shock vein, less than or equal to about 60 μm in width and 1.8 mm in length, in the poikilitic area in the Yamato (Y-) 000047 lherzolitic shergottite. The shock vein occurs only in magnesian Ca-poor clinopyroxene, which may have transformed from orthopyroxene during the pressure increase at the shock event. The shock vein consists of (Mg0.8,Fe0.2)SiO3 pyroxene polymorphs, such as columnar akimotoite, two kinds of pyroxene glasses, dendritic akimotoite, and framboidal pyroxene glass, in the order from the periphery to the center. The compositions and textures suggest that columnar akimotoite in the periphery of the shock vein crystallized from solid-state phase transition of clinopyoroxene during the cooling of the vein, and the remains in the shock vein solidified from shock-produced melt. The glass includes two kinds of massive glass in the vein and framboidal glass in the vein center. The framboidal glass is the most magnesian and may have been vitrified from perovskite crystallized from high-pressure melt produced at high temperature ≥3000 °C and high-pressure 23–40 GPa. Dendritic akimotoites in the vein center metastably crystallized from residual shock melt. The formation sequences of the constituent phases in the shock vein happen in the following order: columnar akimotoites, rim glass, center glass, framboidal glass, and dendritic akimotoites. The increase of the Raman intensity of 660–670 cm−1 in the order of rim glass, center glass, and framboidal glass suggests that the formation of the pyroxene chain proceeds faster in the vein center than in the vein rim due to its slower cooling. The finding of the shock vein consisting merely of high-pressure polymorphs of pyroxene, akimotoite, and framboidal glass (vitrified perovskite) is the first reported among all Martian meteorites.