The Mineralogy of Shock-Induced Melt Vein in Martian Meteorite Zagami: Shock Pressure and Ejection Mechanism

Abstract

The observations by transmission electron microscopy showed that the 125 μm wide shock-induced melt vein in Martian meteorite Zagami consisting of abundant long-shaped stishovite needles plus minor troilite droplets and pyroxenes in the background of silicate glass. This is the first report of Post-stishovite was found in zagami meteorite in the maskelynite outside of shock-induced melt vein. The mineralogy assemblages of melt vein, including stishovite, pyroxene and other high-pressure phases reported by others, indicated the melt-vein crystallization pressure and shock pressure were smaller than 25 GPa. The occurrence of post-stishovite indicated post-stishovite is a production of a solid-state transformation. The Martian meteorite apparently were ejected from Mars during hypervelocity impact event, accelerated to great than the Martian escape velocity (5 km/s) to escape from Martian gravity, and captured by Earth later. The 5 km/s velocity requires a shock pressure of at least 65 GPa in high-pressure ejection mechanism which is inconsistent with Martian meteorite have been subjected to the pressure of less than 25 GPa in general. Although a popular spall mechanism claimed a low-pressure high-velocity ejection, due to the limitation of its simulation calculation code, it is actually still a high-pressure ejection mechanism. The entrainment mechanism, hypervelocity vapor formed in the hypervelocity impact event escapes through cracks and entrains lightly shocked surface rocks to escape from Mars, can explain the pressure range of Martian meteorite very well. However, more detail studies are much needed.

Keywords: Crystallization pressure, Ejection, Martian meteorite, Melt vein, Shock, Shock pressure, Zagami