Textures of Chassignite meteorites: Clues to cumulate formation and early deformation processes on Mars

Chassignite meteorites are martian achondrites that display a magmatic cumulate texture and are mainly composed of olivine with a small amount of poikilitic pyroxene. This study presents a detailed petrological and microstructural analysis using electron backscatter diffraction of all Chassignite meteorites documented up to now, Chassigny, NWA 2737 and NWA 8694, in order to decipher early magmatic, deformation and shock processes. The new results reveal that olivine crystals in Chassignite meteorites initially formed through synneusis, a magmatic process where individual crystals accumulate and aggregate together in specific orientations in the earlier stages of consolidation. Many olivine crystals stick together along (100) crystalline faces as evidenced by the significant concentration of misorientation [100] axes at grain boundaries for all three samples. Synneusis was probably interrupted and progressively replaced by crystal settling until complete crystallisation was achieved. Olivine crystallographic preferred orientations indicate slip on [100](010) and olivine subgrain boundaries reveal high densities of geometrically necessary dislocations of the [100](010) type. The [100](010) slip system is active at high temperatures and low stresses in the Earth’s upper mantle and is often observed in plastically deformed peridotites, though it is not necessarily dominant in terrestrial olivine cumulative rocks. Furthermore, pyroxene crystallographic preferred orientations are consistent with those of olivine ([100] olivine axes parallel to pyroxene [001] axes), supporting the hypothesis of minor plastic deformation of olivine and pyroxene at high temperatures and low stresses. These observations are consistent with possible crystallization and emplacement of the Chassignite magma in the shallow Martian crust. Finally the strong shock events recorded by these meteorites is supported by significant internal deformation of olivine grains, concentration of misorientation axes along their [100] axes, and the activation of the [001](010) slip system leading to formation of olivine subgrain boundaries. Altogether these results are good indicators to decipher primary planetary processes.