Shock-induced phase transformation of anorthitic plagioclase in the eucrite meteorite Northwest Africa 2650

Anorthite is an important constituent mineral in basaltic achondrites from small celestial bodies. Its high-pressure phase transformation in shocked meteorites has not been systematically studied. In this study, we report the diverse phase transformation behaviors of anorthite in a shocked eucrite Northwest Africa (NWA) 2650, which also contains coesite, stishovite, vacancy-rich clinopyroxene, super-silicic garnet, and reidite. Anorthite in NWA 2650 has transformed into anorthite glass (anorthite glassy vein, maskelynite, and glass with a schlieren texture and vesicles), tissintite and dissociated into three-phase assemblage grossular+kyanite+silica glass. Different occurrences of anorthite glass might have formed via the mechanism involving shear melting, solid-state transformation, and postshock thermally melting, respectively. Tissintite could have crystallized from a high-pressure plagioclase melt. The nucleation of tissintite might be facilitated by relict pyroxene fragments and the early formed vacancy-rich clinopyroxene. The three-phase assemblage grossular, kyanite, and silica glass should have formed from anorthitic melt at high-pressure and high-temperature conditions. The presence of maskelynite and reidite probably suggests a minimum peak shock pressure up to 20GPa, while the other high-pressure phases indicate that the shock pressure during the crystallization of shock melt veins might vary from >8GPa to >2GPa with a heterogeneous temperature distribution.