Shock-Induced Phase Transformations in Melt Pockets within Martian Meteorite NWA 4468

Northwest Africa (NWA) 4468 is a 675 g Martian meteorite recovered from the Western Sahara in 2006. It is an olivine basaltic shergottite, comprising clinopyroxene, olivine, maskelynite, chromite, ilmenite, Ca-phosphate polymorphs, troilite, and pyrrhotite. The sample shows both bulk (maskelynite formation) and localized shock effects (pressure-temperature excursions in veins and melt pockets). Here we report the presence of high-pressure polymorphs in melt pockets. We have discovered three phosphate polymorphs in the melt pockets (chlorapatite, merrillite, and tuite). Their crystal structure has been confirmed by Raman analysis. The mineral assemblages of the melt pockets are indicative of solid-state transformations, nucleation, and recrystallization at high temperatures and pressures. The stability fields of these minerals are used to infer the pressure and temperature conditions reached locally in the melt pockets as 23 GPa and 2000°C.