Pyroxene polymorphs in melt veins of the heavily shocked Sixiangkou L6 chondrite

High-pressure pyroxene polymorphs in shock-induced melt veins in the Sixiangkou chondrite include majorite and majorite-pyrope solid solution. Diopside, akimotoite and jadeite were also observed. The diopside co-exists with ringwoodite (+ akimotoite) in coarse-grained fragments of the melt veins. The two mechanisms suggested for the lack of phase transformation of diopside are 1) low local temperature and 2) a very sluggish transformation rate. Majorite and majorite-ringwoodite occur as coarse-grained polycrystalline and fine-grained solid solution; they were formed via solid state transformation and crystallized from a melt under pressure, respectively. Akimotoite co-exists with low-Ca pyroxene in coarse-grained fragments of melt veins. They have nearly identical chemical compositions to the host low-Ca pyroxene, implying a solid state transformation mechanism. Fine-grained jadeite appeared via retrograde transformation of lingunite under moderate post-shock pressure and high temperature conditions.The co-existence of different high-pressure pyroxene polymorphs reflects an inhomogeneous distribution of temperature in these shock-induced melt veins and a kinetic effect of phase transformations of minerals during shock metamorphism.