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

¹ Laboratory for Astrochemistry and Planetary Sciences, Purple Mountain Observatory, Nanjing 210008, P.R. China
² State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing 210093, P.R. China
³ Department of Earth Sciences, Nanjing University, Nanjing 210093, P.R. China

^* E-mail: aczhang{at}pmo.ac.cn

Sixiangkou is a heavily shocked L6 chondrite that contains numerous shock-induced melt veins with varying widths. This paper focuses on pyroxene polymorphs in shock-induced melt veins in the meteorite and discusses their implication to the high pressure and temperature conditions during impact metamorphism. The major high-pressure polymorphs are majorite and majorite-pyrope solid solution. Diopside, akimotoite and jadeite were also observed in melt veins of the Sixiangkou chondrite. Diopside coexists with ringwoodite (+ akimotoite) in coarse-grained fragments of the melt veins. Two mechanisms were proposed for the lack of phase transformation of diopside: a) low local temperature and b) a very sluggish transformation rate. Majorite and majorite-pyrope occur as coarse-grained polycrystalline and fine-grained solid solution. They were formed through solid state transformation and crystallized from a melt under pressures, respectively. Akimotoite coexists 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. Jadeite appeared as fine-grained and was originated through retrograde transformation of lingunite under moderate post-shock pressure and high temperature conditions. The coexistence of different high-pressure pyroxene polymorphs reflects an inhomogeneous distribution of temperature in the shock-induced melt veins and a kinetic effect of phase transformations of minerals during shock metamorphism.

Key-words: Sixiangkou L6 chondrite, heavily shocked, melt veins, akimotoite, phase transformation.