Phase transition processes of olivine in the shocked Martian meteorite Tissint: Clues to origin of ringwoodite-, bridgmanite- and magnesiowüstite-bearing assemblages

Tissint is a heavily shocked olivine-phyric shergottite. We investigated and documented several phase transformations and dissociation phenomena of olivine adjacent to and in many shock-induced melt-pockets of Tissint. Olivine grains entrained in the melt-pockets dissociated to equigranular magnesiowüstite + bridgmanite (latter now vitrified). With approaching to the boundaries between the melt-pockets and host-rocks of Tissint, some olivine grains entrained in the melt-pockets are incompletely dissociated. Spherulitic magnesiowüstite + bridgmanite assemblages occur in the incompletely dissociated olivine. Several residual olivine domains are retained at the center of spherulitic texture. With further approaching to the boundaries between the melt-pockets and host-rocks, some olivine grains entrained in the melt-pockets are segmented with a cellular-like texture, presumably induced by defect arrangement. Many fine-grained magnesiowüstite occurs around the individual “cells” of the segmented olivine grains. Considering the temperature gradient in the melt-pockets and the arrangement of dissociation and segmentation textures, olivine dissociation reaction was presumably initiated by the segmentation of olivine, subsequently followed by magnesiowüstite crystallization around the individual “cells” of the segmented olivine, and, by bridgmanite formation. Subsequently, spherulitic magnesiowüstite + bridgmanite assemblages occur around residual olivine domain, and finally, olivine completely dissociates to equigranular magnesiowüstite + bridgmanite assemblage. Olivine grains adjacent to the melt-pockets transform to randomly oriented polycrystalline ringwoodite assemblages. In rare case, ahrensite occurs in the more iron-rich portions of the olivine grains. With increasing distance from the melt-pockets, sets of ringwoodite lamellae occur in increasing abundance into the interior of the olivine grains. The lamellae consist of almost crystallographically oriented polycrystalline ringwoodite assemblages. The phase transitions from olivine to ringwoodite were presumably promoted by interface-controlled incoherent mechanism. The ringwoodite would be metastable phase which formed in the stability field where olivine dissociates into magnesiowüstite + bridgmanite.