Natural dissociation of olivine to (Mg,Fe)SiO ₃ perovskite and magnesiowüstite in a shocked Martian meteorite

Abstract

We report evidence for the natural dissociation of olivine in a shergottite at high-pressure and high-temperature conditions induced by a dynamic event on Mars. Olivine ) adjacent to or entrained in the shock melt vein and melt pockets of Martian meteorite olivine-phyric shergottite Dar al Gani 735 dissociated into (Mg,Fe)SiO 3 perovskite (Pv)þmagnesiowüstite (Mw), whereby perovskite partially vitrified during decompression. Transmission electron microscopy observations reveal that microtexture of olivine dissociation products evolves from lamellar to equigranular with increasing temperature at the same pressure condition. This is in accord with the observations of synthetic samples recovered from high-pressure and high-temperature experiments. Equigranular O livine is one of the major constituent minerals of terrestrial planets. High-pressure and high-temperature experiments indicate that olivine converts in solid state to its high-pressure polymorphs, ringwoodite subsequent to wadsleyite, with increasing pressure and temperature and finally dissociates at higher pressures to ðMg;FeÞSiO 3 perovskite ðPvÞ þ magnesiowüstite ðMwÞ. It is expected that the dissociation takes place around and below the transition zone of the Earth (1). Seismic observations suggest that ðMg;FeÞSiO 3 Pv and Mw are likely the dominant constituent minerals in the lower mantle. Accordingly, its dissociation mechanism is important to understand the dynamics of the Earth's interior because it affects the physical and chemical properties such as densities and elastic velocities of mantle materials. Since the experimental discovery of the dissociation reaction, many scholars have been searching for evidence for the decomposition reactions in natural samples such as inclusions in diamonds from kimberlites. However, the dissociation of olivine at high-pressure and high-temperature conditions has never been reported from any natural samples so far. Therefore, the dissociation mechanism of olivine in natural samples is still unclear.Planetesimal collision phenomena are recorded in many chondritic, lunar, and Martian meteorites as shock melt veins or melt pockets. Olivine in and around the shock melt veins or melt pockets was converted by the dynamic events in solid-state reactions to its high-pressure polymorphs or its shock-induced monomineralic melt fractionally crystallized to Mg-rich wadsleyite and Fe-rich ringwoodite at high-pressure and high-temperature conditions induced by the dynamic events (2-7). The high-pressure and high-temperature conditions of the dynamic event recorded in Martian meteorites have been estimated by several previous works (8)(9)(10)(11)(12)(13)(14)(15), and the estimated conditions given in some reports are considerably beyond the stability filed of ringwoodite (8, 11). It is likely that olivine dissociated to ðMg;FeÞSiO 3 Pv þ Mw but was overlooked. Accordingly, we carefully scanned olivine grains in the Martian meteorite Dar al Gani (DaG) 735 with a field emission (FE) SEM and laser micro-Raman spectroscopy. Here ...

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