Olivine melting at high pressure condition in the chassignite Northwest Africa 2737

Abstract

Shock-induced melting textures and assemblages in the chassignite Northwest Africa (NWA) 2737 were investigated. NWA 2737 consists mainly of coarse-grained olivine (Fa19.6–20.5) and minor amounts of low-Ca pyroxene, plagioclase (now maskelynite), and chromite. Several melt inclusions formed through pre-igneous activity are enclosed in the olivine grains, and some melt inclusions were melted again by the impact event. Several olivine fragments are entrained in the melt inclusion and dissociated into spherulitic ferroan–periclase and interstitial fine-grained clinopyroxene assemblages. The olivine fragments were melted once by the shock metamorphism, and ferroan–periclase crystallized from the olivine melt. We expect that bridgmanite crystallized from the residual melt and inverted to clinopyroxene through the subsequent high-temperature event, although we cannot rule out the possibility that the ferroan–periclase and clinopyroxene assemblage is the partial melting product of olivine. Parts of the melted olivine in the melt inclusion are mixed with the glass (having low-Ca pyroxene + a small amount of silica components) filling the matrix portion of the melt inclusion. Then, olivine, clinopyroxene, and tridymite crystallized from the mixed melts. We infer that the olivine–clinopyroxene–tridymite assemblage is the crystallization product from the mixed melts during rapid cooling at low- or ambient-pressure conditions. Olivine grains adjacent to the melt inclusion dissociated to clinopyroxene + ferroan–periclase assemblage. As the distance from the melt inclusion increased, polycrystalline olivine crystal assemblage formed. We expect that the polycrystalline olivine assemblage is the remnant of ringwoodite: metastable ringwoodite formed in the olivine grain by the shock metamorphism, and back transformed into olivine again through high temperature and low pressure conditions. Mineral assemblages in and adjacent to the melt inclusion show evidence for high pressure and high temperature conditions and for low pressure and high temperature conditions. The pressure–temperature–time path recorded in NWA 2737 is not simple, i.e., the ejection path from the impact site is complex.