Impact-generated volatile movement and redistribution in the Rose City meteorite

Ordinary chondrites contain textures and mineralogies which indicate a range in intensity of shock and thermal metamorphism. It has been proposed that the metamorphic sequence in ordinary chondrites is the result of thermal metamorphism in ejecta blankets deposited by impact events on the parent meteorite bodies. Because of the association of vapor phase crystallization with recrystallized (thermally-metamorphosed) lunar breccias, a search for vapor phase crystallization was undertaken with the Rose City meteorite (H-group). The Rose City meteorite is a high iron chondrite that underwent brecciation, shock-melting, vaporization and recrystallization in an impact event about 400 million years ago as interpreted from rare gas data. It contains a variety of lithologies: (1) clasts of moderately-shocked H6 lithology, (2) matrix, between clasts, of shock-melted and recrystallized silicate veins, depleted in metal and sulfide but containing numerous mineral fragments and vesicles, (3) matrix veins of metal and sulfide with a cotectic texture, (4) clasts in which the metal-sulfide and silicate portions show a range in degree of shock melting. Many of the larger clasts which are still coherent are surrounded by a band of metal-sulfide with the same cotectic texture common in the matrix metal bands. A striking array of apparently vapor-grown crystals in cavities was found in both the recrystallized matrix and in the clasts of the Rose City meteorite, using the scanning electron microscope and energy dispersive x-ray analyzer. Euhedral to subhedral crystals of troilite, iron, iron-nickel, pyroxene, olivine, and apatite were found attached to the walls of vugs. Similar morphologies arid mineralogies of crystals in vuggy lunar breccias and the Rose City meteorite suggest similar processes caused loss of volatiles plus redistribution of volatiles in both ordinary chondrites and lunar breccias. Cooling of hot ejecta blankets on parent meteorite bodies would include movements of vapors (such as oxides, halides, sulfides, iron) from hotter to cooler areas, and result in eventual loss of noncondensable species and deposition of condensable species in cooler areas. This would explain some of the volatile depletions in ordinary chondrites. Cooling of large thermally-zoned ejecta blankets would partially explain the progressive thermal metamorphism of ordinary chondrites with the more recrystallized meteorites originating in the hotter zones of the ejecta blanket.