COGNATE XENOLITHS IN CHONDRITIC METEORITES - EXAMPLES IN MEZO-MADARAS AND GHUBARA

About 20 per cent of ordinary chondrites contain light grey xenoliths, usually of a more recrystallized type of chondrite, set in darker chondritic hosts. The fayalite contents of olivine in xenoliths and hosts indicate overall similarity in composition between the separate parts of such stones. This relationship, considered generally applicable in xenolithic chondrites, was confirmed by conventional chemical analyses of the xenolith and host portions of Mezö-Madaras and Ghubara. Xenolithic structures in chondrites imply formation of these stones over a definite interval of time. Vein structures predate xenolithic structures in some stones, but in others they were formed later. The host portion of many xenolithic chondrites has itself suffered recrystallization or metamorphism. Relatively uniform oxidation conditions were maintained during evolution of particular xenolithic chondrites. The apparent lack of mechanical mixtures between material belonging to different chemical-mineralogical classes suggests independent origin and evolution of these classes. Metallic particles in Ghubara display a martensitic microstructure and have nickel contents intermediate between those of typical chondritic kamacite and taenite. This stone has either been reheated, or cooled rapidly from the high temperature at which it formed through the duplex region in the system Fe-Ni. © 1968.