MACROCHONDRULES IN SOME CHONDRITES: 2. GENETIC ASPECTS

Abstract

An electron microscopic and microprobe study of the macrochondrules from the chondrites Allende CV3, Krymka LL3.1, Saratov L4, "Velyka Balka" L4-5 and Château-Renard L6 allow to clear some aspects of their formation. The main of them are following: 1) The texture and mineral composition of the majority of the macrochondrules does not differ from those of ordinary chondrules, thus indicating a similarity in formation conditions, 2) It is shown for the first time that the SiO2/MgO and FeO/(FeO+MgO) ratios of the fine-grained rims of the macrochondrules and one`s of chondrules in the Krymka meteorite are well correlated. This indicates to their coexistence in the same dusty environment of the protoplanetary nebula, 3) The unique characteristics of the two firstly found macrochondrules allowed us to detail conditions of their formation and existence in the protoplanetary nebula. A polyzonal graphite-bearing macrochondrule from the Krymka meteorite formed by accretional growth in a mineralogically variable dusty environment. Its complex evolutional history involved remelting and selective evaporation of the macrochondrule surface layer, accretion of an environmental dust, late formation of a fine-grained rim and migratory intrusion of an organic protomaterial and finally a metamorphic event that produced graphite. The melted surface of a macrochondrule from the Allende chondrite was notably affected by high-energy processes and instantaneous cooling in the protoplanetary nebula, 4) The rarity of macrochondrules in chondrites compared to typical chondrules is probably related to their priority destruction. This was caused by permanent movement and collision between them and other solid objects in the pre-agglomeration period of meteorite development.