Petrological Studies on Chondrules in YAMATO-74 Meteorites

Crystallization of chondrule is investigated through the study of Yamato-74191,-74155,-74079,-74371,and -74647 meteorites. Bulk compositions of chondrules are calculated from modal analyses and mineral compositions, and are projected in the MgO-Al_2O_3-SiO_2 system with the compositions of glasses in chondrules, from which the process of crystallization of chondrule is suggested. According to the investigation in the peritectic system, chondrules may be regarded as products of supercooling crystallization. Variety of textures of chondrules, which are classified into porphyritic, barred, radiating, and cryptocrystalline types, is also explained by various degrees of supercooling of droplet (chondrule). Abnormal FeO/FeO+MgO ratios of the olivines, pyroxenes, and glasses, and extraordinary high K_D^ _ values show the change of atmosphere in chondrule. It possibly resulted from a reaction in ambient gas and its diffusion into chondrule. On the basis of mineralogy of chondrules and matrix in the Yamato-74191,a possible process of accretion of chondrules to parent body is discussed. Evidences of recrystallization of chondrules often occurring before accretion are suggested. Most of chondrules have thin rims around them, whose average composition is similar to that of the silicate portion of the L-chondrites. The rims might have been attached to chondrules after recrystallization and before accretion. Distinct shapes of chondrules mean that their accretion speeds to parent body were fairly low.