Formation processes of chondrule rims in the primitive meteorites : Verification of the solar nebula model and the parent body model

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(1) Primitive chondrite meteorites consist mainly of chondrules and fine-grained matrix. Chondrules are commonly surrounded by fine-grained rims. The rims have widely been regarded to have formed by direct accretion of dust onto the surfaces of chondrules, while chondrules were in the solar nebula. We have performed detailed studies of chondrule rims in three CV3 chondrites and found that a certain proportion of chondrule rims in each chondrite consist of hydrous phyllosilicates. The phyllosilicate-rich rims and their host chondrules show much evidence of aqueous alteration. The evidence can not be reconciled with rim formation by direct accretion of dust in the solar nebula, but can be most plausibly explained by meteorite parent-body processes. We concluded that the chondrule/rim assemblages are clasts that formed by fragmentation, due to brecciation, of aqueously altered portions of the parent body. We proposed, for the first time, a model that explains the formation of rims on chon … More drules by parent-body processes.(2) We discovered mineralogical evidence of aqueous alteration and subsequent dehydration in dark inclusions (DIs) in the Mokoia CV3 chondrite. We proposed, about 10 years ago, that DIs are lithic clasts that formed by aqueous alteration and subsequent dehydration on the meteorite parent body. Since then, there has been a controversy on whether DIs formed by accretion of dust in the solar nebula or physical and chemical processing in meteorite parent bodies. The results of our study provide strong support of the parent body origin.(3) The study of four CO3 chondrites has revealed for the first time that dark inclusions (DIs) occur abundantly in CO3 chondrites. The DIs show evidence that they were formed by aqueous alteration and subsequent dehydration of a chondritic precursor. The major element compositions and mineralogy of the DIs suggest that their precursor is a CO chondrite. Thus they probably have a formation history similar to DIs in CV3 chondrites. The CO parent body has previously been regarded to have been dry, homogeneous and unprocessed. However, the present results suggest that the CO parent body was a heterogeneous conglomerate consisting of water-bearing regions and water-free regions, and as it acquired heat source, the water-bearing regions were aqueously altered and subsequently dehydrated.(4) Silicate darkening is known from many metamorphosed chondrites but its true cause has been unknown. The Kobe CK4 chondrite exhibits pronounced silicate darkening of matrix and chondrules. Our study has revealed that the principal cause of the silicate darkening is vesicular olivine that contains high densities of small vesicles and grains of magnetite and pentlandite. The vesicular olivine has not been previously reported in any chondrites. It probably resulted from recrystallization of partially melted matrix olivine by shock.Kobe exhibits light shock effects in olivine ; the degree of shock consistent with such shock effects is too low to explain the olivine melting. We suggest that the vesicular olivine in Kobe was produced by shock metamorphism at a relatively mild shock pressure (<20 GPa) and a high temperature (>600 ℃) Thus, the shock effects in olivine, manifest as fracturing and deformation, were relatively minor, but heating was strong enough to cause partial melting of matrix olivine. We proposed that vesicular olivine can be an indicator of shock events at high temperatures.(5) Dust particles, 20-400 μm in size, accreted onto the Earth with an estimated flux of 〜30000 tons per year. Those collected on the Earth's surface, termed micrometeorites, are predominantly similar in chemistry and mineralogy to hydrated, porous meteorites. This contrasts with the meteorite falls, of which hydrated, porous type comprises only 2.8 %. This large difference in abundance has commonly been attributed to a "filtering" effect of the Earth's atmosphere, that is, porous meteorites are so friable that they would not survive the impact with the atmosphere. Here we report shock-recovery experiments on two porous meteorites, of which one is hydrated and the other is anhydrous. The hydrated meteorite responds to shock with extensive comminution and explosive expansion on pressure release in a much broader pressure range than the anhydrous meteorite. These results indicate that hydrated asteroids probably produce dust particles by mutual collisions at much higher rate than anhydrous asteroids. Thus they explain the different relative abundances of hydrated material in micrometeorites and meteorites, and imply that the abundances are established much before they enter the Earth's atmosphere. Less