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

(1)原始球粒陨石主要由球粒和细粒基质组成。球粒通常被细粒边缘包围。这些边缘被广泛认为是由陨石球粒表面上的尘埃直接吸积而形成的，而陨石球粒则位于太阳星云中。我们对三个CV 3球粒陨石的球粒边缘进行了详细的研究，发现每个球粒陨石中一定比例的球粒边缘由含水层状硅酸盐组成。富含页硅酸盐的边缘和它们的宿主球粒显示出许多水蚀变的证据。这些证据不能与太阳星云中尘埃的直接吸积形成边缘相一致，但可以用陨石母体过程来解释。我们得出的结论是，球粒/边缘组合碎屑，形成的破碎，由于角砾岩，含水蚀变部分的母体。我们第一次提出了一个模型，解释了在chon上形成轮辋的原因。 ...更多信息 通过母体突起形成小核果。(2)我们发现了矿物学证据的水蚀变和随后的脱水在黑暗的包裹体（DI）在Mokoia CV 3球粒陨石。我们提出，大约10年前，DI是岩屑碎屑形成的含水蚀变和随后的脱水陨石母体。从那时起，人们就一直在争论DI是由太阳星云中的尘埃吸积还是陨石母体中的物理和化学过程形成的。本研究结果为亲本起源提供了有力的支持。(3)通过对四种CO_3榴辉岩的研究，首次发现CO_3榴辉岩中存在丰富的暗色包裹体。DI表明，它们是由水蚀变和随后的脱水的一个亲水前体形成的证据。主要元素组成和矿物学的DI建议，其前身是一个CO球粒陨石。因此，它们的形成历史可能与CV 3陨石中的DI相似。CO母体以前被认为是干燥的、均匀的和未加工的。然而，目前的研究结果表明，CO母体是一个由含水区和无水区组成的非均质砾岩，当它获得热源时，含水区被含水蚀变，随后脱水。(4)硅酸盐变黑是已知的许多变质的榴辉岩，但其真正的原因一直不明。科比CK 4球粒陨石表现出明显的基质和球粒的硅酸盐变暗。我们的研究表明，硅酸盐变黑的主要原因是泡状橄榄石，其中含有高密度的小泡和颗粒的磁铁矿和镍黄铁矿。囊状橄榄石以前没有在任何陨石中报道过。科比的橄榄石具有轻微的冲击效应，与这种冲击效应相一致的冲击程度太低，不足以解释橄榄石的熔融。我们认为，科比的泡状橄榄石是在相对较低的冲击压力（<20 GPa）和较高的温度（>600 ℃）下由冲击变质作用产生的。我们提出，泡状橄榄石可以在高温下的冲击事件的指示。(5)尘埃颗粒，20-400 μm大小，在地球上积累的估计流量为每年30000吨。那些收集在地球表面，被称为微陨石，主要是相似的化学和矿物学的水化，多孔陨石。这与陨石福尔斯形成鲜明对比，其中水合多孔型仅占2.8%。这种丰度上的巨大差异通常归因于地球大气层的“过滤”效应，也就是说，多孔陨石是如此脆弱，以至于它们无法在与大气层的撞击中幸存下来。在这里，我们报告两个多孔陨石，其中一个是水合的，另一个是无水的冲击恢复实验。水合陨石对冲击的反应是在比无水陨石更宽的压力范围内进行广泛的粉碎和爆炸性膨胀。这些结果表明，水合小行星可能产生尘埃粒子的相互碰撞的速度比无水小行星高得多。因此，他们解释了微陨石和陨石中水合物质的相对丰度不同，并暗示丰度在它们进入地球大气层之前就已经确定了。少

(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