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

原始陨石球粒边缘的形成过程：太阳星云模型和母体模型的验证

基本信息

批准号：
12440149
负责人：
TOMEOKA Kazushige
金额：
$ 9.15万
依托单位：
Kobe University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
2000
资助国家：
日本
起止时间：
2000 至 2003
项目状态：
已结题

项目摘要

(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

(1)原始球粒陨石主要由球粒和细粒基质组成。球粒通常被细粒的边缘包围。人们普遍认为，这些边缘是由尘埃直接吸积到球粒表面形成的，而球粒位于太阳星云中。我们对三个CV3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都有一定比例的球粒边缘由含水层状硅酸盐组成。富含层状硅酸盐的岩缘和它们的宿主球粒显示出许多水蚀变的证据。证据不能与太阳星云中尘埃直接吸积形成的边缘相一致，但可以用陨石母体过程最合理地解释。我们的结论是，球粒/边缘组合是由母体水中改变部分的角化破碎形成的碎屑。我们首次提出了一个模型，解释了更多规则的父体过程中轮辋的形成。(2)在Mokoia CV3球粒陨石的暗包裹体（DIs）中发现了水蚀变和随后脱水的矿物学证据。早在10年前，我们就提出了岩石碎屑是由水蚀变和随后的脱水在陨石母体上形成的。此后，关于DIs是由太阳星云内尘埃的吸积形成，还是由陨石母体的物理化学过程形成，一直存在争议。我们的研究结果为母体起源提供了强有力的支持。(3)对4个CO3球粒陨石的研究首次揭示了CO3球粒陨石中大量存在暗包裹体（dark inclusions, DIs）。DIs表明它们是由水蚀和随后的球粒体前体脱水形成的。DIs的主要元素组成和矿物学表明它们的前体是CO球粒陨石。因此，它们的形成历史可能与CV3球粒陨石中的DIs相似。CO母体以前被认为是干燥、均匀和未经加工的。然而，目前的研究结果表明，CO母体是由含水区和无水区组成的非均质砾岩，随着其获得热源，含水区发生了水蚀变，随后发生脱水。(4)从许多变质球粒陨石中已知硅酸盐变黑，但其真正原因尚不清楚。神户CK4球粒陨石显示基质和球粒明显的硅酸盐变暗。我们的研究表明，硅酸盐变黑的主要原因是含有高密度的小泡和磁铁矿和镍黄铁矿颗粒的泡状橄榄石。泡状橄榄石以前没有在任何球粒陨石中被报道过。这可能是部分熔融的橄榄石基体在冲击作用下再结晶的结果。神户在橄榄石中表现出光震效应；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化。我们认为神户地区的泡状橄榄石是在相对较轻的冲击压力（<20 GPa）和较高的温度（>600℃）下产生的冲击变质作用，因此橄榄石中的冲击作用相对较小，表现为破裂和变形，但加热强度足以导致基质橄榄石部分熔化。我们提出泡状橄榄石可以作为高温冲击事件的指示物。(5) 20 ~ 400 μm大小的尘埃颗粒以每年约3万吨的速度被吸积到地球上。那些在地球表面收集的陨石，被称为微陨石，在化学和矿物学上与水合多孔陨石非常相似。这与陨星形成鲜明对比，陨星的水合，多孔型只占2.8%。这种数量上的巨大差异通常被归因于地球大气的“过滤”效应，也就是说，多孔的陨石非常脆弱，它们无法在与大气的撞击中幸存下来。本文报道了两种多孔陨石的冲击恢复实验，其中一种是水合的，另一种是无水的。与无水陨石相比，水合陨石对冲击的反应是广泛的粉碎和压力释放时的爆炸膨胀。这些结果表明，含水小行星产生尘埃粒子的速度可能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的不同，并暗示丰度在它们进入地球大气层之前就已经确定了。少