Earliest rock fabric formed in the Solar System preserved in a chondrule rim
Earliest rock fabric formed in the Solar System preserved in a chondrule rim
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DOI：
10.1038/ngeo1120
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发表时间：
2011-04-01
影响因子：
18.3
通讯作者：
Dyl, Kathryn A.
中科院分区：
文献类型：
--
作者：
Bland, Philip A.;Howard, Lauren E.;Dyl, Kathryn A.
岩石组构--颗粒的首选取向--提供了一扇了解岩石形成、变形和压实历史的窗口。球粒陨石是太阳系中最古老的物质之一(1)，它们的组构应该记录了星云和小行星中发生的一系列过程。然而，由于球粒陨石具有丰富的细粒物质，在很大程度上抵制了传统的原位组构分析。在这里，我们使用高分辨率电子背散射衍射来绘制Allende CV碳质球粒陨石中亚微米颗粒的取向图。我们观察了球粒陨石周围的细粒边缘，球粒球粒是在陨石中吸积之前从熔化的水滴中冷却下来的球形颗粒，以及球粒之间的基质材料。尽管基质显示出大块的单轴组构，表明母小行星中存在压缩事件，但我们发现球粒边缘保留了以球粒为中心的球对称组构。我们定义了一种将织物强度与净压缩进行定量关联的方法，并重建了初始边缘孔隙率为70%-80%。我们的计算提供了陨石证据，证明太阳系形成的第一批固体具有高孔隙率，类似于模型和实验室估计(2，3)。我们得出结论，球粒边缘结构形成于星云环境中，因此可能代表了太阳系中第一个岩石组构。
Rock fabrics-the preferred orientation of grains-provide a window into the history of rock formation, deformation and compaction. Chondritic meteorites are among the oldest materials in the Solar System(1) and their fabrics should record a range of processes occurring in the nebula and in asteroids. However, owing to abundant fine-grained material, chondrites have largely resisted traditional in situ fabric analysis. Here we use high-resolution electron backscatter diffraction to map the orientation of submicrometre grains in the Allende CV carbonaceous chondrite. We look at the fine-grained rims surrounding the chondrules-spherical grains cooled from molten droplets before accretion in the meteorite-as well as the matrix material between the chondrules. Although the matrix exhibits a bulk uniaxial fabric indicative of a compressive event in the parent asteroid, we find that the chondrule rims preserve a spherically symmetric fabric centred on the chondrule. We define a method to quantitatively relate fabric intensity to net compression, and reconstruct an initial rim porosity of 70-80%. Our calculations provide meteoritic evidence that the first solids formed in the Solar System accreted with high porosity, similar to modelling and laboratory estimates(2,3). We conclude that the chondrule rim textures formed in a nebula setting and may therefore represent the first rock fabric in the Solar System.