Publication Date:
2011-03-27
Description:
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 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. We conclude that the chondrule rim textures formed in a nebula setting and may therefore represent the first rock fabric in the Solar System. © 2011 Macmillan Publishers Limited. All rights reserved.
Print ISSN:
1752-0894
Electronic ISSN:
1752-0908
Topics:
Geosciences
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