Earliest rock fabric formed in the Solar System preserved in a chondrule rim
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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 System1 and their fabrics should record a range of processes occurring in the nebula and in asteroids, but due to abundant fine-grained material these samples have largely resisted traditional in situ fabric analysis. Here we use high resolution electron backscatter diffraction to map the orientation of sub-micrometre grains in the Allende CV carbonaceous chondrite: the matrix material that is interstitial to the mm-sized spherical chondrules that give chondrites their name, and fine-grained rims which surround those chondrules. Although Allende matrix exhibits a bulk uniaxial fabric relating to a significant compressive event in the parent asteroid, we find that fine-grained rims preserve a spherically symmetric fabric centred on the chondrule. We define a method that quantitatively relates fabric intensity to net compression, and reconstruct an initial porosity for the rims of 70-80% - a value very close to model estimates for the earliest uncompacted aggregates2,3. We conclude that the chondrule rim textures formed in a nebula setting and may therefore be the first rock fabric to have formed in the Solar System.
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Emmerton, S.; Muxworthy, A.; Hezel, D.; Bland, Philip (2011)We have conducted a detailed magnetic study on 45 chondrules from two carbonaceous chondrites of the CV type: (1) Mokoia and (2) Allende. Allende has been previously extensively studied and is thought to have a high ...
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Forman, L.; Bland, Phil; Timms, Nicholas Eric; Collins, G.; Davison, T.; Ciesla, F.; Benedix, G.; Daly, L.; Trimby, P.; Yang, L.; Ringer, S. (2016)The CV3 Allende is one of the most extensively studied meteorites in worldwide collections. It is currently classified as S1—essentially unshocked—using the classification scheme of Stöffler et al. (1991), however recent ...