¹Romy D. Hanna,¹Richard A. Ketcham,²Mike Zolensky,¹Whitney Behr
¹Jackson School of Geosciences, University of Texas, Austin TX 78712, USA
²Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, TX 77058, USA

X-ray computed tomographic scanning of a 44 g Murchison stone (USNM 5487) reveals a preferred alignment of deformed, partially altered chondrules, which define a prominent foliation and weak lineation in 3D. The presence of a lineation and evidence for a component of rotational, noncoaxial shear suggest that the deformation was caused by impact. Olivine optical extinction indicates that the sample can be classified as shock stage S1, and electron backscatter diffraction (EBSD) and electron microscopy reveal that plastic deformation within the chondrules was minimal and that brittle deformation in the form of fracturing, cataclasis, and grain boundary sliding was the dominant microstructural strain-accommodating mechanism. Textural evidence such as serpentine veins parallel to the foliation fabric and crosscutting alteration veins strongly suggest that some aqueous alteration post-dated or was contemporaneous with the deformation and that multiple episodes of fracturing and mineralization occurred. Finally, using the deformed shape of the chondrules we estimate that the strain experienced by Murchison was 17-43%. This combined with the current measured porosity of Murchison suggests that the original bulk porosity of Murchison prior to its deformation was 32.2 – 53.4% and likely at the upper end of this range due to chondrule compressibility, providing a unique estimate of pre-deformation porosity for a carbonaceous chondrite. Our findings suggest that significant porosity loss, deformation, and compaction from impact can occur on chondrite parent bodies whose samples may record only a low level of shock, and that significant chondrule deformation resulting in a chondrite foliation fabric can occur primarily through brittle processes and does not require plastic deformation of grains.

Reference
Hanna RD, Ketcham RA, Zolensky M, Behr W (2015) Impact-induced brittle deformation, porosity loss, and aqueous alteration in the Murchison CM chondrite. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.09.005]
Copyright Elsevier

¹Frans J. M. Rietmeijer
¹Department of Earth and Planetary Sciences, MSC 03-2040, 1-University of New Mexico, Albuquerque, New Mexico, USA

The bulbous Stardust track #80 (C2092,3,80,0,0) is a huge cavity. Allocations C2092,2,80,46,1 nearest the entry hole and C2092,2,80,47,6 about 0.8 mm beneath the entry hole provide evidence of highly chaotic conditions during capture. They are dominated by nonvesicular low-Mg silica glass instead of highly vesicular glass found deeper into this track which is consistent with the escape of magnesiosilica vapors generated from the smallest comet grains. The survival of delicate (Mg,Al,Ca)-bearing silica glass structures is unique to the entry hole. Both allocations show a dearth of surviving comet dust except for a small enstatite, a low-Ca hypersthene grain, and a Ti-oxide fragment. Finding scattered TiO2 fragments in the silica glass could support, but not prove, TiO2 grain fragmentation during hypervelocity capture. The here reported dearth in mineral species is in marked contrast to the wealth of surviving silicate and oxide minerals deeper into the bulb. Both allocations show Fe-Ni-S nanograins dispersed throughout the low-Mg silica glass matrix. It is noted that neither comet Halley nor Wild 2 had a CI bulk composition for the smallest grains. Using the analogs of interplanetary dust particles (IDPs) and cluster IDPs it is argued that a CI chondritic composition requires the mixing of nonchondritic components in the appropriate proportions. So far, the fine-grained Wild 2 dust is biased toward nonchondritic ferromagnesiosilica materials and lacking contributions of nonchondritic components with Mg-Fe-Ni-S[Si-O] compositions. To be specific, “Where are the GEMS”? The GEMS look-alike found in this study suggests that evidence of GEMS in comet Wild 2 may still be found in the Stardust glass.

Reference
Rietmeijer FJM (2015) The smallest comet 81P/Wild 2 dust dances around the CI composition. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12510]
Published by arrangement with John Wiley&Sons