Defining the mechanism for compaction of the CV chondrite parent body

Forman, L.V., Bland, P.A., Timms, N.E., Daly, L., Benedix, G.K., Trimby, P.W., Collins, G.S. and Davison, T.M. (2017) Defining the mechanism for compaction of the CV chondrite parent body. Geology, 45(6), pp. 559-562. (doi:10.1130/G38864.1)

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Abstract

The Allende meteorite, a relatively unaltered member of the CV carbonaceous chondrite group, contains primitive crystallographic textures that can inform our understanding of early Solar System planetary compaction. To test between models of porosity reduction on the CV parent body, complex microstructures within ~0.5-mm-diameter chondrules and ~10-μm-long matrix olivine grains were analyzed by electron backscatter diffraction (EBSD) techniques. The large area map presented is one of the most extensive EBSD maps to have been collected in application to extraterrestrial materials. Chondrule margins preferentially exhibit limited intragrain crystallographic misorientation due to localized crystal-plastic deformation. Crystallographic preferred orientations (CPOs) preserved by matrix olivine grains are strongly coupled to grain shape, most pronounced in shortest dimension <a>, yet are locally variable in orientation and strength. Lithostatic pressure within plausible chondritic model asteroids is not sufficient to drive compaction or create the observed microstructures if the aggregate was cold. Significant local variability in the orientation and intensity of compaction is also inconsistent with a global process. Detailed microstructures indicative of crystal-plastic deformation are consistent with brief heating events that were small in magnitude. When combined with a lack of sintered grains and the spatially heterogeneous CPO, ubiquitous hot isostatic pressing is unlikely to be responsible. Furthermore, Allende is the most metamorphosed CV chondrite, so if sintering occurred at all on the CV parent body it would be evident here. We conclude that the crystallographic textures observed reflect impact compaction and indicate shock-wave directionality. We therefore present some of the first significant evidence for shock compaction of the CV parent body.

Item Type:Articles
Additional Information:This work was funded by the Australian Research Council via the Australian Laureate Fellowship program (FL110100074 to Bland). Collins and Davison were funded by UK Science and Technology Facilities Council grant ST/N000803/1.
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Daly, Dr Luke
Authors: Forman, L.V., Bland, P.A., Timms, N.E., Daly, L., Benedix, G.K., Trimby, P.W., Collins, G.S., and Davison, T.M.
College/School:College of Science and Engineering > School of Geographical and Earth Sciences
Journal Name:Geology
Publisher:Geological Society of America
ISSN:0091-7613
ISSN (Online):1943-2682
Published Online:04 April 2017
Copyright Holders:Copyright © 2017 Geological Society of America
First Published:First published in Geology 45(6):559-562
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher

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