Bland, PA

Prior, DJ

Russell, SS

Watt, LE

Meteoritics and Planetary Science

English

Spiral - Imperial College Digital Repository

OLIVINE IN CV CHONDRITE MATRICES:STRUCTURE, SIZE-DISTRIBUTION AND MORPHOLOGY.  L. E. Watt1, P. A. Bland1, D. J. Prior2 and S. S. Russell3. 1IARC, Dept. Earth Science & Engineering, Imperial College, London SW7 2AZ, UK. (lauren.watt@imperial.ac.uk).  2Dept. Earth & Ocean Sci-ences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK.  3 IARC, Dept. Min., Natural History Museum, London SW7 5BD, UK.  Introduction: The CV3 chondrites were originally divided into the oxidized (CVox) and reduced (CVR) subgroups based principally on their modal metal/magnetite ratios and the Ni con-tent of the metal and sulfides [1].  Later studies [e.g. 2] have shown that the oxidized CV3s can be divided into two additional subgroups, Bali-like (CVoxB) and Allende-like (CVoxA), each showing characteristic alteration features.  These divisions are thought to be correlated to the degree and temperature of aqueous alteration experienced by these meteorites [3]. Methodology: Recent work by Watt et al. [4] has shown that olivine crystallography is related to its formation mechanism, concluding that the short a-axis crystal morphology for Allende (CVoxA) matrix olivine is consistent with a formation by dehydra-tion of phyllosilicates.  In this study we have used electron back-scatter diffraction (EBSD) to analyze olivine in the matrices of a variety of CV3 chondrites, in order to determine if there is a sys-tematic variation between the different subgroups.  We have de-termined grain size distributions, crystal structure data and oli-vine morphology ratios.  Meteorites analyzed include: CVR= Vi-garano, Leoville & Efremovka; CVoxB = Bali, Kaba & Mokoia; CVoxA = Allende, Axtell & ALH84028. Results: Analysis of our data highlights the following: 1. Grain Size Distributions:  Cumulative grain size frequency plots reveal that the CVoxB meteorites have similar distribu-tion trends, while the CVR meteorites appear to differ; CVR distributions can be approximated by an appropriate power law, while CVox distributions cannot. 2. Olivine Crystal Structure:  Where crystal structural data can be determined (i.e. in euhedral, lath shaped olivine), there is a consistent short a-axis crystal morphology for the olivine in the matrices of the CVox meteorites.  This differs to the CVR meteorites, where there is an inconsistent crystal morphology, with examples of short a, b and c axes observed. 3. Olivine Morphology: There is a systematic change in the ratio of euhedral laths (aspect ratio >2) to sub/anhedral grains be-tween the 3 groups; CVR~<20%, CVoxB~20-25%, CVoxA~25-35%. Conclusions:  There is a systematic variation in matrix oli-vine between the different CV3 subgroups, particularly between the CVox and CVR subgroups.  We propose that the variations in olivine characteristics, between the different subgroups, are re-lated to the formation mechanisms by which the olivine has formed.  The occurrence of abundant, euhedral, lath-shaped faya-litic olivine with consistent short a-axis crystal morphology and a characteristic grain size distribution in the matrix of the CVox me-teorites can be attributed to a formation mechanism of dehydra-tion of phyllosilicate [4].  It would appear that the CVR subgroup has largely escaped these processes. References: [1] McSween, H. Y. 1977. GCA 41:1777-1790. [2] Weisberg, M. K. et al. 1997. MAPS 32: A138-139. [3] Krot, A. N. et al. 1998. MAPS 33:1065-1085. [4] Watt, L. E. 2005. LPSC XXXVI: # 1305.  68th Annual Meteoritical Society Meeting (2005) 5224.pdf

Olivine in CV chondrite matrices: Structure, size-distribution, and morphology

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Olivine in CV chondrite matrices: Structure, size-distribution, and morphology

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