Three-dimensional features of chondritic meteorites : applying micro-computed tomography to extraterrestrial material

Almeida, Natasha Vasiliki (2018) Three-dimensional features of chondritic meteorites : applying micro-computed tomography to extraterrestrial material. Doctoral thesis, Birkbeck, University of London.

Abstract

This work examines the application of X-ray computed tomography (XCT) in meteoritics. This powerful technique uses the attenuation of X-rays passing through a sample to map it in three dimensions, allowing for the imaging and quantification of phases and features without the need for destructive sampling. XCT is a novel method with its applications to planetary science only recently recognised and not extensively explored. As such, this study presents two examples of using XCT to both elucidate its potential, and better understand the constituents of chondritic meteorites and the processes experienced on their parent bodies. To test the reliability of XCT, the data are conjoined with standard analytical techniques. Firstly, the 3D fabric and textural properties of 17 L chondrites of varying petrological type and shock stage are described. Specifically, porosity is imaged, quantified and compared with pycnometry data. For each chondrite, the size distribution and orientations of metal grains are reconstructed and correlated with the degree and direction of anisotropy of magnetic susceptibility in the sample. Both porosity and metal grain fabrics reveal trends with progressive thermal and shock metamorphism. The mechanisms accounting for such correlation are explored. Secondly, XCT is used to survey fragments of the Barwell L6 meteorite to identify and locate igneous inclusions. From this data, several inclusions were then subsampled and further geochemically investigated, including oxygen isotopic compositions, hafnium-tungsten systematics, and trace element analysis. Studied inclusions are found to be similar in composition and age to chondrules, but depleted in metal. A possible formation scenario is proposed and the potential link to chondrule formation is discussed. Using these examples, the factors influencing the accuracy of XCT data acquisition and processing are described. The benefits and limitations of the technique, with respect to the analysis of extraterrestrial material and implications for future use, are also considered.

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