title: Dynamic evolution of C-type asteroids inferred from carbonaceous CM chondrites
Creators: name: Floyd, Cameron James
Publisher: name: University of Glasgow
Year: 2024

description: The Mighei-like (CM) group of carbonaceous chondrites are water and carbon-rich meteorites which have been identified as the most chemically primitive chondrites available to study. Their high indigenous water and carbon content has led authors to hypothesise that these meteorites may be at least partly responsible for organic and water delivery to Earth during the early solar system. The CM chondrites are therefore the subject of significant scientific interest.

Spectroscopic analysis has identified C-type asteroids as the likely parent body(ies) for the CM chondrites. Whilst incorporated into these body(ies) the CM chondrites experienced an array of secondary alteration processes including aqueous alteration, brecciation, deformation and space weathering. This thesis seeks to examine the effects deformation, brecciation and space weathering.

For this thesis ten CM chondrites spanning a range of petrologic subtypes were studied. Using a combination of high resolution 2D (SEM and EDS) and 3D (XCT) techniques CM chondrite chondrule sizes and orientations were analysed and relationships between the different deformation and alteration mechanisms investigated.

Chondrule sizes were investigated in ten of the CM chondrites. Analysis was conducted in 2D, following a standardised measurement method developed during this project, and in 3D using established techniques. The limitations and benefits associated with 2D and 3D measurement techniques are discussed and the outcomes of several stereological correction models compared. The results presented in this thesis highlight several challenges associated with the use of stereological correction models and suggests that an adapted version of an existing correction model provides the most reliable 2D-3D correction. 

The results of the chondrule size analysis reveal the CM average chondrule size is significantly smaller than previously reported and more akin to those of CO chondrites. Chondrule sizes, shapes and abundances are also observed to vary between constituent clasts in meteorite breccias. Upon petrologic classification of clasts and lithologies it is found that a negative correlation exists between chondrule size and petrologic subtype, with chondrule sizes increasing with a greater degree of aqueous alteration. It is suggested that this relationship is a consequence of a size sorting process influencing initial chondrule accretion to the parent body followed by a aqueous alteration which varied in intensity as a function of depth. 

3D chondrule orientation analysis was conducted on five CM chondrites to determine the strength and orientation of any chondrule-defined fabric. Results revealed that chondrule defined fabrics are commonplace within the CM chondrites and that 3D techniques are best suited to fabric detection and characterisation. Inter-clast and chondrite variations in chondrule fabrics are observed with instances of both consistency and variability in fabric strength and orientation between clasts. 

Significant chondrule deformation was also observed in all chondrites with chondrules deviating significantly from a compact shape. Chondrule deformation even in chondrites with the weakest fabrics is interpreted as evidence for a prolate original chondrule shape during accretion. This finding reconciles the paradox between low shock stage within the CM chondrites and observable fabrics and alignment. 

During this thesis 3D XCT analysis is shown to be an important and powerful tool for studying CM chondrites. XCT analysis facilitates accurate, ‘true’ values for chondrule size and orientation to be determined and its further use within the CM group is encouraged.

The potential usefulness of the WN etching technique for damage track analysis of space weathered olivine grains is reviewed. It is suggested that future use of the technique on CM chondrite thin sections could yield useful information regarding the accretionary histories of the CM parent body(ies). In particular, such work may improve our understanding of the previously observed inhomogeneous distribution of track-rich grains in the clastic matrix and our understanding of regolith turnover processes acting on the body.