title: Physical and chemical alteration of material from C-Complex asteroids
Creators: name: Jenkins, Laura Elise
Publisher: name: University of Glasgow
Year: 2024

description: C-complex asteroids are the parent bodies of carbonaceous chondrite meteorites and are composed of primitive material that was inherited from a molecular cloud, and condensed out of the early solar nebula (e.g., silicates, metal, sulphides, ices, organics). These asteroids have undergone alteration processes and can contain a variety of secondary phases, many of which are volatile bearing (e.g., serpentine, tochilinite, carbonates). It is important to understand the alteration processes experienced by C-complex asteroids and the products produced. C-complex asteroids are important volatile sources in our Solar System that may have delivered water and organics to Earth during its early development. ‘Mighei-like’ carbonaceous (CM) chondrites have all been aqueously altered and a subset have also undergone subsequent thermal metamorphism (post-hydration heating). Some CM chondrites also contain flattened and aligned chondrules due to deformation. When CM chondrites are exposed to Earth’s atmosphere, they experience terrestrial weathering. To better understand these alteration processes, 13 CM and CM-like chondrites were studied: Allan Hills 83100, Kolang, Lewis Cliff 85311, MacAlpine Hills 88100, Murchison, Murray, Pecora Escarpment (PCA) 02012, PCA 91008, PCA 91084, Shidian, Sutter’s Mill, Winchcombe, &amp; Wisconsin Range 91600. These meteorites were investigated using optical petrography, scanning electron microscopy, electron backscattered diffraction, electron microprobe analysis, transmission electron microscopy, Raman spectroscopy, and X-ray diffraction. The metasomatic mineral, hydroandradite, was found in Kolang and Shidian. This is the first report of meteoritic hydroandradite; it formed from a chemically evolving fluid at &gt;100°C, likely as a product of aqueous alteration. The temperature-timescales of post-hydration heating was investigated through experimentally heating CM chondrite samples as well as by studying naturally heated meteorites. This work established the mechanisms of phase transitions (decomposition of tochilinite, serpentine, calcite, etc.) and the temperatures at which they occur. The temperature-timescales of heating experienced by naturally heated CM chondrites were estimated and found to be quite variable, with temperatures spanning 210-950°C and durations ranging from hours to weeks. The variable timescales experienced indicate that post-hydration heating was unlikely to have been caused by impact heating; solar heating is the most likely cause. Flattened and aligned chondrules in Kolang were studied to better understand physical deformation occurring on C-complex asteroids. There is a lack of evidence for impacts strong enough to flatten chondrules, indicating that chondrules were non-spherical before accretion. The non-spherical chondrules were aligned subsequently, likely by a mild impact event. The extent of which terrestrial weathering may affect a meteorite’s makeup was investigated using the recent CM chondrite fall Winchcombe. Terrestrial calcite, calcium sulphates, and halite were found to have formed rapidly, in the timespan of months, showcasing the reactivity of carbonaceous chondrites under Earth’s humid and oxidizing atmosphere. Various alteration processes experienced by CM and CM-like chondrites, both on their C-complex asteroid parent bodies, and after landing on Earth has been better understood. The reactions and products form these alteration processes have been established.