The aim of this thesis is to comprehensively analyse presolar silicon carbide (SiC) grains from several primitive meteorites in order to investigate their complicated history. During their residence in the interstellar medium, presolar grains are predicted to be affected by many processes which may modify their original elemental and isotopic composition. Presolar SiC grains from three acid residues and two polished meteorite sections were analysed for their carbon, nitrogen and silicon isotope ratios with high spatial resolution, in order to compare the distribution of 14N/15N ratios compared to those found in the literature. As a result of this work, isotopic fractionation effects caused by the distortion of the electric field around the grain topography were identified. These effects have the potential to cause differential transmission of atomic and molecular secondary ions, particularly when small slits and apertures are selected during NanoSIMS analyses. The measured 14N/15N ratios of the presolar SiC grains analysed in this work match well with existing literature data, although many grains cluster at relatively low 14N/15N values. These low ratios do not appear to be the result of either terrestrial contamination or isotopic dilution, and may instead represent real differences between the SiC grain populations of different meteorites. The majority of mainstream SiC grains analysed in this work lie on a slope with a gradient of ~1.3 on a Si 3-isotope plot, in agreement with literature data. SiC grains from the JAMM and JA-MM2 acid residues appear to lie on shallower slopes, although these samples show significant scatter in the data. Neither terrestrial contamination nor isotopic dilution can explain the apparent fractionation of silicon isotopes in these samples. It is possible that these ratios may represent a difference in the Si ratios of grain populations of different meteorites, although fractionation during the sample preparation phase cannot be excluded. Ten presolar SiC grains from the KJG and JA-MM2 acid residues are comprehensively analysed for their trace element compositions using Time-of-Flight Secondary Ion Mass Spectrometry. The majority of analyses are significantly affected by the proximity of neighbouring grains, leading to high background counts which prevent the reliable determination of elemental abundances for many elements. Depth profiles of several elements are determined for two grains from the KJG residue. Each of the measured elements displays approximately homogeneous profiles through the grains, with abundances in agreement with existing literature data. The uniform depth profiles may represent formation in a stellar envelope with a stable composition, although homogenisation by secondary alteration processes cannot be ruled out.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:756851 |
| Date | January 2018 |
| Creators | Clarke, Alex |
| Contributors | Lyon, Ian |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/the-extraction-and-study-of-interstellar-grains(722ee20f-53a4-455c-bba5-144231f0697a).html |
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