Investigating the nakhlite source volcano on Mars via electron backscatter diffraction

Griffin, Sammy (2021) Investigating the nakhlite source volcano on Mars via electron backscatter diffraction. PhD thesis, University of Glasgow.

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The overarching science question being addressed by this thesis is: what can the nakhlite meteorites tell us about their source volcano on Mars? To answer the overriding scientific question I have utilised the crystallographic technique of electron backscatter diffraction (EBSD) to answer several secondary questions: 1) Can EBSD be used to distinguish and identify different igneous units within the nakhlite Martian meteorites; 2) What does the micro-structural information reveal about the individual nakhlites emplacement on Mars; and 3) What crystallographic relationship patterns can be revealed from assessing a significant proportion of the currently identified nakhlite group. Answering these questions can provide critical insight into understanding Amazonian volcanism on Mars and provide further constraints towards locating the nakhlite source location on Mars.

Twenty-one thick sections representing sixteen of the twenty-six (currently identified) nakhlites were analysed in this thesis. The nakhlites represent a series of igneous rocks formed as either several extrusive flows or a series of shallow intrusive sills/dykes during the early-mid Amazonian. The rocks have a basaltic chemical composition, exhibiting high proportions of augite (high Ca-clinopyroxene), and express cumulate shape preferred orientation (SPO). They are the second largest class of identified Martian meteorites and represent the largest group or rocks sourced from a singular unknown location on Mars. Geochemical and isotopic investigations of the nakhlites identify a common parental magma source for all samples. This common parental magma source coupled with a shared ejection age had led to the current hypothesis of the nakhlites forming from a singular unidentified volcano on Mars.

Stitching together multiple electron backscatter (EBSD) maps is a method of EBSD analysis that is becoming more popular with advancing technology. This method enables the collection of statistically relevant datasets [sample dependent but sits at ~300 crystals for the nakhlites (this thesis)] at high spatial resolution (≤4 μm) enabling microstructural analysis to be assessed on the cm scale. However, the stitching of multiple panels can generate artifacts within the dataset. The EBSD data collected in this thesis was used to investigate SPO, crystallographic preferred orientation (CPO), and plastic deformation micro-structure parameters in the form of intra-crystalline misorientation patterns within augite (the dominant mineral phase in all samples).

Results from this thesis indicate that:
1) Microstructural analysis indirectly assessing slip-systems using a statistically relevant number of crystals can distinguish variation in emplacement and subsequent deformation environments between the nakhlite samples. Identifying eight unique environments, ranging from low temperature/high pressure to high temperature/low pressure, where a large proportion of the samples exhibited deformation related to low-moderate temperatures and moderate pressures.
2) Assessment of CPO and SPO within the nakhlites showed common CPO development within all nakhlite samples which varied slightly in strength. These CPO were dominated by low intensity and of weak to moderate strength S (foliation dominated) to LS (a combination of foliation and lineation) CPO often with some expression of weak lineation. These types of CPO are commonly associated with gravity settling environments such as lava lakes, stagnant lava, sills and dykes, where the initial lineation within the magma is overprinted by crystal settling.
3) Modelling of emplacement mechanism endmembers showed crystal settling to be the dominant emplacement mechanism for all assessed nakhlites, agreeing with CPO observations. The crystal settling calculations resulted in unit thicknesses ranging two orders of magnitude from less than one metre to several tens of metres, enabling two distinct groups to be identified: group one being those with magma body thicknesses <10 m and group two with magma body thicknesses <10 m. When considered within the context of known nakhlites ages and the restriction to a localised ejection site on Mars indicate the thickness of the magma bodies to vary randomly over time. When considered against published geochemical data and reported CPO, the calculated magma body thicknesses showed no trends overall showing inherent randomness to the dataset.

Compiling the observed micro-structural CPO and SPO data from the 16 analysed nakhlite meteorites show that the nakhlites source location on Mars represents a diverse magmatic environment, where there are multiple units of variable magmatic thickness formed via a common emplacement process which is dominated by gravitational settling.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Colleges/Schools: College of Science and Engineering > School of Geographical and Earth Sciences
Supervisor's Name: Lee, Professor Martin and Daly, Dr. Luke and Cohen, Dr. Ben
Date of Award: 2021
Depositing User: Theses Team
Unique ID: glathesis:2021-82974
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 16 Jun 2022 15:52
Last Modified: 19 Jan 2023 10:59
Thesis DOI: 10.5525/gla.thesis.82974
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