Doctoral Degrees (Earth Sciences)

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    A phase equilibrium modelling investigation of the consequences of entrainment of components of the source on the crystallisation of mantle-derived magmas in the upper crust, with specific relevance to the petrogenesis of chromitite layers in the Rustenburg Layered Suite of the Bushveld Complex, South Africa
    (Stellenbosch : Stellenbosch University, 2023-03) Otto, Tahnee; Stevens, Gary; Moyen, Jean-Francois; Mayne, Matt; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Thermodynamic modelling techniques offer several advantages over experimental studies in investigations of the partial melting of mafic and ultramafic rocks under upper mantle conditions, but may not be as reliable as experimental studies in accurately predicting rock behaviour. The two main thermodynamic datasets that are in common use for such thermodynamic investigations have different levels of coverage. The Berman (1988) dataset is utilised by the MELTS family of software (Asimow & Ghiorso, 1998; Ghiorso & Sack, 1995; Ghiorso et al., 2002), and along with routines for handling activity-composition relations for solid and liquid solution phases, is embedded within the software. In contrast, the Holland & Powell (2011) dataset is used by a wide range of different software with different computational strategies. Activity-composition relations are handled by published models that are selectable and transparent. One of the programs using the Holland & Powell (2011) dataset, Rcrust (Mayne et al., 2016), has been developed to allow modelling of phase stabilities with changing bulk composition, which makes it particularly powerful for studying processes involving fractionation. In this study, the results of sets of experiments on ultramafic and mafic compositions under upper mantle conditions were compared with outputs from MELTS and Rcrust. Differences between the modelling and experimental results were quantified, and the Holland & Powell (2011) dataset combined with appropriately selected activity-composition models produced the best match with experimental results when melt fraction, melt composition, and the assemblage of coexisting phases were considered. The results demonstrate that modelling is a viable alternative to experimental techniques in investigating melting of the upper mantle. Thermodynamic modelling was used to map the envelope of uncertainty that exists around experimental results due to often unavoidable experimental pitfalls, and demonstrated that factoring in these uncertainties produced a better fit between modelling and experimental results. Next, this study developed a thermodynamic modelling methodology to investigate the stoichiometry of the melting reactions in an eclogite and peridotite as a function of pressure, temperature, and bulk compositional change due to melt loss. Pressure-temperature regions where the partial melting reactions produce peritectic crystals that increased in abundance with melt were identified. The entrainment of these crystals to the magma on segregation produces distinct shifts in the chemistry of the extracted magmas, for both sources. The results illustrate that variable amounts of peritectic crystal entrainment represents a previously unrecognised source of compositional heterogeneity in basaltic and intermediate magmas. The modelled results highlight how peritectic crystal entrainment may be a principal mechanism for creating the magmas that produce layered mafic and ultramafic complexes, because the entrained phases react with melt on ascent to produce magmas with a high crystal load once they reach the upper crust. During the partial melting of peridotite, the most common entrainable peritectic mineral is orthopyroxene, which is Cr2O3- bearing. On ascent, entrained peritectic orthopyroxene reacts to form olivine and chromite, and such magmas will enter high-level magma chambers as a melt-olivine-chromite slurry. Density segregation on intrusion allows the formation of magmatic layering if the magma body has appropriate geometry. Thus, these results provide an explanation for the presence of chromitite seams in layered mafic complexes, as well as average bulk rock Cr2O3 contents well above mantle values in some layered intrusions, such as the Bushveld Complex. Chromitite layers are a consequence of peritectic orthopyroxene entrainment and do not require any complex phase equilibrium magma chamber processes.
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    Phytoplankton and trace metal dynamics in the Southern Ocean
    (Stellenbosch : Stellenbosch University, 2023-03) Viljoen, Johannes Jacobus; Fietz, Susanne; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Marine phytoplankton in the Southern Ocean are essential for the Antarctic food web and help regulate the global ocean biogeochemistry, thereby mediating the warming effect of carbon dioxide (CO2). Yet, spatially and temporally, phytoplankton distribution and their controlling factors are still poorly studied in the Southern Ocean. Owing to low sampling resolution, features such as frontal and island regions have been less sampled. Moreover, the dynamics of trace metals which serve as essential micronutrients has been the least studied in the Southern Ocean. Most studies focussed on iron (Fe) and total chlorophyll-a, few on extended suites of metals and phytoplankton groups. There are still uncertainties how the trace metals Fe, copper (Cu), zinc (Zn), nickel (Ni), cobalt (Co), manganese (Mn), and cadmium (Cd), will affect phytoplankton abundance and community composition, especially in the understudied Southern Ocean during winter. In this thesis Southern Ocean phytoplankton dynamics and the complex interplay with various parameters were elucidated. Through three research papers, phytoplankton distribution was studied utilizing multi-parameter datasets, collected during two research cruises to the Southern Ocean one during summer in the Atlantic sector (0 - 8°E) and another during winter in the Indian sector (30°E). Summer phytoplankton distribution were characterised in the Atlantic Southern Ocean at high resolution across the major zones, within fronts, in sea ice influenced zones (e.g., polynya) and close to Subantarctic islands using a suite of HPLC phytoplankton pigments. Cyanobacteria dominated the Subtropical zone; haptophytes the Subantarctic and Polar Frontal zones while diatoms and haptophytes dominated the Antarctic zone south of the Polar Front. Additionally, distinct communities were observed in frontal, sea ice and near-island regions. For example, phytoplankton abundance in fronts were up to 10-fold higher than in the major zones while haptophytes dominated communities close to the Subtropical Front. Diatoms dominated in the areas affected by recent sea ice melt while Phaeocystis and coccolithophores dominated post-bloom communities. Close to Subantarctic islands, cryptophyte blooms were observed whereas diatoms were dominant further downstream. Results suggest that a combination of nutrient, including trace element supply and mixing regime are essential in controlling the magnitude and composition of blooms close to fronts, sea ice and islands, and in turn, affect Southern Ocean food web activity and potential carbon export. Having noticed the importance of zonal changes and small-scale features, as well as the likely importance of trace element supply, the second and third study included these dynamics. The relationship between phytoplankton dynamics (abundance and community structure) and micronutrients (i.e., trace metals) were investigated for the first time during winter in the Indian sector of the Southern Ocean. This was achieved through two studies using a unique dataset of HPLC chlorophyll-a and accessory pigment concentrations with parallel sampled macronutrients and a suite of dissolved and particulate trace metals and phosphorous concentrations. Results suggest phytoplankton were still active and a dominant contributor to the uptake and remineralisation of trace metals even though their abundance were lower than during summer. Through a suite of metal* calculations, based on the macro- and micronutrient concentrations and the estimated requirements of Southern Ocean phytoplankton, varying degrees of deficiency and potential for co-limiting conditions were proposed. Accordingly, micronutrients are suggested as a major driver of winter phytoplankton abundance and community structure across the Southern Ocean. Some trace metals had stronger relationships with specific phytoplankton groups compared to total phytoplankton abundance. In turn, specific groups, such as diatoms, were confirmed to be major drivers of trace metal dynamics across the transect through preferential uptake. For example, cyanobacteria, a group not considered by previous trace metal studies in this region, was suggested to be dependent and responsible for major uptake of Co and Mn while diatoms were strongly associated with Zn. Ultimately, the preferential uptake by specific phytoplankton groups in deficient conditions, such as diatoms, can aggravate limiting conditions and lead to a change in composition. Therefore, the strong association of specific phytoplankton groups, with different remineralisation lengths, to specific micronutrients during winter would affect the release of these micronutrients for the utilisation by phytoplankton during the following spring and summer seasons. Hence, these two studies yielded new knowledge on phytoplankton-micronutrient dynamics that contributes critical seasonal information for biogeochemical models. Collectively, the research in this thesis demonstrates the importance of understanding not only phytoplankton abundance, but also its community composition and how small- and large-scale changes in the chemical environment, including the availability of trace metals, can influence phytoplankton dynamics.
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    An investigation of the molecular-level mineral chemistry of metal-bearing pyrite and its electrochemical behaviour under flotation related conditions
    (Stellenbosch : Stellenbosch University, 2023-03) Babedi, Lebogang; Von der Heyden, Bjorn; Tadie, M.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Pyrite (FeS₂) is an iron disulphide mineral found in hydrothermal ore deposits, including gold ore deposits, each having its unique physicochemical conditions (e.g., pH, temperature, salinity) at the emplacement site. Pyrite can incorporate several trace elements into lattice sites, making it an n- or p-type semiconductor in nature. Due to this semiconducting variance, numerous investigations have observed diverse flotation responses for pyrite from different ore sources. This study investigates how lattice-incorporated metals (As, Au, Co, and Ni) affect electronic structure and flotation collector (xanthate) response in an alkaline media. This is done by compiling a global dataset of pyrite trace element data to understand its trace element signatures and then utilizing these signatures to guide chemical vapour transport synthesis of high-purity crystals. This work uses X-ray photoelectron spectroscopy and rest potential analysis to investigate the impact of metal geochemical type and concentration on pyrite valence bands and reactivity. Valence band assessments demonstrate that metals in the pyrite lattice shift orbital contributions and Fermi levels depending on geochemical origin and concentration. The metal's nature dictates whether it effects valence band contributions around the Fermi level (Au, Co, Ni) or deeper ones (As). Pyrite's changing chemistry affects its oxidation and interaction with xanthate collector under alkaline conditions. Pyrites (pure, Co, Ni, and Au + Co-bearing) are noble and do not induce mineral surface oxidation, while As- bearing pyrite is the least noble and promotes oxidation. Noble pyrites (pure, Co, and Ni) associated with n-type semiconducting have a weaker collector interaction than the least noble (As-bearing) associated with p-type. The lack of dixanthogen on As-bearing pyrite compared to Co- and Ni-bearing pyrite shows that a greater collector-mineral interaction does not oxidize the collector on the mineral surface. Dixanthogen is present at low Ni concentrations but absent at higher concentrations, showing that collector oxidation on the mineral surface depends on metal concentration. Reactivity and electronic structural trends are correlated. The potency of collector-mineral interactions and the size of Fermi level variations as a function of metal concentration and geochemical nature are comparable. This thesis shows how synthetic minerals grown experimentally may answer important questions regarding the molecular chemistry and reactivity of sulphide minerals like pyrite. This study shows how metals impact pyrite's valence band contributions and how they affect the collector interaction. The behavior of pyrite with a xanthate collector gives important knowledge that may be used to adjust flotation collectors to best show minerals' selectivity and reactivity independent of their semiconducting qualities controlled by chemistry.
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    Magmatic and metamorphic evolution of the Stolzburg Block, Barberton Granitoid-Greenstone Terrain
    (Stellenbosch : Stellenbosch University, 2022-12) Muhlberg, Moritz; Stevens, Gary; Moyen, Jean-Francios; Kisters, Alex; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The processes that formed and shaped the first stable continents in the Archean are poorly constrained for various reasons, including the scarcity of well-preserved, unaltered rock sequences and the lack of modern equivalents of lithologies that are typical for the Archean. This has led to the emergence of various geodynamic models that try to explain the unique features of the Archean crust, but too often rely on assumptions on the thermal state of the crust, in particular of the felsic crust. This thesis is an investigation of the igneous and metamorphic history of the Stolzburg Block, an area comprised of tonalite-trondhjemite-granodiorite (TTG) and amphibolite-facies supracrustal rocks, located in the Barberton Granitoid Greenstone Terrain (BGGT) in South Africa. The thermal history of a well-preserved piece of felsic Archean crust is studied through a combination of field work and geochemical, geochronological and petrological analyses on TTG and related rocks, with the aim of providing robust constraints on the thermal state of the Archean crust to improve geodynamic models. Uranium-lead (U-Pb) thermochronology on apatite grains has revealed four distinct groups of 207Pb/206Pb ages that are present throughout the Stolzburg Block. The three oldest groups of U-Pb apatite ages overlap with the time of emplacement of TTG plutons (~3450 Ma), of regional metamorphism (~3230-3200 Ma) and of intrusion of granitic plutons (~3105 Ma), while the ~2820 Ma group indicates a previously undiscovered thermal event. The preservation of the ~3450 Ma apatite ages indicates that the Stolzburg Block has not been heated above ~400- 500 °C after the emplacement of TTG plutons – at least not for a prolonged period. Trace-element and strontium isotope analyses of the dated apatite grains show homogenous compositions regardless of age and 87Sr/86Sr ratios that are within uncertainty identical to the initial 87Sr/86Sr ratio of the respective bulk rock, and these features are being interpreted as primary igneous signatures. This indicates that the different groups of U-Pb apatite ages are not the result of new growth but rather of partial resetting of the U-Pb systematics through heating. Zirconium-in-titanite and titanium-in-zircon thermometry gives temperatures of ~700 °C for magmatic titanite and ~650 °C for metamorphic titanite and zircon from TTG samples, which is in the same range as the reported metamorphic conditions recorded by the associated supracrustal rocks at ~3230-3200 Ma. While no chronological constraints are available for the temperature estimates of the TTG rocks, a shared history between TTG plutons and greenstones can be inferred from the intrusive relationship between the two. The preservation of ~3450 Ma U-Pb signatures in apatite grains shows that, regardless of the timing of heating to ~650 °C, the heating was very short-lived (< 1 million years). This work demonstrates that the felsic Archean crust – at least the section preserved in the BGGT – was relatively cool and stable, and that any heating of the TTG plutons was short- lived. The results of this study argue against a prolonged radiogenic heating of TTG crust and a partial convective overturn of the Archean crust, as proposed by some geodynamic models.
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    Understanding Southern Ocean phytoplankton ecophysiological response to iron availability
    (Stellenbosch : Stellenbosch University, 2022-04) Singh, Asmita; Ryan-Keogh, Thomas; Fietz, Susanne; Thomalla, Sandy; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Over the past century, climate change has been of growing concern, due to its ecological and economic global impact. The Southern Ocean buffers the impacts of climate change by accounting for a significant proportion of the total oceanic uptake of CO2. Phytoplankton primary production and carbon export (the biological carbon pump) plays an important role in the Southern Ocean carbon cycle and despite their ability to remove a significant amount of the global organic carbon flux each year, it is often constrained by the availability of light and nutrients. The micronutrient iron is particularly important in the production of key metabolic and photosynthetic proteins in phytoplankton and its scarcity in the Southern Ocean dictates its role as a key driver of variability in Southern Ocean productivity. A greater understanding of the response of Southern Ocean phytoplankton to seasonal and regional variability to their environmental drivers (with a focus on iron in particular) is thus required in order to increase the accuracy in assessing and predicting the impact of climate change. Active chlorophyll-a fluorescence is a non-invasive, powerful instantaneous tool, which can assess the phytoplankton photosynthetic efficiency in response to potential environmental drivers and in particular under stressful growth conditions, i.e. under iron limitation. However, our understanding of the seasonal cycle of photophysiological responses of phytoplankton to iron and other biogeochemical drivers remains limited, primarily due to the prevalence of experiments and measurements only being conducted in summer. In this research, three individual studies of phytoplankton photophysiology across all seasons of the Atlantic Southern Ocean were used to investigate physical and biogeochemical drivers of inter-zonal, inter-annual and intra-seasonal variability in phytoplankton photophysiology (Fv/Fm) in summer. Results from this study point to a combination of drivers (notably sea surface temperature, macronutrients and community structure) that elicit simultaneous and oftentimes antagonist responses in Fv/Fm, making it difficult to ascertain one dominant driver over another. In addition, the degree of iron stress in the Sea-Ice Zone in autumn, and the full zonal extent of the open Atlantic Southern Ocean in spring and winter, was determined using a series of short-term (24 hrs) in situ iron addition experiments. Key results suggest that phytoplankton in the Sea-Ice Zone of Dronning Maud Land are not iron-limited in autumn, and presumably have a sufficient year-round supply of iron potentially from shallow topography. However, both winter and spring showed some positive responses to iron addition with zonal variability being linked to the timing of the incubation experiments relative to the seasonal cycle of the mixed layer depth, highlighting the importance of convective overturning as a dominant seasonal iron supply mechanism. The research presented in this thesis contributes to a greater understanding of the complex interplay of multiple drivers of phytoplankton photosynthesis across the seasonal cycle. It is recommended that future research continues to address seasonal and regional variability in phytoplankton photophysiology but with a focus on resolving the relationship between multiple drivers in line with anticipated climate-mediated adjustments in environmental conditions.