- ItemConstraints on the size and temperature of magma batches that constructed the Peninsula pluton, based on the metamorphic record of metasedimentary xenoliths(Stellenbosch : Stellenbosch University, 2023-03) Muller, Jared; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: This study aimed to investigate the metamorphic histories of nine greenschist facies xenolith samples from the Malmesbury Group metasediments, which have an estimated age of deposition of approximately 575 to 550 Ma. These samples are hosted in the Peninsula Pluton granitoids (537.8 ± 1.6 Ma to 532.7 ± 1.9 Ma) of the Cape Granite Suite. The goal of the study was to put constraints on the size and temperature of the magma batches that formed the Peninsula Pluton granitoids. To achieve this, various techniques including petrography, field evidence, scanning electron microscopy, phase equilibria modelling using the freeware "Rcrust" and thermal modelling using the freeware "Energy2D" were employed to establish peak metamorphic mineral assemblages and constrain peak metamorphic conditions. The peak metamorphic assemblages for the xenoliths were determined to be quartz + biotite + plagioclase + cordierite ± K-feldspar ± ilmenite ± muscovite. The results of the bulk rock and mineral composition analyses revealed that the metasedimentary xenoliths of this study plot within a normal range of the Malmesbury group sediments and metasediments. Temperatures of formation for the magma batches that constructed the Peninsula Pluton have been noted to be between 800 and 900°C. Ti-in-biotite thermometry was used to constrain the temperature of peak metamorphism for the xenoliths of this study to be in the range of 665 to 725°C. Phase equilibria modelling revealed that the different xenoliths experienced peak temperatures between 530 and 730°C and pressures between 0 and 4.5 kbar. This has important implications for background magma temperature in the pluton and the size and temperature of the magma batches that entrained the xenoliths. Obviously if the magma arrived in the upper crust at ~ 700°C, batches could conceivably be any size. However, there is strong evidence for the Peninsula Pluton magmas having arisen by fluid absent melting involving biotite at >850°C. Two-dimensional conductive thermal modelling was used to explore the ranges of background magma temperature and magma batch sizes and temperatures that would be consistent with the xenoliths recording only amphibolite facies temperatures. The results indicate that the background magma temperature cannot be higher than the maximum temperatures recorded by the xenoliths. Additionally, if the magma batches that sampled the xenoliths and introduced them into the pluton were at 850°C, they could not have been thicker than 1m if intruded as 10m wide sills. Similarly, magma batches arriving at 750°C could not have been thicker than 2m if intruded as 10m wide sills. These size and temperature estimates contribute to the body of evidence supporting the incremental growth model of granitic plutons, as well as providing insight into the size and temperature of magma batches that were assimilated to form the Peninsula Pluton granitoids. The assimilation of these batches, as well as their frequency, can be better understood considering these data.
- ItemEvaluating groundwater storage changes under drought in the Western Cape using satellite and in-situ data(Stellenbosch : Stellenbosch University, 2023-03) Nenweli, Ritshidze; Chow, Reynold; Watson, Andrew; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: The Western Cape is a water scarce province of South Africa and has become more reliant on groundwater over the last few decades due to frequent droughts, which are anticipated to worsen in the future. While data on surface water use and storage levels in reservoirs are easily accessible throughout the year, the impact on groundwater storage at the provincial scale has not been evaluated yet. However, lack of monitoring boreholes, discontinuous measurements, unavailable monitoring data, and installation of boreholes without proper licensing has made it difficult to manage groundwater sustainably in the region. Remote sensing from National Aeronautics and Space Administration’s Gravity Recovery and Climate Experiment (GRACE) provides continuous and freely accessible datasets through which trends and variations in groundwater storage can be estimated. This research aims to understand how GRACE and Global Land Data Assimilation System (GLDAS) data can be used to monitor groundwater storage in the Western Cape. In-situ groundwater storage declined prominently in the Western Cape during droughts (2003-2006, 2009-2011, 2015-2018 and 2017-2019). While some aquifers are recovering, others (e.g., Vanhynsdorp Aquifer) are still declining, possibly from over-abstraction for irrigation. GRACE TWS (JPL-M) anomalies showed positive moderate correlation (r = 0.69) with in-situ groundwater measurements from the Adelaide Subgroup Aquifer possibly because of the unit’s large areal extent, unconfined nature, and large groundwater storage fluctuations. While the Table Mountain Group Upper Aquifer Unit (TMG UAU) and Cape Flats Aquifer both showed significant positive correlations with GLDAS CLSM (catchment land surface model) groundwater storage of 0.83 and 0.73, respectively; possibly because they are unconfined and their fast response to precipitation. GRACE TWS anomalies have the potential to monitor groundwater storage of the Adelaide Subgroup Aquifer, while GLDAS CLSM groundwater storage data has the potential to monitor groundwater storage of the unconfined TMG UAU and Cape Flats Aquifer. GRACE and GLDAS CLSM groundwater storage data have the potential to monitor groundwater in other parts of South Africa or data scarce regions of Africa to ensure that future generations have access to groundwater.
- ItemImpact of trace metals (Cu, Cd, Zn) on phytoplankton in open and coastal oceans: a Southern African perspective(Stellenbosch : Stellenbosch University, 2023-03) Wium, Emtia; Fietz, Susanne; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: This study reflects on the understanding of marine trace metal concentrations and their impacts on growth of marine phytoplankton. The aim was to better understand the oceanic concentrations of copper (Cu), cadmium (Cd) and zinc (Zn) and their potential toxicity effects on phytoplankton in general, and the ubiquitous Nannochloropsis sp. The data for Paper 1 was obtained from previously published literature and focussed on (1) oceanic Cu, Cd and Zn concentrations worldwide, and in seawater near Southern Africa, (2) toxicity threshold concentrations of Cu and Cd for marine phytoplankton, and (3) threshold concentrations of Cu, Cd and Zn for phytoplankton used in incubation studies, and were used to determine the risk of metal toxicity in oceans around Southern Africa, at less than or equal to 200m water depth. Paper 1 shows the concentrations for Cu, Cd and Zn in the coastal water around Southern Africa, at aforementioned depths, were 1.1 – 240 nM, 0.20 – 180 nM and 0.40 – 8300 nM, respectively. These concentrations vary worldwide, as metal concentrations in seawater are influenced by a number of factors, e.g., human pressure, distance to shore, water depth, resident biological community and chemical-physical interaction. Paper 1 showed a range of threshold concentrations of Cu, Cd, and Zn on phytoplankton species and revealed that picoeukaryotes, nanoplankton, cyanobacteria, dinoflagellates and diatoms may be at risk of trace metal toxicity in coastal seawater around Southern Africa. Paper 1 showed the need for further studies and easily accessible data to better understand the impact of high Cu, Cd and Zn concentrations on marine phytoplankton. Therefore, in Paper 2, a key phytoplankton in the marine realm was selected, the ubiquitous Nannochloropsis sp. Paper 1 furthermore served to inform experiments of Paper 2, where 0 – 9.0 μM Cu, 0 – 5.5 μM Cd and 0 – 20 μM Zn were investigated. The parameters tested on Nannochloropsis sp. in the incubation experiments were dry weight biomass, chlorophyll-a (chl-a) concentration, chl-a content, density of single living cells, percent contributing dead cells, growth rate of single living cells, and trace metal and nitrate change in the solution. It was established that 0.94 – 1.7 μM Cu caused decline in chl-a concentrations and chl-a content, while increasing the dead cell contribution. A range of 4.0 – 5.5 μM Cd decreased the biomass. The Zn range of 4 – 11 μM decreased the chl-a concentration, while 11 – 20 μM Zn decreased the biomass, single cell density and growth rate. Combining the results of Paper 1 and Paper 2, it was established that Nannochloropsis sp. may face toxicity risks in the coastal oceans around Southern Africa, especially if future concentrations increase. This study therefore highlighted the importance of trace metal monitoring and in maintaining phytoplankton growth in the marine environments.
- ItemApplying phase equilibria modelling to igneous systems by coupling trace element partitioning and accessory phase saturation to compositionally variable thermodynamic modelling in Rcrust(Stellenbosch : Stellenbosch University, 2023-03) Hoffman, Sean; Mayne, Matthew Jason; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: Phase equilibria modelling techniques can constrain the equilibrium conditions of major rock forming minerals, however current thermodynamic databases and activity-composition models cannot accommodate minor and trace elements as chemical components in modelled systems. This thesis presents the integration of trace element partitioning routines between melt and solid phases in the thermodynamic modelling tool Rcrust in order to effectively estimate the trace element composition of stable phases when melt is present. Additionally, trace elements occur in large abundances in certain accessory phases, for which we do not currently have activity-composition models. These trace element-rich accessory phases may have significant influence on the trace element chemistry of a magmatic system. For this constraint, accessory phase saturation routines for apatite and monazite, two commonly occurring accessory phases in granitic rocks, are newly integrated in this thesis to the existing Rcrust modelling tool. This provides a novel methodology for approximating trace element distribution among thermodynamically constrained phases with thermodynamically unconstrained accessory phases, apatite and monazite. The results highlight the importance of major element components that also contribute to accessory phase formation, such as calcium in apatite (which can accommodate a non-trivial quantity of the available calcium) and the resultant effect on phase equilibria of major phases. Saturating apatite can alter phase equilibria, mainly for calcium-bearing phases, that is equivalent to up to 20 °C change in temperature. A case study of the Peninsula Pluton granodiorite, South Africa, is used to validate the use of accessory phase saturation and trace element partitioning with phase equilibria modelling. The model predicts LREE concentrations of apatite and monazite that match the natural sample at temperatures below 725 °C, corroborating previous findings for emplacement P-T constraints. However, phase assemblages predicted through phase equilibria modelling show that the rock equilibrated to P-T below the emplacement estimates of previous studies. By modelling both major and accessory phases, constraints are provided on the formation and crystallisation of crustal magmas which has important implications on geothermobarometry and thermochronology in the crust.
- ItemFluorescence detection of trace aluminium using a sequential injection analyser(Stellenbosch : Stellenbosch University, 2022-12) De Jongh, Tara; Roychoudhury, Alakendra N.; Samanta, Saumik; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: Analytical instruments are either expensive to operate, prone to interferences, require a separate offline preconcentration step, or are not suitable for use onboard a ship when measuring dissolved Aluminium (Ald; < 0.2 µm fraction) in seawater. Thus far, Ald measurements have been performed most commonly using flow injection analysis (FIA), as it combines preconcentration, fluorometry and spectroscopy in a single manifold. Despite FIA’s compact and comprehensive design, the manifold has several drawbacks, including its laminar flow regime and the peristaltic pump, which causes tube stretching and generates measurement drift. As a result of these limitations, a third-generation flow analyser was developed, known as a miniaturized two-line sequential injection analyser (miniSIA-2). The miniSIA-2 incorporates similar design features to that of FIA, however, it operates on a bidirectional flow regime, enabling reverse fluid flow and encourages turbulence to allow complete sample and reagents reaction, while reducing the reagent consumption and waste generation. Thus far, the miniSIA- 2 has only been used to measure trace concentrations of Zinc (Zn) (Grand et al., 2011; Grand et al., 2016) and Iron (Fe) (Oliveira et al., 2015; Hatta et al., 2018) in seawater. This project therefore involves a novel and systematic approach to measuring Ald in seawater through fluorescence detection by an advanced miniSIA-2 manifold of Grand et al., (2016). The systematic approach involved integrating the experimental procedures outlined by Hatta et al., (2018) and the experimental protocols of Grand et al., (2011 & 2016) first and second design of Znd for measurements of Ald. This led to a method approach initiated with optimizing the batch method, followed by the Stop in Holding coil (SHC) procedure and finally incorporating a Solid Phase extraction (SPE) step with the SHC to ensure a fully automated instrument for measuring Ald in seawater. However, this project focused on developing and applying the well- established batch method from Hydes & Liss, (1976). The method details the development of a two-step method. Initially, a basic analytical setup was established by systematically adjusting parameters to achieve measurements at relatively higher Ald concentrations. During this step, a limit of detection (LOD = 9 nM) was achieved. The above-mentioned analytical setup was then used to achieve instrumental detection limits similar to the lowest Ald concentrations measured in global oceans (0.1 nM). In this second instant a lower LOD (7 nM) was achieved, however, the improvement was much less than the initial aim. This was the result of the instruments fluctuating baseline values, high Relative Standard Deviation (RSD), and the manifold’s inability to self-regulate internal temperatures during analysis time causing a weak peak configuration that resembled that of photobleaching and quenching.