School of Earth Sciences - Theses
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Further development and application of the speleothem U-Pb geochronometer
Speleothems are valuable archives recording information on past climates and processes of landscape evolution. In fact, the high preservation potential of speleothems and their ubiquity across the globe are key attributes and speleothems are now extensively used in palaeoclimate modelling as complements to the more traditional marine and ice core climate records. The real ’engine’ of speleothems science, however, lies in the ability to reliably and robustly place these archives into chronological context. Historically this has been facilitated by the highly successful U-Th geochronometer, which can accurately date speleothems that formed within the last 650 thousand years. However, many speleothems – and by extension their proxy archives – are far older than this 650 ka limit and so require a significantly longer-lived radiometric methodology: the U-Pb geochronometer has recently filled this void. The U-Pb geochronometer is well known in the geological sciences, but its application to the analytically-challenging medium of speleothems has been relatively recent. Modern mass spectrometry methods now allow the accurate and precise analysis of the extremely low-levels of U and Pb within speleothems, thus providing access to such ’deep-time’ (i.e. multi-million years and further) palaeoclimatic and landscape evolution archives. Despite these significant advances, there remain fundamental impediments to the whole-scale production of speleothem U-Pb ages. This thesis investigates some of these shortfalls with the ultimate aim of advancing the speleothem U-Pb methodology and application such that future speleothem-based studies may more rapidly and more reliably place their reconstruction models into a more robust chronology. The first half of this thesis addresses two fundamental limitations of the speleothem U-Pb method: the large expenditure of time and effort required for producing calcite U-Pb ages, and the potential limitations in the correction for initial isotopic disequilibrium effects in the U-Pb decay chain. To address the former, I provide new software for the rapid production of publication-quality U-Pb isochron figures and ages. Additionally, a new chemical separation (i.e. chromatography) procedure for generating high-purity U and Pb fractions is developed. This new ’stacked resin’ protocol can produce mass spectrometry-ready U and Pb sample aliquots in a single working day – roughly half the time of the previous method – and so accelerates the acquisition of U-Pb ages. In terms of subsequent data deconvolution, the largest hurdle to obtaining accurate speleothem U-Pb ages arguably relates to the uncertainty added from having to estimate an initial isotopic disequilibrium value after the system has returned to equilibrium. As a potential remedy to this problem, this research program develops a framework in which to calculate appropriate disequilibria values for the production of disequilibrium-corrected U-Pb ages. The second part of this thesis is designed to highlight new applications of the calcite U-Pb chronometer made possible by the methodological advances documented in the first half. This research program investigates the utility of speleothems as proxies for regional uplift rates in karst terranes. This is accomplished by producing chronologies for 120 speleothems from the Buchan karst along southeast Australia’s passive margin. The results of this study indicate that SE Australia experienced a renewed period of uplift occurring at a maximum rate of 76 +/- 7 m / Ma beginning at least 3.5 million years ago. This speleothem-derived uplift rate is consistent with other independent regional uplift estimates thus adding credibility to the utility of speleothems as proxies for uplift. The relatively straight-forward method developed here is likely applicable in many future karst studies. Additionally, this research program also investigated the potential applicability of U-Pb geochronology to another form of carbonate archive – stromatolites. The methodological advances described in this thesis were applied to stromatolites from Lake Turkana, Kenya, and the palaeo-lake Bungunnia in southern Australia. The results of this experiment suggest that stromatolites contain too much inherited Pb that simply overwhelms the radiogenic ’age signal’ and so are unlikely to be successful candidates for U-Pb dating, at least for stromatolites formed during much of the Cenozoic period.
The structure, rheology, and rupture mechanics of seismogenic faulting in continental lithosphere
This thesis explores relationships between the structure, rheology, and rupture properties of seismogenic faulting in continental lithosphere from four different perspectives. First, I derive a scaling relationship that links the spacing between two nearly parallel strike-slip faults to the frictional strength, fault width, and lower crust viscosity for strike-slip shear zones. Based on the scaling law, I estimate a possible range of lower crust viscosity in the San Andreas Fault system (California), the Marlborough Fault Zone (New Zealand) and central Tibet (China). Second, using numerical modelling methods, I explore how lower crust rheology contrast may affect the long-term evolution of a major plate boundary fault. The case study is based on the San Andreas Fault, which is found to vary dipping angles (~50-90 degree) along strike. The moderately dipping strike-slip fault is not consistent with Anderson faulting theory. This inconsistency may be reconciled if there are lateral variations in the lower crustal rheology across the fault plane that decrease fault dip with time. Third, in addition to strike-slip faults in active tectonic settings, my study also includes reverse faults in stable cratonic regions, especially for the cratonic areas of western and southern Australia. I apply statistical methods to investigate the co-seismic slip distribution of 11 surface-rupturing earthquakes in Australian stable regions and provide a link between the shape and characteristics of co-seismic slip distributions and the geophysical properties of the host crust. Fourth, using a comprehensive geophysical survey with the co-located 13GA-EG1 and 12GA-AF3 seismic reflection profiles and magnetotelluric profiles and regional gravity and magnetic maps in the Nullarbor Plain (Australia), I find that faults initiated back to the Proterozoic could still be reactivated in a Cenozoic convergent setting, especially for those major faults cutting to deep crust. Those deep-penetrating faults at terrane boundaries could be a potential channel for fluids to pass through, and thus further weakened by the fluids, which is revealed by high-conductivity anomaly in magnetotelluric profiles. The last research chapter of this thesis addresses a technical issue in the particle-in-cell finite element method, which is widely used in geodynamic numerical modelling. As mixing materials with contrasting viscosity within one element results in stress fluctuations, I assess different smoothing methods to reduce the spurious stress in mixed-material elements.
A taxonomic revision of the tertiary echinoid genus monostychia
For over 100 years the genus Monostychia (Echinoidea: Clypeasteroida) and its type species M. australis Laube, 1869 have been a taxonomic home for a wide range of genera and species with the commonality of a rounded to pentagonal, discoidal test and a submarginal periproct. The specimens comprising this group are all extinct and from the Tertiary strata of southern Australia. While there have been a few minor species identified beyond M. australis, notably M. etheridgei Woods, 1877 and P. loveni (Duncan, 1877), it has been clear to many researchers that the variability remaining in M. australis was representative of numerous other taxa awaiting discovery. Recent taxonomic works on the Clypeasteroida suggested that the number of interambulacral plates on the oral surface of the test of some species was a useful diagnostic character. Of interest were the plates that first come into contact with the periproct. However, there appeared little evidence in the literature that it had been established that the number of such plates remained constant with test length and age, or that the variability in each taxon, of those plate numbers, has been determined. Without understanding those two issues the utility of plate numbers was questionable. This study set out to resolve some of those issues for Monostychia and its relatives. It was found that the number of interambulacral and ambulacral plates on the oral surface was fixed and did not change with increasing test length and therefore there was potential utility for plate numbers as a taxonomic tool. However, there was substantial variability in the numbers. As a result, the use of plate numbers in the paired interambulacra, paired ambulacra, and ambulacrum III on the oral surface appears to have limited utility at genus level. At the species level, however, such numbers can be quite useful diagnostically, particularly when paired interambulacral, paired ambulacral and ambulacrum III plate numbers are used in combination. The plates that first come into contact with the periproct was shown to have little value taxonomically at the genus or species level within the monostychioids, largely because most species had the same plate number dominating, but also because of the variability. At subfamily level the taxonomic value of this feature is yet to be established. A previously unreported structural feature was identified in many of the specimens. This was a thin circumferential wall of stereom present on the right-hand side of the test, lying half way between the marginal and central buttressing. It was a form of intestinal buttressing referred to hereafter as the intermurum. Its presence enabled the establishment of a new subfamily, Monostychinae in the family Arachnoididae. Four genera have been placed in the Monostychinae; Monostychia Laube, 1869, Quinquestychia gen. nov., Rotundastychia gen. nov. and Deltoidstychia gen. nov. A key to these genera is provided. Several species have been established and others redescribed in this study. In Monostychia there are seven species; M. australis Laube, 1869, M. macnamarai Sadler et al. 2017, M. alanrixi Sadler et al. 2017, M. merrimanensis Sadler et al. 2019, M. etheridgei Woods, 1877, M. glenelgensis Sadler et al. 2019 and M. robheathi sp. nov. A new genus, Quinquestychia gen.nov., has also been established. It contains four species: Q. mannumensis sp. nov., Q. gigas sp. nov., Q. berylmorrisae sp. nov. and Q. robertirwini (Sadler et al. 2017). The last of these species was published as a Monostychia earlier in this study but reassigned later on the basis of further data. A second new genus, Rotundastychia gen. nov., has also been established. It contains three species: R. pledgei sp. nov., R. aquilaensis sp. nov. and R. eyriei sp. nov. A third new genus, Deltoidstychia gen. nov., has also been established. It currently contains a single species, D. erioaster sp. nov. In addition to the above, two other new genera were established but they do not belong in the subfamily Monostychinae. Instead they are tentatively placed in the subfamily Ammotrophinae. The first of these is Obscurostychia gen. nov. with two new species: O. spirographica sp. nov. and O. curtus sp. nov. Keys to all the genera discussed above that contain more than one species have been provided.
Radiocarbon dating of Kimberly rock art
Throughout the world, ancient rock art records some of the earliest attempts at complex human communication. However, constraining the age of older rock art has remained a largely intractable scientific problem thereby limiting our ability to integrate rock art into the rest of the archaeological record. Researchers have studied the globally significant Aboriginal rock art in the Kimberley region of Western Australia for more than 40 years and have comprehensively documented a sequence of rock art stylistic periods. It has long been thought that the oldest styles in this sequence date back to the Pleistocene but only two such dates, relating to identifiable motifs, have been published and both are problematic. The surviving pigment in paintings of all but the most recent Kimberley rock art style contains no material that can be radiometrically dated. There are, however, mineral accretions and mud wasp nests in the same Kimberley rock shelters that house rock art and, occasionally, these under or overlie paintings. This study explores the development of radiocarbon dating techniques to reliably date remnant mud wasp nests found to be in contact with rock art. Recently constructed mud wasp nests were collected and analysed to understand the source and age of carbon-bearing material they contain. Unburned plant material and charcoal were found in similar volumes, but charcoal is the carbon-bearing constituent most likely to provide a reliable radiocarbon age for old nests. Old wasp nests were analysed using a wide range of techniques to determine how taphonomic processes alter their physical and chemical composition. These results guided experimentation with pretreatment methods designed to remove sources of carbon contamination while preserving as much of the carbon in the original charcoal as possible. A total of 120 old mud wasp nests were prepared for radiocarbon dating of which 75 contained sufficient carbon for measurement. The distribution of the 75 ages indicated nests were built quasi-continuously over, at least, the last 20,000 years. The motifs in contact with the 75 nests were classified into one of the six main Kimberley rock art stylistic periods by two subject matter experts. Just 3 nests overlay motifs from each of the Cupules and Wanjina periods suggesting only that some motifs in these styles are older than 7,200 years and 500 years, respectively. The 4 dates available for each of the Static Polychrome and Painted Hand periods permit a very tentative hypothesis for their chronology while the 16 dates relating to Irregular Infill Animal Period (IIAP) motifs and the 20 dates for Gwion motifs provide a more secure estimate. The concise hypothesis proposed for the chronology of the Kimberley rock art styles is that the IIAP style was in use from at least 17,000 to 13,000 years ago. It was followed by the Gwion period from 13,000 – 12, 000 years ago and then the Static Polychrome period 11,000 to 9,000 years ago. The Painted Hand period followed at around 8,500 to 9,000 years ago.
Sub-meter scale lithological heterogeneity and its influence on the CO2 trapping capacity in CO2 storage reservoirs
Lithological heterogeneity at mm- to cm-scale exists in the form of sedimentary structures such as cross bedding and planar bedding in heterogeneous siliciclastic reservoirs. At this scale, sedimentary structures typically consist of two lithologies with significant differences in their porosity, permeability, capillary entry pressure and mineral composition. The lithology with high porosity, high permeability, low capillary entry pressure and a high abundance of quartz and feldspar characterizes the reservoir rock and is conducive to fluid flow. On the other hand, the lithology with low porosity, low permeability, high capillary entry pressure and a high abundance of clay and carbonate minerals characterizes the intraformational baffle and acts as flow barrier. The presence of both lithologies at mm- to cm-scale govern local fluid flow and geochemical reactions. This is especially important in CO2 storage reservoirs where CO2 flow and mineralization might be significantly affected. Hence, it is necessary to account for lithological heterogeneity at sub-meter scale in regional scale dynamic simulations. However, intraformational baffles are typically neglected in static reservoir models primarily because small scale intraformational baffles are not commonly resolved by wireline logs. Therefore, their lithological properties are not accounted for in reservoir models. This study addresses the characterization and incorporation of intraformational baffles in reservoir models so that the reservoir scale estimation of carbon mineral trapping capacity can be improved. Firstly, rocks from the CO2CRC’s Otway Research Facility were characterised using a range of sample and data analysis methods. The variability in porosity, permeability, capillary entry pressure, grain size and mineral composition was used to classify five homogeneous rock type classes: coarse sandstone, fine sandstone, siltstone, mudstone and carbonate-cemented sandstone. A new workflow was developed where homogeneous rock types and composite rock types consisting of two lithologies characteristic of intraformational baffles were represented. Rock properties for composite rock types were derived so that mm-scale heterogeneity can be upscaled for integration in coarser discretised reservoir models. Lithotype logs of composite rock types were derived and used to build a high resolution 2D static reservoir model of the site. Multiphase fluid flow and reactive transport simulations were run on 120 and 8 realizations, respectively, to develop a detailed understanding of CO2 flow and geochemical reactions at mm-scale. It was found that the capillary entry pressure characteristic of the reservoir rock significantly governed the range of turning point saturations while the amount of clay and carbonate content of the baffle governed the amount of carbon mineralized. Reactive transport simulations were also run on two 2D reservoir models to quantify the error associated with the estimation of carbon mineral trapping at reservoir scales if intraformational baffles are not accounted for in dynamic simulations. The results show that the neglection of intraformational baffles in reservoir rocks might lead to an underestimation of mineral trapping capacity by nearly 2.5 times.
The relationship between climate and mechanisms of tropical cyclone formation
A major uncertainty in future projections of tropical cyclone (TC) frequency is due to inadequate understanding of the atmospheric mechanisms leading to a reduction in the TC formation in warmer climates. Although the recently proposed “marsupial pouch theory,” of TC formation indicates that a semi-enclosed recirculating region known as a “pouch” within large-scale disturbances provides necessary conditions for TC formation, it is important to link the frequency of the pouch environments to the evolving climate conditions. Therefore, this thesis examines the changes in the marsupial pouch TC formation environments and their relationship to the large-scale environmental conditions in the current climate and idealized warmer climates. Here we use ERA-interim reanalysis data and high-resolution Australian Community Climate and Earth-System Simulator (ACCESS) climate model simulations of the current climate and idealized warmer climates (aqua-planet simulations) with two fundamentally different TC tracking schemes. The first scheme is the Okubo-Weiss Zeta Parameter (OWZP), a phenomenon-based tracking scheme that detects TC-favorable locations within marsupial pouches using resolution-independent thresholds. The other scheme is the Commonwealth Scientific and Industrial Research Organization (CSIRO), a traditional TC tracking scheme that uses resolution-dependent thresholds. Environmental and structural composite analysis of developing and non-developing tropical depressions (TDs) 48 hours before TD formation, which are detected using the OWZP scheme in the reanalysis, showed that developing circulations have a strong protective layer around the core from the lower to mid troposphere that protects it from external disruptive influences. The relative importance of the environmental variables influencing tropical storm (TS) formation varies across ocean basins due to differences in the large-scale disturbances and surrounding environmental conditions. Statistical TS prediction schemes are also developed using the environmental conditions of developing and non-developing TDs. This analysis notes that random forests and support vector machine algorithms have higher accuracy than decision trees (DTs). Additionally, the hybrid approach of DTs and Markov decision process is proposed which indicates that developing TDs have a higher likelihood of remaining in more favorable environmental conditions than non-developing TDs. In current climate simulation using the climate model, the TCs detected within marsupial pouches are a subset of the traditionally detected TCs. Also, the OWZP scheme performs better in representing the TC frequency statistics and has stronger relationships with the surrounding environmental conditions in the current climate than the CSIRO scheme. In idealized aquaplanet model simulations, we observe both reduced marsupial pouch environments and traditionally detected TCs with increasing sea surface temperatures (SSTs). The increased saturation deficit and increased stability explain the reduction in the frequency of formations with increased SST. The present study also notes a decrease in the frequency of low-intensity storms and an increase in the frequency of intense storms with increasing SSTs. In a drier and more stable atmosphere, the initial vortices need to be stronger for TC formation to occur, with higher low-deformation vorticity and higher upward mass flux. This research indicates that the marsupial pouch theory may be a fundamental paradigm of TC formation due to its better relationships with the large-scale environmental conditions in different climates.
Modelling the atmospheric influence of coral reef-derived dimethyl sulfide
Dimethyl sulfide (DMS) is a naturally occurring aerosol precursor gas which plays an important role in the global sulfur budget, aerosol formation and climate. While DMS is produced predominantly by phytoplankton, recent observational literature has suggested that corals and their symbionts produce a significant amount of DMS, which is currently unaccounted for in modelling studies. It has further been hypothesised that the coral reef source of DMS may modulate the climate. In this thesis, two atmospheric models coupled to online chemistry and aerosol schemes were used for the first time to explore the influence of coral reef-derived DMS on atmospheric composition and meteorology across temporal and spatial scales. A simple non-varying representation of coral reef-derived DMS was developed and added to a common DMS surface water climatology. By comparing the differences between simulations with and without coral reef-derived DMS, the role of coral reef-derived DMS was quantified. The Australian Community Climate Earth System Simulator coupled to the United Kingdom Chemistry and Aerosol model (ACCESS-UKCA) was used to quantify the influence of coral reefs at the global scale. ACCESS-UKCA was evaluated against satellite observations and other global climate models and the sensitivity of aerosol, clouds and radiation to large scale perturbations of DMS was tested. ACCESS-UKCA was found to have similar biases and DMS sensitivity compared to other models and it was estimated that marine DMS contributes 0.45K cooling to the present climate. The influence of coral reef-derived DMS on global to regional scale climate was then investigated. In the Maritime Continent-Australian region, where the highest density of coral reefs exist, a small decrease in nucleation and Aitken mode aerosol was found when coral reefs were removed from the system. However, these small responses were found to have no robust effect on global or regional climate. The Weather Research Forecast model coupled to the CBMZ-MOSAIC (Carbon Bond Mechanism Z - Model for Simulating Aerosol Interactions and Chemistry) chemistry-aerosol scheme (WRF-Chem) was then used to study the same question at higher spatial and temporal scales. WRF-Chem was run to coincide with an October 2016 field campaign over the Great Barrier Reef, Australia, against which the model was evaluated. After halving the DMS surface water climatology, the model performed well for DMS and sulfur processes, though aerosol number was overestimated. The inclusion of coral reef-derived DMS resulted in no compositional change in sulfate aerosol mass or total aerosol number. No direct or indirect aerosol effects were detected. Throughout this work, the complexities of the aerosol-climate system have been emphasised and the limitations of current modelling capabilities highlighted. In conclusion, while total marine DMS was found to have an important climatic influence, this thesis has found no robust link between coral reef-derived DMS and climate or weather. Thus, these results do not support hypotheses around the ability of coral reefs to modulate global or regional climate.
A study of atmospheric oxidation chemistry in Australasia using MAX-DOAS measurements
Oxidation reactions provide the fundamental mechanism for chemical cycling of atmospheric constituents. Understanding the key chemical and meteorological factors determining atmospheric oxidation chemistry has important implications for air pollution and climate change. With strong but isolated urban pollution sources and endemic plants known to emit high levels of volatile organic compounds (VOCs), Australasia is a fascinating place to study atmospheric oxidation chemistry in a range of remote, coastal and urban environments. To date however, Australia is vastly under represented in the observational atmospheric chemistry literature. In this thesis, the passive solar multi-axis differential optical absorption spectroscopic (MAX-DOAS) technique is used to study key molecules in the atmospheric oxidation cycle including nitrogen dioxide (NO2), formaldehyde (HCHO), nitrous acid (HONO), glyoxal (CHOCHO) and iodine monoxide (IO). In particular, the role of these molecules in forming hydroxyl radicals (OH) and ozone (O3), two key daytime tropospheric oxidants, is studied using urban measurements at Broadmeadows, Victoria and at Garden Island in Western Australia. In the first comprehensive study demonstrating the MAX-DOAS technique in Australasia, the technique is verified using a range of analysis sensitivity studies, inter-instrument comparison and validation against in-situ and remote sensing methods. This includes the first long term MAX-DOAS-satellite comparison in the Southern Hemisphere, where MAX-DOAS measurements were in excellent agreement with Tropospheric Monitoring Instrument (TROPOMI) results in Melbourne, Australia and at Lauder in New Zealand. The HONO observed at Broadmeadows was consistently at daytime concentrations exceeding what is expected given the known mechanisms. The maximum daytime HONO levels correlated with soil moisture levels indicating that soil-based emissions may play a role in the missing HONO source. The exponential dependence on temperature observed for HCHO at Broadmeadows suggests that the primary formaldehyde source there is oxidation of biogenic VOCs. In contrast, glyoxal appears to be more dependent on biomass burning or anthropogenic emissions. Chemical trajectory modelling studies at Garden Island suggest that isoprene oxidation is expected to be the dominant HCHO source, while anthropogenic emissions are expected to be the dominant CHOCHO source. At Broadmeadows, HONO photolysis was found to be the greatest boundary-layer OH source in all months, contributing on average between 45-50 % of OH production. HCHO and ozone (O3) were often present at sufficient levels in summer to be commensurate with HONO as surface OH producers. The vertical profiling capability of the MAX-DOAS technique showed that while HONO typically dominated the OH production close to the surface, ozone and formaldehyde photolysis were the dominant mechanisms at higher altitudes. At Garden Island, a background marine boundary layer level of IO was detected which did not appear to have a locally significant O3-destruction orOH-formation role, but nevertheless provide evidence of ubiquitous iodine chemistry in the marine boundary layer. Using the ratio of HCHO to NO2, significant VOC emissions on hot summer days (as indicated by high observed HCHO levels), were found to shift the ozone production regime from VOC-limited to NOx-limited. The ozone production regime was found to be mostly NOx-limited at Garden Island. This has important implications for air pollution, for while NO2 was not found in exceedance of guideline pollution levels, O3 smog could still be reduced on hot days by curbing nitrogen oxide emissions.
The nature and extent of reactions between CO2 - rich water and fractured continental flood basalts
Continental flood basalts have been considered as an unconventional reservoir for geological carbon storage where vesiculated basalt intervals serve as reservoirs and massive basalt zones as a caprock. However, the presence of fractures in the massive layer may lead to CO2 leakage. The objective of this study is to understand the reactions of CO2-saturated fluid on fractured basalt and the respective changes in the fracture and adjacent connected pore network geometry at an early stage of the evolving geochemical system. Batch reaction experiments, high-resolution sample characterisation, and modeling are used to better evaluate basalts as CO2 storage reservoirs. The nature and extent of initial reactions between CO2-rich water and fractures including the connected pore network were determined. Reactions of CO2-saturated water with powdered massive (MB) and with vesicular basalts (VB) was studied at three different P/T conditions to understand the mobilisation of ions from crystalline basalt at far-from-equilibrium conditions. Reaction path modeling was undertaken for experiments involving MB to estimate the amount of dissolved and precipitated minerals. Mobilisation of ions was unexpectedly low at high experimental pressure and temperature conditions suggesting concurrent mineral precipitation, which was confirmed by the model. Secondary minerals formed a coating on the surface of the primary minerals, which controlled the primary mineral dissolution rate. The nature of secondary minerals was studied in a separate experiment involving basalt wafers at pressure and temperature conditions representing a depth of approximately 800 m. Mostly clay minerals and zeolites were observed at the surface of the wafers and in suspension, which agreed with the change in water composition. Certain clay minerals and zeolites were highly supersaturated based on the outlet fluid composition. Siderite (FeCO3) was close to equilibrium. The volume of dissolved and precipitated minerals was quantified in a third experiment using an artificially fractured basalt core at pressure and temperature conditions representing a depth of approximately 800 m. Net mineral precipitation occurred and the pore network structure changed, while only a very minor reduction in the net connected pore network volume was observed. These observations provide evidence for concurrent dissolution and precipitation processes leading to complex changes in the pore network geometry In conclusion, precipitation of secondary silicate minerals dominants at an early stage of CO2 -saturated water and basalt reaction and the presence of fractures in basalt enhances the geochemical reactions.
Neoproterozoic to Permian evolution of the Cape Fold Belt, South Africa: Constraints on sediment provenance and orogenesis from high-precision 40Ar/39Ar dating of detrital and metamorphic micas
The Permian Cape Fold Belt extends 1300 km along the western and southern coastal margins of South Africa. It comprises complexly deformed rocks of the lower-Palaeozoic Cape Supergroup, and Mesozoic parts of the Karoo Supergroup. Large-scale thrusts expose portions of the underlying Saldania Belt; a low-grade metamorphic belt intruded by granites of the ca. 550-500 Ma Cape Granite Suite. Both the Cape Fold Belt and Saldania Belt are segments of ancient continent-scale orogenic systems. The former is thought to be a portion of the Permian Gondwanides Orogen that extended from the Sierra de la Ventana Fold Belt of Argentina, across southern Africa, and into the Falkland (Malvinas) Islands, and Ellsworth-Whitmore Mountains of Antarctica, whereas the latter is considered one of the many Neoproterozoic-Cambrian Pan-African/Brasilliano terranes developed during the amalgamation of west Gondwana. However, fragmentation of Gondwana and separation of the Cape Fold Belt from its neighbouring terranes during the Cretaceous has provided major challenges in understanding both the geodynamic evolution of the poorly exposed Saldania Belt and the mechanics by which the Cape Orogen formed within the Gondwana interior. Gondwanan tectonic models often rely on geochronological provenance studies to not only link sedimentary sources and sinks, but to also correlate sedimentary successions in separated terranes. Previous geochronological provenance studies on the Cape Fold Belt have utilised U-Pb dating of detrital zircons to suggest that sediments of the Saldania Belt and Cape Supergroup were largely sourced from Mesoproterozoic rocks of the Namaqua-Natal Metamorphic Belt to the immediate north and underlying the Cape Fold Belt, as well as undifferentiated Pan-African and/or Brasilliano terranes. However, as zircon is able to survive orogenic recycling and long-distance transport, U-Pb detrital zircon studies have been unable to identify the most recent and proximal sources of sediments. In addition to having only broadly defined sediment provenance, the timing and extent of orogenesis during the Pan-African and Permian periods are poorly constrained. The timing of deformation in the Saldania Belt is only defined relative to the Cape Granite Suite, whereas the Cape Orogeny has been dated in a handful of limited 40Ar/39Ar studies. Early studies attempting to constrain Cape deformation utilised bulk mineral aliquots that yielded largely discordant 40Ar/39Ar age spectra, which the authors interpreted to represent multiple phases of deformation. A more recent study performed 40Ar/39Ar dating of single mica grains and proposed a bi-model evolution for the Cape Orogeny; however, this is based on only eight analyses. In this study, high precision 40Ar/39Ar geochronology is used to constrain the ages of individual detrital and metamorphic micas from low-grade rocks, as well as localised zones of variable deformation intensity in the southern Cape Fold Belt branch. Fundamental to this study, was the collection of detailed structural observations, petrographic and mineral chemistry data used to delineate detrital and neocrystallised mica age populations, from partially reset, altered, and/or complexly intergrown micas. A total of 648 individual mica grains were dated from 57 samples representing a variety of relatively undeformed and deformed Cape Supergroup and eastern Saldania Belt sediments, including crenulated metasediments, axial planar cleavages, thrust planes in duplex structures, and major shear zones. Only seven samples, collected from zones of intense deformation and focused fluid flow, yielded reproducible mica ages indicating that Cape Orogenesis was most pronounced at 257-248 Ma. These samples were located along, or in close proximity to the Worcester and Kango Fault systems, which are considered major decollement structures responsible for thin-skinned deformation of the Saldania Belt and Cape Supergroup sediments inland from the Gondwana margin. Biotite fusion ages of 272-270 Ma from a metamorphosed mafic dyke in the Kaaimans Inlier provide possible evidence for an earlier onset of Cape Orogenesis, preserved only in the Saldania basement. Samples from less deformed zones contained partially recrystallised detrital mica grains, mixed detrital and neocrystallised mica grains, and/or complex micas hosting clay and chlorite intergrowths, resulting in a spread of apparent ages older than the Cape Orogeny. 40Ar/39Ar detrital mica age populations were defined for a number of Saldania Belt and Cape Supergroup samples; these data were integrated with published U-Pb detrital zircon ages for provenance analysis. Detrital muscovite and zircon ages for the Lime Bank sequence and possibly part of the Kleinrivier sequence (Gamtoos Inlier) suggest exclusive provenance from the Mesoproterozoic Namaqua-Natal Metamorphic Belt or the similar-aged Maud Belt of Antarctica. In contrast, detrital ages for the Groenefontein and Huis River formations (Upper Cango Caves Group, Kango Inlier) indicate a source that experienced early Pan-African Orogenesis (580-550 Ma), such as the Sor Rondane Mountains or the Dronning Maud Land sector of the East African-Antarctic Orogen. The Cango Caves Group was folded prior to deposition of the overlying Kansa Group, which hosts abundant 530-510 Ma zircons. This suggests that deformation of the Cango Caves Group is Pan-African in age (i.e. 550-530 Ma) - possibly related to tectonic loading of the Kaaimans Inlier to the south, or the western Saldania Belt and Gariep Belt to the west. Later deformation of the Kango Inlier folded both the Cango Caves and Kansa Group, after which the conglomeratic Schoemanspoort Formation was deposited. A tightly constrained detrital mica age population of 510-500 Ma in the Schoemanspoort Formation represents either cooling/exhumation of the source terrane during the late stages of Pan-African tectonism or a younger tectonic and thermal pulse in the source area. These events could have occurred in the proximal Kaaimans Inlier, and may have been responsible for the combined folding of the Cango Caves and Kansa Groups. In the Kaaimans Inlier, 40Ar/39Ar incremental step-heating of muscovite from a pegmatitic vein suggest that parts of the Saldania Belt were affected by a post-Pan-African Ordovician thermal overprint. This overprint, identified in other Cape Granite Suite intrusives by previous studies, may represent a raised geothermal gradient in the basement as a result of Cape Supergroup sediment load and mantle flow coupled to far-field subduction along the Proto-Andean margin, and/or asthenospheric upwelling in the southern East African-Antarctic Orogen to the east of the Cape Fold Belt. 40Ar/39Ar analysis of individual mica grains from the Cape Supergroup reveals a dominant Ordovician (490-465 Ma) detrital muscovite population, suggesting provenance from orogenic belts possibly associated with the aforementioned Ordovician thermal overprint in the Cape Fold Belt basement; i.e. the Famatinian Orogen of western Argentina and/or the East African-Antarctic Orogen. Lesser detrital muscovite populations of 650-500 Ma and >730 Ma corroborate previous zircon provenance studies suggesting Pan-African and Namaqua-Natal Metamorphic Belts sources, respectively. The sediment provenance investigations presented in this study provide insights into the evolution of late Neoproterozoic to Devonian sedimentation in the Cape Fold Belt, and enable correlation of orogenic terranes spanning South American, African and Antarctic. In addition, well-constrained deformation ages from intensely deformed metasediments and major shear zones permitted formulation of a tectonic model for the Cape Orogeny, considering the onset, duration of deformation, and structural development of the Cape Fold Belt along the southwest Gondwana margin.
The origin, composition, and evolution of the Kimberley kimberlites (South Africa)
Kimberlites are deeply derived (i.e., >150 km), small-volume igneous bodies that have been emplaced on all continents throughout the last 2.8 billion years. The typical volcanic expression of kimberlites is a deep irregular root zone and/or feeder dyke system connected to a regular steeply dipping and outwardly tapering pipe-like diatreme, which may be overlain by a crater and extrusive material (when not removed by erosion). The crater and diatreme facies contain pyroclastic rocks, which transition into coherent (sub-volcanic) rocks in the root zone. Kimberlites are of economic value as the major host of gem quality diamonds at the Earth’s surface. They also hold great scientific significance, as the deepest derived melts to reach the surface, with entrained mantle material that provides some of our best information on the structure, composition, and evolution of the sub-continental lithospheric mantle. However, despite over a century of dedicated research, numerous aspects of kimberlite petrogenesis remain poorly understood and contentious. One central issue that this project has addressed is the composition and evolution of kimberlite melts. The composition of kimberlite melts remain poorly constrained because: (1) rocks emplaced near the surface are prone to deuteric and hydrothermal alteration; (2) they have been contaminated by the physical incorporation of xenocrystic and xenolithic material; (3) their parental magmas have been modified by interaction with and partial assimilation of mantle and crustal material; and (4) they undergo syn-emplacement differentiation. Therefore, in this study exceptionaly ‘fresh’ rocks from the well studied kimbeley cluster (the type locality) were examined to gain further insights into kimberlite melt compositions. Better constraints on kimberlite melt compositions are pivotal if we are to move toward a comprehensive understanding of the petrogenesis of these enigmatic rocks. The studied samples derive from the Kimberley cluster (South Africa), which lies within the Western terrane of the Kaapvaal carton. This cluster constitutes the type locality of kimberlites, containing five major kimberlite pipes (The Kimberley mine, De Beers, Dutoitspan, Wesselton, and Bultfontein), numerous smaller pipes, and abundant dyke/sill complexes (e.g., Benfontein, Wesselton Floors, Wesselton Water Tunnels). The Kimberley cluster has been dated by various geochronological techniques yielding emplacement ages of ~80-90 Ma. To provide new insights into the composition and evolution of kimberlite melts, a detailed petrographic study of sub-volcanic (hypabyssal) coherent kimberlites was conducted. This included the investigation of mineralogy, mineral zonation, inclusion populations (mineral, melt, and fluid), and textural relationships between phases, utilizing a range of microscopy techniques. This petrographic data formed the basis of targeted geochemical analysis by electron microprobe. A study of olivine compositions across multiple intrusions of the Kimberley cluster shows that olivine, more than any other mineral, provides the most complete record of kimberlite evolution. This study showed that pre-ascent metasomatism of the lithosphere by kimberlite melts is wide-spread, and that so-called ‘xenocrystic’ olivine is not directly representative of the wider lithosphere due to metasomatism of the conduit by previous pulses of kimberlite magmatism. The composition of kimberlitic liquidus olivine overlaps that of olivine from other mantle-derived carbonate-bearing magmas (orangeites, ultramafic lamprophyres, melilitites), with low Mn/Fe and Ca/Fe, and moderate Ni/Mg ratios. It is suggested that these compositions are typical of olivine in equilibrium with melts derived from carbonate-rich peridotite sources. Compositional zonation patterns indicte that olivine crystallises throughout magma ascent and that its crystallisation continues after emplacement into the upper crust. Magmatic olivine (i.e. crystallised from the kimbelrite magma) displays distinct generations of crystallisation, with increasing Mg, Ca, and Mn contents interpreted as the result of fractional crystallisation and increasing oxygen fugacity (fO2). The stability of olivine at sub-solidus conditions implies that secondary melt inclusions cannot trap primitive melts, but rather evolved residual fluids. Although olivine provides a wealth of information on early kimberlite melt evolution and metasomatism of the surrounding lithosphere, details about the later stages of kimberlite melt evolution are evident in other magmatic groundmass phases. Therefore, detailed petrographic and mineral chemical studies were undertaken on all mineral consistents from a suite of samples from the exceptionally fresh De Beers dyke to determine the crystallisation sequence. In turn, this data yielded insights into melt evolution from the perspective of multiple different magmatic phases. The early stages of kimberlite crystallisation (i.e., olivine, Cr-spinel, Mg-Ilmenite, rutile) are defined by decreasing Mg/Fe ratios. This subsequently reverses (i.e., increasing Mg/Fe) during later groundmass crystallisation, which is attributed to increasing fO2. Comparison with published data shows that the melt parental to early crystallising phases in this dyke are indistinguishable from those in the root-zone intrusions of the Kimberley cluster, meaning that not all dykes are the crystalisation product of magmas that underwent pre-emplacement fractionation. To gain additional insights into the very late stages of kimberlite melt evolution further detailed petrographic and mineral chemical studies were conducted on late-stage groundmass phases (i.e., apatite and mica) from samples of different root zone intrusions and dyke/sill complexes in the Kimberley area. Despite the early crystallising phases (i.e., olivine, Cr-spinel, Mg-ilmenite) being compositional indistinguishable in dykes/sills and root zone kimberlites, the compositions of apatite appear to be controlled by the style of magma emplacement. Apatite from dykes/sills is Si-rich and Sr-poor, whereas apatite in root zone intrusions show the opposite features. The high Si content of apatite in dykes/sills is attributed to the coupled incorporation of silica and a carbonate ion for phosphorus, reflecting higher CO2 contents in the melts parental to dykes/sills. The high Sr content of apatite in root zone intrusions likely requires crystallisation from, or overprinting by, hydrous fluids. These features indicate that dyke/sill kimberlites have higher CO2/H2O ratios than the magma that produced root zone intrusions. This is consistent with petrographic observations, whereby dykes/sills are enriched in carbonates, may contain dolomite, and have lower abundances of serpentine, mica, and monticellite than root-zone kimberlites. These differences in CO2/H2O ratios of the crystallised melt are attributed to differences in emplacement style, whereby a rapid decrease in pressure in root zone kimberlites leads to exsolution of a (CO2-rich) fluid phase, possibly caused by breakthrough to the surface. The knowledge gained through detailed petrographic and mineral chemical studies led to the development a new quantitative method for reconstructing the composition of kimberlite melts. This model allowed for constraints to be placed on the composition of primitive kimberlite melts and their evolution, as they incorporate and assimilate xenocrystic material, undergo fractional crystallisation, and post emplacement alteration. The results of this modelling indicate that the melt parental to the Bultfontein kimberlite was transitional between silicate and carbonate. This composition is consistent with experimental constraints on the amount of CO2 that can be dissolved into kimberlite melts in the upper crust. The reconstructed primitive melt composition is in equilibrium with asthenospheric source rocks. Based on constraints from experimental studies, Kimberley kimberlites could have been produced by ~0.5% melting of carbonated lherzolite in the upper asthenosphere (i.e., 6.0-8.6 GPa and ~1400-1500 degrees Celsius).
An Investigation into the Potential of Pliocene Speleothems from the Nullarbor Plain, Southwest Australia, to Preserve Interpretable Palaeoclimate Signals
The Pliocene has been identified as a key time interval from which estimates of future climate scenarios can be made. At present there is a significant paucity of Pliocene climate data from the Southern Hemisphere, and moreover, from terrestrial sources therein. This thesis aims to address both of these scarcities through applications of both traditional and novel techniques of palaeoclimate analysis to speleothems from the Nullarbor Plain, southwest Australia. This thesis focuses on two primary stalagmites, as well as several additional speleothems, that grew during the Pliocene as revealed by U-Pb dating. It provides one of the first detailed studies of speleothems of a greater antiquity than the 500 kyr dating limit previously imposed by the U-Th method. This study has provided invaluable insights into the climate of the Nullarbor Plain during the Pliocene, through the application of conventional stable isotope analyses, trace element analyses, fluid-inclusion analyses, clumped-isotope analyses, and modern precipitation isotope analyses. For speleothems of such antiquity, a multi-proxy approach grounded in a site-specific modern isotope study, has provided the means for delineating the key drivers of the geochemical variations present. The use of both traditional and novel speleothem proxies provides key information regarding both temperature and precipitation dynamics in the Nullarbor region during the Pliocene. Conventional stable isotope and trace element records from the speleothems reflect changes in precipitation above the cave and the overlying vegetation dynamics. Modern precipitation analyses enable a comprehensive understanding of the influences on isotopic variation in current precipitation, with applications to the interpretation of the speleothem data. This study indicates that during the Pliocene, precipitation was significantly higher in the Nullarbor region, suggestive of an increase in moisture sourced from NCBs and the Southern Ocean. The increased precipitation resulted in significantly higher vegetation cover compared to present, as supported by both the carbon isotope and trace element signals. These conclusions are in agreement with both modelled estimates of vegetation and precipitation for the Nullarbor Plain, and also with studies of speleothem-derived pollen data from the region. Palaeotemperature estimates were derived from combined fluid-inclusion and clumped-isotope analyses; indicating a temperature range of 18.8 'C +/-1.8 'C to 21.4 'C +/-1.3 'C. While the individual methods provided several additional estimates, the uncertainties associated with each limit the reliability of the absolute temperatures. However, they signify significant temperature variations within the Pliocene, indicating that despite being a period of overall global warmth in comparison to the present day, the Nullarbor region experienced significant fluctuations in average temperatures. This study has identified several areas for focus in future research in order to further develop the novel application of speleothem research to the more distant past, while providing unique and important information regarding the climate dynamics of the Nullarbor Plain during the Pliocene.