School of Earth Sciences - Theses

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    Neoproterozoic marine oxygenation and environmental evolution
    O'Connell, Brennan ( 2022)
    The Tonian and Cryogenian periods (1000–635 Ma) of the Neoproterozoic era (1000–541 Ma) witnessed changes in Earth’s oxidation state, the breakup of the supercontinent Rodina, massive swings in oceanic dissolved carbon, and two extreme global glaciations. The evolving environment set the stage for advances in ecosystem complexity and the evolution of multicellular animals in the late Neoproterozoic. Despite recent advances in understanding environmental broad-scale changes through time, an in-depth understanding of links between environment, ocean oxygenation, global glaciation, and early life is still lacking. This thesis examines Neoproterozoic sedimentary environments of the Adelaide Fold Belt, South Australia. Links are explored between nearshore to deep marine sedimentation, ocean oxygenation, environments of early microbial life, and paleoenvironmental change/environmental evolution. Sedimentology, stratigraphy, and petrology reveal insights into mixed carbonate-siliciclastic beach-barrier systems (Burra Group), backreef peritidal settings (Angepena Formation), and deep–shallow ramp settings (Trezona Formation). Coupled sedimentological and biological (stromatolite) data are explored to understand links between environmental change and biologic change. Geochemical data from major/trace element data, including rare earth element data are coupled with sedimentary facies to provide a window into the paleo-redox scape and ocean oxygenation. Cryogenian oceans were iron-rich and stratified suboxic/oxic-ferruginous. The earliest sites of marine oxygenation were Cryogenian tidal flats at the interface of the atmosphere and ocean ~650 Ma. In slightly younger settings ~650–640 Ma, giant iron-oxide-rich ooids with micrite hematite tidal rhythmite(?) couplets record rapid ooid precipitation and short-term redox variability facilitated by tidal mixing in a stratified suboxic/oxic ferruginous Cryogenian ocean.
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    The Impacts of Atmospheric Rivers in Australia and New Zealand
    Reid, Kimberley Jane ( 2022)
    Atmospheric Rivers (ARs) are narrow filaments of strong water vapour transport in the lower troposphere. They are responsible for about 90% of the meridional moisture transport over the globe and are comparable in magnitude to water transport in the largest terrestrial rivers on Earth. Globally, ARs can be associated with numerous hazards including heavy rainfall, floods, blizzards, landslides, strong winds and polar heatwaves. For these reasons, it is imperative for scientists to understand and ultimately be able to predict AR behaviour in the present and future climate. Atmospheric River research has been concentrated in the Northern Hemisphere particularly over North America and Europe. This is the first thesis to explore AR impacts over Australia. We developed a global AR identification algorithm and tested the sensitivity of the AR frequency results to the input dataset parameters including resolution, regridding method and moisture transport threshold. The results showed that the combination of low moisture thresholds and restrictive geometric requirements can lead identification algorithms to miss the strongest ARs especially in the Pacific Ocean. Additionally, the resolution and regridding method of the input data, and the order of regridding and calculating moisture flux, can all impact the final AR frequency results. We applied this identification algorithm to the European Centre for Medium-Range Weather Forecasts reanalysis (ERA5) and used this new dataset to analyse AR impacts over Australia and New Zealand (NZ). It was found that nine of the ten most expensive floods in NZ between 2007-2017 were associated with AR events and seven to ten of the ten wettest rainfall days at eleven different stations occurred at the same time as an AR over that station. In Australia, ARs contribute about 10-20% of annual rainfall except in the Murray-Darling Basin region, in the southeast, where ARs are associated with approximately one-third of the mean annual rainfall. Similarly, 30-40% of the heaviest rainfall days over southeast Australia occur during the passage of an AR. Using composites of the vertical structure of ARs in ERA5, we showed that the intensity of ARs that form at tropical and subtropical latitudes is driven by the wind component of the moisture flux, while ARs that form in the extratropics are strengthened by an increase in the specific humidity component of the moisture flux. Following widespread flooding over eastern Australia associated with persistent and high integrated water vapour transport (IVT), we evaluated and assessed future IVT changes in the latest generation of the Coupled Model Intercomparison Project (CMIP6) global climate models. We found that daily IVT would likely become more extreme over Sydney – Australia’s most populated city. This thesis fills a key regional gap in AR science and furthers understanding of extreme rainfall over Australia. It also contributes to our understanding of AR structure and potential response to climate change, which is valuable globally. This thesis provides the foundation for future AR work in the areas of climate change impacts and forecasting extreme rainfall over Australia and New Zealand.
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    Age and Petrogenesis of Kimberlites and Related Rocks from Finland
    Dalton, Hayden Boyd ( 2022)
    Kimberlites are rare, small volume ultramafic igneous rocks found on every continent on Earth, with eruption ages spanning almost 3 billion years. These rocks are derived from the deepest magmas that reach Earth’s surface (>150-200 km) and provide unique insights into the nature of the convecting mantle. In addition, their cargo of entrained mantle xenocrysts (including diamonds) and xenoliths permit examination of the subcontinental lithospheric mantle (SCLM), while ‘deep’ diamonds give insights into the composition of the underlying asthenospheric mantle. Despite their significant scientific and economic importance, and decades of research, particularly in regions of southern Africa, North America and Siberia, questions remain as to the petrogenesis of kimberlites. Contention persists around the depth of origin of kimberlites, melting trigger(s) including tectonic settings, and the composition and evolution of kimberlite melts during their ascent. To provide new insights on these issues, this study presents a comprehensive petrographic, geochemical and geochronological investigation on samples from three occurrences of kimberlite and related magmatism in Finland, comprising the Lentiira-Kuhmo cluster of olivine lamproites, Kuusamo cluster of kimberlites and ultramafic lamprophyres (UMLs) and the Kaavi-Kuopio kimberlites. Finland represents an optimal location for testing various petrogenetic models, particularly regarding the links to geodynamic processes as the tectonic evolution of the Baltic Shield and its role in supercontinent cycles are well constrained. This work presents the first petrological account of the Kuusamo kimberlites, revealing that they represent highly differentiated magmas with scarce olivine macrocrysts and other mantle-derived xenocrysts. These characteristics contrast with the neighbouring Kaavi-Kuopio kimberlites, which are inferred to have crystallised from less differentiated magmas that were modified by mantle assimilation, as evidenced by correlations between the Mg# of xenocrystic (mantle-derived) olivine cores and the composition of magmatic olivine rims, spinel, and groundmass modal mineralogy. New radiometric ages show that at least ~100 Myr separates the emplacement of the Kuusamo kimberlites (~735-750 Ma) from those at Kaavi-Kuopio (~625-585 Ma). These new age data also indicate temporal overlap between ultramafic lamprophyre magmatism at Kuusamo and the eruption of olivine lamproites at Lentiira-Kuhmo (~1180-1220 Ma), some 100 km to the northeast. As part of this geochronological investigation, the robustness of Rb-Sr phlogopite, U/Pb perovskite and 40Ar/39Ar phlogopite dating methods were evaluated by applying multiple geochronometers to individual intrusions. It is evident that each radiometric system can yield both precise and accurate emplacement ages, with important caveats regarding best practice and interpretation. Radiogenic isotope data (Sr-Nd-Hf) indicates that the olivine lamproites and UMLs were contemporaneous, but have distinct source compositions. The highly unradiogenic Nd-Hf isotope compositions of the former are consistent with derivation from the metasomatised SCLM whereas the UML compositions suggest they were sourced from predominantly asthenospheric melts that were modified by (up to 15%) incorporation of enriched SCLM components. The Mesoproterozoic timing of their emplacement suggests that eruption of the olivine lamproites and UMLs was facilitated by the extensional regime associated with the separation of Baltica from Laurentia. The Kuusamo and Kaavi-Kuopio kimberlites were also emplaced at a time of supercontinent disruption. The Kuusamo eruptions occurred as the break-up of Rodinia was initiated, while the Kaavi-Kuopio rocks were emplaced as Rodinia break-up was completed, contemporaneous with the formation of the Central Iapetus large igneous province. In keeping with their petrographic disparities, the Sr-Nd-Hf isotopic composition of these kimberlites indicates that they were sourced from distinct source regions in the convective mantle. The homogenous composition of the Kuusamo rocks overlaps the prominent PREMA-like signature of kimberlites globally, whereas the Kaavi-Kuopio samples exhibit an extreme range in Hf isotope compositions with a temporal trend from PREMA-like towards lower epsilon Hf(i) values in younger kimberlites. Isotopic modelling suggests that this temporal enrichment of the kimberlite source region was due to increasing entrainment (of up to 10%) of subducted material. These findings are consistent with mounting evidence for subducted material being an important source ‘pollutant’ for kimberlites globally and a petrogenetic link with supercontinent cycles and/or the large mantle plumes that initiate supercontinent disintegration.
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    Generating Improved Regional Inventories, Using Remote sensing and Inverse Modelling
    Shahrokhishahraki, Nasimeh ( 2022)
    Air pollution is among the most crucial environmental concerns in the world. Releasing toxic and dangerous gases and particulate matter into the air causes various types of human diseases. Moreover, air pollution negatively impacts plants, animals, and even buildings, especially monuments and historical places. This study investigates carbon monoxide (CO) and oxides of nitrogen (NO2), as typical air pollutants in urban areas. Improving air quality forecasts can be achieved by improving the modelling system and its inputs. Among such inputs, emission inventories (EIs), which provide emission rates for various pollutants in space and time, have a significant impact but are often associated with large uncertainties. Most cities do not have reliable atmospheric emission inventories. Different methodologies can be utilised to improve the existing EIs. One of these is to try to update the inventories to match measurements of concentration, usually termed an inverse method. This study generates urban scale emission data for Tehran, using globally accessible datasets and applies satellite measurements and an inverse approach to refine emission inventories. A downscaling approach is used to apply the Emissions Database for Global Atmospheric Research (EDGAR), Gridded Population of the World and Fossil Fuel Data Assimilation System to generate high-resolution EIs. The resultant urban scale inventory (also called as `prior EIs') is applied to run an offline WRF-CMAQ Modelling System (also called as `forward model') to simulate the concentrations of gaseous air pollutants. The modelled concentrations during August 2018, November 2018, February 2019 and May 2019 are compared with the TROPOspheric Monitoring Instrument (TROPOMI) satellite data, and surface observations (split into `road' or `city' type stations) to assess the performance of the system. The model overestimates the amplitude of NO2 tropospheric column satellite data by a factor of ~1.2-2.1, while it overestimates the amplitude of CO column data with a ratio of ~0.61-0.88. Comparison with surface measurements shows that the model with the prior EIs overestimates diurnal cycle of NO2 concentrations in both city and road types during all studies months with more than 0.70 correlation, between less than 1 ppb for road sites in August, up to around 70 ppb for city sites in November and February. For CO there is no similar pattern for the diurnal cycle of city sites and road sites. In city sites, except August, the model overestimates the CO with a bias of 0.05-0.8 ppm, while the negative bias values in road sites (-0.1 to -1.4 ppm) indicate that the model underestimates the observations. Analysis of concentration structures and meteorological fields suggests the modelling system is good enough to apply in an inverse system. A regional-scale inverse-modelling system has been established using remote sensing to regenerate emission data for Tehran. A Python-based four-dimensional variational (Py4DVar) data assimilation approach is applied to refine CO-EIs (`posterior EIs') for Tehran. This system applies the TROPOMI retrievals and two forms of allowed emissions variations: One with fixed temporal variation (FTV) and the other with emissions varying by diurnal categories (CTV). The posterior results show that the model captures the amplitude of CO column data with ratios of 0.94-1.02 through the CTV scheme and 0.75-0.98 through the FTV scheme. Comparisons with surface measurements also indicate that the posterior shows some improvements in simulating observations but not in all cases. Although we solve for aspects of the lateral boundary condition, our results are sensitive to their prior values. This suggests that future studies should use larger domains where boundary condition influence can be reduced. Similarly, the inability to match simultaneously the surface and column-integrated observations is most likely a manifestation of the longstanding problems in vertical tracer transport. Finally the temporal limitation of the satellite data we used limits improvement at unobserved times of the day. Notwithstanding these weaknesses, the system has already provided some useful information on the local EI and the approach shows considerable promise in regions lacking good local inventories.
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    Marine Carbonates as Paleoredox Proxies: Links between Ocean Redox and Key Neoproterozoic and Paleozoic Evolutionary Events
    Shuster, Alice Mary ( 2021)
    The emergence of complex life on Earth is associated with a long-term rise in oxygen concentrations in Earth’s atmosphere and oceans. Fluctuations of surficial oxygen levels are suggested to have played a large role both in evolutionary diversification and in mass extinctions. It has become increasingly evident that the mid-Palaeozoic was not as oxic as previously thought, and it was not until the Late Palaeozoic or even Mesozoic that Earth’s surface systems reached present oxygen levels. This thesis has utilised marine carbonates to develop complexity in our understanding of ocean oxygenation, marine conditions and the evolution of life in the Late Neoproterozoic and early Phanerozoic. Marine chemical conditions may be reflected in the record of marine carbonate mineralogy. The reason for the abundance of dolomite in Earth's early geological record compared to modern environments remains contentious, but could be linked to seawater composition. This study provides new insight into this Precambrian “dolomite problem” by revisiting one of the most controversial dolomite localities, the Beck Spring Dolomite, of Death Valley, USA. Consistent with some previous studies, petrographic evidence indicates that although the Beck Spring Dolomite now consists almost entirely of dolomite, it was originally precipitated largely as aragonite and high-Mg calcite. Depositional constituents (microbialites and ooids) were likely originally aragonitic, and early marine length-fast cements (now dolomite) are suggested to have precipitated as high-Mg calcite then replaced syntaxially by dolomite. Based on petrographic and geochemical evidence, marine dolomitization was the dominant synsedimentary diagenetic process in the unit, and for the most part, involved syntaxial and mimetic replacement. A length-slow fibrous dolomite generation was precipitated during the later stages of marine diagenesis as a primary marine dolomite cement. The trace metal geochemical composition of well-preserved marine components, especially dolomite marine cements, reveals information about redox conditions in this Tonian shallow seawater. In terms of rare earth element geochemistry, the Beck Spring Dolomite has no significant Ce anomaly, and a ubiquitous positive Eu anomaly, consistent with widespread oceanic anoxia during deposition. Furthermore, the relatively low levels of iron and chalcophile elements ii Marine Carbonates as Paleoredox Proxies: Links Between Ocean Redox and Key Neoproterozoic and Palaeozoic Evolutionary Events Co, Cu, Pb and Zn in marine components compared to other Neoproterozoic carbonates suggest euxinic conditions (both anoxic and sulphidic) prevailed during deposition of the Beck Spring Dolomite. This petrographic and geochemical evidence suggests ocean anoxia plays a significant role in promoting marine dolomite precipitation. Methods established in the Beck Spring Dolomite case study were utilised to explore Paleozoic ocean redox. Rare earth and trace element compositions of well-preserved marine cements from the Mid-Late Devonian Canning Basin Reef Complexes record three intervals of marine anoxia, with Ce anomalies ranging from present day oceanic values to almost a negligible anomaly over an approximately 15 My period (from Late Givetian to Middle Famennian), linked to episodes of the drawn-out Late Devonian Mass Extinction. Fluctuating, shallow-water redox conditions during the Late Devonian in the Canning Basin could be related to the interaction of two distinct water masses: a shallow, oxygenated water body, and a deep, anoxic (possibly ferruginous and/or euxinic) water mass. Related to the unstable marine redox and mass extinction events in the Late Devonian, the Canning Basin presents a diverse range of stromatolites from the mid- Frasnian until the mid-Famennian including deep-water varieties and the Fe microstromatolite Frutexites. Petrographic analysis shows that these deep-water stromatolites consist partly of organisms which actively precipitated Fe oxides. SEM microscopy reveals a variety of Fe-oxide-rich skeletal structures, including sheaths, elongated gelatinous crusts, aggulated spheroids and Fe-rich walls and tubes which make up Frutexites. Non-skeletal laminated varieties consist predominantly of microbially precipitated micrite and spar but also have Fe-oxide rich laminae, possibly mediated by another Fe-precipitating bacterium or underdeveloped Frutexites. It is likely that these organisms precipitated Fe-oxide and trapped minor detrital phases possibly as a result of an iron-oxidising metabolism. Trace metal and rare earth element work in marine cements shows that anoxic and possibly ferruginous deep waters upwelled onto the Lennard Shelf during the same interval as the deep water stromatolite development, suggesting ocean anoxia as the source of the iron.
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    Stratigraphic and structural evolution of the Gippsland Basin, Late Cretaceous to Miocene, Australia
    Mahon, Elizabeth ( 2021)
    Deposition in the Gippsland Basin from the Late Cretaceous to the Miocene was characterised by extensive, wave-dominated shorelines in front of lower coastal plain peatlands. Using depositional architecture evident on seismic data, these deposits have been interpreted to consist of 23 discrete packages. Shoreface morphology ranges from progradational beaches to large, aggradational beach-barriers. While some of these beach-barrier-coastal plain units are progradational, on a multi-million-year timescale they retrograde. Transgression occurred from the Late Cretaceous to the Oligocene, with the main driver for this transgression likely basin subsidence. Despite large changes in paleoclimate, basin tectonics and ocean chemistry, the depositional style remains remarkably consistent. The Gippsland Basin experienced compressional tectonics which resulted in large anticlines forming across the basin. The timing of onset of compressional tectonics in the basin has been revised based on measurements of syn-tectonic sediment thickness changes across structures. These measurements indicate extensional growth faulting was occurring from the Late Cretaceous until the Late Eocene, and compressional structures did not begin growing until the Eocene-Oligocene transition. This research has brought the previously interpreted date for the onset of compressional tectonism in the Gippsland Basin forward approximately 10-20 Ma, from the previously interpreted early to mid-Eocene, to the Eocene-Oligocene transition. From the Palaeocene-Eocene transition to the Eocene-Oligocene transition a series of large channels incised into the top Latrobe Group - the Tuna and Marlin Channels. These channels have previously been interpreted as forming via fluvial processes associated with tectonic uplift. However, this research has shown that tectonic uplift occurred after channel incision, indicating uplift did not contribute to channel down- cutting. Additionally, well data reveals a marine origin for channel fill sediments, and seismic data indicates channels are located seaward of coeval palaeoshorelines. This suggests these channels formed in a submarine environment, with the close proximity of channel heads to the shorelines indicating they were shelf-incising. A second pulse of compressional tectonics occurred in the mid Miocene, which measurements across structures indicate primarily affected the present-day onshore area. This episode of tectonic uplift corresponds to the previously documented unconformity at ~10 Ma. From the early Miocene onwards, shorelines become progradational and regressive. This is interpreted to be the result of compressional tectonics and global icehouse conditions.
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    Novel proxies study of long-term climate variability and karst evolution using speleothems from southern Australia
    Weij, Rieneke Petronella ( 2021)
    This research program aimed to produce robust and reliable speleothem chronologies for the Naracoorte Caves in South Australia which could then be used in studies of palaeoclimate and karst evolution. The thesis is structured around three sub-themes and the main findings of each are outlined below. The first component evaluates the utility of speleothem age frequency distributions for palaeoclimate assessment. Age distributions of this type can be used as proxies for past climate change, where the peaks in age density are linked to a controlling climatic parameter (e.g., temperature or precipitation). There remain, however, considerable gaps in our understanding of how best to use speleothem age frequency distributions in this way. To address these issues, a synthetic age dataset was modelled by randomly generating U-Th ages based on a known climatic forcing, followed by sampling from this dataset under varying conditions. The model shows that periodic Quaternary climate fluctuations can be recovered from age frequency distributions with a minimum sample size of 120–150 radiometric ages. This study provides a much-needed statistical framework for the use of age frequency distributions relevant to speleothem palaeoclimate studies — and one which is also beneficial for the radiocarbon and zircon dating communities. The second theme concerns the antiquity of the Naracoorte Cave Complex (NCC) in southern Australia. Caves are unique archives of past environmental and climatic conditions and may also act as important fossil repositories, as is the case for the World Heritage listed NCC. In these circumstances, understanding the timing of initial cave development and opening can shed light on the potential antiquity of the fossil deposits (and thus guide excavation), but these geomorphological processes remain challenging to constrain. This study places robust temporal constraints on the onset of cave and entrance development of the NCC by utilising an extensive campaign of U-Th and U-Pb dating of speleothems. Additionally, speleothem charcoal and pollen concentrations were used as novel indicators of cave openness. The key finding is that caves can be twice as old as their surface expression. These techniques provide important new tools for a range of disciplines interested in the timing and extent of cave opening, e.g., palaeontology, palaeoanthropology and archaeology. The final theme concerns the palaeoclimatic history of semi-arid southern Australia. Changes in the hydroclimate during the Quaternary remain poorly constrained for the southern Australian semi-arid subtropics. In this study, changes in southern Australia’s hydroclimate were reconstructed for the Late Quaternary using an age frequency distribution and pollen-based climate reconstruction from U-Th dated speleothems. This study represents the largest geochronological dataset from a single cave province in the Southern Hemisphere. Collectively, these reconstructions demonstrate orbitally-paced speleothem growth within the 100-ka period linked to changes in moisture availability that consistently lag maximum interglacial temperatures by ~25 ka over the last three glacial-interglacial cycles. The results show that times of highest moisture availability occurred during parts of the glacials, rather than interglacials periods, which implies that, in the semi-arid subtropics, temperature and moisture availability were strongly decoupled.
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    Recent Trajectories in Urban Emissions from Remote Sensing Proxies
    Luqman, Muhammad ( 2021)
    The United Nations Framework Convention on Climate Change (UNFCCC) motivates monitoring fossil fuel emission variations. This thesis studies the trends in urban fossil fuel emissions of Carbon Dioxide (CO2) and their drivers by developing an algorithm for delineating urban boundaries to generate masks for population and CO2 emissions and applies bottom-up techniques to approximate on-road emissions. The algorithm named BUNTUS (Built-up, Nighttime Lights, and Travel time for Urban Size) incorporates three remote sensing datasets; built-up area extracted from Landsat imagery, nighttime lights datasets, and travel time rasters. These three datasets are combined through a ruleset to produce an urban boundary. BUNTUS made 125 city boundaries, out of which 91 cities are studied. There are two opinions about the impact of urbanization on fossil fuel emissions intensification. First, some scientists consider that developing cities produce more fossil fuel emissions due to the migration of the rural population into cities. On contrary, the second opinion says that once a city develops economically, it becomes a low-emissions city. To test the role of urbanization in emissions intensification, this study quantified the urban emissions of 91 cities by taking ODIAC (Open-source Data Inventory for Anthropogenic CO2) as CO2 emission proxies and LandScan’s raster for population count. We find that urban CO2 emissions are increasing everywhere but that the dominant drivers differ according to development. Urban emissions are increasing along with urban populations, but per capita emissions are often growing more slowly than their national counterparts. A cluster analysis of factors shows that developing countries are dominated by cities with the rapid area and per capita CO2 emissions increases. Cities in the developed world, by contrast, show slow area and per capita CO2 emissions growth. The thesis also updates an existing global CO2 emissions inventory (FFDAS), improving its resolution and sectoral detail. FFDAS employs a data assimilation approach and yields globally gridded datasets of CO2 emissions. This study can serve as a foundation to study emissions trends in cities and on-road emissions inventory development.
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    Exploring the source attribution of environmental Pb – development of a continental-scale Pb isotope regolith map of Australia
    Desem, Candan Ulgen ( 2021)
    The element Pb is a particularly useful tracer as it is both a toxin of environmental concern and is often associated with mineralisation. As a result, Pb concentration data are often collected as part of environmental contamination and mineral exploration studies. Pb isotopic analysis, however, offers an important advantage in discriminating between different sources of Pb. The Pb-isotopic signature of regolith is a function of contributions from bedrock geology (including mineralisation) and exogenous inputs such as windblown dust, groundwater and anthropogenic contamination (industry, mine wastes, paints, transport, agriculture, waste handling and residential inputs). Pb-isotopic analysis allows us to estimate the relative contributions from each of these components and therefore provides an important tracing tool in both mineral exploration and environmental contamination studies. The spatial distribution of Pb in the regolith profile is also a function of the source of Pb (exogenous versus endogenous), depth and grain-size providing further diagnostic tools. Finally, Pb isotopic signatures in the continental crust show large natural variations which are transferred to the regolith and thus provide bedrock signatures that can be easily distinguished. Naturally occurring Pb has four stable isotopes with the following crustal abundances: 204Pb (1.36%), 206Pb (25.42%), 207Pb (21.11%) and 208Pb (52.10%). 206Pb is the decay product of 238U, 207Pb is the decay product of 235U and 208Pb is the decay product of 232Th. In contrast 204Pb is non-radiogenic and is often used as a stable reference isotope relative to which the accumulation of the other radiogenic isotopes over time can be quantified. Importantly, in mineralised systems, once an ore rich in Pb (e.g., galena, PbS) forms, the isotopic composition of its contained Pb does not evolve any further, as it does not contain significant amounts of U or Th. The Pb signature is thus ‘frozen in’ to the ore and will reflect the time at which the Pb was incorporated. As a consequence of these geological processes most ore deposits, and the products derived from them (leaded petrol, paint etc) have distinct Pb isotopic signatures that enable ‘fingerprinting’ of anthropogenic Pb contamination in the environment. In order to determine whether Pb-isotope signatures in the regolith are anomalous, an understanding of background Pb-isotopic signatures is critical. The use of large Pb-isotope datasets, however, has until recently been limited due to constraints associated with the availability and accessibility of lower cost, high-precision and high-throughput analyses. The advent of instrumentation such as Sector-Field Inductively Coupled Plasma Mass Spectrometers (SC-SF-ICP-MS) allowing rapid Pb-isotope analysis, now enables us to revisit the utility of Pb isotopes in this regard. Exploring the use of this new technology in the Australian context is a direct aim of this research. To this end, a continental-scale Pb isotope regolith map of Australia has been produced as part of this study, using catchment outlet (~ floodplain sediment) samples from Geoscience Australia’s National Geochemical Survey of Australia (NGSA) dataset. The dataset contains samples from 1300 locations across Australia obtained at a sampling frequency of approximately 1 sample/5200 km2 and ultimately covering ca. 81% of the continent. The Pb-isotope map is based on the coarse grain-size fraction (less than 2mm) of the top outlet sediment (0 – 10 cm depth) samples. Leaching techniques have been employed to separate loosely bound Pb (e.g. anthropogenic contamination) from Pb structurally bound in minerals (i.e. from underlying geology, mineralisation, or their weathering products). An analytical framework has also been established to achieve ‘fit-for-purpose’ data quality, with the elimination of the Pb-separation chemistry step required by conventional Multicollector-Inductively Coupled Plasma-Mass Spectrometer (MC-ICP-MS) or Thermal Ionisation Mass Spectrometer (TIMS) analysis combined with the high-throughput required in order to process large sample sets (> 1500 samples in this case). A sequential leach protocol (‘P618’) originally developed at CSIRO (Carr et al., 2011) has been utilised in which an ammonium acetate leach is followed by an aqua-regia (HNO3-HCl) digest. Pb isotope signatures were acquired using a Nu Instruments Attom single-collector sector-field ICP-MS (SC-SF-ICP-MS) with data quality comparisons also made with MC-ICP-MS and Quadrupole-ICP-MS, in which this research has demonstrated that both precision and accuracy achieved on a SC-SF-ICP-MS is greater than that achievable on a Quad-ICP-MS. The aims of this program were to (i) compare Pb signatures released using the different leaches, (ii) examine whether soil Pb isotope signatures can identify bedrock geology and metallogenic provinces as well as (iii) investigate the presence and extent of anthropogenic contamination across the continent. The data, in particular across older terranes, reveal a dominant contribution of Pb to the regolith from the underlying geology with major crustal elements well defined. A variety of exogenous Pb inputs from point sources of pollution are also apparent. The continent-wide Pb isotope regolith map of Australia demonstrates that the top coarse aqua-regia digested fraction of regolith samples in Australia, are dominated by Pb from the catchment bedrock geology. The influence and prominence of bedrock geologic Pb signatures are more easily visible in older terranes, such as the Archaean Yilgarn Craton and Proterozoic North Australia Craton of Australia where radiogenic signatures have had more time to establish. Apparently meaningful Pb-Pb isochrons on these top coarse fraction regolith samples demonstrate not only the data quality obtained from the Attom SC-SF-ICP-MS but also that the dominant bedrock geologic Pb-isotope signature is carried to shallow depths within the regolith profile in these areas. Visibly distinct Pb-isotope patterns which correlate with the major crustal elements and geological regions of Australia, further support the idea that the Pb-isotope signatures at shallow depths across Australia are governed by large-scale Pb reservoirs. At the smaller scale, variation in grain-size and depth of Pb-isotope signatures provide information as to how the Pb-isotopic signature varies within the regolith profile. Throughout this study, variation in Pb isotopic signatures with depth and grain-size have been found to be more pronounced where there is a distinct exogenous Pb input. In areas where a significant exogenous Pb input has not been identified (by means of an anomalous Pb isotopic signature), it appears there is some level of homogenisation of Pb-isotopic signatures within the regolith profile. Finally, this analytical program has revealed areas of anomalous Pb isotopic signatures in the regolith which would be useful targets for future investigation as the source of these signatures has not yet been determined.
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    Petrological and geochemical constraints on the source to surface evolution and emplacement style of the Lac de Gras kimberlites, Canada
    Tovey, Madeline Hannah ( 2021)
    Kimberlites originate from the deepest-derived magmas on Earth and are characterised by ultrabasic, H2O and CO2-rich, and silica-poor compositions. These magmas entrain and transport mantle material (sometimes including diamonds) during their ascent to the Earth’s surface, before intruding the upper crust, or erupting explosively to form deep (2-3 km) conical diatremes. With the exception of one Quaternary occurrence, active kimberlite magmatism has not occurred since ~30 Ma, and surface deposits are often eroded. Assimilation of mantle material, crustal contamination and post-emplacement hydrothermal alteration modify the compositions of kimberlites during emplacement, hindering attempts to constrain original melt compositions. There is also uncertainty about the factors that control the emplacement style of these magmas and whether melt compositions have any influence. To improve constraints on the composition and evolution of kimberlite melts and their mode of emplacement, 30 coherent intrusive and extrusive kimberlites (CK), and two volcaniclastic kimberlites (VK) from the Lac de Gras (LDG) field, Northwest Territories, Canada were studied using petrographic and geochemical methods. Olivine rim and chromite compositions show that kimberlites at LDG derive from a range of primitive melt compositions. Increasing age-corrected Nd-Hf isotope ratios with time correlate directly with olivine rim Mg# [100xMg/(Mg+Fe2+)] compositions and inversely with chromite Ti# [100xTi/(Ti+Al+Cr)] compositions for central LDG kimberlites. These correlations indicate that melt compositional variations stem from partial melting of an evolving kimberlite source due to progressive assimilation of less refractory, deeply-subducted crustal material. These relationships are not observed when considering all the LDG kimberlites. This is attributed to decoupling of the kimberlite source and primitive melt compositions for all the LDG kimberlites by assimilation of laterally heterogenous mantle material, as indicated by a strong correlation between olivine rim and olivine core compositions, which are considered to be proxies for the compositions of primitive melt and entrained lithospheric mantle material, respectively. Different initial epsilon Nd and Hf, and olivine rim and chromite compositions for extrusive pipe-filling CK and intrusive kimberlite dykes from different LDG localities indicate derivation from different primary melt compositions. However, at some localities (e.g., Diavik), intrusive and extrusive kimberlites feature indistinguishable olivine and chromite compositions, indicating similar primitive melt compositions. These results indicate that primitive melt compositions may control the emplacement style of some, but not all, kimberlite magmas at LDG. Greater modal abundances of groundmass phlogopite and monticellite and lower groundmass abundances of carbonate for the extrusive versus intrusive kimberlites are attributed to greater volatile exsolution during the ascent and higher energy emplacement of the extrusive kimberlites. Greater SiO2, MgO and NiO, and lower incompatible element (i.e., TiO2, Nb, Ta, REE) whole-rock compositions for the extrusive versus the intrusive kimberlites cannot be explained by mixing lithospheric mantle or crustal compositions with reconstructed primitive kimberlite melt compositions, suggesting that these processes were not responsible for the different degrees of volatile exsolution evident in these kimberlites. Explosive emplacement of gas-rich magma excavated pipes at LDG prior to the emplacement of pipe-filling CK, which suggests that pipe-filling CK might reflect the waning stages of volcanic eruptions initiated by the explosive emplacement of a gas-rich dyke tip (VK) followed by the emplacement of melt-rich tails (pipe-filling CK). Further work is required to test the potential genetic relationship between CK and VK at LDG. Primitive melt composition, geological setting, the availability of water to trigger phreatomagmatic eruptions and/or magma segregation during ascent are suggested to influence the emplacement style of kimberlite magmas.