School of Earth Sciences - Theses
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ItemAge and Petrogenesis of Kimberlites and Related Rocks from Finland( 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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ItemNovel proxies study of long-term climate variability and karst evolution using speleothems from southern Australia( 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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ItemAge, origin and extent of the Newer Volcanic Province in the Melbourne region, Australia( 2019)Abstract Intraplate basaltic volcanism is present on every continent, untethered to any specific tectonic setting. The ca. 4.6 Ma – 5 ka Newer Volcanic Province (NVP) in south-eastern Australia is a chemically and morphologically diverse intraplate basaltic province. Its diversity, preservation and accessibility make it an ideal natural laboratory for investigating the causes and evolution of intraplate magmatism worldwide. Previous studies in the NVP have put forward competing magmatic models, but their validity is dependent on a limited geochronological dataset. Furthermore, previous geochronology has seldom been supported by geochemical or geomorphological studies on the same volcanic products, such that there is often a disconnect between the absolute ages of NVP rocks and the extent and composition of their associated lava flows or eruption points. This study utilised a holistic approach to lava flow mapping, with a focus on the diagnostic petrographic and geochemical features of individual basaltic lava flows selected for geochronology. The current study area of Melbourne, which is located at the eastern margin of the NVP, was selected for its age range and geochemical complexity, rivalling those of the entire NVP. Drill core was utilised to trace lava flows at depths of up to 80 m, thereby unravelling complex flow networks and facilitating the construction of a detailed lava flow map for the Melbourne area. Trace element geochemistry was utilised to distinguish between petrographically similar flows, and to investigate the eruption of three chemically distinct magma batches erupted consecutively from Mount Fraser. This mapping and geochemical work is complemented by new, high-precision 40Ar/39Ar age constraints, spanning ~7.9 Ma – 0.8 Ma. The new geochronological constraints on Melbourne lava flows reveal that the earliest activity (~7.9 – 3.8 Ma) was dominated by small-volume eruptions that predominantly produced alkali basalts. From oldest to youngest, these included the Bald Hill, Mount Ridley, Tullamarine, Crowe Hill, Spring Hill, Summerhill Rd, Redstone Hill and Aitken Hill lava flows. After ~3.8 Ma, large-volume eruptions dominated, with Fenton Hill, Mount Kororoit, Tulloch Hill and Mount Fraser producing lavas generally of tholeiitic composition. This progression from alkali basalt to tholeiitic volcanism over time is contrary to the purported province-wide progression from tholeiitic to alkali-rich lavas, indicating either that geochemical evolution in Melbourne was distinct from that of the NVP, or that the conclusions of province-wide studies are based on an unrepresentative sample set. An age of 7.931 +/- 0.038 Ma for the Bald Hill Lava Flow far exceeds the generally accepted maximum age of NVP activity (ca. 4.6 Ma). This lava flow, along with those erupted from Pretty Sally, Green Hill and Mount Cooper, is also geochemically distinct from other Victorian NVP products, but is not dissimilar to lavas of the Cosgrove Leucitite suite, the purported products of a long-lived mantle plume. This raises the possibility that the ‘Cosgrove Plume’ traversed the latitude of Melbourne just after 7.9 Ma, casting doubt on its possible role in the initiation of NVP activity some 3.3 million years later. 11 of the 38 samples selected for 40Ar/39Ar dating produced concordant results, with the remainder exhibiting varying degrees of discordance. The underlying causes of this discordance and implications for accurate age determinations are examined and modelled in age spectra and inverse isochron space. A correlation is found between the proportion of radiogenic 40Ar (40Ar*) released and the nature of discordance exhibited by a sample. 39Ar recoil is suggested as the most likely cause of discordance in high-40Ar* samples, whereas low-40Ar* samples exhibit discordance consistent with the modelled effects of mass-dependent fractionation. Based on modelling results, isochron rotation is the main impact of isotopic disturbance on an inverse isochron plot (39Ar/40Ar vs 36Ar/40Ar), leading to a negative correlation between 40Ar/39Ar ages and (40Ar/36Ar)i values. A new framework for the treatment of 40Ar/39Ar data from basaltic rocks is submitted, optimising the interpretation of inverse isochrons and informing the allocation of age constraints. Finally, a new method of 40Ar/39Ar data treatment, here named the multi-isochron approach to 40Ar/39Ar dating, is introduced. A multi-isochron regression utilises the combined output of all possible inverse isochrons from data of a single aliquant to determine its ideal isochron and eruption age. This method has the potential of transforming 40Ar/39Ar data treatment, allowing eruption ages to be calculated even when isotopic disturbance is severe, and to be reported in cases where this was previously not possible.