School of Earth Sciences - Theses

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    An Investigation of Fission Tracks in Monazite: Development of a New Ultra-Low Temperature Fission Track Thermochronometer
    Jones, Sean Curtis ( 2022)
    Monazite, a rare-earth element (REE) phosphate mineral, is found as an accessory in a variety of rock types. Suitable uranium and thorium content make it a useful mineral for isotopic and chemical dating using the (U-Th)/He and U-Th-Pb methods. However, unlike other uranium-bearing minerals, apart from a few reconnaissance studies, its potential for fission-track dating has not been systematically investigated. Earlier studies produced very young ages suggesting that fission tracks may be annealed at very low temperatures. This study explores the fission track properties of monazite and presents the findings of a new track etching protocol and thermal annealing experiments. These are accompanied by a case study in SW Japan, demonstrating how a new fission track thermochronometer can be applied in a young and small orogenic belt. The previously reported concentrated (12M) HCl etchant at 90 degrees C for 45 min was found to cause grain loss from epoxy mounts and high degrees of grain erosion. Therefore, in efforts to reduce these hindrances, an alternative etching protocol of 6M HCl at 90 degrees C for 60 – 90 min has been established for monazite after testing several alternative etchants. However, it was found in an isothermal annealing experiment that ~4 percent annealing occurs after one hour exposure to this etching temperature. Thus, a key concern is that some track annealing could occur during track etching before the etchant reaches the track ends. To investigate this, possibility the application of focused ion beam scanning electron microscopy was used to mill progressively into implanted 252Cf fission tracks after slight etching, followed by an etch-anneal-etch experiment. Results showed that the etchant penetrated to the track ends in <15 min, suggesting less than ~1 percent of fission track length reduction is likely to occur during etching. Other etching experiments performed show that crystal settling during standard epoxy mounting means that (100) faces are preferentially displayed so that subsequent experiments were weighted towards this orientation. The size and shape of well-etched spontaneous fission track openings in monazite were also constrained to be rhombic in shape. Average rhombic etch pit diameters Dpc and Dpb, parallel to the crystallographic c- and b-axes on (100) faces are 0.81 +/- 0.20 micrometer and 0.73 +/- 0.26 micrometer, respectively. An angular distribution experiment on (100) faces found that spontaneous fission tracks initially etch anisotropically, being preferentially revealed at an azimuth of 90 degrees to the crystallographic c-axis up to ~60 min of etching. As etching continues, however, the distribution becomes progressively more uniform and is essentially isotropic by 90 min. Electron microprobe analyses also showed a correlation between etching rate and elemental composition, with over-etched grains tending to have higher U and Th concentrations, also suggesting a radiation damage effect. A series of isochronal laboratory annealing experiments were then performed on collimated 252Cf fission tracks implanted into monazite crystals on both (100) and ~(001) faces over 1, 10, 100 and 1000 hour schedules at temperatures between 30 degrees C and 400 degrees C. In all cases, the mean equivalent confined track length was always less than that in unannealed control samples. Monazite fission track annealing also appears to be anisotropic, with tracks on surfaces perpendicular to crystallographic c-axis consistently annealing faster than those parallel to the (100) face. To investigate how mean track lengths decreased as a function of time and temperature, one parallel and two fanning Arrhenius models were fitted to the empirical dataset. The temperature limits of the monazite partial annealing zone (MPAZ) were defined as length reductions to 0.95 (lowest) and 0.5 (highest) for these experiments. Extrapolation of the laboratory experiments to geological timescales indicates that for a heating duration of 107 years, estimated temperature ranges of the MPAZ are -71 to 143 degrees C (both +/- 6-21 degrees C, 2 standard errors) for the best fitting linear fanning model (T0 = infinity). If a monazite fission-track closure temperature is approximated as the mid-point of the MPAZ, it is estimated that the closure temperature (Tc) for fission tracks in monazite ranges between ~45 and 25 degrees C over geological timescales of 106 – 107 yrs, making this system potentially useful as an ultra-low temperature thermochronometer. Even ambient surface temperatures remain well within the MPAZ over these time scales. The final chapter of this study presents a low-temperature thermochronology study of Cretaceous granitoid samples from the Ryoke belt, located in eastern Yamaguchi and Nara Prefectures, SW Japan. Historically, low-temperature thermochronology techniques such as apatite fission track (AFT) and apatite (U-Th-Sm)/He (AHe) have been limited in their applicability to uncover the neotectonic evolution of Japan. This is predominantly due to the young age and small amount of total denudation the Japanese island arc has experienced since initiation of uplift. However, the monazite fission track (MFT) system provides an opportunity for the first time to directly analyse the neotectonic and denudation history of this area. Zircon (U-Th)/He (ZHe), AFT and AHe data and modelled thermal histories reveal Late Cretaceous - Pliocene cooling related to paleo-Izanagi and Pacific plate subduction along the eastern Eurasian continental margin. MFT dating reveals Plio – Pleistocene central ages interpreted to reflect elastic loading caused by Philippine Sea plate subduction since the Middle - Late Miocene, along with Quaternary collision of NE and SW Japan at the Itoigawa-Shizuoka Tectonic Line (ISTL). Estimated denudation rates based on MFT dating are in the order of 0.10 – 0.47 mm/yr and 0.15 – 0.56 mm/yr in the eastern Yamaguchi and Nara Prefectures, respectively, which are in accord with estimated rates calculated using geomorphological and altitude dispersion methods. No relationship with topography or geomorphological factors has been established to explain the higher denudation rates in the Nara Prefecture. Instead, differences are likely to reflect variations in the tectonic regime, timing of uplift and uplift mechanisms of the two regions.
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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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    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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    The structure and evolution of the northern Australian margin: Insights from the Papuan Fold and Thrust Belt, Papua New Guinea
    Mahoney, Luke George ( 2021)
    The Papuan Fold and Thrust Belt (PFTB) in Papua New Guinea (PNG), located on the leading edge of the northern Australian continental margin, has been subject to complex tectonism as a result of its location throughout much of the Cenozoic between the obliquely converging Australian and Pacific plates. The remoteness and inhospitable terrain characterising the PFTB make it one of the least well-known fold and thrust belts on Earth. The architecture of the northern Australian continental margin has been affected by both extensional and compressional tectonic forces, which first formed, and subsequently deformed, the Papuan Basin in the period from the early Mesozoic through to the present-day. Defining the geology, structure and evolution of the PFTB and Papuan Basin is central to our understanding of the geological and tectonic evolution of the northern Australian margin. In this thesis, a multidisciplinary approach is used to investigate the evolution of the PFTB, Papuan Basin and northern Australian continental margin. Field mapping and structural analysis within the remote Western Fold and Thrust Belt (WFTB) provide significantly improved constraints on the geology, structure and evolution of the fold belt. New geological constraints acquired over > 100 km of traverses suggest that the exposed Cenozoic Darai Limestone has very low shortening between ~ 12-22% yet structures in the Muller Range are elevated up to 7 km above regional. Structural work utilising regional-scale geological observations suggest that the inversion of pre-existing rift architecture on the northern Australian continental margin is the primary influence on the evolution of the area. The huge structural relief is produced by both tectonic inversion on deep-rooted normal faults and their linkage to the surface via triangle zones that form within the incompetent Mesozoic passive margin sedimentary sequence. Local- and regional-scale heterogeneities within the northern Australian continental margin, such as accommodation-zones and transfer structures are now expressed in the fold belt structure as discontinuities and cross-cutting structural features that are recognised throughout the PFTB. The 2018 Mw 7.5 PNG Highlands earthquake and aftershock sequence has provided an unprecedented opportunity to observe and analyse the crustal processes that have ultimately controlled the evolution of the PFTB. Seismological, GPS and remote sensing data offer constraint on the complex nature and spatiotemporal distribution of crustal deformation during the event, revealing that the PFTB experienced up to 1.2 m of uplift and ground deformation over 7,500 km2. Remarkable spatial and morphological similarities exist between the distribution of coseismic ground deformation associated with the event, and the less-inverted and uninverted extensional architecture that is well-constrained in the foreland across the Stable Platform. This suggests that the 2018 Highlands earthquake sequence was related to tectonic inversion along a previously unidentified extensional fault system beneath the PFTB, indicating the northern Australian passive margin has had a primary control on the evolution of structural styles observed throughout the PFTB. New low-temperature thermochronology data from extensive field surveys in the Muller Range were combined with legacy data in modern thermal history modelling tools to investigate the thermotectonic evolution of the WFTB and Papuan Basin. In particular, the Late Cretaceous to Oligocene history of the region is largely unknown due to the absence of a continuous stratigraphic record. Thermal history models based on these data suggest two major Cenozoic cooling episodes. The youngest, and best constrained, is clearly recorded in the stratigraphic record and relates to Neogene collision at the northern margin of the Australian continent. An older episode of comparable or greater magnitude occurred in the Eocene to Oligocene and may relate to the removal of 1,500-3,000 m of Late Cretaceous to Eocene stratigraphic section across the Muller Range prior to the widespread deposition of the shelfal Darai Limestone. It is suggested that extension along major faults beneath the Muller Range accommodated sedimentation from the Late Cretaceous to the Eocene, consistent with long-lived extensional structures observed in the foreland across the Stable Platform. The selective removal of this sequence across the Muller Range suggests it was uplifted in the Eocene to Oligocene, possibly in part facilitated by the inversion of extensional faults in the Muller Range area. This inversion is interpreted to have resulted from the Eocene to Oligocene collision of the expansive Sepik Terrane to the northwest of the PNG margin, an interpretation that has significant implications for the tectonic evolution of PNG and Southeast Asia. The studies presented in this thesis provide several key insights that significant advance our understanding of the geological, structural and tectonic evolution of the PFTB, Papuan Basin and northern Australian margin. An ongoing theme relates to the complex interplay between spatial variations in the architecture of the margin and spatial and temporal variations in the compressional stress field associated with an evolving tectonic setting between the Australian and Pacific plates.