School of Earth Sciences - Theses
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ItemCryogenian iron formations: glaciation and oxygenation( 2018)The Cryogenian Period (720–635 Ma) experienced extreme glaciations broadly coincident with a transformation of the Earth’s surface oxidation state, supercontinent breakup, and the evolution of complex animal multicellularity. However, the cause-and-effect relationships of these events are unresolved. The Cryogenian ice ages, known as ‘Snowball Earth' events, would have placed important constraints on the biosphere, and it remains unclear what role global refrigeration played in setting the stage for eukaryotic diversification and the origin of animals. The Cryogenian also experienced the deposition of iron-rich marine chemical sediments (iron formations), representing the first episode of global iron formation deposition in over one billion years. This shift in iron cycling highlights complexities in seawater chemistry and oxidation state during this time, and these iron formations offer valuable insights into Cryogenian palaeoenvironments. Iron formations from Cryogenian glacial successions in Namibia, USA and Australia were studied in order to investigate Cryogenian iron formation genesis and elucidate the relationships between glaciation, ocean chemistry, oxygenation and biotic evolution. In-depth sedimentology, stratigraphy and petrography reveals that these iron formations are intimately associated with Sturtian glacial sediments and are interpreted have been deposited in a range of glaciomarine environments. Geochemical analysis of these chemical sediments permits the reconstruction of Cryogenian ocean chemistry and the synglacial palaeoredox landscape. Multiple geochemical proxies, including rare earth element and iron isotope systematics, indicate widespread marine anoxia with increasing seawater oxidation with proximity to the ice shelf grounding line. A genetic model is proposed whereby the mixing of oxygenated glacial fluids with ferruginous seawater led to the deposition of iron formations in glacial successions during the Cryogenian. Atmospheric oxygen trapped in glacial ice was likely an important oxidant source, delivered to Cryogenian glaciomarine environments via subglacial meltwater outwash. This meltwater supply may have been crucial in establishing oxygenated marine habitats for eukaryotes, including early animals, during Snowball Earth. Multi-million-year oxidation of the oceans via this mechanism may have also set the stage for a Neoproterozoic marine oxygenation event.
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ItemIsotopic disequilibrium in granitic systems: the origins of heterogeneity in granites and implications for partial melting in the crust and petrogenetic models( 2017)Unravelling the processes involved in granite magmatism is essential to our understanding of the continental crust, its formation and evolution. Radiogenic isotope systems are commonly employed to this end, but the behaviour of these systems may not be as simple as is often assumed. Understanding the causes of isotopic complexity in granite suites is the aim of this research. By modelling the disequilibrium partial melting of isotopically heterogeneous protoliths the different compositions of the melt, source and restite for a range of hypothetical scenarios have been calculated. Results demonstrate that the melt produced may have Sr, Nd and Hf isotope compositions distinct from both the protolith and restite. A corollary is that restite-bearing magmas may exhibit different isotope compositions than their melts, a feature which should be preserved as a difference between the Hf isotope compositions of bulk-rock samples and their magmatic zircon populations. The same modelling also suggests that a single source rock can produce melts with diverse isotope compositions. The predictions of this modelling have been tested by analysing S- and I-type granites from the Lachlan Fold Belt, southeastern Australia, including iconic examples of restite-bearing rocks. Comparisons of Hf isotope compositions between bulk-rocks and their magmatic zircons reveal discrepancies (ΔεHfbulk-zircon) ranging from -0.6 to +2.5 ε units for I-type granites. This intra-sample Hf isotopic heterogeneity is interpreted to represent disequilibrium between the melt and restite assemblage. The ΔεHfbulk-zircon values are consistent with calculated ΔεHfmagma-melt values (from -4.2 to +7.4) based on the disequilibrium amphibole dehydration melting of 0.5-1.0 Ga meta-igneous protoliths. S-type granites also record differences between their bulk-rock and magmatic zircon Hf isotope compositions; however, the disparity is more subtle. Both positive and small negative ΔεHfbulk-zircon values are observed, consistent with modelling the partial melting of isotopically heterogeneous meta-sedimentary protoliths. In addition to low-temperature granites, case studies of two high-temperature I-type granitoid suites (Boggy Plain and Wallundry) have also been conducted. Both display a weak coupling between geochemical parameters that have been interpreted previously to indicate the involvement of assimilation and fractional crystallisation (AFC) processes. Positive ΔεHfbulk-zircon values obtained in the Boggy Plain Suite support the existing petrogenetic model in which basaltic melt becomes variously contaminated by material derived from the continental crust. The positive value is explained by retention of earlier-crystallised, more radiogenic phases in isotopically evolved, more felsic samples. In contrast, the Wallundry Suite is characterised by negative ΔεHfbulk-zircon values caused by the presence of unmelted components of its contaminant. A complex interplay of contamination, crystallisation, melt segregation and interaction between magma batches is required to account for the Wallundry Suite isotope data. The results of this study indicate that disequilibrium partial melting can produce within-suite isotopic variability without recourse to assimilation or mixing processes (1) in mafic to felsic samples caused by the progressive separation of melt from its isotopically distinct restite assemblage; and (2) via the extraction of multiple batches of isotopically distinct melts produced from a single source as anatexis proceeds. Furthermore, the isotope variation resulting from restite unmixing may be distinguished from magma mixing by decoupling of the Rb-Sr, Sm-Nd and Lu-Hf isotope systems. Importantly, the isotopic discrepancy between bulk-rock granite samples and their magmatic zircon populations suggests that the most mafic bulk-rock granite samples of a given suite, not magmatic zircon, provide the most accurate estimate of source rock Hf isotope compositions. This raises concerns regarding the ubiquitous use of zircon Hf isotope data to constrain crustal growth models.
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ItemNeoproterozoic seas: ocean chemistry and marine carbonate mineralogy( 2014)The step-wise oxygenation of the ocean-atmosphere system is arguably one of the most profound processes in Earth history, affecting most surficial Earth processes. The last major oxygenation of the oceans is believed to have occurred in the Neoproterozoic Oxygenation Event (~800-540 Ma), and is implicated as a trigger for the rise of animal life. However, the timing of this event is not well constrained, both due to geochronological problems with Neoproterozoic stratigraphy; and because of the inherent uncertainty in ocean oxygenation proxies. Furthermore, there is now evidence for a more complex Neoproterozoic ocean chemical history, including return to strongly anoxic and ferruginous conditions. An additional complication in the understanding of Precambrian marine environments is the abundance of dolomite in Proterozoic successions. A recently discovered series of dolomitic reef complexes in the Neoproterozoic Adelaide Fold Belt, Australia, and Otavi Belt, Namibia, improve our understanding of Precambrian marine conditions. Stratigraphic and petrological analysis suggests that synsedimentary marine dolomite precipitation was pervasive within these reefs. Newly described dolomite cements have optical properties, chemical zonation and cathodoluminescent characteristics indicating that they were direct marine precipitates. Dolomite precipitation during marine diagenesis in these reef complexes suggests that the oceans of the Cryogenian were chemically different to those of the Phanerozoic. Marine dolomite precipitation appears to be linked to anoxic, magnesium-rich ocean conditions. These newly documented primary marine dolomite cements preserve information about conditions in the parent seawater via their petrographic properties and geochemistry. Being constrained by sedimentology, carbonate geochemistry provides a window into Cryogenian ocean chemistry and structure. Geochemical results reveal a pronounced chemical stratification where a thin veneer of oxic surface waters existed above a peritidal redoxcline with anoxic, strongly ferruginous seawater at depth. These conditions describe a ferro-sulfidic ocean and encompass some of the most extreme anoxia yet documented during the late Precambrian. A return to Archean-like ocean conditions at this time suggests large-scale disruption of the ocean system during the Neoproterozoic. These conditions may be linked to extreme climatic fluctuations at this time, perhaps induced by ocean stratification in this Neoproterozoic ‘Stagnant Earth’. When analysed in stratigraphic framework, variations in carbonate mineralogy provide a record of ocean oxygenation during the Neoproterozoic. New sedimentological and stratigraphic constraints for the Namibian Otavi Belt provides a context for this variation and has also led to the discovery of new Cryogenian reef complexes. When correlated with the Adelaidian succession, the distribution of marine cements in these sequences reflects changing seawater conditions. Pre-Sturtian, Neoproterozoic oceans precipitated both dolomite and aragonite and developed widespread marine anoxia prior to glaciation. Interglacial Cryogenian oceans were extremely anoxic and ferruginous, with widespread dolomite precipitation. In contrast, late Cryogenian and Ediacaran oceans hosted abundant aragonite precipitation recording a gradual decline in marine dolomitisation. The deepening of the oceanic chemocline during this interval suggests that these seas were likely to have been moderately oxygenated, paving the way for the large-scale radiation of animal life.
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ItemExploring the palaeoclimate potential of South East Australian speleothems( 2013)The paucity of palaeoclimatic data existing for the Southern Hemisphere and the regional bias of new data from the Northern Hemisphere has meant conclusions regarding the global response to the numerous climatic events of the last 20 kyr is both widely disputed and poorly understood. Despite being one of the Southern Hemisphere’s largest landmasses, Australia in particular displays a limited pool of palaeoclimatic information and the production of a new, robust record providing an insight into the response and timing of key climatic events is paramount to generating a more comprehensive characterisation and improved understanding of palaeoclimate in this region. Speleothems (cave deposits) are valuable archives of palaeoclimate variation, characterised by their extensive growth intervals and large geographic extent. They contain a multitude of ‘proxy’ records both directly and indirectly linked to climatic fluctuations and are typically robust, displaying high preservation potential with no post-depositional alteration. Key to their success is their amenability to radiometric dating, allowing the establishment of robust and reliable chronologies to which their multi-proxy records can be anchored. Consequently speleothems provide a clear opportunity to explore and expand palaeoclimatic knowledge at sites across the globe. This thesis describes and explains the use of state of the art technology to exploit relatively recent advances in U-series dating to construct reliable and detailed records of south east Australia’s response to palaeoclimatic fluctuations over the last 50 kyr using samples collected from cave sites from across the states of Victoria and New South Wales. 28 speleothem samples have been analysed in terms of both their coincident growth intervals and stable isotope variation to provide records with palaeoclimatic implications at a range of time scales. The production of a chronological template of speleothem growth intervals has enabled the assessment of south east Australia’s response to some of the key local and global millennial scale climatic events of the last glacial to interglacial transition and detailed stable isotope analysis of selected samples have been interpreted with the aid of a thorough cave monitoring programme, identifying increased variability in the region’s climate during the late Holocene. The palaeoclimatic records developed in this thesis represent a significant step forward in Southern Hemispheric palaeoclimatology. These records offer valuable new data for both palaeoclimatologists exploring south east Australia’s past climate and those investigating climatic fluctuations at a hemispheric to global scale. The high resolution and robust chronology of the records produced means that they provide a benchmark to which future records might be anchored.
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ItemGeochronology and geochemistry of Cenozoic volcanism in relation to epithermal gold mineralisation in western Java, Indonesia( 2014)Western Java in Indonesia is a well known gold deposit district that contains a number of world class ore deposits. The district hosts mainly low-sulphidation epithermal gold deposits and the most important gold deposits occur in the Pongkor, Cibaliung, Cikotok, and Papandayan districts. Although Java has a long record of volcanic activity, little is known of its pre-Pleistocene arc history, particularly in relation to the timing and geochemical evolution of the volcanic successions associated with ore mineralisation. Moreover, much of the available geochronological data for volcanic rocks in the region are based on imprecise K-Ar results. Therefore a combination of 40Ar/39Ar dating techniques and elemental geochemical methods (major and trace element analyses) have been utilised in the current project. Most of volcanic samples from the current study are characterised by enriched LILE and LREE compositions, which are characteristic of calc-alkaline arcs. However, Papandayan basaltic samples exhibit depleted LREE contents, typical of island arc tholeiites. The more enriched LILE and LREE compositions present in some Pongkor samples and Papandayan intrusive rocks, are indicative of high-K calc-alkaline and shoshonite arcs. Trends in Nb/Y, Th/Nb, Ce/Yb, and Ce/La(N) ratios reflect temporal evolution of the arcs, from: i) a primitive arc (low Nb/Y, Th/Nb, Ce/Yb and high Ce/La(N) ratios) characterised by the tholeiite basaltic samples; ii) an evolved arc (high Nb/Y, Th/Nb, Ce/Yb and low Ce/La(N) values) typified by the high K – shoshonite volcanic samples; and iii) a mature arc (with intermediate Nb/Y, Th/Nb, Ce/Yb and Ce/La(N) values) represented by the calc-alkaline volcanic samples. Trace element signatures of La and Ce suggest a broadly similar magma source for all rocks in the region. 40Ar/39Ar dating of volcanic rocks that host gold mineralisation are representative of the western Java magmatic arc, give the oldest age of ca. 18 Ma for the Papandayan district; ages ranging from ~11 Ma to ~9.5 Ma for the Cibaliung district, from ~18 Ma to ~4.5 Ma for the Cikotok district, and the youngest ages from 2.74 ± 0.03 Ma to ca. 2 Ma for the Pongkor district. Adularia crystallisation that is associated with western Java gold mineralisation shows 40Ar/39Ar ages from the oldest to most recent: ca. 18 Ma for the Papandayan district; from 12.44 ± 0.19 Ma to 9.39 ± 0.75 Ma for the Cibaliung district; 5.36 ± 0.46 Ma and 3.43 ± 0.04 Ma for the Cikotok district; and from 2.02 ± 0.03 Ma to 1.80 ± 0.03 Ma for the Pongkor district. Some host volcanic rocks have been affected by hydrothermal alteration, which has resulted in partial resetting of ages towards the time of gold mineralisation. Variation of the mineralisation ages suggests multiple generation of adularia growth in the mineralised veins. The dating results suggest that the magmatic arc across western Java are most likely linked to Southeast Asia tectonic evolution, from Early Miocene counter clock wise rotation of Kalimantan to Late Miocene – Pliocene subduction of the Eurasian continental plate beneath the Indo-Australian oceanic plate. The new and existing age data allow for a reconstruction of the western Java magmatic arc, with three main events identified: an Early Miocene primitive tholeiite arc (20 – 18 Ma), a Middle Miocene mature calc-alkaline arc (13 – 9 Ma) and a Late Miocene – Pliocene evolved high-K calc-alkaline and shoshonitic arc (7 – 2 Ma). The reconstruction indicates that the Early Miocene Papandayan low sulphidation epithermal system (with some indications of high sulphidation activity) is related to a basement comprising thinned island arc crust. In contrast the Miocene – Pleistocene low-sulphidation epithermal mineralisation system of the Cibaliung, Cikotok and Pongkor districts, which is associated with a calc-alkaline arc, was constructed on Sundaland continental crust.