School of Earth Sciences - Theses
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ItemThe role of soil microorganisms in the solubilisation of secondary lanthanide phosphate minerals in weathered granite( 2022)During the weathering of granite, a major component of Earth’s continental crust, many elements are redistributed from metastable minerals into new, more stable ones, a critical process in soil formation. Microorganisms are key drivers of mineral weathering, but the mechanisms by which they promote mineral dissolution and the connections between microbial metabolisms and element redistribution are poorly understood. Understanding the biogeochemical controls involved in the remobilisation of rare earth elements (REEs, ‘lanthanides’) from relatively insoluble minerals to uptake for cofactors of lanthanide-dependent microbial metabolism is an important and topical area of study. This thesis was developed around investigating highly insoluble secondary lanthanide phosphate mineral formation and dissolution in weathered granite profiles. Prior to soil formation, phosphate liberated by rock weathering is often sequestered into highly insoluble lanthanide phosphate minerals. Dissolution of these minerals is critical for releasing phosphate into the biosphere. For decades, a question has prevailed regarding how highly insoluble secondary lanthanide phosphate minerals dissolve during rock weathering. It was hypothesised that microorganisms play a role in this process; however, REEs were considered biologically inert at the time. Recently, following the discovery of a lanthanide-dependent methanotroph, studies have shown that lanthanide-dependent methanol oxidation is common in many environments, including soils, but the mechanisms responsible for lanthanide acquisition and uptake from the environment remain poorly understood. The thesis reviews and discusses the major outstanding questions and mechanisms involved in the biogeochemical mobilisation of lanthanides in weathered granite (Chapter One). Field emission-SEM/EDX (scanning electron microscopy with energy dispersive X-ray) and mass spectrometry were used to study primary and secondary mineral formation and dissolution in granite material while genome-resolved metagenomics, enrichment, isolation and bioleaching were utilised to study the capability of the microbial community to break down REE minerals and utilise lanthanides (Chapter Two). The process of secondary lanthanide phosphate formation and dissolution was investigated in different granite types of variable mineralogy. The phenomenon of REE mobilisation, precipitation of nanocrystalline REE/P minerals, and ultimately REE/P mineral dissolution, during granite weathering was documented in three major granitic types (I-, A-, and S-type). It was documented that the dissolution of highly insoluble secondary REE/P minerals, as well as monazite, can precede soil formation and occurs in soils (Chapter Three). Using genome-resolved metagenomics alongside enrichment and isolation methods, the work aimed to characterise the microbial community of a weathered granite profile, to detect the presence of lanthanide-dependent genes, and to assess their potential REE/P- bioleaching capabilities. One well-developed granite weathering profile in which lanthanum phosphate formation and dissolution were documented was selected for further biological analysis. Genome-resolved metagenomic analyses revealed microbes including Verrucomicrobia, Acidobacteria, Gemmatimonadetes, and Alphaproteobacteria with the capacity to utilise lanthanide-dependent methanol oxidation (XoxF) are prevalent in the region where insoluble lanthanide-phosphate minerals dissolve. Conserved hypothetical proteins and transporters not previously associated with xoxF systems were also identified across diverse phyla suggesting lanthanide usage is prevalent and highlights targets for further investigation. Biosynthetic gene clusters of predicted siderophores containing xoxF systems were identified and may be involved in dissolving REE/P minerals (Chapter Four). Enriched microbial co-cultures capable of producing siderophores were capable of dissolving lanthanum from highly insoluble rhabdophane in the laboratory at essentially circumneutral pH suggesting dissolution mechanisms independent of organic acids (Chapter Five). This study's genome-resolved metagenomic analysis identifies mechanisms in moderately weathered granite that may promote REE/P mineral dissolution and general rock weathering. The study also successfully demonstrates a new in situ sampling approach for enriching and isolating microorganisms capable of solubilising lanthanum from highly insoluble lanthanide phosphate minerals. The findings and methodology of this study have relevance and application suitable for experimental studies looking to describe new lanthanide-based systems, express and characterise novel siderophore gene clusters, improve lanthanide recovery technology, and further describe the microbial controls on mineral weathering prior to soil formation.
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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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ItemGabbro magmatism in the Lachlan Orogen, southeastern Australia: implications for mafic–felsic associations and granitoid petrogenesis( 2016)The nature and role of mafic endmembers in granitoid petrogenesis is poorly constrained in the Lachlan Orogen of southeastern Australia. Most previous studies have focussed on intermediate–felsic magmas and attempted to model the nature of their primitive precursors. Ordovician–Devonian magmatic rocks of gabbroic composition are exposed as rare volumetrically minor intrusions throughout the Lachlan Orogen, and are temporally and spatially associated with more voluminous granitoids. A detailed study of tholeiitic and alkali gabbros exposed in the Kuark Zone of eastern Victoria provides new insights into gabbro petrogenesis and the source regions of such magmas which in turn have implications for the generation of granitoids, particularly I- and A-type magmas, throughout the Lachlan Orogen. The Arte Igneous Complex and Scrubby Flat Gabbro are poorly exposed mafic intrusions spatially associated with A-, I- and S-type magmas. Although major- and trace-element variations do not always show clear evidence for a geochemical link between these units, variations in Sr, Nd and Hf isotope compositions indicate a shared source for at least some of the magmas. Alkali gabbros in particular preserve considerable Sr-Nd isotopic heterogeneity ranging greater than 10 epsilon Nd units within a small geographic area interpreted to represent the root of the Arte intrusion. It is proposed herein that magmatic differentiation occurred via fractional crystallisation and cumulate processes; however, it is argued that much of the Sr-Nd isotopic variation was inherited from a heterogeneous source region. A model involving a small degree (<20%) of partial melting of greenstone basement can explain the variation within the alkali gabbro of the Arte Igneous Complex. Subsequent higher degrees of partial melting (>30%) can explain the more voluminous, less heterogeneous tholeiitic gabbros. Spatially associated A- I- and S-type granitoids and the gabbros is more cryptic, some geochemical and Sr-Nd isotope links are apparent. Importantly, the I-type intrusions are interpreted to have been derived magmas fromed by partial melting of more intermediate compositions within greenstone basement (c.f. ultramafic to mafic for the gabbros), thus they share a similar heterogeneous source region with the gabbro rocks. In contrast, the S-types intrusions have a more complex link to the gabbros and are interpreted to have been derived via partial melting and assimilation of Ordovician turbidites by tonalite magmas of the Arte Igneous Complex. Comparison of the magmatic rocks of the Kuark Zone with other twelve of the 20 known Lachlan Orogen gabbros reveals similar isotopic heterogeneity. This requires that the source heterogeneity is present on a local- and regional-scale. Cambrian greenstone basement exposed in rare fault-bounded belts throughout the Lachlan Orogen have the same isotopic heterogeneity. Moreover, the same heterogeneity is observed in I-type granitoids of the Lachlan Orogen. The implication is that I-type magmas may also be generated by partial melting of greenstone basement rocks, thus both gabbroic and I-type magmas image their source region. There is a correlation between the age of gabbros and Sr-Nd isotope values, with younger gabbros characterised by on average more isotopically juvenile compositions. The ca. 380 Ma Bingie Bingie Suite that approachs compositions of depleted mantle. A number of the gabbros exhibit arc-like trace-element characteristics, however, given that the greenstones were generated in a Cambrian arc environment, these signatures may be inherited from their source (greenstones) rather than the gabbro magmas themselves being generated in a subduction zone setting. The chemical characteristics of most of the gabbros are consistent with the partial melting of greenstone basement in a back arc basin setting under extension. Influx of new mantle-derived magma is only likely to have occurred to produce the youngest mafic rocks (e.g., Mount Buller Igneous Complex). The results of this study provide new insights into the source regions for mafic intrusions of gabbro/diorite composition in the Lachlan Orogen. In light of this new information, these insights present an opportunity to re-examine the petrogenetic models for I- and A-type granitoids in particular.