title: Uptake of iron and its effect on grain refinement of pure magnesium by zirconium

authors: Cao, Peng

subjects: grain refinement

abstract: The uptake of iron by molten magnesium from uncoated new mild steel crucibles at temperatures 680°C, 730°C, and 780°C has been investigated. It was shown that the uptake of iron was sluggish at 680°C and the use of 0.05% zirconium addition could effectively suppress the increase in iron content within the first 2 h of holding at temperature. Rapid and severe uptake of iron was observed at 780°C. As a consequence, it was found that the grain refinement of pure magnesium achieved by 1% zirconium addition nearly vanished after 60 min hold at 780°C due to the depletion of soluble zirconium. The uptake of iron at 730°C was conspicuous but it was still controllable by use of 0.05% zirconium addition within the first 60 min of holding at temperature. The work conducted using an aluminium titanite crucible and a boron nitride coated mild steel crucible at 730°C further confirmed the highly detrimental influence of the uptake of iron on the grain refinement of pure magnesium by zirconium. The characteristic zirconium rich coring structures developed from circular to rosette like when the melt was held at 730°C in an uncoated mild steel crucible, while no such evolution was observed when held in an aluminium titanite crucible at the same temperature. Recommendations to minimise the consumption of zirconium by the uptake of iron were made based on the results obtained from this investigation. The mechanism of grain refinement of magnesium by a low concentration of zirconium is discussed.

text: Uptake of iron and its effect on grain refinement of pure magnesium by zirconium The uptake of iron by molten magnesium from uncoated new mild steel crucibles at temperatures 680°C, 730°C, and 780°C has been investigated. It was shown that the uptake of iron was sluggish at 680°C and the use of 0.05% zirconium addition could effectively suppress the increase in iron content within the first 2 h of holding at temperature. Rapid and severe uptake of iron was observed at 780°C. As a consequence, it was found that the grain refinement of pure magnesium achieved by 1% zirconium addition nearly vanished after 60 min hold at 780°C due to the depletion of soluble zirconium. The uptake of iron at 730°C was conspicuous but it was still controllable by use of 0.05% zirconium addition within the first 60 min of holding at temperature. The work conducted using an aluminium titanite crucible and a boron nitride coated mild steel crucible at 730°C further confirmed the highly detrimental influence of the uptake of iron on the grain refinement of pure magnesium by zirconium. The characteristic zirconium rich coring structures developed from circular to rosette like when the melt was held at 730°C in an uncoated mild steel crucible, while no such evolution was observed when held in an aluminium titanite crucible at the same temperature. Recommendations to minimise the consumption of zirconium by the uptake of iron were made based on the results obtained from this investigation. The mechanism of grain refinement of magnesium by a low concentration of zirconium is discussed.

year: 2004-05

title: Sedimentology and Petrology of Miocene Cold-Seep Carbonates in Southern Hawke's Bay: Geological Evidence for Past Seabed Hydrocarbon Seepage

authors: Troup, Melissa Jane

subjects: cold seep carbonate

abstract: This study focuses on the five sites of Early Miocene aged cold seep carbonate deposits (Wanstead, Wilder Road, Haunui, Ugly Hill, and Ngawaka) in the southern East Coast Basin, North Island, New Zealand, which archive paleo-hydrocarbon seafloor seepage in a convergent margin setting. Seep deposits are hosted within calcareous slope mudstones of the Ihungia Fm and provide an important analogue to modern seep sites in the East Coast Basin. Individual fossilised seep sites have revealed, through mapping and logging, differences in outcrop extent, morphology, and lithology, while petrology has uncovered a complex array of textures, fabrics, and mineralogies within the methane derived authigenic carbonates (MDAC). Fabrics are diverse and often comprise chaotic multi-phase assemblages of various mineralogies in original and/or altered states. Enclosed within the carbonates are the fossilised remnants of chemosynthesis-based communities, particularly macro-invertebrates, which includes lucinid and vesicomyid clams, modiolid mussels, corals, gastropods, and worm tubes. These fossil types, and their distribution throughout the deposit, not only reflect the variations in fluid conditions but also the maturity of the seep system. Veining and (micro)brecciation are common at a variety of scales with multiple generations of cements forming thick fill sequences representing cyclic episodes of fracturing and sealing through carbonate formation.

Detailed analysis of MDAC deposits has led to the identification of thirteen mineral fabrics, four fracturing and/or corrosion phases, and two alteration phases. Relationships between the various fabrics and phases has led to the elucidation of a generalised paragenetic sequence while appreciating that some local variation occurs. The paragenetic sequence is divided into two diagenetic stages: an early stage that includes all mineralisation and events that occurred in the (near) seafloor environment, and a late stage that includes all mineralisation and events that occurred in the burial realm.

Early diagenesis (8 events) involved precipitation of mainly aragonite phases, including clotted and peloidal microcrystalline carbonate (micarb), fibrous and radiating crystal forms (often hydrocarbon inclusion-bearing) occasionally encase thrombolites between successive isopachous generations, and anhedral crystal masses and layers. Early destructive episodes of fracturing, (auto)brecciation, and dissolution via corrosive fluids significantly modified the deposits, ultimately increasing surface area available for further mineral growth. Late diagenesis (9 events) occurred within the burial realm and resulted in the addition of late stage pore and fracture occluding low magnesium calcite (LMC) and some minor dolomite and silica forms. Some of the early metastable aragonite forms remained pristine and unaltered, while others underwent varying degrees of fabric retentive to destructive neomorphic change generally resulting in stable LMC mineralogies. Later uplift and exhumation brought the carbonate deposits into their present-day positions.

Distinctly depleted stable δ13C isotope signatures indicate carbonate carbon was largely sourced from thermogenic methane, with some of the migrating fluids likely affected by methane pool oxidation, and mixing with marine water and residual CO2. Stable δ18O isotope signatures suggest that the ascending hydrocarbon-rich fluids experienced slight temperature fluctuations, possibly due to contact with warmer advecting fluids, and were also influenced by local gas hydrate formation and dissociation events. Preliminary lipid results importantly confirm the presence of microbes which carry out the anaerobic oxidation of methane (e.g. PMI and biphytanic diacid) and sulphate reduction (e.g. iso-and anteiso fatty acids).

The complexity revealed within these cold seep carbonates, and more particularly their cement stratigraphy, reflects the temporally and spatially dynamic nature of the fluids reaching the paleo sea floor of the East Coast Basin. Many of the features observed within these southern East Coast Basin cold seep deposits have a striking similarity with the Miocene deposits of the Italian Apennines, while others, particularly the occurrence of microbial mats (thrombolites), are an interesting feature more atypical of worldwide occurrences.

text: Sedimentology and Petrology of Miocene Cold-Seep Carbonates in Southern Hawke's Bay: Geological Evidence for Past Seabed Hydrocarbon Seepage This study focuses on the five sites of Early Miocene aged cold seep carbonate deposits (Wanstead, Wilder Road, Haunui, Ugly Hill, and Ngawaka) in the southern East Coast Basin, North Island, New Zealand, which archive paleo-hydrocarbon seafloor seepage in a convergent margin setting. Seep deposits are hosted within calcareous slope mudstones of the Ihungia Fm and provide an important analogue to modern seep sites in the East Coast Basin. Individual fossilised seep sites have revealed, through mapping and logging, differences in outcrop extent, morphology, and lithology, while petrology has uncovered a complex array of textures, fabrics, and mineralogies within the methane derived authigenic carbonates (MDAC). Fabrics are diverse and often comprise chaotic multi-phase assemblages of various mineralogies in original and/or altered states. Enclosed within the carbonates are the fossilised remnants of chemosynthesis-based communities, particularly macro-invertebrates, which includes lucinid and vesicomyid clams, modiolid mussels, corals, gastropods, and worm tubes. These fossil types, and their distribution throughout the deposit, not only reflect the variations in fluid conditions but also the maturity of the seep system. Veining and (micro)brecciation are common at a variety of scales with multiple generations of cements forming thick fill sequences representing cyclic episodes of fracturing and sealing through carbonate formation.

Detailed analysis of MDAC deposits has led to the identification of thirteen mineral fabrics, four fracturing and/or corrosion phases, and two alteration phases. Relationships between the various fabrics and phases has led to the elucidation of a generalised paragenetic sequence while appreciating that some local variation occurs. The paragenetic sequence is divided into two diagenetic stages: an early stage that includes all mineralisation and events that occurred in the (near) seafloor environment, and a late stage that includes all mineralisation and events that occurred in the burial realm.

Early diagenesis (8 events) involved precipitation of mainly aragonite phases, including clotted and peloidal microcrystalline carbonate (micarb), fibrous and radiating crystal forms (often hydrocarbon inclusion-bearing) occasionally encase thrombolites between successive isopachous generations, and anhedral crystal masses and layers. Early destructive episodes of fracturing, (auto)brecciation, and dissolution via corrosive fluids significantly modified the deposits, ultimately increasing surface area available for further mineral growth. Late diagenesis (9 events) occurred within the burial realm and resulted in the addition of late stage pore and fracture occluding low magnesium calcite (LMC) and some minor dolomite and silica forms. Some of the early metastable aragonite forms remained pristine and unaltered, while others underwent varying degrees of fabric retentive to destructive neomorphic change generally resulting in stable LMC mineralogies. Later uplift and exhumation brought the carbonate deposits into their present-day positions.

Distinctly depleted stable δ13C isotope signatures indicate carbonate carbon was largely sourced from thermogenic methane, with some of the migrating fluids likely affected by methane pool oxidation, and mixing with marine water and residual CO2. Stable δ18O isotope signatures suggest that the ascending hydrocarbon-rich fluids experienced slight temperature fluctuations, possibly due to contact with warmer advecting fluids, and were also influenced by local gas hydrate formation and dissociation events. Preliminary lipid results importantly confirm the presence of microbes which carry out the anaerobic oxidation of methane (e.g. PMI and biphytanic diacid) and sulphate reduction (e.g. iso-and anteiso fatty acids).

The complexity revealed within these cold seep carbonates, and more particularly their cement stratigraphy, reflects the temporally and spatially dynamic nature of the fluids reaching the paleo sea floor of the East Coast Basin. Many of the features observed within these southern East Coast Basin cold seep deposits have a striking similarity with the Miocene deposits of the Italian Apennines, while others, particularly the occurrence of microbial mats (thrombolites), are an interesting feature more atypical of worldwide occurrences.

year: 2010

title: Visualising categorical data: Linguistic case studies from te Reo Māori and New Zealand English

authors: Trye, David

subjects: visualisation

abstract: Categorical variables are prevalent in real-world datasets across numerous domains, yet few visualisation techniques accommodate them effectively. This is especially true of datasets comprising three or more categorical variables, termed multivariate categorical data. Visualising such data is challenging due to the lack of inherent ordering of nominal categories, the so-called ‘curse of dimensionality’, and the potential variability in the number of categories per variable. Corpus linguistics, which involves the study of large digital collections of naturally occurring language, serves as the primary application domain in this thesis. This domain was chosen because it is rich in multivariate categorical data and, at the same time, is often visualised using only basic techniques.

This thesis contributes to the area of categorical data visualisation in several ways. First, we propose a taxonomy of techniques for visualising categorical data, highlight limitations of existing solutions, and identify relevant analysis tasks. Building on this foundation, the thesis introduces novel techniques and enhancements for visualising datasets involving multiple categorical variables. We focus on adapting the layout and interactive capabilities of an existing technique that uses a matrix of heatmaps to represent pairwise category intersections. These modifications show that directly visualising statistical test results for categorical data can be beneficial for exploring bivariate patterns and associations. Furthermore, we contribute the design, implementation and evaluation of a novel technique called MultiCat, which is not restricted to pairwise intersections but rather facilitates analysis of relationships among multiple variables simultaneously. Both these techniques are interactive and offer greater scalability than existing alternatives, thereby affording new possibilities for analysing multivariate categorical data. However, since categorical variables can occur within more complex data structures, we also consider their presence in networks and hypergraphs, which require specialised methods.

To demonstrate the application of these techniques, we draw on two linguistic case studies that focus on languages of special significance in Aotearoa New Zealand. Addressing the low-resource status of Māori, the country’s Indigenous language, we first contribute two related Twitter datasets—a monolingual Māori corpus and a mixed–language Māori–English corpus—together with an architecture for differentiating Māori and English words. Our initial case study uses the monolingual Māori corpus and proposed visualisation techniques to investigate grammatical possession in Māori, offering fresh insights into the linguistic practices of contemporary speakers. The second case study uses networks and hypergraphs with categorical attributes to explore Māori loanword co-occurrence in New Zealand English newspaper articles. We find that loanwords tend not to occur in isolation and that New Zealanders are still importing new (unlisted) borrowings from Māori.

Ultimately, the techniques developed in this thesis have broad applications both within and beyond the corpus linguistics community. By enabling more effective visualisation and analysis of multivariate categorical data, this research has the potential to facilitate deeper insights into domains as diverse as education, healthcare, business and science.

text: Visualising categorical data: Linguistic case studies from te Reo Māori and New Zealand English Categorical variables are prevalent in real-world datasets across numerous domains, yet few visualisation techniques accommodate them effectively. This is especially true of datasets comprising three or more categorical variables, termed multivariate categorical data. Visualising such data is challenging due to the lack of inherent ordering of nominal categories, the so-called ‘curse of dimensionality’, and the potential variability in the number of categories per variable. Corpus linguistics, which involves the study of large digital collections of naturally occurring language, serves as the primary application domain in this thesis. This domain was chosen because it is rich in multivariate categorical data and, at the same time, is often visualised using only basic techniques.

This thesis contributes to the area of categorical data visualisation in several ways. First, we propose a taxonomy of techniques for visualising categorical data, highlight limitations of existing solutions, and identify relevant analysis tasks. Building on this foundation, the thesis introduces novel techniques and enhancements for visualising datasets involving multiple categorical variables. We focus on adapting the layout and interactive capabilities of an existing technique that uses a matrix of heatmaps to represent pairwise category intersections. These modifications show that directly visualising statistical test results for categorical data can be beneficial for exploring bivariate patterns and associations. Furthermore, we contribute the design, implementation and evaluation of a novel technique called MultiCat, which is not restricted to pairwise intersections but rather facilitates analysis of relationships among multiple variables simultaneously. Both these techniques are interactive and offer greater scalability than existing alternatives, thereby affording new possibilities for analysing multivariate categorical data. However, since categorical variables can occur within more complex data structures, we also consider their presence in networks and hypergraphs, which require specialised methods.

To demonstrate the application of these techniques, we draw on two linguistic case studies that focus on languages of special significance in Aotearoa New Zealand. Addressing the low-resource status of Māori, the country’s Indigenous language, we first contribute two related Twitter datasets—a monolingual Māori corpus and a mixed–language Māori–English corpus—together with an architecture for differentiating Māori and English words. Our initial case study uses the monolingual Māori corpus and proposed visualisation techniques to investigate grammatical possession in Māori, offering fresh insights into the linguistic practices of contemporary speakers. The second case study uses networks and hypergraphs with categorical attributes to explore Māori loanword co-occurrence in New Zealand English newspaper articles. We find that loanwords tend not to occur in isolation and that New Zealanders are still importing new (unlisted) borrowings from Māori.

Ultimately, the techniques developed in this thesis have broad applications both within and beyond the corpus linguistics community. By enabling more effective visualisation and analysis of multivariate categorical data, this research has the potential to facilitate deeper insights into domains as diverse as education, healthcare, business and science.

year: 2024