title: Volcanology and secondary alteration of the 1.6 Ma Ngaroma Ignimbrite, Upper Waipari Valley

authors: Ross, ChrisAnne

subjects: Taupo Volcanic Zone (TVZ)

abstract: The 1.6 Ma Ngaroma eruption from the Mangakino Volcanic Centre (MVC) is the first known rhyolitic event of the Taupo Volcanic Zone (TVZ), and variations of primary components recorded throughout the Ngaroma Ignimbrite provide insight(s) into pre-eruptive processes and post-emplacement mechanisms.

A field study of the Ngaroma Ignimbrite was undertaken proximal to the source caldera (~25 km) around the township of Ngaroma. Two primary facies (a lithic- rich and lithic-poor facies) and four subfacies have been identified and are presented in four stratigraphic logs, which vary both vertically and laterally throughout the field area. The facies reflect syn- and post-deposition process that are identifiable from one another based on macroscopic observations including changes in primary componentry, the degree of welding and nature of secondary alteration. Variations in depositional structures and textures highlight the intense welding, limited incorporation of vent-derived lithics, and the absence of a pumice/ash fall deposit underlying the ignimbrite, which suggest that the parent pyroclastic flow formed from the immediate collapse of a highly explosive, caldera-forming eruption column.
Petrographic observations and mineralogic analyses involving optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) record high concentrations of sanidine and cristobalite, followed by clay minerals (kaolinite), which suggest the deposit endured significant post-emplacement processes such as vapour-phase alteration and devitrification. The ignimbrite matrix varies from a eutaxitic texture comprising (a) visible, well-formed, but devitrified, glass shards typically 300 to 800 μm, to (b) a fine-grained, homogenous, optically unresolvable medium; the latter can be resolved under SEM and comprises intergrown <3 μm- sized K-feldspar (sanidine), amorphous/nano-crystalline silica (cristobalite ± tridymite), and pore cavities.

Geochemical analyses of major and trace elements on bulk ignimbrite and pumice were determined using x-ray fluorescence (XRF) spectrometry and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and on individual minerals by electron microprobe analysis (EPMA). Samples used in these geochemical analyses have been pervasively altered, and that due to the lack or absence of primary material (fresh glass and pumice), some of the patterns observed here are presumed to in part reflect post alteration processes (vapour-phase alteration and devitrification) and therefore, are of limited use for understanding primary magmatic processes. The major and selective trace elements of bulk rock ignimbrite and pumice confirm a rhyolitic composition and are consistent with the effect of syn- and post-emplacement alteration processes. Geochemical signatures of rare earth elements (REE) observed on multielement diagrams are associated with subduction related magmas and indicative of crustal contamination. Major element compositions of both feldspar and pyroxene phenocrysts range from An₁₅₋ An₂₈ and Or₅₋Or₁₂ and from En₃₂ to En₄₁, respectively, and are typical composition of rhyolitic magmas.

text: Volcanology and secondary alteration of the 1.6 Ma Ngaroma Ignimbrite, Upper Waipari Valley The 1.6 Ma Ngaroma eruption from the Mangakino Volcanic Centre (MVC) is the first known rhyolitic event of the Taupo Volcanic Zone (TVZ), and variations of primary components recorded throughout the Ngaroma Ignimbrite provide insight(s) into pre-eruptive processes and post-emplacement mechanisms.

A field study of the Ngaroma Ignimbrite was undertaken proximal to the source caldera (~25 km) around the township of Ngaroma. Two primary facies (a lithic- rich and lithic-poor facies) and four subfacies have been identified and are presented in four stratigraphic logs, which vary both vertically and laterally throughout the field area. The facies reflect syn- and post-deposition process that are identifiable from one another based on macroscopic observations including changes in primary componentry, the degree of welding and nature of secondary alteration. Variations in depositional structures and textures highlight the intense welding, limited incorporation of vent-derived lithics, and the absence of a pumice/ash fall deposit underlying the ignimbrite, which suggest that the parent pyroclastic flow formed from the immediate collapse of a highly explosive, caldera-forming eruption column.
Petrographic observations and mineralogic analyses involving optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) record high concentrations of sanidine and cristobalite, followed by clay minerals (kaolinite), which suggest the deposit endured significant post-emplacement processes such as vapour-phase alteration and devitrification. The ignimbrite matrix varies from a eutaxitic texture comprising (a) visible, well-formed, but devitrified, glass shards typically 300 to 800 μm, to (b) a fine-grained, homogenous, optically unresolvable medium; the latter can be resolved under SEM and comprises intergrown <3 μm- sized K-feldspar (sanidine), amorphous/nano-crystalline silica (cristobalite ± tridymite), and pore cavities.

Geochemical analyses of major and trace elements on bulk ignimbrite and pumice were determined using x-ray fluorescence (XRF) spectrometry and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and on individual minerals by electron microprobe analysis (EPMA). Samples used in these geochemical analyses have been pervasively altered, and that due to the lack or absence of primary material (fresh glass and pumice), some of the patterns observed here are presumed to in part reflect post alteration processes (vapour-phase alteration and devitrification) and therefore, are of limited use for understanding primary magmatic processes. The major and selective trace elements of bulk rock ignimbrite and pumice confirm a rhyolitic composition and are consistent with the effect of syn- and post-emplacement alteration processes. Geochemical signatures of rare earth elements (REE) observed on multielement diagrams are associated with subduction related magmas and indicative of crustal contamination. Major element compositions of both feldspar and pyroxene phenocrysts range from An₁₅₋ An₂₈ and Or₅₋Or₁₂ and from En₃₂ to En₄₁, respectively, and are typical composition of rhyolitic magmas.

year: 2020

title: Exuberance in British share prices during the railway mania of the 1840s: Evidence from the Phillips, Shi and Yu Test

authors: Oxley, Les

subjects: explosive behaviour

abstract: In this study, we empirically investigate evidence of explosive behaviour in the British share prices of canals, railways and waterworks in the nineteenth century using the right-tailed unit root test of Phillips, Shi and Yu (2015, PSY). Of particular interest to the Railway Mania in the 1840s, our results provide evidence of exuberance in share prices of railways during the most remarkable events in history. In addition, we find signs of exuberance in canals and waterworks share prices. Our findings will be of great interest to both economic historians and scholars who are interested in the rage of speculation in historical share prices during the Railway Mania

text: Exuberance in British share prices during the railway mania of the 1840s: Evidence from the Phillips, Shi and Yu Test In this study, we empirically investigate evidence of explosive behaviour in the British share prices of canals, railways and waterworks in the nineteenth century using the right-tailed unit root test of Phillips, Shi and Yu (2015, PSY). Of particular interest to the Railway Mania in the 1840s, our results provide evidence of exuberance in share prices of railways during the most remarkable events in history. In addition, we find signs of exuberance in canals and waterworks share prices. Our findings will be of great interest to both economic historians and scholars who are interested in the rage of speculation in historical share prices during the Railway Mania

year: 2017-04

title: Ethics in Maori research: Working paper

authors: Cram, Fiona

subjects: Maori

abstract: When we think about what we are doing as researchers, one of our main tasks is to acquire knowledge. For some researchers their task begins and ends there. Knowledge is viewed as cumulative, that by adding to some knowledge pool we will one day be able to put the component parts together and discover universal laws. Many researchers also assume that the knowledge they have collected is objective, value-free and apolitical. This is part of psychologists’ ‘physics envy’. A Maori view of knowledge is very different from this. For Maori the purpose of knowledge is to uphold the interests and the mana of the group; it serves the community. Researchers are not building up their own status; they are fighting for the betterment of their iwi and for Maori people in general.

text: Ethics in Maori research: Working paper When we think about what we are doing as researchers, one of our main tasks is to acquire knowledge. For some researchers their task begins and ends there. Knowledge is viewed as cumulative, that by adding to some knowledge pool we will one day be able to put the component parts together and discover universal laws. Many researchers also assume that the knowledge they have collected is objective, value-free and apolitical. This is part of psychologists’ ‘physics envy’. A Maori view of knowledge is very different from this. For Maori the purpose of knowledge is to uphold the interests and the mana of the group; it serves the community. Researchers are not building up their own status; they are fighting for the betterment of their iwi and for Maori people in general.

year: 1993