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    Inter- and intra-crystal quartz δ18O homogeneity at Okataina volcano, Aotearoa New Zealand: Implications for rhyolite genesis
    (Elsevier B V, Amsterdam, 2022-01) Sas M; Shane P; Kawasaki N; Sakamoto N; Zellmer GF; Yurimoto H
    The sources and processes involved in the genesis of the voluminous rhyolitic magmas of cataclysmic caldera-forming eruptions, and the intervening lower-volume intra-caldera extrusions, have been subject to much debate. To better understand generation of high-volume and low-volume silicic eruptions within a single volcanic centre, and how they may differ, we examined ten volumetrically varied high-SiO2 rhyolite eruptions from the Okataina Volcanic Centre (OVC) in Aotearoa New Zealand. The OVC is one of the world's most recurrently active silicic volcanoes. In the last ~600 ky, the OVC was the focus of three known caldera-forming events and numerous intermittent dome-building and fissure eruption episodes, with rhyolitic eruption activity as recent as 1314 CE. To elucidate how mass contributions from the mantle and crust may have fluctuated over the lifespan of the OVC magmatic system, oxygen isotopic ratios (δ18O) of quartz in rhyolites were investigated for the first time at inter-crystal and intra-crystal scales. Quartz crystals from four eruption episodes (two caldera-forming events, Utu, ~557 ka, Rotoiti, ~45 ka, and two intra-caldera dome-building events, Rotoma, ~9.5 ka, and Kaharoa, ~0.7 ka) yielded intra-crystal δ18O isotopic homogeneity (±0.23‰, 2sd) based on secondary ion mass spectrometry (SIMS). These samples also display inter-crystal and inter-unit homogeneity within slightly lower precision (7.6 ± 0.5‰, 2sd). Whole-crystal quartz from the same four units, as well as six other units (two intra-caldera dome-building episodes, Okareka, ~21.8 ka, Whakatane, ~5.5 ka, three pre-Rotoiti extra-caldera domes, Round Hill, Haparangi, Kakapiko, and one immediately post-Rotoiti eruption, Earthquake Flat), were then examined using high-precision laser fluorination. Single crystals also yielded mostly homogenous ratios with average δ18O = 7.6 ± 0.5‰ (2sd), which is consistent with intra-crystal SIMS analyses, albeit for a larger set of samples. Stable and radiogenic isotope mixing models using the newly obtained δ18O ratios demonstrate that OVC rhyolites can be produced by ≥25% assimilation of a regional (Torlesse-like) metasedimentary endmember by a depleted mantle source with slightly variable amounts of subduction flux, and that any incorporation of hydrothermally altered material to the system is limited to <5% in caldera and intra-caldera eruptions. The δ18O records of the OVC are among the most homogenous currently known and indicate stable and consistent mantle and crustal contributions across the lifespan of the magmatic system, with assimilation largely occurring prior to segregation of rhyolitic melts within the silicic reservoir. This isotopic homogeneity may be due to a relatively high-volume and constant magma flux at the OVC, which contrasts to other rhyolitic caldera volcanoes with greater isotopic variability.
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    Intra-caldera rhyolitic eruptions : lithostratigraphy and pyroclast textures to reconstruct the ~1314 CE Kaharoa eruption of Mt Tarawera, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand
    (Massey University, 2022) Todde, Andrea
    Rhyolitic eruptions are commonly sourced from silicic caldera systems and display a great variety of eruptive styles and magnitudes, ranging from the extrusion of lava domes to catastrophic caldera-forming eruptions. The different types of eruptions associated with rhyolitic volcanism can result in severe impacts to the environment and society, varying from a local to a global scale. Yet, due to their typically longer recurrence times compared to volcanic events of less evolved magmas, only a very limited number of rhyolitic eruptions have been documented in historical accounts or recorded through geophysical monitoring. This lack of direct records limits the current understanding of the main processes controlling the dynamics of rhyolitic volcanism and hinders the construction of robust eruption scenarios. This study presents new insights into the eruptive behaviour of rhyolitic eruptions using the Kaharoa eruption from the Taupō Volcanic Zone (TVZ) of New Zealand as a case study. The TVZ is one of the most frequently active regions of rhyolitic volcanism on Earth with the 1314±12 CE Kaharoa eruption being the most recent rhyolitic event in the TVZ. This eruption is sourced from multiple vents along the Tarawera dome complex, within the Okataina caldera system, and erupted up to 9 km3 of magma. By investigating the Kaharoa pyroclastic succession, this research contributes to constraining the key factors controlling the dynamics of moderate- to large-scale rhyolitic eruptions occurring in intra-caldera settings. The approach used in this research combines geological field investigations and quantification of the sedimentological and componentry characteristics of the deposits with the analyses of single-clast features (e.g., bulk density and textures of vesicles in pumice clasts). Field-stratigraphic relationships of 24 lithostratigraphic units for the Kaharoa deposit elucidate the intra-eruption chronology, placing time constraints on the numerous, discrete explosive episodes, while revaluating previous stratigraphic schemes. From variations within the stratigraphy in sedimentary structures, grain size and particle content of the pyroclastic beds, an array of deposit types is identified and linked to temporal changes in transport and depositional patterns as well as eruptive styles. Five distinct explosive eruptive phases are established for the Kaharoa eruption. These include multiple phases of repeated subplinian-type, fall-dominated episodes, alternating with phases characterised by overall sustained pyroclastic density currents and episodes of ash emission. A final sixth phase places the main lava dome building sequence within the proposed reconstruction and eruption model. Following constraints on the eruption from field-derived data, an in-depth investigation of the Kaharoa pumice microtextures is performed using Scanning Electron Microscopy, which revealed complex and anisotropic vesicle textures. To characterise the observed complex vesicle features of the Kaharoa pumices, a methodology is developed providing guidelines for the 2D quantification of tube-like vesicles. The integration and interpretation of the pumice textural results along the stratigraphic sequence indicates that the main processes that regulate the evolution of the magma during ascent in the shallow conduit region are magma shearing, bubble coalescence and outgassing. These factors provide bounding conditions for magma ascent dynamics and indicate cyclical variation in the eruption behaviour. Furthermore, by combining textural, sedimentological and componentry data, this study suggests that the inferred dike-shaped geometry of the conduit, together with conduit-vent wall instabilities, are primary factors in controlling: (i) the intrinsic responses of the magma to ascent and decompression to the surface and (ii) the characteristics of the ejected gas-pyroclasts mixtures, influencing the transportsedimentation regime. The dynamic magma-conduit interrelationships ultimately govern the changes in eruptive styles and overall dynamics of the Kaharoa eruption. This research defines a framework to relate depositional and textural characteristics to eruptive processes and provides critical insights into the types of eruptive sequence and eruptive style changes of intra-caldera rhyolitic eruptions. The scenario depicted for the Kaharoa eruption highlights the complex episodic, multi-phase, explosive to dome-forming nature of dike-fed rhyolitic eruptions. Furthermore, it provides crucial information for scenario-based volcanic hazard assessments of a Kaharoa-type eruption at Okataina and at other rhyolitic centres within the TVZ. Finally, comparing with available datasets from other volcanic events of similar magnitudes, magma composition, and geological settings, this research suggests that this type of rhyolitic eruption behaviour is common at other silicic caldera systems worldwide, making it of great relevance for future volcanic hazard studies.