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Item Unravelling magma generation, storage, and ascent processes from the crystal cargo and their host lavas : a case study of Taranaki Volcano, New Zealand : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Sciences at Massey University (Manawatū campus), Palmerston North, New Zealand(Massey University, 2021) D'Mello, NessaMt. Taranaki is a back-arc stratovolcano situated in the North Island of New Zealand, c. 400 km west of the Hikurangi trench. This thesis investigates the Holocene lavas of Taranaki volcano to constrain the processes of melt generation and evolution before these magmas were erupted. The crystal cargo carried in these eruptives is dominated by plagioclase, clinopyroxene, and amphibole, which with exception of the crystal rims are in chemical disequilibrium with the carrier melt represented by the groundmass. Compositional overlap of mineral data between the crystals in lavas and from xenoliths, along with numerous glomerocrysts and fractured crystals, indicate an antecrystic or xenocrystic origin of the crystals. Thermohygrometric data derived from chemical equilibrium between crystal rims and groundmass reveal hot and occasionally H2O-undersaturated felsic melts (55–68 SiO2 wt%). These cannot be related to deep crustal hot zones and are thus interpreted to be sourced from subduction melange diapirs that rise through the mantle wedge. Repeated, heterogeneous intrusion of these diverse melts into the crust, dominated by plutonic rocks of the 120 Ma Median Batholith, results in a subvolcanic crystal mush zone. The melts entrain mafic crystals in various proportions (40–55 vol.%), reducing magmatic silica contents by 5–11 wt%. Mineral phases display a complex and varied history observed through elemental concentration maps that suggest repeated resorption and recrystallisation in varied environments. These crystals are not exclusive to lavas of any stratigraphic unit and can be found within the same sample over very short length scales (within centimetres from each other). Eruption-triggering injections pick up this complex crystal cargo. Amphiboles break down during ascent and develop distinct reaction rims that vary little in width (± 20%, 1σ, on average) within individual thin sections but show a large variation between samples from different lava flows, from less than 5 to more than 450 µm. The combined evidence indicates that Taranaki lavas are a product of high temperature, aphyric to sparsely phyric subduction melange partial melts, and remobilised colder, mafic mush zone crystals. Together they ascend through the shallow crust on timescales of the order of hours to days.Item Olivine-hosted MIs as recorders of processes and conditions of slab dehydration and magmatic differentiation in the subduction zones of northern Japan : 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, 2021) Brahm Scott, RaimundoSubduction zones are the controlling mechanisms of geochemical cycling between the Earth’s crust and mantle. Transfer of incompatible trace elements and volatile species from the subducted slab to the mantle wedge by an H₂O-rich component triggers hydrous melting of the metasomatized mantle. This mechanism produces the magmatism that gives rise to volcanic arcs across the globe. The analysis of basic volcanic products from arc settings thus provides a window to study the processes involved in arc magmatism, from the mass transfer of slab to mantle wedge, partial melting, transport, differentiation and storage. In particular, olivine-hosted MIs (OHMIs) provide a window to assess for early magmatic processes without the interference of the crystal cargo, as they are potentially less affected by differentiation processes. OHMIs have the additional advantage of having the potential to retain the volatile signatures (H₂O, CO₂, S, Cl and F) which are usually lost through degassing during ascent and eruption. The subduction zones of northern Japan make for an excellent natural laboratory for the study of arc variations as they have been well characterized by seismic tomography work, making for more accurate estimations of the geometry and PT conditions of the subducted slab. There is a significant variation in the distance of the volcanic front to the trench, and in addition there is back-arc magmatism, with one volcano located extremely far from thetrench. This produces a broad distribution of PT conditions of the slab below each volcano, making it feasible to study across-arc variations related to progressive subduction. This thesis addresses the problem of post-entrapment processes that can make the recovery of initial OHMI compositions difficult. In particular, the identification of complete Fe-Mg re-equilibration of long stored antecrystic olivine hosts impedes the recovery of the syn-entrapment melt composition from traditional reverse crystallization and diffusion models. The MushPEC algorithm is developed, a novel tool to recover original MI compositions from a set of cogenetic re-equilibrated OHMIs that evolved through simple fractional or equilibrium crystallization. Next, the storage conditions and differentiation process recorded by MI populations from five arc-front samples of northern Japan are addressed. Homogeneous olivine compositions with a wide range of SiO₂ contents indicate that most olivine-hosts in arc front systems of northern Japan are antecrystic and have been completely re-equilibrated. Application of the MushPEC algorithm shows that the differentiation trends that the MI follow within each sample are incompatible with simple fractional/equilibrium crystallization and require boundary layer fractionation, where interstitial differentiated melts formed in highly crystallized solidification fronts are progressively extracted and incorporated into the main un-differentiated magmatic body. The estimated P-T-H₂O conditions of the MIs agree with boundary layer fractionation processes, promoted in hydrous melts stored at >100 MPa, as proposed by previous studies. Further, the processes of material release from the slab to the source mantle are addressed, and how they vary with increased P-T conditions across the arc. Across-arc variations of large ion lithophile (LILE), rare earth (REE), high field strength elements (HFSE) and volatile ratios are correlated with the estimated PT conditions of the slab and the expected metamorphic reactions that release components enriched in trace and volatile elements. Constant LILE/HREE and volatile/HREE ratios in the arc front indicate homogeneous slab component compositions along the arc. Progressive enrichment of these ratios towards the back-arc indicates a new liquid source enriched in most LILEs and volatiles. This increase coincides with the expected initiation of the antigorite breakdown reaction in lithospheric serpentinites, providing enriched liquids involved in back-arc magmatism. Constant (Pb, Sr)/HREE from arc to back-arc points to lawsonite breakdown occurring at all depths. Progressive LREE/HREE increase towards the back-arc is explained by increasing LREE mobility with increasing T, which controls the solubility of LREE-rich accessory minerals (e.g., allanite). Finally, the composition of the slab liquid depleted of the LILE and halogen components released by serpentinites when serpentine is exhausted at c. 8 GPa, and strong enrichment of HFSE elements indicates the participation of supercritical liquids at these extreme depths of the slab. This thesis highlights the importance of assessing the re-equilibration of OHMIs during long storage times, a process that is seldom discussed in the literature, the widespread occurrence of boundary layer fractionation in arc magmatic systems that needs to be taken into account when modelling differentiation processes in hydrous magmas, as application of simple fractional crystallization models will result in estimations of inaccurate magmatic conditions. This work also highlights the importance of the hydrated lithospheric mantle on the production of back-arc magmatism in island arc systems and the potential of halogen volatile phases on the tracking of slab fluid compositions.Item Deciphering magmatic processes in response to growth and destruction at Taranaki Volcano, New Zealand : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Sciences at Massey University (Manawatū campus), Palmerston North, New Zealand(Massey University, 2021) Zemeny, AlizTaranaki Volcano is an atypical back-arc andesitic stratovolcano located on the Taranaki Peninsula of the North Island in New Zealand. Volcanism started c. 200 kyr ago and the edifice went through at least 14 sector collapse events with the same number of corresponding growth cycles, expanding the surrounding volcanic apron over its lifetime, which presently is populated and farmed. Previous studies focussed on the modern edifice (<14 ka), the tephra deposits (<29 ka), and the volcaniclastic stratigraphy over the 200 kyr volcanic history. However, there is a significant knowledge gap in relation to the evolution of the Taranaki volcanic system during successive edifice growth cycles (i.e. inter-collapse states) and the response of the magmatic system to unloading of the edifice. In order to unravel the subaerial and subvolcanic aspects of these growth cycles, the sedimentary textures, lithologies and stratigraphy of the volcaniclastic mass-flow deposits were investigated in the southwestern sector of the Taranaki ring plain, which provides a nearly continuous stratigraphic record of the time period of c. 65-34 ka that comprises three edifice regrowth phases. Volcaniclastic mass-flow deposits were the focus of this study, providing an opportunity to explore sedimentological and geochemical characteristics of eruptive periods of Taranaki Volcano, as there are no proximal sites available close to the modern edifice. Due to the well-exposed volcaniclastic successions along the coastline of Taranaki Volcano, a classification framework was established for the globally applicable categorization of volcaniclastic mass-flow deposits in ring plain settings. Additionally, the development of the stratigraphic model of the time period c. 65-34 ka highlighted the high frequency of widely distributed volcanic mass-flow events, approximately occurring 4-5 times in every 4-10 kyr. As these deposits encompass the characteristics of eruptive periods, vesicular pyroclasts were analysed in order to investigate the time related aspects of the Taranaki magmatic system during edifice growth cycles. Based on the analysis of 220 lapilli-sized pyroclasts, whole-rock compositions were reproduced by a mixing model, indicating that the volcanic rocks originate from melt-mush mixing processes. The mixing ratios varied within the individual growth cycles and further revealed that the melt-mush ratios define the produced whole-rock compositions, where the assimilant endmember is a primitive mush and the melt endmember is a trachyandesitic ascending melt. The temporal variation of the pyroclast geochemistry showed that within the inter-collapse states (i.e. growth cycles), the range of bulk rock compositions display a broadening pattern over time towards pre-collapse states. This chemostratigraphic pattern was attributed to edifice loading affecting crustal magmatic processes over time. Whole-rock geochemical results demanded a detailed investigation of the crystal mush, from which Taranaki Volcano is fed, producing the basaltic to trachyandesitic magmas. The textural and chemical analyses of the Taranaki crystal cargo revealed reoccurring and specific crystal patterns and proved the antecrystic origin of the majority of the clinopyroxene, plagioclase and amphibole crystals. The observations highlighted that the Taranaki magmatic plumbing system involved repeated magma recharge, melt-mush mixing and crystal convection processes affecting the produced magmas within the time period of c. 65-34 ka. Crystallisation conditions (i.e. P-T-H₂O) were estimated applying thermobarometric modelling on clinopyroxene and amphibole phenocrysts. Results of the clinopyroxene rim equilibration modelling suggested source depths of approximately 26-12 km (±7.5), which outline the mid- to lower-crustal regions and further indicate polybaric rim crystallization processes. In addition, hygrometry approximations indicated that within the individual growth cycles, melts with various properties (2.9-3.7 to 3.9-4.8 wt% H₂O) arrived at different regions of the crystal mush at mid-crustal depths. Altogether, textural, chemical and hygrothermobarometric analyses outlined the spatiotemporal complexity of the Taranaki magmatic plumbing system and the connected magmatic processes of andesitic volcanism. The interconnected sedimentological and geochemical studies of this research provided an understanding of mid-crustal melt-mush mixing processes producing the Taranaki magmas within a complex, interconnected vertical mush domain affected by the temporal influence of edifice loading and unloading during consecutive edifice growth cycles.Item Understanding magmatic processes and their timescales beneath the Tongariro Volcanic Centre through microanalytical investigations of the tephra record : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Sciences at Massey University, (Manawatū Campus), Palmerston North, New Zealand(Massey University, 2020) Lormand, CharlineThe Tongariro Volcanic Centre (TgVC) is a complex volcanic system located at the southern end of the Taupo Volcanic Zone in New Zealand, and has produced historical explosive eruptions of different eruptive styles. Its three ski fields and its iconic Tongariro Alpine Crossing attract more than 130,000 visitors annually. The last eruption occurred in 2012 on the northern flank of Tongariro, at the Te Maari vent. Due to the lack of precursory activity, this eruption could have turned into a tragedy if it had happened during day time. Previous studies have focused on the TgVC phenocrysts, which do not provide insights into shallow magmatic processes, essential to mitigate the resulting volcanic hazards. To understand magma ascent processes and their associated timescales, the textures and compositions of the micrometre-sized crystal cargo (i.e. microlites and micro-phenocrysts) carried during explosive eruptions are investigated, along with their conditions of crystallisation [i.e. P-T-X(H₂O)], which are constrained using hygrothermobarometry and MELTS modelling. Glass shards from five tephra formations spanning from c. 12 ka BP to 1996 AD, associated with explosive eruptions ranging from Strombolian to Plinian in style, are studied here. High resolution images and chemical maps of the tephras and the crystals are acquired using scanning electron microscopy (SEM) and secondary ion mass spectrometry. The variety of disequilibrium textures and compositions found in the micro-phenocrysts (< 100 μm) indicates multiple events of magma mixing, magma recharge, pressure fluctuations, and suggests an antecrystic origin. Crystal size distribution (CSD) of 60,000 microlites (< 30 μm) of plagioclase and pyroxene are generated from back-scattered-electron (BSE) images using a semi-automatic method developed here to undertake this study, employing the Weka Trainable Segmentation plugin to ImageJ. Combined with a well-constrained growth rate, crystallisation times are derived and indicate that microlites crystallised 2 to 4 days before the eruption, regardless of the eruption style. Microlite crystallisation occurred between mid-crustal depths and the surface (average of c. 4 km), at unusually high temperature for arc magmas of intermediate composition (average of 1076 °C), and at low water contents (average of 0.4 wt%). Considering the inferred depths and the crystallisation times of 2 to 4 days, ascent rates of only up to 9 cm s⁻¹ prior to shallow water exsolution are calculated. Vent exit velocities are not exceeding 27 m s⁻¹ after complete water exsolution, too slow to feed explosive eruptions characterised by supersonic exit velocities. This research proposes a new conceptual model for the magmatic plumbing system beneath TgVC, where the microlitic crystal cargos result from multiple intrusions of aphyric melts through dykes, which most of the time stall and evolve at depth as deep as the mid-crust. Eventually, a magma injection percolates through previous intrusions and entrains crystals of differing textures and histories. Dykes feeding volcanism funnel into a narrow cylinder towards the surface, allowing acceleration and triggering explosive eruptions. Therefore, the conduit geometry at TgVC is a key controlling factor on the explosivity, with narrower conduits resulting in more explosive eruptions, suggesting that volatile-poor magmas can still trigger explosive eruptions. This study supports that vertical foliation of the igneous upper crust is consistent with dyking and thus may be more common than typically acknowledged.Item Melt generation, storage and ascent below Tongariro Volcanic Complex, Southern Taupo Volcanic Zone : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Manawatu, New Zealand(Massey University, 2018) Arpa, Maria Carmencita B.The study of Tongariro Volcanic Complex in New Zealand gives an opportunity to view arc magmatism from a setting where the classic arc structure is overprinted by the regional tectonic setting. Instead of viewing the volcano (and associated magmatic processes) as a component of a volcanic arc to determine the origin of andesitic magmas, focus was given on magmatic processes within the volcanic complex. Processes within the plumbing system of the volcanic complex and their implications on andesitic magmatism and volcanic hazards were determined by tracking magma, of selected eruptive products, from their reservoirs to the surface. By focusing on processes that may determine the petrological characteristics of specific deposits (from known eruptions), the influence of local structures associated with eruptive centres within the complex and the diversity of resultant eruption styles may be interpreted as magmatic processes are evaluated. The deposits for this study are from the last 16 ka history of Tongariro, majority are from the last 10 ka. These are from known eruptions and the deposits were mapped, dated and studied by previous researchers. Lava flow eruptions are from Te Maari and Red Crater, and Plinian to vulcanian eruptions are represented by the Mangamate Tephra and Ngauruhoe deposits. For each eruptive deposit, whole rock major, trace and isotope compositions were determined. Groundmass and mineral components were analysed for major elements. Major element and volatile (H2O, CO2, S, Cl) compositions of melt inclusions in component olivine and pyroxene crystals were also determined. The deposits from the recent history of Tongariro Volcano can be related to a common source. The basalts can differentiate to more evolved andesitic to dacitic compositions by crystallization and/or melting. Magmatic differentiation takes place in different reservoirs, at different depths, within the complex. Differences were observed in the volatile contents of the magmas and these may be related to magma storage and ascent processes. Magmatic processes for the deposits in this study, interpreted from compositions, considered and are consistent with eruption styles.Item Geological evolution and magmatic models for spatially and temporally variable modes of distributed volcanism, Jeju Island, Republic of Korea : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand(Massey University, 2012) Brenna, MarcoDispersed volcanism in intraplate settings produces volcanic fields that may erupt over millions of years to produce hundreds to thousands of scoria and spatter cones, tuff cones and maars, as well as lava flows. Many aspects of this globally ubiquitous process are poorly known, ranging from the tectonic drivers to the mechanisms controlling magma accumulation and ascent. To investigate magma generation processes leading to a variety of individual eruption types at volcanic fields and to understand the spatio-temporal evolution of these whole-systems, knowledge of the geochemical and petrological properties of erupted products must be linked to the geologic and tectonic framework. This study was based on detailed stratigraphic sampling of small- (<0.01 km3) and large-volume (>1 km3) eruptive sequences in the Jeju Island Volcanic Field, Korea at both individual exposed eruption centres and from deep drill cores. This island is the subaerial representation of a volcanic field developed above continental crust over the last 1.8 Ma. Pyroclastic and lava samples were analysed for whole-rock major-, trace-elements and Sr-Nd-Pb isotopes, and for mineral compositions and Sr-Nd-Pb isotopes. The Jeju magmatic system started with small-volume alkali basaltic eruptions sourced at mantle depths equivalent to c. 2.5 GPa in partially hydrous peridotite. These magmas passed through the crust and erupted rapidly, with minor modification. Intrusions and eruptions accommodated regional tectonic strain, and excess melts became stalled to fractionate toward trachyte compositions in both the lower and upper crust. Trachyte erupted sporadically, with the first episode at c. 750 ka. After this, the system started to erupt with volumetric rates two orders of magnitude higher. This accelerated magma production involved alkali basalt melts derived from greater depths/pressures (3.5 GPa) than earlier, along with subalkali basalts derived from c. 2.5 GPa. Despite prevailing extensional tectonics in the Ryukyu Volcanic Arc and strain accommodation at Jeju, further magmas accumulated and evolved to trachyte compositions at lower crustal depths and erupted in a second episode c. 25 ka ago. Small-volume eruptions of rapidly rising primitive alkali basalt also continued throughout the life of the field, and potentially interacted with shallower reservoirs of subalkali magmas to generate bimodal volcanism. Depending on magma volumes, intrusion and plumbing complexities, these generated a range from simple volcanic structures to complex multiple-episode and/or multiple vent eruptive centres at the surface. The geochemical data collected revealed how seemingly simple monogenetic eruptions can be fed by complex and distinct magmatic entities. The same was valid for the entire field, where magma source and evolution conditions vary over time. The variety in volcanic activity is a function of magma types influenced by prior mantle modification events, as well as local and distal tectonic stresses and strain arrangements. This study showed that it is ultimately the site and spatial pattern of melting and melt-production rate that determines the final surface morphology, elevation and spatial distribution of magma types in a volcanic field.