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Oxygen isotopes are useful for tracing interactions between magmas, crustal rocks and surface-derived waters. We use them here to consider links between voluminous silicic magmatism and large-scale hydrothermal circulation in New Zealand's central Taupō Volcanic Zone (TVZ). We present >350 measurements of plagioclase, quartz, hornblende and groundmass glass δ18O values from 40 eruptions from three discrete magmatic systems: Ōkataina and Taupō calderas, and the smaller Northeast Dome system. For each mineral, mean δ18O values vary by ~1% (δ18Oplag = +6.7-7.8%, δ18Oqtz = +7.7-+8.7%, δ18Ohbl = +5.4-+6.4%, δ18Oglass = +7.1-+8.0%), and inter-mineral fractionations mostly reflect high-temperature equilibria. Outliers (e.g., ~+6% or >+10% plagioclase) represent contaminants incorporated on short-enough timescales to preserve disequilibrium (~102 yrs for plagioclase). Melt δ18O values calculated from phenocrysts are ~+7.3-+8.0%. Where multiple magmas were involved in the same eruption their δ18Omelt values are indistinguishable, implying that their parental mushes were isotopically well-mixed. However, small (≤0.5%) but consistent δ18Omelt value gradients occur over millennial timescales at Ōkataina and Taupō, with short-term ~0.4-0.5% decreases in δ18Omelt values over successive post-caldera eruptions correlating with increases in 87Sr/86Sr. These changes reflect tens of percent assimilation of a mixture of hydrothermally altered silicic plutonic material and higher-87Sr/86Sr greywacke. These examples represent the first evidence for assimilation of altered crust into TVZ magmas. The subtle and short-lived isotopic signals of these interactions are only recognized through the high temporal resolution of the TVZ eruptive record and complementary radiogenic isotope data. Similar interactions may have been obscured in other nominally highor normal-δ18O magmatic systems. [ABSTRACT FROM AUTHOR]

Our understanding of magmatism in the New Zealand North Island subduction system is limited by sparse knowledge of the structure and composition of the deep arc crust, particularly in the northern Taupo Volcanic Zone (NTVZ), where there are multiple major tectono-magmatic transitions impacting the crust. Herein, we use detailed textural and micro-chemical analyses from multiple crystalline phases and melt inclusions to probe the magmatic processes occurring in the lower crust that ultimately control the early evolution of magmas beneath Pūtauaki (Mt. Edgecumbe) in the NTVZ. Here, two-pyroxene-plagioclase glomerocrysts in dacite lavas have textures, compositions, and phase associations consistent with a lower crustal cognate cumulate origin. Importantly, the glomerocrysts record a multi-stage evolution in the lower crust. Elevated 87Sr/86Sr isotope ratios (0.7058–0.7062) in high-An (up to An92) plagioclase, along with relatively low Mg/Fe ratios in olivine and pyroxene within the glomerocrysts, record the deep assimilation of isotopically enriched lower crust and the significant fractionation of Mg-rich phases, near the base of the crust. The evolved melts appear to have then ascended into the lower crust and subsequently crystallized the gabbroic mineral assemblage preserved in the xenoliths. Two pyroxene thermobarometry indicates that crystallization/equilibration occurred at temperature between 926 and 1001 °C and pressure between 4 and 7 kbars, consistent with crustal depths of 15–26 km. Melt inclusions hosted within pyroxenes in glomerocrysts are rhyodacitic to rhyolitic (68–74 wt% SiO2) indicating that fractionation of the glomerocryst assemblage resulted in the production of melts. We propose that these represent melts that ascend into the upper crust and fractionate to produce rhyolites that erupt from the large calderas of the central TVZ (CTVZ). Our findings also indicate that crustal architecture and magma pressure–temperature–time pathways beneath the NTVZ are similar to magmatic systems in the STVZ, but distinct from those beneath the large rhyolitic systems in the central TVZ. These differences may arise from variations in mantle flux and rates of extension along the arc. [ABSTRACT FROM AUTHOR]

Arc magmas typically contain phenocrysts with complex zoning and diverse growth histories. Microlites highlight the same level of intracrystalline variations but require nanoscale resolution which is globally less available. The southern Taupo Volcanic Zone (TVZ), New Zealand, has produced a wide range of explosive eruptions yielding glassy microlite-bearing tephras. Major oxide analyses and textural information reveal that microlite rims are commonly out of equilibrium with the surrounding glass. We mapped microlites and microcrysts at submicron resolution for major and trace element distributions and observed three plagioclase textural patterns: (1) resorption and overgrowth, (2) oscillatory zoning, and (3) normal (sharp) zoning. Pyroxene textures are diverse: (1) resorption and overgrowth, (2) calcium-rich bands, (3) hollow textures, (4) oscillatory zoning, (5) sector zoning, (6) normal zoning and (7) reverse zoning. Microlite chemistry and textures inform processes operating during pre-eruptive magma ascent. They indicate a plumbing system periodically intruded by short-lived sub-aphyric dykes that entrain microantecrysts grown under diverse physico-chemical conditions and stored in rapidly cooled, previously intruded dykes. Changes in temperature gradients between the intrusion and the host rock throughout ascent and repeated magma injections lead to fluctuations in cooling rates and generate local heterogeneities illustrated by the microlite textures and rim compositions. Late-stage degassing occurs at water saturation, forming thin calcic microcryst rims through local partitioning effects. This detailed investigation of textures cryptic to conventional imaging shows that a significant proportion of the micrometre-sized crystal cargo of the TVZ is of antecrystic origin and may not be attributed to late-stage nucleation and growth at the onset of volcanic eruptions, as typically presumed. [ABSTRACT FROM AUTHOR]

We consider here the validity, accuracy, and precision of rhyolite-MELTS modelling in inferring the pre-eruptive magma storage conditions for the caldera-forming 25.5 ka Oruanui and 232 CE (1.72 ka) Taupō eruptions at Taupō volcano, New Zealand as proposed by Pamukçu et al. (2020: Contrib Mineral Petrol 175: 48). There are four major issues with the modelling as used. First, the model is inappropriately applied for the Taupō event as this magma does not contain phenocrystic quartz. Second, the products of both eruptions, as is typical for virtually all Taupō Volcanic Zone rhyolites, contain plagioclase but lack sanidine, leading to increased model errors beyond those stated. Third, temperatures utilised for modelling crystallisation histories for both magmas are in conflict with independent measures of magmatic temperatures for these rocks. Fourth, glass compositions for each of these eruptions individually fall within analytical uncertainty yet yield model pressures that vary over hundreds of MPa, in part due to contrasting analytical protocols. We conclude that rhyolite-MELTS is too imprecise (errors of ± ~ 100 MPa or more for the methods applied) for quartz + 1 feldspar systems like at Taupō volcano specifically and the Taupō Volcanic Zone in general to be applied with confidence to deriving relative or absolute depths of magma storage. [ABSTRACT FROM AUTHOR]

The Taupo Volcanic Center [TVC; Taupo Volcanic Zone (TVZ), New Zealand] is home to the two most recent silicic caldera-forming eruptions in the TVZ—the giant (530 km3 DRE) Oruanui supereruption and the large (35 km3 DRE) Taupo eruption. This offers a unique opportunity to investigate similarities and differences between large and supersized volumes of rhyolitic magma at the TVC and more generally. We combine information from crystal textures (crystal size distributions), crystal compositions (Ti-in-quartz diffusion chronometry), and glass compositions (rhyolite-MELTS geobarometry) to investigate the crustal storage conditions, longevity, and eruption of these magmas. Results show differences between the storage depths of the Oruanui and Taupo magmas consistent with the differences in their glass silica contents but strikingly similar, short crustal residence times for the final crystal-poor, eruptible magma bodies (decades to centuries). Differences in groundmass textures between the two systems also likely reflect differences in ascent durations, decompression paths, and/or amounts of undercooling experienced by these systems due to the differences in their storage depths. Finally, comparing the evolution of storage depths and magma compositions at the TVC with those in a preceding central TVZ ignimbrite 'flare-up' supports the idea that the modern TVC may be flaring up and highlights the evolution of magma storage depths with increasing system maturity in this volcanic region. [ABSTRACT FROM AUTHOR]

We present a non-stationary epidemic-type aftershock sequence (ETAS) model in which the usual assumption of stationary background rate is relaxed. Such a model could be used for modeling seismic sequences affected by aseismic transients such as fluid/magma intrusion, slow slip earthquakes (SSEs), etc. The non-stationary background rate is expressed as a linear combination of B-splines, and a method is proposed that allows for simultaneous estimation of background rate as well as other ETAS model parameters. We also present an extension to this non-stationary ETAS model where an adaptive roughness penalty function is used and consequently provides better estimates of rapidly varying background rate functions. The performance of the proposed methods is demonstrated on synthetic catalogs and an application to detect earthquake swarms (possibly associated with SSEs) in Hikurangi margin (North Island, New Zealand) is presented. [ABSTRACT FROM AUTHOR]

Subduction-related Quaternary volcanic rocks from Solander and Little Solander Islands (46°34′S, 166°53′E), south of mainland New Zealand, are amphibole- and plagioclase-phyric trachyandesites–andesites. The Solander and Little Solander Island trachyandesites–andesites have a narrow range of SiO2 (60·26–62·19 and 58·20 wt % respectively), and high incompatible element concentrations (e.g. Sr 931–1265 ppm, Ba 619–769 ppm and Sr 2200–2269 ppm, Ba 733–798 ppm respectively). They have geochemical affinities with modern adakites (e.g. high Sr/Y ∼66–105 and ∼132–146, and depleted Y ∼11–16 ppm and 16–17 ppm, respectively). Isotopically similar porphyritic quench-textured enclaves reflect mixing with intermediate (basaltic andesite) magmas with high incompatible element concentrations. The presence of porphyritic and equigranular enclaves, disequilibrium phenocryst textures and compositions, and cumulate nodules is consistent with the Solander sample suite having evolved in an open crustal magma storage system through combined crystal fractionation and mafic magma recharge. Mixing with incoming batches of hotter and more mafic magma is marked by the appearance of oscillatory- and simple-zoned amphibole and sieve-textured plagioclase phenocrysts with An-rich rim overgrowths. High XMg clinopyroxene crystals in the Little Solander trachyandesites indicate the late influx of more mafic compositions. Although fractionation is clearly an important process in producing diversity in the Solander magmas, mafic magma recharge is suggested to be responsible for the enrichment in incompatible element concentrations observed in the more evolved Solander and Little Solander Island rocks. The concave-up rare earth element patterns of the Solander and Little Solander trachyandesites and andesites are consistent with significant amphibole fractionation of hydrous magmas at crustal depths outside the garnet stability field. [ABSTRACT FROM PUBLISHER]

Regular large-scale edifice collapse and regrowth is a common pattern during the long lifespans of andesitic stratovolcanoes worldwide. The >130 k.y. history of Mount Taranaki, New Zealand, is punctuated by at least 14 catastrophic collapses, producing debris avalanche deposits of 1 to >7.5 km³. The largest of these sudden events removed as much as one-third of the present-day equivalent cone. The resulting deposits show similar sedimentary and geomorphic features, suggesting similar proto-edifice characteristics, failure trigger mechanisms, and runout path conditions. Each collapse was followed by sustained renewed volcanism and cone regrowth, although there are no matching stepwise geochemical changes in the magma erupted; instead a stable, slowly evolving magmatic system has prevailed. Last Glacial climatic variations are also uncorrelated with the timing or magnitudes of edifice collapse. We demonstrate here that, if the magmatic composition erupted from stratovolcanoes is constant and basement geology conditions are stable, large-scale edifice collapse and the generation of catastrophic debris avalanches will be governed by the magma supply rate. Using a mass balance approach, a volume-frequency model can be applied to forecasting both the probable timing and volume of future edifi ce failure of such stratovolcanoes. In the Mount Taranaki case, the maximum potential size of a present collapse is estimated to be 7.9 km³, while the maximum interval before the next collapse is <16.2 k.y. The current annual collapse probability is ~0.00018, with the most likely collapse being a small one (<2 km³). [ABSTRACT FROM AUTHOR]

Timescales of magma chamber assembly and recharge are investigated here by applying 1D and 2D diffusion modeling techniques to high-resolution maps of titanium in quartz from a large-volume ignimbrite eruption in the Taupo Volcanic Zone, New Zealand. We compare quartz zonation patterns and associated diffusion timescales from the ∼340 ka Whakamaru super-eruption (magma volume ∼1000 km3) with the Younger Toba Tuff super-eruption, 74 ka (2000 km3), Sumatra, and the smaller volume ∼50 ka Earthquake Flat eruption (10 km3), Okataina Caldera Complex, New Zealand. Two principal timescales are presented: that of chamber recharge and eruption triggering events, and that of magma generation (involving long-term assembly, stirring and reactivation). Synchrotron micro-X-ray fluorescence maps of core–rim quartz transects provide a high-resolution record of magma chamber conditions throughout quartz crystallization. Quartz crystals from the Whakamaru magma display complex zonation patterns indicating fluctuating pressure–temperature conditions throughout the crystallization history. Toba and Earthquake Flat, in contrast, display simple quartz-zoning patterns and record slightly longer periods of crystal residence in the chamber that fed the eruption. We apply Lattice Boltzmann 2D diffusion modeling to reconstruct the timescales of quartz crystal zonation, accounting for crystal boundary complexities. Quartz crystal orientation is also accounted for by using geometry constraints from the synchrotron data. Our calculations suggest that crystal-mush reactivation for the main Whakamaru magma reservoir occured over a period of the order of 103–104 years. Both the Earthquake Flat and Toba eruptions experienced a significant recharge event (causing a temperature and pressure change), which occurred within ∼100 years of eruption. In comparison, the complex Whakamaru quartz zoning patterns suggest that the magma body experienced numerous thermal and compositional fluctuations in the lead-up to eruption. The final magma recharge event, which most probably triggered the eruption, occurred within ∼10–60 years of the eruption. Even though the volume of these systems spans two orders of magnitude, there does not appear to be a relationship between magma volume and diffusion timescale, suggesting similar histories before eruption. [ABSTRACT FROM AUTHOR]

Taranaki (Mt. Egmont) in the western North Island of New Zealand is a high-K andesite volcano with an eruptive history extending over more than 200 kyr. In general, petrological research has concentrated on the post-10 ka record of the modern edifice. This study focuses on the earlier history, which is recorded in 11 major pre-7 ka debris avalanche deposits. Each of these formed as a result of a catastrophic collapse of the edifice of the time. The clast assemblages of these deposits provide insights into the chemical compositions of magmas erupted during the earlier stages of activity of the volcano and form the basis for a new chemo-stratigraphic analysis of the pre-10 ka volcanic succession. Sample suites from the studied debris avalanche deposits show a progressive enrichment in K2O and large ion lithophile elements (LILE), reflecting a gradual evolution to high-K andesite. The early magmatic system (pre-100 ka) produced a wide range of compositions including relatively primitive basalts and basaltic andesites. These rocks contain phenocryst assemblages that indicate crystallization within the lower crust or mantle, including a broad range of clinopyroxene compositions, high-Al2O3 hornblende, olivine and phlogopite. A higher proportion of high-silica compositions in the younger sample suites and the appearance of late-stage, low-pressure mineral phases, such as high-TiO2 hornblende, biotite and Fe-rich orthopyroxene, reflect a gradual shift to more evolved magmas with time. These new data are interpreted to reflect a multi-stage origin for Taranaki andesites. Parental magmas were generated within a lower crustal ‘hot zone’, which formed as a result of repeated intrusions of primitive melts into the lower crust. The geochemical and mineralogical evidence indicates that prior to 100 ka this zone was relatively thin and cold, so that primitive magmas were able to rise rapidly through the crust without significant interaction and modification. As the hot zone evolved, larger proportions of intruded and underplated mafic material were partially remelted, and interaction of these melts with fractionating mantle-derived magmas generated progressively more K- and LILE-enriched compositions. A complex and dispersed magma assembly and storage system developed in the upper crust where the hot-zone melts were further modified by fractional crystallization and magma mixing and mingling. [ABSTRACT FROM PUBLISHER]

Cathodoluminescence (CL) zoning in quartz crystals from rhyolitic pumices in two ignimbrite members of the ~340-ka Whakamaru super-eruption deposits, Taupo Volcanic Zone, New Zealand, is investigated in conjunction with the analysis of Ti concentration in quartz to reconstruct the history of changing magma chamber conditions and to elucidate the eruption-triggering processes. CL intensity images are taken as a proxy for Ti concentration and thus temperature and/or pressure and/or compositional variations during crystal growth history. Estimates of the maximum temperature changes (i.e., assuming other factors influencing Ti uptake remain constant) are made using the TitaniQ geothermometer based on the Ti concentration in quartz. These results are reviewed in comparison with Fe-Ti oxide, feldspar-melt and amphibole geothermometry. Core-to-rim quartz Ti profiles record a marked change in conditions (temperature increase and/or pressure decrease and/or change in melt composition) causing and then following a significant resorption horizon in the outer parts of the crystals. Two alternative models that could explain the quartz Ti zonation invoke a temperature increase caused by mafic recharge and/or a pressure decrease due to magma ponding and re-equilibration at shallow crustal levels. Concomitant changes in melt composition and Ti activity may, however, also have strongly influenced Ti uptake into the quartz. Some crystals also show other marked increases in CL brightness internally, but any accompanying magmatic changes did not result in eruption. Diffusion modelling indicates that this significant change in conditions occurred over ~10-85 years prior to caldera-forming eruption. This rapid thermal pulse or pressure change is interpreted as evidence for open-system processes, and appears to record a magma chamber recharge event that rejuvenated the Whakamaru magma system (melt-dominant magma plus crystal mush), and potentially acted as a trigger for processes that led to eruption. [ABSTRACT FROM AUTHOR]

The Auckland volcanic field is a Quaternary monogenetic basaltic field of 50 volcanoes. Rangitoto is the most recent of these at ~500 year BP and may mark a change in the behaviour of the field as it is the largest by an order of magnitude and is unusual in that it erupted magmas of alkalic then subalkalic basaltic composition in discrete events separated by ≤50 years. Major and trace element geochemistry together with Sr-Nd and U-Th-Ra isotopes provides the basis for modelling the melting conditions that brought about the eruption of two chemically different lavas with very little spatial or temporal change. Sr-Nd isotopes suggest that the source for both eruptions is similar with a slight degree of heterogeneity. The basalts show high Th-excess compared with comparable continental volcanic fields. We show that the alkalic basalts give evidence for lower degrees of partial melting, higher amounts of residual garnet, a longer melting column and lower melting and upwelling rates compared with the subalkalic basalts. The low upwelling rates (0.1-1.5 cm/year) modelled for both magmas do not suggest a plume or major upwelling in the mantle region beneath Auckland; therefore, we suggest localised convection due to relict movement from the active subduction system situated 400 km to the southeast. A higher porosity for the initial alkalic basalt is based on Ra-excesses, suggesting movement of melt by two different porosities: the initial melt travelling in fast high porosity channels from greater depths preserving a high Th-excess and the subsequent subalkalic magma travelling from a shallower depth through lower porosity diffuse channels preserving a high Ra-excess; this creates a negative array in (Ra/Th) versus (Th/U) space previously only seen in mid ocean ridge Basalt data. This mechanism suggests the Auckland volcanic field may operate by the presence of discrete melt batches that are able to move at different depths and speeds giving the field its erratic spatial and temporal pattern of eruptions, a type of behaviour that may have implications for the evolution of other continental volcanic fields worldwide. [ABSTRACT FROM AUTHOR]

Previous U-Pb zircon dating of the Pomona Island Granite (PIG) pluton (South Island, New Zealand) yielded either Permo-Carboniferous or Late Jurassic ages for five samples essentially indistinguishable in their field, petrographic, and geochemical characteristics. Detailed cathodoluminescence imaging and LA-ICP-MS dating of zircon in new and previously dated samples reveal that portions of the pluton contain either delicately oscillatory-zoned Late Jurassic zircon grains with rare Permo-Carboniferous cores, or Permo-Carboniferous grains with ubiquitous but thin Late Jurassic rims. Based on zircon dissolution-overgrowth textures, zircon rim and core trace element compositions, and the limited extent of sub-solidus rock recrystallisation textures, the bipartite age distribution is unlikely to reflect variable Pb-loss or metamorphic re-equilibration. Magmatic Zr-saturation temperatures were ≥851°C for samples dominated by Jurassic zircon and ≤809°C for samples with a predominance of Permo-Carboniferous zircon. Together, these data are consistent with PIG magmas having been derived from partial melting of a Permo-Carboniferous felsic igneous source at variable temperature wholly in the Late Jurassic (157 ± 3 Ma). The lowest temperature melts would have been incapable of dissolving significant amounts of pre-existing zircon and consequently generated inheritance-rich magmas, with the very thin rims on the pre-existing zircon grains the only evidence of the Late Jurassic magmatic age. As the partial melting temperature increased and nearly all pre-existing zircon grains dissolved into the magma, an inheritance-poor batch of melt was generated, which precipitated new zircon grains upon crystallisation. Concentrations of major and many trace elements in both magma batches may have been buffered by retention of residual quartz and feldspar in the source, which would explain the limited geochemical differences between inheritance-rich and inheritance-poor portions. [ABSTRACT FROM AUTHOR]

The Whakamaru eruption is the largest-volume eruption known to have originated from the hyper-productive Taupo Volcanic Zone, New Zealand. Major, minor and trace element concentrations of plagioclase crystals and cathodoluminescence images, used as a proxy for Ti concentrations in quartz crystals, have been used to explore their chemical zonation. Three plagioclase populations are identified. Group 1 crystals are characterized by inherited cores of composition An45–60, Ba 115–650 ppm and La 3–9 ppm, rims of c. An30, Ba 450–800 ppm and La 7–10 ppm and the presence of a thin overgrowth rim on several crystals cores. Group 2 crystals are oscillatory-zoned plagioclases of composition An30–40, Ba 450–730 ppm and La 8·5–9·5 ppm. Group 3 plagioclase crystals have cores of An25–35 and rims of An20–25 and low Sr contents (280–480 ppm). From the chemical composition of these plagioclase crystals, four physicochemically distinct rhyolitic melts are identified: (1) an andesitic progenitor melt in which the cores of Group 1 crystals crystallized; (2) a greywacke melt or greywacke protolith melt responsible for narrow overgrowth rims on Group 1 crystal cores; (3) melt derived from the rejuvenation of a mature crystal mush body from which Group 3 plagioclase crystals crystallized; (4) a final, rhyolitic melt created by the amalgamation of varying proportions of the andesitic, greywacke-derived and rejuvenated melts with subsequent, open-system fractional crystallization of a plagioclase-dominant crystal assemblage. Cathodoluminescence imaging of quartz crystals reveals complex zonation, the result of a dynamic crystallization history from potentially polygenetic sources. Diffusion modelling of the greyscale intensity of cathodoluminescence images (as a proxy for Ti content) for a selection of bright core–rim interfaces of quartz crystals suggests that renewed quartz growth at the rim zones occurred <300 years (peak likelihood 50–70 years) prior to and continued towards the climactic eruption. This is consistent with timescales of <280 years determined from core–rim interfaces of Group 1 plagioclase crystals, suggesting that the magma chamber was ephemeral, derived from mixing of magmas from multiple sources shortly prior to eruption. This study adds to a growing body of evidence for the ephemeral nature and geologically rapid mixing and mobilization of liquid silicic magma bodies leading to supereruptions, compared with the timescales of hundreds of thousands of years required to accumulate the precursor magma and crystals. [ABSTRACT FROM AUTHOR]

U-Pb zircon age spectra from a metasedimentary xenolith and a young dacite provide evidence for rapid episodes of crustal melting and eruption at Taupo volcano, as well as unexpected crustal structure beneath the Taupo Volcanic Zone. Ages from the xenolith in a 28 ka rhyolite dome match those from Early Cretaceous Pahau Terrane graywackes that crop out >75 km east of the volcano and were underthrust and partially melted during the penecontemporaneous accretion process. Boundaries between graywacke terranes in the North Island are thus more complex than implied from map views. In contrast, ages from zircons in the 20 ka Omega dacite match the nearby surficial graywacke of the Jurassic Kaweka Terrane (55% of grains), Quaternary plutonic sources (39%), and Pahau Terrane graywacke (6%). Five of 72 grains analyzed contain graywacke cores and Pleistocene rims. Survival of euhedral xenocrystic zircons through the generation processes for Omega dacite magma refl ects wholesale melting of crustal protoliths (including varieties not previously considered for petrogenetic modeling), open-system behavior, and crustal melts being generated and erupted within ∼1-10 yr. [ABSTRACT FROM AUTHOR]

Slab-derived aqueous fluid components (Ba, Cl) correlate well with oxygen fugacity, and other well-defined characteristics of silicic magmas in the Taupo Volcanic Zone (TVZ) between a cold-wet-oxidizing magma type [R1: amphibole ± biotite; high Sr, low Zr and FeO*/MgO, depleted middle rare-earth elements (MREE)] and a hot-dry-reducing magma type (R2: orthopyroxene ± clinopyroxene; low Sr, high Zr, and FeO*/MgO, less depleted MREE). Oxygen fugacity was obtained from analysis of Fe–Ti oxides and ranges between −0.04 and +2.1 log units (ΔQFM, where QFM = quartz + fayalite + magnetite buffer) and is positively correlated with the bulk-rock Ba/La ratio, indicating that slab-derived fluid is the oxidizing agent in the rhyolites. Chlorine contents in hornblende also correlate with the bulk-rock Ba/La ratio. Hence, high-fluid flux typically correlates with the R1 and low-fluid flux with R2 rhyolite magma types. A geochemical evolution and distribution can be tracked in time and space throughout the central region of the TVZ from 550 ka to present and has revealed two distinct magmatic cycles that vary in length. The first cycle included widespread R1 type magmatism across the central TVZ beginning ca. 550 ka and was directly associated with previously unreported dome-building and ignimbrite-forming volcanism, and led to a voluminous (>3,000 km) ignimbrite ‘flare-up’ between ca. 340 and 240 ka. The second cycle began roughly 180 ka, erupting ca. 800 km of magma, and continues to the present. The duration, rate, and composition of magma production within these cycles appears to be governed by the flux of fluid released from the subducting slab, while the distribution of magmas may be governed more by extension along the central rift axis. Shorter cycles have also been identified and are unrelated to subduction processes, but occur following large, caldera-forming events. [ABSTRACT FROM AUTHOR]

The ∼140 Ma Halfmoon Pluton of Stewart Island, New Zealand, provides direct evidence for a number of physico-chemical processes that operate at depth within active arc settings. It is characterized by a sequence of mingled mafic sheets and enclaves that are preserved within intermediate–felsic host-rocks. Way-up structures and textures are consistent with a younging direction to the south, and allow a south-dipping magma chamber ‘stratigraphy’ to be constructed. The similarity in modal mineralogy, geochemistry, and isotopic signature between the various components, and their proximity in space and time, indicates that the system can be considered as a single composite pluton with discrete portions active at different times. Interpretation of mingling structures and textures in the field, combined with detailed geochemical and geochronological analyses, has allowed the identification of several physical and chemical processes operating within the chamber including: (1) multiple mafic magma pulses that were extensively mingled and sometimes physically mixed at the exposed level and at depth; (2) the development of mineral fabrics (e.g. aligned plagioclase and hornblende crystals) and alignment of mafic enclaves caused by processes of crystal accumulation, magmatic flow and compaction; (3) shortening of the chamber during crystallization as a result of magmatic loading; (4) fractional crystallization of the intermediate–felsic host magma that was interrupted periodically by mafic magma intrusion; (5) incremental assembly of the pluton from the base up; (6) limited chemical transfer between the mingled mafic sheets and enclaves and the surrounding intermediate–felsic host-rocks. Variations in morphology, chemistry and texture of the mingled mafic rocks relative to their position within the magmatic ‘stratigraphy’ indicate that processes operating within the chamber varied in both space and time. Mineral assemblages, chemical characteristics, primitive isotopic signatures and a lack of zircon inheritance indicate that the amphibole-rich, calc-alkaline Halfmoon Pluton was emplaced into a juvenile arc setting and experienced no contamination with ancient crustal materials. A model of pluton construction and evolution is presented that shows that the Halfmoon Pluton consisted of at least two adjacent magma pods that grew incrementally by episodic replenishments of mafic magma into a magma chamber of evolving intermediate–felsic composition. Data are consistent with a model whereby hydrous amphibole-rich basaltic magmas ponded at the crust–mantle interface and episodically rose, injected and mingled with an overlying intermediate–felsic magma chamber that itself represented the fractionated product of the same basaltic mantle melts. [ABSTRACT FROM PUBLISHER]

Crater Hill is a small volume alkali olivine basalt volcano in the Auckland volcanic field. Crater Hill consists of a sequence of pyroclastic and effusive eruptive units of which the earliest have low silica, ferromagnesian elements and Mg/Fe ratios, high incompatible elements and are more silica undersaturated while the last material to be erupted has higher silica, ferromagnesian elements and Mg/Fe ratios but relatively low incompatible elements. Through the sequence, Mg-number changes from 59 to 67 and LaN/LuN decreases by a factor of 3. This systematic compositional variation is interpreted to be the result of clinopyroxene ± spinel fractionation at pressures of at least 1.4–1.9 GPa, from a primary magma generated by small-degree partial melting in the garnet peridotite stability field (>2.5 GPa). Fractionation occurred where early crystals grew and accumulated along the conduit walls. The rising magma evolved along a polybaric liquid line of descent until it encountered lithosphere cold enough to chill the dike margin. Above this depth, further cooling resulted only in the growth of suspended phenocrysts in a magma separated from the country rock by a chilled margin. This process is observed in the Auckland volcanic field because the rate of magma production is very small allowing compositional features to be preserved that would be overwhelmed in a larger scale magmatic system. [ABSTRACT FROM AUTHOR]

Mineral assemblages in volcanic rocks record both pre-eruptive conditions and changes experienced by magma as it rises. Titanomagnetite in andesitic magmas is especially sensitive to changes in temperature and oxygen fugacity immediately prior to and during eruptions. Two end-member eruption states can be distinguished by examining titanomagnetite textures in erupted rocks. Slow-ascent eruptions—characterized by near-stagnant magma bodies and slow effusion of lava domes—show solid-state exsolution of titano-hematite/ilmenite lamellae within titanomagnetite hosts. By contrast, fast-ascent eruptions—characterized by rapid chilling of magma in sub-Plinian eruptions—contain titanomagnetites without such exsolution features. This mineralogical distinction is particularly useful in examining very fine-grained distal tephra layers where other characteristic properties of the two eruptions types are not present. Such tephra records in lake deposits typically provide the most precise long-term eruption records from andesitic volcanoes. Using an example from Mount Taranaki, New Zealand, we show that by classifying eruption styles within such sequences, the underlying magmatic system processes at a volcano can be elucidated and separated from other environmental factors such as vent/crater configuration. [ABSTRACT FROM AUTHOR]

Large pyroclastic rhyolites are snapshots of evolving magma bodies, and preserved in their eruptive pyroclasts is a record of evolution up to the time of eruption. Here we focus on the conditions and processes in the Oruanui magma that erupted at 26.5 ka from Taupo Volcano, New Zealand. The 530 km3 (void-free) of material erupted in the Oruanui event is comparable in size to the Bishop Tuff in California, but differs in that rhyolitic pumice and glass compositions, although variable, did not change systematically with eruption order. We measured the concentrations of H2O, CO2 and major and trace elements in zoned phenocrysts and melt inclusions from individual pumice clasts covering the range from early to late erupted units. We also used cathodoluminescence imaging to infer growth histories of quartz phenocrysts. For quartz-hosted inclusions, we studied both fully enclosed melt inclusions and reentrants (connecting to host melt through a small opening). The textures and compositions of inclusions and phenocrysts reflect complex pre-eruptive processes of incomplete assimilation/partial melting, crystallization differentiation, magma mixing and gas saturation. ‘Restitic’ quartz occurs in seven of eight pumice clasts studied. Variations in dissolved H2O and CO2 in quartz-hosted melt inclusions reflect gas saturation in the Oruanui magma and crystallization depths of ∼3.5–7 km. Based on variations of dissolved H2O and CO2 in reentrants, the amount of exsolved gas at the beginning of eruption increased with depth, corresponding to decreasing density with depth. Pre-eruptive mixing of magma with varying gas content implies variations in magma bulk density that would have driven convective mixing. [ABSTRACT FROM AUTHOR]

The caldera-forming 26·5 ka Oruanui eruption (Taupo, New Zealand) erupted ∼530 km3 of magma, >99% rhyolitic, <1% mafic. The rhyolite varies from 71·8 to 76·7 wt % SiO2 and 76 to 112 ppm Rb but is dominantly 74–76 wt % SiO2. Average rhyolite compositions at each stratigraphic level do not change significantly through the eruption sequence. Oxide geothermometry, phase equilibria and volatile contents imply magma storage at 830–760°C, and 100–200 MPa. Most rhyolite compositional variations are explicable by ∼28% crystal fractionation involving the phenocryst and accessory phases (plagioclase, orthopyroxene, hornblende, quartz, magnetite, ilmenite, apatite and zircon). However, scatter in some element concentrations and 87Sr/86Sr ratios, and the presence of non-equilibrium crystal compositions imply that mixing of liquids, phenocrysts and inherited crystals was also important in assembling the compositional spectrum of rhyolite. Mafic compositions comprise a tholeiitic group (52·3–63·3 wt % SiO2) formed by fractionation and crustal contamination of a contaminated tholeiitic basalt, and a calc-alkaline group (56·7–60·5 wt % SiO2) formed by mixing of a primitive olivine–plagioclase basalt with rhyolitic and tholeiitic mafic magmas. Both mafic groups are distinct from other Taupo Volcanic Zone eruptives of comparable SiO2 content. Development and destruction by eruption of the Oruanui magma body occurred within ∼40 kyr and Oruanui compositions have not been replicated in vigorous younger activity. The Oruanui rhyolite did not form in a single stage of evolution from a more primitive forerunner but by rapid rejuvenation of a longer-lived polygenetic, multi-age ‘stockpile’ of silicic plutonic components in the Taupo magmatic system. [ABSTRACT FROM AUTHOR]

The South Auckland Volcanic Field is a Pleistocene (1·59–0·51 Ma) basaltic intraplate, monogenetic field situated south of Auckland City, North Island, New Zealand. Two groups of basalts are distinguished based on mineralogy and geochemical compositions, but no temporal or spatial patterns exist in the distribution of various lava types forming each group within the field: Group A basalts are silica-undersaturated transitional to quartz-tholeiitic basalts with relatively low total alkalis (3·0–4·6 wt %), Nb (7–29 ppm), and (La/Yb)N (3·4–7·6); Group B basalts are strongly silica-undersaturated basanites to nepheline-hawaiites with high total alkalis (3·3–7·9 wt %), Nb (32–102 ppm), and (La/Yb)N (12–47). Group A has slightly higher 87Sr/86Sr, similar ɛNd, and lower 206Pb/204Pb values compared with Group B. Contrasting geochemical trends and incompatible element ratios (e.g. K/Nb, Zr/Nb, Ce/Pb) are consistent with separate evolution of Groups A and B from dissimilar parental magmas derived from distinct sub-continental lithospheric mantle sources. Differentiation within each group was controlled by olivine and clinopyroxene fractionation. Group B magmas were generated by <8% melting of an ocean island basalt (OIB)-like garnet peridotite source with high 238U/204Pb mantle (HIMU) and enriched mantle (EMII) characteristics possibly inherited from recycled oceanic crust. Group A magmas were generated by <12% melting of a spinel peridotite source also with HIMU and EMII signatures. This source type may have resulted from subduction-related metasomatism of the sub-continental lithosphere modified by a HIMU plume. These events were associated with Mesozoic or earlier subduction- and plume-related magmatism when New Zealand was at the eastern margin of the Gondwana supercontinent. [ABSTRACT FROM PUBLISHER]

Juvenile ejecta from the September and October 1995 eruptions of Ruapehu volcano, New Zealand, indicate that mixing occurred between relatively higher- and lower-temperature (high-T and low-T) andesitic magmas. Compositional zonations in clinopyroxene phenocrysts provide direct evidence for a pre-eruption crystal–melt mush chamber containing low-T magma, and elucidate the processes of magma mixing and eruption, following the injection of high-T magma. Many phenocrysts with Fe-rich cores derived from low-T magma have extremely reverse zoned mantles around slightly resorbed cores. Mg-value [100Mg/(Mg + Fe)] increases from 65–70 to ∼85 over a short width (<20 μm) at the inner edge of the mantle, usually accompanied by an abrupt change of Cr2O3 and Wo contents. After the first extreme reverse zoning, high Mg-value in the crystal mantle usually continues over several tens to 200 μm in width, with intermittent reversals, and then Mg-value decreases towards the outer rim. The high Mg-value (∼85) is the same as the core compositions of phenocrysts derived from the high-T magma, suggesting that these extremely zoned phenocrysts were surrounded immediately by invading high-T magma, as opposed to mixed melt. This does not indicate a simple melt-mixing process between crystal-rich magmas, but is interpreted as evidence for mixing in a mush chamber. It is suggested that high-T magma was injected into a mushy low-T magma, then the denser and hotter high-T magma displaced the lighter and cooler interstitial melt in the mush. In addition, the high-T magma could thermally erode crystals in the mush network. Hence, eroded crystals from the mush were captured by the high-T magma. Repeated reverse zonations in the crystal mantles, and variable mantle widths, indicate that injection of high-T magma intermittently continued, both to form and to enlarge magma pockets in the mush. In the process, melt mixing occurred owing to convection and/or conjunction and coalescence of the magma pockets. The mixing between the magma pockets and interstitial melt of the mush gave rise to normal zoning in the mantles of contained phenocrysts. Zoning in the outermost rims of pyroxenes differs between the September and October scorias. Almost all of the October phenocrysts, but few of the September phenocrysts, have thin (several micrometres) reverse zoned rims. Therefore we suggest that the October eruptions may have been triggered by intensive injection just before the eruption, whereas the September eruptions may have been triggered by either smaller-scale injections or coalescence of magma pockets. These differences in presumed triggering process are consistent with geophysical observations before and during the 1995 eruptions. [ABSTRACT FROM PUBLISHER]

Outlines an alternative technique for directly measuring the parent-magma compositions of layered intrusions using the compositions of rehomogenized melt-inclusions from early-forming cumulus minerals in Greenhills Complex, New Zealand. Greenhills Complex's geology; Determination of the parent-magma composition; Platinum-group element mineralization.

White Island is an active composite stratovolcano in the Bay of Plenty, New Zealand, that comprises many small volume (<0·1 km3) andesite–dacite lava flows and pyroclastic deposits with phenocryst contents of ∼15–44%. Minor high-Mg basaltic andesite explosive eruptions, such as those of 1976–1992, may have occurred at intervals throughout the history of White Island, but are rarely preserved. These alternate with major episodes of andesite–dacite lava extrusion. The high-Mg magmas form by hydrous melting of mantle, metasomatized by fluids from the dehydrating slab at the slab–mantle wedge interface, that rise rapidly to shallow magma chambers (2–7 km?) where limited mixing and contamination occurs before eruption. Some of this magma remains in the magma chamber where it interacts with the crystal mush, from which it inherits phenocrysts, to form so-called ‘dirty’ lavas. Total phenocryst content of these lavas is correspondingly higher. As more magma is intruded into the chamber, the heat flux will increase and melt fraction will eventually rise to the surface to form high-silica andesite–dacite magma (‘clean’ lavas) with fewer inherited phenocrysts. Similar multi-magma chamber plumbing systems, with complex evolution involving fractionation and contamination, probably occur in most andesite–dacite arc volcanoes. [ABSTRACT FROM PUBLISHER]

Egmont Volcano (Mt Taranaki) is located 140 km west of the Taupo Volcanic Zone (TVZ) the principal locus of volcanic activity in the North Island of New Zealand, and is on of four closely associated Quaternary andesitic volcanoes in Taranaki province. Taranak eruptives are enriched in K and other large ion lithophile elements compared with thei counterparts at Ruapehu in the southern TVZ, with the youngest Egmont andesites being th most K rich. Egmont andesites are invariably fractionated but isotopic information indicate that, unlike those at Ruapehu, they have not extensively assimilated enriched crust. Ti/Zr, Ba/La, Ce/Pb, and K/Rb ratios indicate that a more depleted mantle wedge and compositionally different slab-derived fluids were involved in the generation of Taranaki primary magmas. Magmas parental to Egmont eruptives were relatively undersaturated, hydrous, high-Mg basalts generated by low degrees of partial melting in a depleted mantle wedge fluxed by deep slab fluids. Fractionation of these magmas at the base of the crust produced basaltic andesite and extensive ultramafic cumulates. Plagioclase fractionation was suppressed by high aH2o. Rising geothermal gradients eventually resulted in partial anatexis of amphibolitic underplated crust, and interaction of basaltic andesites with these melts led to progressively more K-rich compositions. [ABSTRACT FROM AUTHOR]

Beneath subduction zone volcanoes, the parental magmas are thoroughly blended during temporary storage in the crust via repeated heating and mixing from new inputs of magma. To obtain a clearer picture of magma assembly, we employed a sub-crystal-scale Sr-isotope analysis, coupled with textural and geochemical data, on plagioclase phenocrysts in basaltic andesite and andesite erupted from the Tongariro volcanic complex, New Zealand. Deposits from two neighbouring vents were examined: (1) 0.3–3.4 ka lava flows of the polygenetic Red Crater volcano and (2) 26 ka pyroclastics of the monogenetic Pukeonake scoria cone. Despite the differences in eruptive styles and history of the two volcanoes, the plagioclase growth histories are similar. One common phenocryst type comprises of a resorbed core zone (variously, ~ An50–70) surrounded by a sieve-textured mantle/rim of higher An (~ 80), Fe and Mg contents. Other phenocrysts have resorbed calcic cores (~ An70–90). In situ 87Sr/86Sr analyses reveal a wide isotopic range (~ 0.7044–0.7060 for pre-1.8 ka Red Crater; ~ 0.7044–7057 for post-1.8 ka Red Crater; ~ 0.7047–0.7055 for Pukeonake). Most of the values are higher than that of the host bulk rock (87Sr/86Sr ~ 0.7049–0.7051 for pre-1.8 ka Red Crater; 0.7045–0.7046 for post-1.8 ka Red Crater; 0.7048 for Pukeonake). The variation within some individual phenocrysts is comparable to that of the entire population. The higher 87Sr/86Sr ratios are related relic phenocryst cores, indicating that their parental source was more radiogenic than the host magma. In contrast, their sieve-textured mantle/rim zones have lower 87Sr/86Sr ratios (~ 0.7045–0.7049), similar that of their host rock. These features record magma mixing. Less than 10% of phenocryst interiors are cognate (autocrysts) with the host rock based on their high 87Sr/86Sr ratio. Some phenocrysts are likely to be xenocrysts derived from the disintegration of deep crustal meta-igneous rocks. However, based on textures consistent with growth from a melt, the majority are antecrysts from intrusive forerunners and/or crystal mush zones, entrained during eruption or storage. Overall, the andesites are a product of crystal-poor magmas of mantle or lower crustal origin carrying phenocrysts from mostly crustal sources. The magma system likely comprised of a series of semi-isolated melt pods or crystal mush zones in a larger network of intrusions of various ages and states of solidification, possibly hosted in country rock of various compositions. Resorption of the sieve-textured mantle zones on the phenocrysts and growth of a normally zoned outermost rim represents a final period of fractional crystallization in the newly homogenised melt following the ascent and intrusion of mafic magma. Hence, the final tipping point to eruption may have involved crystallization-induced volatile exsolution and degassing. [ABSTRACT FROM AUTHOR]

Examines changes in seismic Anisotropy after eruption of Mount Ruapehu in New Zealand. Methods in monitoring volcanoes for future eruption; Observation of shear-waveforms; Interpretation of the magma injection of Ruapehu.