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Arc magma generation and evolution are affected by transcrustal processes, such as assimilation and fractional crystallisation, and source processes like slab influx into the mantle wedge. However, the relative contribution of these mechanisms remains contentious. The Pleistocene to Holocene Taupo Volcanic Zone (TVZ) is unusually dominated by felsic volcanism with more than 95% of the total eruptive volume corresponding to rhyolitic magmas. Despite the relatively thin crustal basement (∼16–30 km), combined wall rock assimilation and fractional crystallization (AFC) of primary basalt has previously been presented as a necessary means to generate the chemical characteristics of these magmas. This petrogenetic model is here assessed using a new, internally consistent set of major and trace element concentrations, and Sr-Pb isotope data from a suite of basaltic to rhyolitic samples that are representative of the entire TVZ with respect to geographical distribution and age. The dataset was modelled for AFC using the energy- and mass-constrained Magma Chamber Simulator (MCS) and the local Permian to Early Jurassic Torlesse composite terrane basement used as the principal assimilant. Regardless of the intensive parameters employed, the energy- and mass-constrained models presented here fail to realistically reproduce the combined major oxide, trace element, and isotope systematics of the sample set. The Pb isotope ratios of TVZ basalts to rhyolites reveal a broadly linear trend ranging from an unradiogenic (206Pb/204Pb: 18.77) to a more radiogenic crustal endmember (206Pb/204Pb: 18.84). This trend can be reproduced by a composite mélange dominated by the Torlesse terrane (representative of tectonically eroded forearc crust), with some contribution of subducting sediments (GLOSS-II), interacting with the sub-arc mantle to generate the source of the magmas observed in the TVZ. This model is also consistent with Sr-Pb isotope systematics. At New Zealand's active margin, tectonic erosion has been reported based on geophysical data. Therefore, it is proposed that slab input can have a stronger relevance in the magma generation of the TVZ than previously thought. The broad compositional variations in the TVZ may result from source contamination by a subduction mélange, with differentiation processes within the overriding crust being subordinate. [ABSTRACT FROM AUTHOR]

Diopside-rich rocks (diopsidites) are interlaminated with nephrite in boulders derived from metasomatic contacts developed between Pounamu Ultramafic meta-serpentinite and country rock Alpine Schist, Westland, New Zealand. Petrographic textures indicate that parental tremolite rock, formed by metasomatic diffusion during metamorphism, has been intensely deformed and recrystallised to alternating semi-nephrite and nephrite domains during development of a secondary crenulation cleavage. Nephrites are subsequently sequentially overprinted by porphyroblastic tremolite, diopside, then further tremolite. Crystallisation is controlled by fluctuating activities of SiO2, CaO and H2O in associated fluids. Pervasive dissolution of nephritic tremolite and crystallisation of diopside generates diopsidites containing accessory epidote, uvarovite and zincian chromite formed in equilibrium with H2O-rich fluids. Diopsidites are in turn overgrown by coarse grained (in places > 50 cm long) diopside crystals, interpreted to have infilled an extension fracture that formed during ongoing uplift of the Southern Alps. [ABSTRACT FROM AUTHOR]

A new phase equilibria geobarometer determines magmatic storage and crystallization conditions, including pressure, temperature, oxygen fugacity ( $${f_{{{\text{o}}_2}}}$$ ), and the presence of a fluid phase for glass-bearing rocks containing the assemblage plagioclase + pyroxene(s). This newly developed geobarometer can better constrain crystallization conditions of shallow (< 500 MPa; <~ 20 km), glass-bearing andesites to dacites. The geobarometer utilizes rhyolite-MELTS to determine crystallization conditions in natural pumice and scoria samples. The validity of the geobarometer is tested by comparing it to results from experiments. Uncertainties are assessed using Monte Carlo simulations. We apply the geobarometer to the plag + opx + cpx-bearing system of Mt. Ruapehu, in the southern Taupo Volcanic Zone, New Zealand. The samples from Mt. Ruapehu are tested from ~ 5 to ~ 400 MPa and from super-liquidus to 90% crystalline (~ 1200 to ~ 700 °C). Mt. Ruapehu serves as a methodological testing ground for the geobarometer, and results from our geobarometer agree with recent Mt. Ruapehu studies. Results show a distribution of crystallization pressures ranging from 50 to 150 MPa (~ 2.0 to 5.9 km) for different eruptions, with modes of 110 MPa (~ 4.3 km) and 130 MPa (~ 5.1 km). These are consistent with field interpretations of different eruptive styles based on juvenile clast textures and previous knowledge of the magma plumbing system. Mt. Ruapehu magmas are fluid saturated, with $${f_{{{\text{o}}_2}}}$$ of ΔQFM ~ + 1 (NNO). [ABSTRACT FROM AUTHOR]

The textural variation and compositional zoning of plagioclase in pre-historic and historic basaltic andesite lava flows from Ngauruhoe volcano reveals extensive crystal recycling from a multi-level magma system. Most phenocrysts have a calcic (~ An 80–90 ) resorbed core with diffuse or no zonation that is depleted in Fe and Mg. Some cores display patchy zonation from replacement by high An crystallization prior to resorption. The cores are mantled by oscillatory-zoned rims of lower An content (< An 60 ), and are enriched in Fe and Mg. Rim zones vary in relative thickness and textural complexity, and include sieve-textured bands, and/or cyclic calcic growth following dissolution events. A subordinate crystal population display similar features, but lack a resorbed core. These latter crystals display overall rimward enrichment in An, Fe and Mg. The resorbed cores crystallized from magmas more mafic than those erupted at Ngauruhoe, and slow cooling and prolonged storage resulted in loss of An zoning patterns and depletion of Fe and Mg by diffusion. These crystals are likely to have originated from deep cumulates or intrusions, and were subsequently entrained in ascending magmas. Patchy-textured cores were produced during decompression in a water under-saturated magma and staged ascent. The diversity in crystal cores reflect different conduits and ascent histories. The crystal rims grew in a more differentiated magma reservoir, and are in equilibrium with the erupted melt. Most of the zoning patterns in the rim zone require water pressure and/or temperature changes. These changes could have been caused by convective self-mixing in a closed system and/or the intrusion of hydrous melts of similar bulk composition. Other crystals display rimward elemental enrichments consistent with mafic recharge. Previously reported rimward enrichment in 87 Sr– 86 Sr compositions can be explained by the re-cycled origin of the crystal cores and progressive crustal assimilation at shallower depths in the magma system where subsequent crystal growth occurred. A steady-state balance of intrusion and crystallization in the deep crust, and remobilization and entrainment of co-magmatic crystals in newly formed and ascending magmas has produced relatively monotonous batches of basaltic andesite throughout the volcano's life. Trace element zonation (Fe and Mg) in the plagioclase mostly reflects variable diffusive loss through the crystal history, and the patterns in different crystals are highly variable. This, along with uncertainties in estimating trace element partitioning coefficients, highlights the difficulty of reconstructing melt compositions and subterranean residence time in such magmas. [ABSTRACT FROM AUTHOR]

The Kidnappers [~1200 km³ dense rock equivalent (DRE)] and Rocky Hill (~200 km³ DRE) calderaforming events in the Taupo Volcanic Zone, New Zealand, were erupted in close succession from the Mangakino volcanic centre. They have identical radiometric ages at ~1 Ma, yet erosion along the contact between the two deposits suggests that some years to decades separated the two eruptions. Field constraints and the similarities of crystal textures and compositions and glass chemistries of both eruption deposits demonstrate that they came from one overall magmatic system with a common crystal mush source. However, second-order variations in these parameters confirm that the Kidnappers and Rocky Hill deposits represent distinct events and are not the products of a single zoned magma chamber. The systematically zoned Kidnappers fall deposits provide evidence for the tapping of three discrete magma bodies, whereas the succeeding Kidnappers ignimbrite is compositionally more diverse. The transition from fall to flow deposition marks a change in the style of caldera collapse and the simultaneous evacuation of discrete but compositionally diverse melts, each of which underwent a distinct evolution and was held at slightly different P-T conditions prior to eruption. Contrasting plagioclase and orthopyroxene zonation patterns are present in pumices originating from three discrete magma bodies. Less evolved mafic melts interacted with the system, which mobilized portions of the final erupted melt through heating and volatile or chemical exchange in the mush. The two largest Kidnappers melt-dominant bodies were re-tapped in modified form, or re-established from their common mush source, prior to the Rocky Hill event. Rocky Hill pumices contain common, fluid-affected antecrystic crystal clots derived from chamber wall material. Amphibole compositions from each eruption reflect melt evolution processes and, in particular, the contemporaneous crystallization of biotite and breakdown of orthopyroxene. Plagioclase and orthopyroxene from Rocky Hill pumices share common zonation patterns with those from the two largest magma bodies in the Kidnappers. The rapid production of new meltdominant bodies and the triggering of the Rocky Hill eruption reflect the ability of the magmatic system to rejuvenate on a geologically short timescale. The Mangakino centre did not follow a typical cycle of decreased activity after the supervolcanic Kidnappers event, instead producing a second caldera-forming eruption, within years to decades from the same system. [ABSTRACT FROM AUTHOR]

Mangakino, the oldest rhyolitic caldera centre delineated in the Taupo Volcanic Zone of New Zealand, generated two very large (super-sized) ignimbrite eruptions, the 1.21 ± 0.04 Ma >500 km Ongatiti and ~1.0 Ma ~1,200 km Kidnappers events, the latter of which was followed after a short period of erosion by the ~200 km Rocky Hill eruption. We present U/Pb ages and trace-element analyses on zircons from pumice clasts from these three eruptions by Secondary Ion Mass Spectrometry (SIMS) using SHRIMP-RG instruments to illustrate the evolution of the respective magmatic systems. U-Pb age spectra from the Ongatiti imply growth of the magmatic system over ~250 kyr, with a peak of crystallisation around 1.32 Ma, ~100 kyr prior to eruption. The zircons are inferred to have then remained stable in a mush with little crystallisation and/or dissolution before later rejuvenation of the system at the lead-in to eruption. The paired Kidnappers and Rocky Hill eruptions have U-Pb zircon ages and geochemical signatures that suggest they were products of a common system grown over ~200 kyr. The Kidnappers and Rocky Hill samples show similar weakly bimodal age spectra, with peaks at 1.1 and 1.0 Ma, suggesting that an inherited antecrystic population was augmented by crystals grown at ages within uncertainty of the eruption age. In the Kidnappers, this younger age peak is dominantly seen in needle-shaped low U grains with aspect ratios of up to 18. In all three deposits, zircon cores show larger ranges and higher absolute concentrations of trace elements than zircon rims, consistent with zircon crystallisation from evolving melts undergoing crystal fractionation involving plagioclase and amphibole. Abundances and ratios of many trace elements frequently show variations between different sectors within single grains, even where there is no visible sector zoning in cathodoluminescence (CL) imaging. Substitution mechanisms, as reflected in the molar (Sc + Y + REE)/P ratio, differ in the same growth zone between the sides (along a-axis and b-axis: values approaching 1.0) and tips (c-axis: values between 1.5 and 5.0) of single crystals. These observations have implications for the use of zircons for tracking magmatic processes, particularly in techniques where CL zonation within crystals is not assessed and small analytical spot sizes cannot be achieved. These observations also limit applicability of the widely used Ti-in-zircon thermometer. The age spectra for the Ongatiti and Kidnappers/Rocky Hill samples indicate that both magmatic systems were newly built in the time-breaks after respective previous large eruptions from Mangakino. Trace element variations defining three-component mixing suggest that zircons, sourced from multiple melts, contributed to the population in each system. [ABSTRACT FROM AUTHOR]

The Taupo Volcanic Zone (TVZ), New Zealand is a region of voluminous and frequent rhyolitic volcanism and widespread geothermal activity. Additionally, the hydrothermal systems of the TVZ contain relatively high concentrations of base and precious metals. Here we present an extensive dataset of major element, volatile, and trace element (including Pb, Zn, As, Mo, Cu) abundances in melt inclusions, pumice glasses and minerals from eight eruptions within the Okataina Volcanic Center (OVC) of the TVZ to investigate the behavior of metals during melt evolution. The high-SiO2 rhyolites of the OVC contain high concentrations of volatiles (&leq;6 wt % H2O, &leq;0·25 wt % Cl) and underwent significant degassing prior to and during eruption. The OVC melts contain moderate concentrations of metals (11–24 ppm Pb, 20–50 ppm Zn, 2–7 ppm As, <2·5 ppm Mo, <∼5 ppm Cu). Ferromagnesian minerals (amphibole, biotite and orthopyroxene) in the OVC pumice have high concentrations of Zn (&leq;1500 ppm), and plagioclase and biotite contain moderate amounts of Pb (&leq;11 ppm). The melt inclusion and pumice glass trace element data reveal complex histories of magma mixing and mingling prior to eruption; however, discrete melt batches are easily identified based on trace element geochemistry. Variations in incompatible trace elements within these melt batches suggest that the OVC rhyolites underwent at least ∼20–25% fractional crystallization during quartz crystallization and melt inclusion entrapment (at pressures of ∼100–200 MPa) and little to no crystallization (&leq;5%) during ascent and eruption. Comparison of melt inclusion metal and incompatible element (e.g. U) concentrations reveals that melt Pb, Mo and As increase, whereas melt Zn decreases, during fractional crystallization at depth (∼100–200 MPa). These observations can be explained by minor partitioning of the metals Pb, Mo and As into the fractionating minerals and stronger partitioning of Zn into the ferromagnesian phases, supported by calculated metal D values and analyzed metal concentrations in OVC minerals. Interestingly, throughout both deep, vapor-saturated crystallization and during extensive degassing during magma ascent and eruption (as recorded by pumice glasses), the metals analyzed here do not appear to partition appreciably into the vapor. We propose that the lack of volatility of the metals analyzed in this study can be attributed to a combination of several factors. First, vapor–melt partitioning requires the presence of ligands—commonly Cl, S and OH—with which the metals may complex. Given the low Cl/H2O ratios in the OVC melts and the extensive degassing of H2O compared with Cl, it seems likely that the rhyolites would have exsolved H2O-rich vapor with insufficient Cl to transport metals (in particular Pb and Zn) into the vapor phase, either at depth or during magma ascent. Second, the overall small volumes of vapor present during crystallization at pressures of 100–200 MPa would have impeded significant vapor–melt partitioning of the metals. Finally, the estimated very rapid ascent of the OVC melts from depths of 4–8 km suggests that there was insufficient time at low pressure for diffusion of metals out of the melt. These results imply that there may be an indirect connection between the rhyolites and the metals of the hydrothermal systems of the TVZ. As the metals, and other species such as Cl, remain in the rhyolitic magmas upon eruption, they are available in the large volumes of rhyolite emplaced in the upper crust of the TVZ for leaching by heated meteoric waters. [ABSTRACT FROM AUTHOR]

Abstract: Crystallisation-age spectra have been obtained by SIMS techniques (SHRIMP-RG) on zircons from altered volcanic units penetrated by drillholes at Kawerau Geothermal Field in the central Taupo Volcanic Zone (TVZ), New Zealand. Drillholes penetrate 700–1300m of volcanic rocks and sediments before reaching the basement Mesozoic greywacke. Twenty-seven samples of altered volcanic lithologies and two surficial, fresh rock units have been studied in order to constrain ages of the major stratigraphic units. Within the volcanic/sedimentary pile the oldest in-situ ignimbrites that can be widely correlated have ages of ~1.45Ma. Between them and the basement greywacke is a variable thickness of sediments, mostly greywacke gravels and minor volcaniclastic units, reflecting localised basinal deposition associated with strike–slip faulting. Two ignimbrites within this sequence yield age estimates of c. 2.4 and 2.1Ma, consistent with these being distal southern Coromandel Volcanic Zone deposits, pre-dating TVZ activity. Below the regional marker plane of the 0.32Ma Matahina ignimbrite, three main ignimbrite groups occur, with ages around 1.45Ma, 1.0Ma and 0.6–0.5Ma, which are separated by sediment-dominated intervals and andesite volcanics. All of these ignimbrites represent marker horizons from other volcanic centres and do not reflect the presence of local magmatic heat sources. Numerous bodies of coherent rhyolite, previous labelled as Caxton and Onepu rhyolites, have been intersected at all pre-Matahina ignimbrite levels (including within the basement greywacke) and reflect earlier local magmatic heat sources. Our geochronological data resolve these rock bodies into three packages. The youngest is represented by the surficial rhyodacite Onepu domes, 40Ar/39Ar dated at 0.138±0.007Ma. U–Pb ages on zircons from dome material yield a spectrum that can be matched (consistent with petrography) with two dikes intersected at 880m and 2.67km depth, and with an estimated age of 0.15±0.01Ma (Onepu Formation). The older two packages consist of older crystal rich (~15%) and younger crystal-poor (~5%) rhyolite, here grouped as Caxton Formation and with eruption/intrusion age of 0.36±0.03Ma. The shallowest Caxton rhyolite bodies are interpreted to be domes, whilst deeper intersections are inferred to be sills based on the lateral extents relative to thicknesses. Net subsidence rates inferred from depths to key units do not reflect the present-day situation. Modern rates of subsidence (2±1mm/yr) associated with TVZ rifting processes can have been active for no more than ~50,000years, based on elevation differences of the Matahina ignimbrite top surface. An inferred change in intrusion geometry from sill (Caxton) to dike (Onepu) indicates a change in principal stress orientations reflecting onset of the modern Whakatane Graben. This change is dated at ~0.37Ma in coastal sedimentary sequences 23km to the north of Kawerau, consistent with our age data. Although previously interpreted to be a long-lived system, the modern Kawerau Geothermal Field is a Holocene entity reflecting the rejuvenation of magmatic heat flux associated with Putauaki volcano superimposed on an area of multiple reactivated fault structures, sporadic magmatism and variable rates of subsidence. [Copyright &y& Elsevier]

Ruapehu, New Zealand’s largest active andesite volcano, is located at the southern tip of the Taupo Volcanic Zone (TVZ), the main locus of subduction-related volcanism in the North Island. Geophysical data indicate that crustal thickness increases from <25 km within the TVZ to 40 km beneath Ruapehu. The volcano is built on a basement of Mesozoic meta-greywacke, and geophysical evidence together with xenoliths contained in lavas indicates that this is underlain by oceanic, meta-igneous lower crust. The present-day Ruapehu edifice has been constructed by a series of eruptive events that produced a succession of lava flow-dominated stratigraphic units. In order from oldest to youngest, these are the Te Herenga (250–180 ka), Wahianoa (160–115 ka), Mangawhero (55–45 ka and 20–30 ka), and Whakapapa (15–2 ka) Formations. The dominant rock types are plagioclase- and pyroxene-phyric basaltic andesite and andesite. Dacite also occurs but only one basalt flow has been identified. There have been progressive changes in the minor and trace element chemistry and isotopic composition of Ruapehu eruptive rocks over time. In comparison with rocks from younger formations, Te Herenga eruptive rocks have lower K2O abundances and a relatively restricted range in major and trace element and Nd–Sr isotopic composition. Post-Te Herenga andesites and dacites define a Sr–Nd isotopic array that overlaps with the field for TVZ rhyolites and basalts, but Te Herenga Formation lavas and the Ruapehu basalt have higher 143Nd/144Nd ratios. The isotopic, and major and trace element composition of Te Herenga andesite can be replicated by models involving mixing of an intra-oceanic andesite with a crustal component derived from a meta-igneous composition. Post-Te Herenga andesites show considerable variation in major and trace element and Sr and Nd isotopic compositions (87Sr/86Sr ranges from 0·7049 to 0·7060 and 143Nd/144Nd from 0·51264 to 0·51282). The range of compositions can be modeled by assimilation–fractional crystallization (AFC) involving meta-greywacke as the assimilant, closed-system fractionation, or by mixing of intra-oceanic andesite or basalt and a meta-greywacke crustal composition. Plagioclase and pyroxene compositions vary over wide ranges within single rocks and few of these have compositions consistent with equilibration with a melt having the composition of either the host-rock or groundmass. The 87Sr/86Sr compositions of plagioclase also vary significantly within single whole-rock samples. Glass inclusions and groundmasses of andesitic rocks all have dacitic or rhyolitic major and trace element compositions. The application of various mineral geothermometers and geobarometers indicates pre-eruption temperatures between 950 and 1190°C and pressures ranging from 1 to 0·2 GPa. These pressure estimates are consistent with those obtained from xenolith mineral assemblages and geophysical information. Plagioclase hygrometry and the paucity of amphibole are indications that melts were relatively dry (< 4 wt % H2O). Magmas represented by Ruapehu andesites were dacitic or rhyolitic melts carrying complex crystal and lithic cargoes derived from the mantle and at least two crustal sources. They have evolved through a complex interplay between assimilation, crystal fractionation, crustal anatexis and magma mixing. Parental magmas were sourced in both the mantle and crust, but erupted compositions very strongly reflect modification by intracrustal processes. Geochemical variation in systematically sampled lava flow sequences is consistent with random tapping of a complex plumbing system in which magma has been stored on varying time scales within a plexus of dispersed reservoirs. Each magma batch is likely to have had a unique history with different sized ... [ABSTRACT FROM AUTHOR]

Abstract: Granitoid clasts in pyroclastic deposits of the 0.7ka (Kaharoa) eruption from the intra-caldera Tarawera volcano of the Okataina Volcanic Centre (OVC), New Zealand, provide an insight to the plutonic processes beneath one of the most productive Quaternary rhyolite centers on Earth. SIMS U–Th data for 79 granitoid zircon crystals from six clasts reveal a unimodal age spectrum yielding a weighted average model age of 208±4ka (MSWD=1.2; n=72). The remaining crystals are at secular equilibrium and U–Pb analyses indicate that a few of these outliers date back to ~750ka, a period significantly longer than the known volcanic record at OVC (probably ~550ka). In contrast, zircon crystallization in co-erupted pumice and lava of the 0.7ka Kaharoa event, and that of the three preceding rhyolite eruptions, occurred mostly during 0–50ka, reflecting a separate magmatic history. Brittle deformation features, incipient alteration, and low-δ18O whole-rock compositions (+3‰) are consistent with a shallow solid intrusion that has interacted with meteoric hydrothermal fluids. However, δ18O SIMS analyses of zircons (+5.4±0.2‰; n=11) are consistent with magmatic compositions, and thus meteoric interaction occurred post-emplacement. The Kaharoa granitoids contrast with those ejected in the ~40–60ka caldera-forming Rotoiti event that were partly molten and display zircon age spectra indistinguishable from those for co-erupted pumices, suggesting the latter were derived from contemporaneous crystal mush. The 0.7ka Kaharoa case shows that zones of the magmatic system remained solid despite frequent and voluminous magma production that included caldera-forming events. Such long-lived sub-solidus zones in magma systems would act as barriers to melt connectivity and interaction with country rock, but are also a potential source for antecrysts/xenocrysts in subsequent eruptions. This exemplifies how plutonic and volcanic evolution can diverge even in close proximity. [Copyright &y& Elsevier]

We obtained U-Th disequilibrium age data on zircons from each of the four rhyolite eruptions that built Tarawera volcano in the last 22 ka within the Okataina Volcanic Center (OVC), caldera, New Zealand. Secondary ion mass spectrometry analyses on unpolished euhedral crystal faces that lack resorption features show that crystal growth variously terminated from near-eruption age to ~100 ka prior to eruption. Age-depth profiling of crystals reveals long periods of continuous (~34 ka) and discontinuous growth (~90 ka). Growth hiatuses of up to ~40 ka duration occur, but do not all relate to obvious resorption surfaces. Age differences up to similar magnitude are found on opposing faces of some crystals suggesting episodes of partial exposure to melts. These features are best explained by periodic, complete, or partial, sub-solidus storage and/or inclusion in larger crystal phases, followed by rapid liberation prior to eruption. This is supported by high abundances of U and Th (~500 − >2,000 ppm) in some zircons consistent with periods of high crystallinity (>70%) in the magmatic system, based on crystal/melt partitioning. Contemporaneous but contrasting rim-ward trends of these elements within crystals, even in the same lava hand sample, require synchronous growth in separate melt bodies and little connectivity within the system, but also significant crystal transport and mixing prior to eruption. Many crystals record continuity of growth through the preceding ~60 ka OVC caldera-collapse and subsequent eruptions from Tarawera. This demonstrates a decoupling between eruption triggers, such as shallow crustal extension and mafic intrusion, and the crystallization state of the OVC silicic magmatic system. The data highlights the need to distinguish between the time for accumulation of eruptible magma and the long-term magma residence time based on the age of crystals with high closure temperatures, when assessing the potential for catastrophic eruptions. [ABSTRACT FROM AUTHOR]

The Taupo Volcanic Zone (TVZ), located in the North Island of New Zealand, represents part of a magmatic arc that is at present undergoing active extension. Around 0·9 Myr ago, an acceleration in rifting was followed by a progressive transition in the composition of volcanic products (until ~0·7 Ma) from typical arc-type andesite into overwhelmingly large, caldera-forming rhyolitic eruptions with subordinate basalt and dacite in the Central TVZ. Despite an obvious compositional gap in the erupted products in the Central TVZ within the last 0·7 Myr (little to no erupted products with SiO2 contents between 55 and 65 wt %), phenocryst minerals (plagioclase, amphibole, pyroxene) show an uninterrupted compositional record that suggests crystallization from a continuum of melt compositions. Coupled with radiogenic isotope evidence, the whole-rock and mineral chemistry data are consistent with magmatic differentiation controlled by crystal fractionation of primary mantle-derived magmas accompanied by some assimilation of local wall-rocks. In the Southern TVZ and in the early part of the Central TVZ, magmatic differentiation was dominated by the lower crustal evolution of relatively dry (~1 wt % H2O) arc basalts, crystallizing a pyroxene–plagioclase-dominated assemblage. However, the conditions of crystallization in the lower crust appear to have changed within the last million years in the Central TVZ, with amphibole and oxides appearing earlier in the crystallization sequence. In this framework and using numerical simulations coupling crystallization kinetics and multiphase fluid dynamics of magma reservoirs, we show that melts extracted from crystal mushes within an optimal ‘extraction window’ (~50 and 80 vol. % crystals) match those erupted at the surface. Lower crustal mushes fed by basalt with 1 wt % H2O (dominated by a pyroxene–plagioclase assemblage) release andesitic melts at the extraction window. These melts then erupt at the surface to form the observed andesitic part of the arc. With a slightly higher water content (~2 wt %) in the basalt, the melt composition at the extraction window from lower crustal mushes is dacitic rather than andesitic. Although some dacitic melts will reach the surface, most will be trapped in the upper crust and crystallize to form a silicic mush. Extraction of the interstitial liquid after >50% crystallization from this upper crustal reservoir produces the large volumes of rhyolitic magma erupted over the past 0·7 Myr (>4000 km3 from ignimbrite-forming eruptions). [ABSTRACT FROM AUTHOR]

Abstract: Two mineralogically and chemically distinct rhyolite magmas (T1 and T3) were syn-erupted from the same conduit system during the 21.9ka basalt intrusion-triggered Okareka eruption from Tarawera volcano, New Zealand. High spatial resolution U–Th disequilibrium dating of zircon crystals at the ~3–5μm scale reveals a protracted yet discontinuous zircon crystallization history within the magmatic system. Both magma types contain zircon whose interiors predate the eruption by up to 200ka. The dominant age peak in the T1 magma is ~30ka with subordinate peaks at ~45, ~75, and ~100ka, whereas the T3 magma has a dominant zircon interior age peak at ~90ka with smaller modes at ~35 and ~150ka. These patterns are consistent with isolated pockets of crystallization throughout the evolution of the system. Crystal rim analyses yield ages ranging from within error of the eruption age to at least ~90ka prior to eruption, highlighting that zircon crystallization frequently stalled long before the eruption. Continuous depth profiling from crystal rims inward demonstrates protracted growth histories for individual crystals (up to ~100ka) that were punctuated by asynchronous hiatuses of up to 30ka in duration. Disparate zircon growth histories can result from localized thermal perturbations caused by mafic intrusions into a silicic reservoir. The crystal age heterogeneity at hand-sample scale requires considerable crystal transport and mixing. We propose that crystal mixing was achieved through buoyancy instabilities caused by mafic magma flow through crystal mush. A terminal pre-eruptive rejuvenation event was capable of mobilizing voluminous melts that erupted, but was too short (<102–103 years) to result in extensive zircon growth. The contrasting, punctuated zircon histories argue against closed-system fractional crystallization models for silicic magmatism that require protracted cooling times following a mostly liquid starting condition. [Copyright &y& Elsevier]

The Rotoiti (~120 km) and Earthquake Flat (~10 km) eruptions occurred in close succession from the Okataina Volcanic Centre at ~50 ka. While accessory mineral geochronology points to long periods of crystallization prior to eruption (10-10 years) and separate thermal histories for the magmas, little was known about the rates and processes of the final melt production and eruption. Crystal zoning patterns in plagioclase and quartz reveal the thermal and compositional history of the magmatic system leading up to the eruption. The dominant modal phase, plagioclase, displays considerable within-crystal zonation: An, ~40-227 ppm MgO, 45-227 ppm TiO, 416-910 ppm Sr and 168-1164 ppm Ba. Resorption horizons in the crystals are marked by sharp increases (10-30%) in Sr, MgO and X that reflect changes in melt composition and are consistent with open system processes. Melt inclusions display further evidence for open system behaviour, some are depleted in Sr and Ba relative to accompanying matrix glass not consistent with crystallization of modal assemblage. MI also display a wide range in XHO that is consistent with volatile fluxing. Quartz CL images reveal zoning that is truncated by resorption, and accompanied by abrupt increases in Ti concentration (30-80 ppm) that reflect temperature increases ~50-110°C. Diffusion across these resorption horizons is restricted to zones of <20 μm, suggesting most crystallization within the magma occurred in <2000 years. These episodes are brief compared to the longevity (10-10 year) of the crystal mush zones. All textural and compositional features observed within the quartz and plagioclase crystals are best explained by periodic mafic intrusions repeatedly melting parts of a crystal-rich zone and recharging the system with silicic melt. These periodic influxes of silicic melt would have accumulated to form the large volume of magma that fed the caldera-forming Rotoiti eruption. [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]

At Tucker Hill, in Central Otago, New Zealand, a series of pseudotachylyte veins are hosted in quartzofeldspathic schist. Chilled margins, microlites, flow banding, and the crystallisation of mineral phases absent from the host rock provide unequivocal evidence for melting during pseudotachylyte formation. Whole rock analyses of pseudotachylyte reveal c. 3× enrichment of K2O, Ba, and Rb, and similar depletion of Na2O, CaO, Sr, and Eu, as compared to host schist. Formation age of pseudotachylyte is 95.9±1.8 Ma as measured by total fusion 40Ar/39Ar analyses. Stepwise heating of pseudotachylyte matrix yields an excellently defined 40Ar/39Ar plateau age of 96.0±0.3 Ma. These well-defined ages are attributed to the presence of potassium feldspar, low abundance of inherited lithic material from the host rock, and few fluid inclusions containing extraneous Ar. We propose that formation of these pseudotachylyte veins was related to Cretaceous extensional uplift and exhumation of the Otago Schist. [ABSTRACT FROM AUTHOR]

The ice active protein profile of New Zealand snow tussocks Chionochloa macra and C. rigida consisted of ice nucleation activity but no antifreeze or recrystallization inhibition activity. The ice nucleation activity was similar in the two species, despite them being collected at different altitudes and at different times. The activity is intrinsic to the plant and is associated with the surface of the leaves. Snow tussocks collect water from fog. Nucleation sites on the surface of their leaves may aid the efficiency of this process.