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We use comprehensive geochemical and petrological records from whole-rock samples, crystals, matrix glasses and melt inclusions to derive an integrated picture of the generation, accumulation and evacuation of 530 km 3 of crystal-poor rhyolite in the 25.4 ka Oruanui supereruption (New Zealand). New data from plagioclase, orthopyroxene, amphibole, quartz, Fe–Ti oxides, matrix glasses, and plagioclase- and quartz-hosted melt inclusions, in samples spanning different phases of the eruption, are integrated with existing data to build a history of the magma system prior to and during eruption. A thermally and compositionally zoned, parental crystal-rich (mush) body was developed during two periods of intensive crystallisation, 70 and 10–15 kyr before the eruption. The mush top was quartz-bearing and as shallow as ~3.5 km deep, and the roots quartz-free and extending to >10 km depth. Less than 600 year prior to the eruption, extraction of large volumes of ~840 °C low-silica rhyolite melt with some crystal cargo (between 1 and 10%), began from this mush to form a melt-dominant (eruptible) body that eventually extended from 3.5 to 6 km depth. Crystals from all levels of the mush were entrained into the eruptible magma, as seen in mineral zonation and amphibole model pressures. Rapid translation of crystals from the mush to the eruptible magma is reflected in textural and compositional diversity in crystal cores and melt inclusion compositions, versus uniformity in the outermost rims. Prior to eruption the assembled eruptible magma body was not thermally or compositionally zoned and at temperatures of ~790 °C, reflecting rapid cooling from the ~840 °C low-silica rhyolite feedstock magma. A subordinate but significant volume (3–5 km 3 ) of contrasting tholeiitic and calc-alkaline mafic material was co-erupted with the dominant rhyolite. These mafic clasts host crystals with compositions which demonstrate that there was some limited pre-eruptive physical interaction of mafic magmas with the mush and melt-dominant body. However, the mafic magmas do not appear to have triggered the eruption or controlled magmatic temperatures in the erupted rhyolite. Integration of textural and compositional data from all available crystal types, across all dominant and subordinate magmatic components, allow the history of the Oruanui magma body to be reconstructed over a wide range of temporal scales using multiple techniques. This history spans the tens of millennia required to grow the parental magma system (U–Th disequilibrium dating in zircon), through the centuries and decades required to assemble the eruptible magma body (textural and diffusion modelling in orthopyroxene), to the months, days, hours and minutes over which individual phases of the eruption occurred, identified through field observations tied to diffusion modelling in magnetite, olivine, quartz and feldspar. Tectonic processes, rather than any inherent characteristics of the magmatic system, were a principal factor acting to drive the rapid accumulation of magma and control its release episodically during the eruption. This work highlights the richness of information that can be gained by integrating multiple lines of petrologic evidence into a holistic timeline of field-verifiable processes.

The transport and degassing pathways of volatiles through large silicic magmatic systems are central to understanding geothermal fluid compositions, ore deposit genesis, and volcanic eruption dynamics and impacts. Here, we document sulfur (S), chlorine (Cl), and fluorine (F) concentrations in a range of host materials in eruptive deposits from Taupō volcano (New Zealand). Materials analysed are groundmass glass, silicic melt inclusions, and microphenocrystic apatite that equilibrated in shallow melt-dominant magma bodies; silicic melt and apatite inclusions within crystal cores inferred to be sourced from deeper crystal mush; and olivine-hosted basaltic melt inclusions from mafic enclaves that represent the most primitive feedstock magmas. Sulfur and halogen concentrations each follow distinct concentration pathways during magma differentiation in response to changing pressures, temperatures, oxygen fugacities, crystallising mineral phases, the effects of volatile saturation, and the presence of an aqueous fluid phase. Sulfur contents in the basaltic melt inclusions (~ 2000 ppm) are typical for arc-type magmas, but drop to near detection limits by dacitic compositions, reflecting pyrrhotite crystallisation at ~ 60 wt. % SiO2 during the onset of magnetite crystallisation. In contrast, Cl increases from ~ 500 ppm in basalts to ~ 2500 ppm in dacitic compositions, due to incompatibility in the crystallising phases. Fluorine contents are similar between mafic and silicic compositions (< 1200 ppm) and are primarily controlled by the onset of apatite and/or amphibole crystallisation and then destabilisation. Sulfur and Cl partition strongly into an aqueous fluid and/or vapour phase in the shallow silicic system. Sulfur contents in the rhyolite melts are low, yet the Oruanui supereruption is associated with a major sulfate peak in ice core records in Antarctica and Greenland, implying that excess S was derived from a pre-eruptive gas phase, mafic magma recharge, and/or disintegration of a hydrothermal system. We estimate that the 25.5 ka Oruanui eruption ejected > 130 Tg of S (390 Tg sulfate) and up to ~ 1800 Tg of Cl, with potentially global impacts on climate and stratospheric ozone.

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 high‐ or normal‐δ18O magmatic systems. Plain Language Summary Heat coming from large sub‐volcanic magma systems drives convection of surface‐derived waters through the upper few kilometers of Earth's crust, forming hydrothermal systems. The nature and depth of the interface between hydrothermal systems and their underlying magmatic heat sources are often not well constrained. High temperature alteration of rocks by surface‐derived waters lowers rock oxygen isotope (18O/16O) ratios, which can thus be used to track infiltration of water into the crust. We measured the 18O/16O ratios of minerals from the products of 40 young eruptions from the highly active Taupō Volcanic Zone, New Zealand, to assess whether the erupted magmas had melted and incorporated rocks that were altered in this way. At both Taupō and Ōkataina volcanoes, we observe periods of subtle (but statistically significant) progressive lowering of magma 18O/16O ratios over successive eruptions, suggesting that their magmatic systems at times overlapped and interacted with overlying hydrothermal systems. At both volcanoes, these reductions occurred as the magmatic system was rebuilt to shallow levels in the crust following very large (caldera‐forming) eruptions. The subtle and short‐lived signals of these interactions are only recognized here because of the unusually high eruption frequencies of Taupō and Ōkataina volcanoes. Key Points Large silicic mushes in the Taupō Volcanic Zone are isotopically well‐mixed with respect to oxygen but show subtle temporal δ18O variations Temporal trends in melt δ18O values reflect transient interactions with altered and unaltered assimilants after caldera collapses Muted (sub‐permil) melt δ18O value variability reflects limited isotopic contrasts between magmas and country rocks in this setting

Chemical anomalies in polar ice core records are frequently linked to volcanism; however, without the presence of (crypto)tephra particles, links to specific eruptions remain speculative. Correlating tephras yields estimates of eruption timing and potential source volcano, offers refinement of ice core chronologies, and provides insights into volcanic impacts. Here, we report on sparse rhyolitic glass shards detected in the Roosevelt Island Climate Evolution (RICE) ice core (West Antarctica), attributed to the 1.8 ka Taupō eruption (New Zealand)—one of the largest and most energetic Holocene eruptions globally. Six shards of a distinctive geochemical composition, identical within analytical uncertainties to proximal Taupō glass, are accompanied by a single shard indistinguishable from glass of the ~25.5 ka Ōruanui supereruption, also from Taupō volcano. This double fingerprint uniquely identifies the source volcano and helps link the shards to the climactic phase of the Taupō eruption. The englacial Taupō-derived glass shards coincide with a particle spike and conductivity anomaly at 278.84 m core depth, along with trachytic glass from a local Antarctic eruption of Mt. Melbourne. The assessed age of the sampled ice is 230 ± 19 CE (95% confidence), confirming that the published radiocarbon wiggle-match date of 232 ± 10 CE (2 SD) for the Taupō eruption is robust.

Supereruptions (>10 15 kg ≈ 450 km 3 of ejected magma) have received much attention because of the challenges in explaining how and over what time intervals such large volumes of magma are accumulated, stored and erupted. However, the processes that follow supereruptions, particularly those focused around magmatic recovery, are less fully documented. We present major and trace-element data from whole-rock, glass and mineral samples from eruptive products from Taupo volcano, New Zealand, to investigate how the host magmatic system reestablished and evolved following the Oruanui supereruption at 25.4 ka. Taupo’s young eruptive units are precisely constrained chronostratigraphically, providing uniquely fine-scale temporal snapshots of a post-supereruption magmatic system. After only ~5 kyr of quiescence following the Oruanui eruption, Taupo erupted three small volume (~0.1 km 3 ) dacitic pyroclastic units from 20.5 to 17 ka, followed by another ~5-kyr-year time break, and then eruption of 25 rhyolitic units starting at ~12 ka. The dacites show strongly zoned minerals and wide variations in melt-inclusion compositions, consistent with early magma mixing followed by periods of cooling and crystallisation at depths of >8 km, overlapping spatially with the inferred basal parts of the older Oruanui silicic mush system. The dacites reflect the first products of a new silicic system, as most of the Oruanui magmatic root zone was significantly modified in composition or effectively destroyed by influxes of hot mafic magmas following caldera collapse. The first rhyolites erupted between 12 and 10 ka formed through shallow (4–5 km depth) cooling and fractionation of melts from a source similar in composition to that generating the earlier dacites, with overlapping compositions for melt inclusions and crystal cores between the two magma types. For the successively younger rhyolite units, temporal changes in melt chemistry and mineral phase stability are observed, which reflect the development, stabilisation and maturation of a new, probably unitary, silicic mush system. This new mush system was closely linked to, and sometimes physically interacted with, underlying mafic melts of similar composition to those involved in the Oruanui supereruption. From the inferred depths of magma storage and geographical extent of vent sites, we consider that a large silicic mush system (>200 km 3 and possibly up to 1000 km 3 in volume) is now established at Taupo and is capable of feeding a new episode or cycle of volcanism at any stage in the future.

Deposits of highly vesicular pumice that blanket submarine volcanoes are often attributed to explosive eruptions. Density and textural analysis of clasts dredged from the submarine Macauley Volcano, southwest Pacific Ocean, however, reveal an eruptive style that is neither explosive nor effusive, with clasts instead forming from buoyant detachment of a magma foam. Many submarine caldera volcanoes are blanketed with deposits of highly vesicular pumice, typically attributed to vigorous explosive activity 1 , 2 , 3 , 4 . However, it is challenging to relate volcanic products to specific eruptive styles in submarine volcanism 5 , 6 . Here we document vesicularity and textural characteristics of pumice clasts dredged from the submarine Macauley volcano in the Kermadec arc, southwest Pacific Ocean. We find that clasts show a bimodal distribution, with corresponding differences in vesicle abundances and shapes. Specifically, we find a sharp mode at 91% vesicularity and a broad mode at 65–80%. Subordinate clasts show gradients in vesicularity. We attribute the bimodality to a previously undocumented eruptive style that is neither effusive nor explosive. The eruption rate is insufficient to cause magma to fragment explosively, yet too high to passively feed a lava dome. Instead, the magma foam buoyantly detaches at the vent and rises as discrete magma parcels, or blebs, while continuing to vesiculate internally. The blebs are widely distributed by ocean currents before they disintegrate or become waterlogged. This disintegration creates individual clasts from interior and rim fragments, yielding the bimodal vesicularity characteristics. We conclude that the generation and widespread dispersal of highly vesicular pumice in the marine environment does not require highly explosive activity.

Pyroclastic deposits from four caldera volcanoes in the Kermadec arc have been sampled from subaerial sections (Raoul and Macauley) and by dredging from the submerged volcano flanks (Macauley, Healy, and the newly discovered Raoul SW). Suites of 16–32 mm sized clasts have been analyzed for density and shape, and larger clasts have been analyzed for major element compositions. Density spectra for subaerial dry-type eruptions on Raoul Island have narrow unimodal distributions peaking at vesicularities of 80–85%, whereas ingress of external water (wet-type eruption) or extended timescales for degassing generate broader distributions, including denser clasts. Submarine-erupted pyroclasts show two different patterns. Healy and Raoul SW dredge samples and Macauley Island subaerial-emplaced samples are dominated by modes at ~80–85%, implying that submarine explosive volcanism at high eruption rates can generate clasts with similar vesicularities to their subaerial counterparts. A minor proportion of Healy and Raoul SW clasts also show a pink oxidation color, suggesting that hot clasts met air despite 0.5 to >1 km of intervening water. In contrast, Macauley dredged samples have a bimodal density spectrum dominated by clasts formed in a submarine-eruptive style that is not highly explosive. Macauley dredged pyroclasts are also the mixed products of multiple eruptions, as shown by pumice major-element chemistry, and the sea-floor deposits reflect complex volcanic and sedimentation histories. The Kermadec calderas are composite features, and wide dispersal of pumice does not require large single eruptions. When coupled with chemical constraints and textural observations, density spectra are useful for interpreting both eruptive style and the diversity of samples collected from the submarine environment.

The construction of a free-standing stone wall was a significant occasion in Londinium's history, remarkable for the quantity of masonry used and for the continuing additions to the defences over at least three identifiable phases. Since the local geology in the London Basin does not offer suitable building stone, Londinium's walls offer an exceptional example by which to examine the logistics of construction and the transportation of materials in the context of Romano-British building projects. We examine the sources of the materials used, their transport and the scale of labour and investment involved in the construction of the Landward Wall using an energetics-based methodology. Finally, we provide new insights into Londinium's Landward Wall and the socio-economic and practical implications of its construction. Supplementary material is available online (https://doi.org/10.1017/S0068113X21000088) and comprises technical data related to the architectural energetics.

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.