Search

New geochemical data, including Sr–Nd–Pb isotopes for whole-rock and groundmass samples, are reported for edifice-forming eruptives at Tongariro volcano, New Zealand, which span its ~ 350 ka to late Holocene history. Tongariro eruptives are medium-K basaltic-andesites to dacites (53.0–66.2 wt% SiO2) that evolved via assimilation-fractional crystallisation (AFC) processes partly or mostly in the uppermost 15 km of the crust. When ordered chronologically using a high-resolution 40Ar/39Ar-dated eruptive stratigraphy, the compositional data show systematic 10–130 kyr cycles. Mafic replenishment events inferred from MgO values occurred at ~ 230, ~ 151, ~ 88 and ~ 56 ka and in the late Holocene, with high-MgO flank vents erupting at ~ 160, ~ 117, ~ 35 and ~ 17.5 ka. Cycles in Sm/Nd, 87Sr/86Sr, 143Nd/144Nd and Pb isotopic ratios, which are decoupled from MgO, K2O and Rb/Sr cycles, indicate periods of prolonged crustal residence of magmas from ~ 230 to ~ 100 ka and ~ 95 to ~ 30 ka. AFC modelling shows that intermediate and silicic melt compositions, with r-values between 0.1 and 1, are needed to reproduce Tongariro compositional arrays. AFC models also indicate that ~ 20% of the average Tongariro magma comprises assimilated (meta)sedimentary basement material. Locally, Tongariro and adjacent Ruapehu volcanoes attain their most crust-like 87Sr/86Sr and 143Nd/144Nd compositions at ~ 100 and ~ 30 ka, paralleling with zircon model-age crystallisation modes at the rhyolitic Taupō volcano ~ 50 km to the NNE. These coincidences suggest that the timing and tempo of magma assembly processes at all three volcanoes were contemporaneous and may have been tectonically influenced since at least 200 ka.

Volatile measurements in mineral-hosted sealed melt inclusions, and open-ended embayments, have previously been used to study magma ascent dynamics in large rhyolitic eruptions. However, despite occurring more frequently, smaller-volume explosive events remain under-studied. We present magmatic volatile data from quartz-hosted melt inclusions and embayments for eight post-25.4 ka rhyolitic eruptions at Ōkataina Volcanic Centre, Aotearoa New Zealand. Seven originated from within the main caldera, and the other erupted from the associated Ōkareka Structural Embayment. Melt inclusions preserve volatile contents of 2.92–5.82 wt% H2O and 13–126 ppm CO2, indicating pre-eruptive storage depths of 4.5–7.4 km, with younger eruptions being more shallow. The lack of correlation between H2O, CO2, inclusion size or distance to the crystal rim suggests magma bodies experienced variable degrees of degassing during magma storage, with some amount of post-entrapment volatile modification prior to and concurrent with final magma ascent. Diffusion modelling of measured H2O gradients in melt embayments indicates ascent rates of 0.10–1.67 m.s−1 over time spans of 20–230 min for the intra-caldera events. In contrast, ascent rates for the eruption from the Ōkareka Structural Embayment may be more rapid, at 1.59–4.4 m.s−1 over a time span of 22–34 min. Our findings imply that the final, pre-eruptive magma movement towards the surface could be less than a few hours. Comparisons with published data for caldera-forming explosive events reveal no clear relationships between final ascent rate, eruption size or initial volatile content, implying that other factors besides eruption volume control rhyolite magma ascent.

No description supplied

This work has been supported by the Marsden Fund grant VUW0813 (Royal Society of New Zealand to C.J.N.W.), a James Cook Fellowship (Royal Society of New Zealand) to C.J.N.W., and the ECLIPSE Programme, funded by the N.Z. Ministry of Business, Innovation and Employment. G.F.C. is supported by a NERC Standard Grant (NE/T000317/1), M.L.M. is supported by an NSF CAREER grant (EAR 2042662) and S.J.B. acknowledges Marsden Fund grant VUW1627.

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 ( 130 Tg of S (390 Tg sulfate) and up to ~ 1800 Tg of Cl, with potentially global impacts on climate and stratospheric ozone.

Taupō volcano (New Zealand) is distinguished as the source of Earth's youngest supereruption (∼25.5 ka), with Lake Taupō occupying the resulting caldera. Taupō has also produced eruptions of a wide...

Silicic caldera volcanoes are frequently situated in regions of tectonic extension, such as continental rifts, and are subject to periods of unrest and/or eruption that can be triggered by the interplay between magmatic and tectonic processes. Modern (instrumental) observations of deformation patterns associated with magmatic and tectonic unrest in the lead up to eruptive events at silicic calderas are sparse. Therefore, our understanding of the magmatic-tectonic processes associated with volcanic unrest at silicic calderas is largely dependent on historical and geological observations. Here we utilize existing instrumental, historical and geological data to provide an overview of the magmatic-tectonic deformation patterns operating over annual to 104 year timescales at Taupō volcano, now largely submerged beneath Lake Taupō, in the rifted-arc of the Taupō Volcanic Zone. Short-term deformation patterns observed from seismicity, lake level recordings and historical records are characterized by decadal-scale uplift and subsidence with accompanying seismic swarms, ground shaking and surface ruptures, many of which may reflect magma injections into and around the magma reservoir. The decadal-scale frequency at which intense seismic events occur shows that ground shaking, rather than volcanic eruptions, is the primary short-term local hazard in the Taupō District. Deformation trends near and in the caldera on 101–104 yr timescales are atypical of the longer-term behavior of a continental rift, with magma influx within the crust suppressing axial subsidence of the rift basin within ∼10 km of the caldera margin. Examination of exposed faults and fissures reveals that silicic volcanic eruptions from Taupō volcano are characterized by intense syn-eruptive deformation that can occasionally extend up to 50 km outside the caldera structure, including ground shaking, fissuring and triggered fault movements. We conclude that eruption and unrest scenarios at Taupō volcano depend on the three-way coupling between the mafic-silicic-tectonic systems, with eruption and/or unrest events leading to six possible outcomes initially triggered by mafic injection either into or outside the magma mush system, or by changes to the tectonic stress state.

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 Pamukcu 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.

Andesites erupted from Tongariro volcano, North Island, New Zealand contain feldspathic and quartzose xenoliths derived from basement rocks. New major oxide, trace element and Sr-Nd-Pb isotopic data indicate that both the Waipapa and Kaweka (meta)sedimentary terranes are represented in erupted xenoliths, rather than only the Kaweka terrane as previously thought. Xenolith mineral assemblages differ from their likely source materials, notably through the lack of white mica, illite, chlorite and quartz, which is reflected in contrasting chemical and isotopic compositions. Major and trace element data indicate that most xenoliths underwent bulk mass decreases of about 50% when pyrometamorphosed at temperatures of ~800–980 °C, similar to typical Tongariro magma temperatures of ~800–1000 °C. Bulk Eu concentrations were retained (in restitic plagioclase); however, other rare earth elements are commonly lower in xenoliths than in protoliths. In xenoliths, the 143Nd/144Nd ratios of protoliths were also retained, which indicates that xenoliths were derived from the Kaweka and Waipapa terranes in subequal amounts. Reductions in 87Sr/86Sr ratios by up to 0.003 in xenoliths, relative to their likely protoliths, were accompanied by decreases in Rb/Sr ratios from 0.1–0.8 down to

EnVision is a proposed orbiter mission aiming at determining the nature and current state of Venus' geological evolution and its relationship with the atmosphere, to understand how and why Venus and Earth evolved so differently. It is is one of two M5 mission concepts in Phase A study with a final down-selection expected in June 2021. EnVision’s overall science goals are- to characterise the sequence of events that generated the regional and global surface features of Venus, and characterize the geodynamics framework that controls the release of internal heat over Venus history; - to search for ongoing geological processes and determine whether the planet is active in the present era;- to characterise regional and local geological units, to better assess whether Venus once had condensed liquid water on its surface and was thus perhaps hospitable for life in its early history.EnVision will deliver new insights into geological history through complementary imagery, polarimetry, radiometry and spectroscopy of the surface coupled with subsurface sounding and gravity mapping; it will search for thermal, morphological, and gaseous signs of volcanic and other geological activity; and it will trace the fate of key volatile species from their sources and sinks at the surface through the clouds up to the mesosphere.EnVision’s science payload consists of VenSAR, a dual polarization S-band radar also operating as microwave radiometer, three spectrometers VenSpec-M, VenSpec-U and VenSpec-H designed to observe the surface and atmosphere of Venus, and the Subsurface Radar Sounder (SRS), a High Frequency (HF) sounding radar to probe the subsurface. These are complemented by a radio science investigation which achieves gravity mapping and radio occultation of the atmosphere, for a comprehensive investigation of the Venusian surface, interior and atmosphere and their interactions.

Step leaching of fine-grained refractory inclusions from Allende reveals the presence of large 84Sr anomalies., The 87Rb-87Sr radiochronometer provides key insights into the timing of volatile element depletion in planetary bodies, yet the unknown nucleosynthetic origin of Sr anomalies in Ca-Al–rich inclusions (CAIs, the oldest dated solar system solids) challenges the reliability of resulting chronological interpretations. To identify the nature of these Sr anomalies, we performed step-leaching experiments on nine unmelted CAIs from Allende. In six CAIs, the chemically resistant residues (0.06 to 9.7% total CAI Sr) show extreme positive μ84Sr (up to +80,655) and 87Sr variations that cannot be explained by decay of 87Rb. The extreme 84Sr but more subdued 87Sr anomalies are best explained by the presence of a presolar carrier enriched in the p-nuclide 84Sr. We argue that this unidentified carrier controls the isotopic anomalies in bulk CAIs and outer solar system materials, which reinstates the chronological significance of differences in initial 87Sr/86Sr between CAIs and volatile-depleted inner solar system materials.

We present new nucleosynthetic, radiogenic and stable Sr isotopic data from fifteen previously studied CAIs from the Allende CV3 meteorite, including the highly altered Curious Marie inclusion. We use double-spike TIMS techniques to determine the degrees of isotopic mass fractionation, and also present internally normalised data for the same sample digestions to permit comparisons with previous studies and couple these isotopic data with Rb, Sr, Eu and Th abundance data to consider the origins and relationships of the isotopic variations documented here. Analysed CAIs display elevated μ^(84)Sr anomalies of +58 ppm to +287 ppm, with variability far outside of analytical uncertainties (13 ppm 2 s.d.). We cannot tell at present whether these variations arise from heterogeneities in p-process ^(84)Sr or in the other non-radiogenic isotopes of Sr (^(86)Sr, ^(88)Sr) that are produced by the main s-process, weak s-process, and r-process. All inclusions fall on an offset mass-dependent fractionation line in three-isotope space (δ^(88/86)Sr vs δ^(84/86)Sr) identical within error to that previously defined by bulk undifferentiated meteorites, and have a total range of δ^(88/86)Sr of ∼5.3 ‰ (+1.67 ‰ to -3.67 ‰), reflecting kinetic isotope effects during partial condensation/evaporation and/or low-temperature alteration processes. CI-normalized Sr/Th ratios in our CAIs correlate with normalized Eu/Th ratios with a ∼1:1 relationship, regardless of texture or Sr-isotopic values. This indicates that Sr and Eu had similar condensation behaviors with Eu condensing as Eu^(2+) and having the same chemical behavior in minerals as Sr^(2+) under conditions relevant to CAI formation in the solar nebula. Rb/Th ratios are highly variable: fine-grained CAIs display elevated Rb/Th ratios, consistent with the introduction of Rb into the CAIs by alkali-rich secondary alteration fluids. The μ ^(84)Sr anomalies measured in our CAIs are similar (in magnitude) to those found in carbonaceous chondrites that formed in the outer part of the solar system. A way to reconcile this observation with the formation of CAIs near the Sun would be if the inventories of Sr and other refractory elements in carbonaceous chondrites are dominated by a cryptic refractory dust component (CRD) that was formed early and near the Sun, and was subsequently transported outwards to the carbonaceous chondrite-forming region.

The 2·08 Ma, ∼2500 km3 Huckleberry Ridge Tuff (HRT) eruption, Yellowstone, generated two fall deposits and three ignimbrite members (A, B, C), accompanying a ∼95 x 65 km caldera collapse. Field data imply that the pre-A fall deposits took weeks to be erupted, then breaks of weeks to months occurred between members A and B, and years to decades between B and C. We present compositional and isotopic data from single silicic clasts (pumice or fiamme) in the three ignimbrite members, plus new data from co-eruptive mafic components to reconstruct the nature and evacuation history of the HRT crustal magmatic complex. Geochemical data, building on field characteristics, are used to group nine silicic clast types into seven compositional suites (A1-A3; B1; C1-C3) within their respective members A, B and C. Isotopic data are then added to define four magmatic systems that were tapped simultaneously and/or sequentially during the eruption. Systems 1 and 2 fed the initial fall deposits and then vented throughout member A, accompanied by trace amounts of mafic magma. In member A, volumetrically dominant system 1 is represented by a rhyolite suite (A1: 73·0–77·7 wt % SiO2, 450–1680 ppm Ba) plus a distinct low-silica rhyolite suite (A2: 69·2–71·6 wt % SiO2, >2500 ppm Ba). System 2 yielded only a low-Ba, high-silica rhyolite suite (A3: 76·7–77·4 wt % SiO2, ≤250 ppm Ba). Glass compositions in pumices from systems 1 and 2 show clustering, indicative of the same multiple melt-dominant bodies identified in the initial fall deposits and earliest ignimbrite. Member B samples define suite B1 (70·7–77·4 wt % SiO2, 540–3040 ppm Ba) derived from magmatic system 1 (but not 2) that had undergone mixing and reorganisation during the A: B time break, accompanying mafic magma inputs. Mafic scoriae erupted in upper member B cover similar compositions to the member A clasts, but extend over a much broader compositional range. Member C clast compositions reflect major changes during the B: C time break, including rejuvenation of magmatic system 2 (last seen in member A) as suite C3 (75·3–77·2 wt % SiO2, 100–410 ppm Ba), plus the appearance of two new suites with strong crustal signatures. Suite C2 is another rhyolite (74·7–77·6 wt % SiO2, with Ba decreasing with silica from 2840 to 470 ppm) that defines magmatic system 3. Suite C2 also shows clustered glass compositions, suggesting that multiple melt-dominant bodies were a repetitive feature of the HRT magmatic complex. Suite C1, in contrast, is dacite to rhyolite (65·6–75·0 wt % SiO2, with Ba increasing with silica from 750 to 1710 ppm) that defines magmatic system 4. Compositions from magmatic systems 1 and 2 dominantly reflect fractional crystallization, but include partial melting of cumulates related to earlier intrusions of the same mafic magmas as those syn-eruptively vented. Country rock assimilation was limited to minor amounts of a more radiogenic (with respect to Sr) evolved contaminant. In contrast, systems 3 and 4 show similar strongly crustal isotopic compositions (despite their differences in elemental composition) consistent with assimilation of Archean rocks via partial melts derived from cumulates associated with contrasting mafic lineages. System 3 links to the same HRT mafic compositions co-erupted in members A and B. In contrast, system 4 links to olivine tholeiite compositions erupted in the Yellowstone area before, sparsely during, and following the HRT itself. All four magmatic systems were housed beneath the HRT caldera area. Systems 1 and 2 were hosted in Archean crust that had been modified by Cretaceous/Eocene magmatism, whereas systems 3 and 4 were hosted within crust that retained Archean isotopic characteristics. The extreme compositional diversity in the HRT highlights the spatial and temporal complexities that can be associated with large-volume silicic magmatism.

We integrate field observations with geochemical data from quartz-hosted melt inclusions to develop a hydrogen diffusion-based method to shed light on timescales and conduit processes during the opening stages of two supereruptions. We focus on the initial explosive phases (first ~5% of the total magma volume erupted) from: (1) the episodic Oruanui eruption, Taupo NZ (530 km3, 25.4 ka) and (2) the continuous Bishop Tuff eruption, Long Valley (650 km3, 0.765 Ma). Quartz-hosted melt inclusions for both eruptions exhibit coherent trends for major and trace elements (e.g. Al2O3, SiO2, Nb, U, Rb). However, in the first three phases (of ten) in the Oruanui eruption there is significant variability in measured H2O values (2.6–5.8 wt%), particularly in phases 1 and 3, each of which reflects the opening stages of two respective inferred vent sources. Phase 3 inclusions also record greater scatter in Li concentrations. In contrast, melt inclusions from the first two Bishop Tuff fall layers yield a narrower range of H2O concentrations (4.0–5.8%, with 90% between 4.5 and 5.8 wt%). We interpret the scatter in measured H2O concentrations to represent changes during slow magma ascent or partial stalling within the conduit at lower pressures. Using a diffusion model, we estimate that 61% of the Oruanui melt inclusions experienced modification of original H2O concentrations due to slow ascent in the conduit system prior to eruptive quenching, with the longest timescales (>5 days) mostly in phase 3 inclusions. In contrast, only 33% of melt inclusions from the initial Bishop fall layer (F1) experienced diffusive losses from their starting concentrations, implying that most magma spent less than a day ascending from the storage region at the eruption onset. Similarities between the Oruanui and the previously studied Huckleberry Ridge Tuff include comparable extents and distribution of diffusive losses (seen as H and Li scatter), and field evidence for episodic activity. Our observations imply that in these two systems, the first-erupted magmas rose from reservoirs with low degrees of overpressure and that magma mobilization was controlled by external factors (e.g. tectonics) rather than by strong volatile over-pressures. In contrast the Bishop eruption may well have been initiated by overpressure, leading to continuous, escalating activity.

Taupō volcano, New Zealand, is a large caldera volcano that has been highly active through the Holocene. It most recently erupted ∼1800 years ago but there have been multiple periods of historic volcanic unrest. We use seismological and geodetic analysis to show that in 2019 Taupō underwent a period of unrest characterised by increased seismic activity through multiple swarms and was accompanied by ground deformation within the caldera. The earthquakes, which include non-double-couple events, serve to outline an aseismic zone beneath the most recent eruptive vents. This aseismic zone is coincident with an inflating source, based on forward modelling of ground deformation data. We infer that this aseismic and deforming region delineates the location of the present day magma reservoir that is ≥250 km3 in volume and has a melt fraction of > 20–30%, inhibiting seismic activity. Our analysis shows that the 2019 unrest at Taupō was volcanic in nature and origin, demonstrating that this is an active and potentially hazardous volcano, and that improving our monitoring and understanding of its behaviour is important.

Tarawera volcano (New Zealand) is volumetrically dominated by rhyolitic lavas and pyroclastic deposits, but the most recent event in AD 1886 was a basaltic Plinian fissure eruption. In March 2019 a swarm of at least 64 earthquakes occurred to the NE of Tarawera volcano, as recorded by the New Zealand Geohazard Monitoring Network (GeoNet). We use seismological analysis to show that this swarm was most likely caused by a dyke that intruded into the brittle crust between depths of 8–10 km and propagated toward Tarawera volcano for 2 km at a rate of 0.3–0.6 m s−1. We infer that this was a dyke of basaltic composition that was stress-guided toward Tarawera volcano by the topographic load of the volcanic edifice. Dyke intrusions of this nature are most likely a common occurrence but a similar process may have occurred during the 1886 eruption with a dyke sourced from some lateral distance away from the volcano. The 2019 intrusion was not detected by InSAR geodesy and we use synthetic models to show that geodetic monitoring could only detect a ≥6 m wide dyke at these depths. Improvements to geodetic monitoring, combined with detailed seismological analysis, could better detect future magmatic intrusions in the region and serve to help assess ongoing changes in the magmatic system and the associated possibilities of a volcanic event.

Phosphine (PH3) is proposed to be a possible biomarker in planetary atmospheres and has been claimed to have been observed in the atmosphere of Venus, sparking interest in the habitability of Venus’s atmosphere. Observations of another biomarker, methane (CH4), have been reported several times in the atmosphere of Mars, hinting at the possibility of a past or present biosphere. The Atmospheric Chemistry Suite on the ExoMars Trace Gas Orbiter has a spectral range that includes several absorption lines of PH3 with line strengths comparable to previously observed CH4 lines. The signature of PH3 was not observed in the 192 observations made over a full Martian year of observations, and here we report upper limits of 0.1–0.6 ppbv.

Ruapehu (150 km3 cone, 150 km3 ring-plain) and Tongariro (90 km3 cone, 60 km3 ring-plain) are iconic stratovolcanoes, formed since ���230 and ���350 ka, respectively, in the southern Taupo Volcanic Zone and Taupo Rift. These volcanoes rest on Mesozoic metasedimentary basement with local intervening Miocene sediments. Both volcanoes have complex growth histories, closely linked to the presence or absence of glacial ice that controlled the distribution and preservation of lavas. Ruapehu cone-building vents are focused into a short NNE-separated pair, whereas Tongariro vents are more widely distributed along that trend, the differences reflecting local rifting rates and faulting intensities. Both volcanoes have erupted basaltic andesite to dacite (53���66 wt.% silica), but mostly plagioclase-two pyroxene andesites from storage zones at 5���10 km depth. Erupted compositions contain evidence for magma mixing and interaction with basement rocks. Each volcano has an independent magmatic system and a growth history related to long-term (>104 years) cycles of mantle-derived magma supply, unrelated to glacial/interglacial cycles. Historic eruptions at both volcanoes are compositionally diverse, reflecting small, dispersed magma sources. Both volcanoes often show signs of volcanic unrest and have erupted with a wide range of styles and associated hazards, most recently in 2007 (Ruapehu) and 2012 (Tongariro).

In this presentation we will describe recent work to model upward and downward fluxes of solar and thermal radiation in the atmosphere of Venus using the NEMESIS radiative transfer and retrieval tool (Irwin et al., JQSRT, 109, 1136, 2008). Using a plane-parallel matrix operator multiple-scattering model we simulate the internal 3D radiation field within Venus’ atmosphere and compare our simulations with the observations of the Pioneer Venus and Venera 13 and 14 entry probes. Such simulations allow us to assess the availability of sunlight and the visibility of the sun azimuth direction in the cloud layer for potential balloon missions, and also enables us to predict at what altitude the surface will become visible for probes descending on dayside. A reanalysis of the Venera 13 and 14 radiance spectra observations will be used to reassess earlier estimates of cloud structure and water vapour abundance. Such modelling also allows us to simulate the visible appearance of Venus’ atmosphere during the descent of a probe mission as will be shown.

If Venus were a newly discovered exoplanet, it would be one of the most Earth-like yet identified. Its similarity in size, bulk density and cloud top temperatures give no clue to the hellish temperatures at its surface. Venus hosts an array of geological features as complex as Earth, but without its organisation, and sustains a chemically reactive atmosphere, but without life. Proposed in response to ESA’s M5 call, with enabling support from NASA, EnVision is currently in Phase A study for the next medium-class mission opportunity proceeding towards a final mission selection in summer 2021 for a launch in 2032. Here we discuss the science questions motivating the mission in more detail. Building on discoveries from Magellan, Venus Express and Akatsuki, EnVision will focus on three overarching questions: Is Venus geodynamically active? How did Venus arrive at its current state? How does the Venus climate work and how do the interior, surface and atmosphere interact? 1 – Is Venus geodynamically active? Venus should be geologically active today and its globally young surface implies extensive volcanic resurfacing, but whether this happened in episodic global events or in a continuous process of small-scale resurfacing is uncertain. Constraining the rate of volcanic and tectonic activity can reveal whether Venus occupies one of these end members, or lies somewhere on the spectrum between. Magellan radar image mosaic of a 550 km wide region of Lada Terra (47°S, 25°E) showing a system of radar-bright and dark lava flows breaching a ridge belt and ponding in a large radar-bright deposit covering 100,000 km². Ammavaru, the source caldera, lies 300 km to the west (JPL/Caltech). Understanding the tectonic regime driving resurfacing and heat loss is as important as knowing the rate of activity. The surface appears partitioned into areas of low strain bounded by narrow high strain margins. The abundance of steep slopes and landslides implies active uplift in these high strain areas, but existing data provide no constraint on the frequency of landslides or rates of tectonic movement. Are the low strain regions actively created and destroyed, like Earth’s oceanic plates, or simply mobilised locally? What is the significance of the global network of elevated rift systems and linear lowland wrinkle-ridged plains? Unique to Venus are coronae, quasi-circular volcano-tectonic features, typically 100–500 km across. Are they the surface expression of plumes, or magmatic intrusions, or subduction zones? The processes of weathering, mass wasting and aeolian transport are critical for understanding both the geological history and climatic evolution of Venus. Venus Express found anomalously high IR emissivities near suspected active volcanoes, interpreted as fresh, unweathered lava flows, but both their mineralogy and the weathering processes involved are unknown. Magellan detected several, probably impact-related, dune fields at the limit of its resolution, and found indirect evidence for globally distributed smale-scale ripples. 2 – How Did Venus Arrive at its Current State? The cratering record reveals that the Venus surface is, on average, under 1 Ga old. While some areas are likely active today, other regions, e.g. the tessera highlands, may be considerably older. Magellan data imply a variety of age relationships and long-term activity, with a non-random association between geological features and elevation, e.g. the uplands are consistently more deformed than the lowlands. Impact craters themselves show alteration to dark floors, perhaps from airfall deposits or magmatic resurfacing. Densely fractured plains (right) abutting tessera (bottom); both are embayed by younger plains (dark areas). A steep-sided volcanic dome (upper left) may be the source of the plains or unrelated to them, and older or younger than the tesserae. Magellan image at 46°N, 360°E, after Basilevsky and McGill (2007). Constraining the history is critical to understanding not only when and how resurfaced, but whether that activity has changed systematically through time. Were the plains formed in a short period by massive outpourings or by many thousands of small flows over their entire history? Or did many different mechanisms – including sedimentary processes – operate at different times and places? Globally, Venus exhibits perhaps even more tectonic deformation than Earth: what is its role in resurfacing the planet and have the regimes of tectonic deformation changed over time? How did tesserae, especially, accumulate their extraordinary degree of deformation? The interior of Venus is probably Earth-like, but not the same; its core size is poorly constrained by Magellan gravity data. These data are consistent with an organised pattern of mantle convection but lack the resolution necessary to connect it with geological-scale features. 3 - How does the Venus climate work and how do the Interior, Surface and Atmosphere Interact? How and why the atmospheres of two Earth-like planets evolved so differently is one of the many compelling reasons to study Venus, directly addressing the question of how our own world became habitable. Understanding its evolution depends on knowing the exchanges between its interior, surface and space. The lower/middle clouds of Venus as imaged by Akatsuki IR2 camera. Dark areas represent thicker clouds, which may represent volcanic plumes of ash or sulphate particulates. (JAXA/ISAS/DART/Damia Bouic) Models suggest that the clouds are maintained by a constant input of H2O and SO2, both of which vary considerably, perhaps because of volcanic emissions. Elevated D/H ratios indicate the loss of early oceans, but the ‘starting’ mantle D/H ratio is unknown. Identification of granites in the ancient highlands would support this inference, but the direct detection and characterisation of volcanic volatiles is key to inferring past climate evolution. Beyond the specific investigations outlined above, however, the most important unknown factor in determining the present day state of Venus and its atmosphere, and how it arrived at that state, is the interaction between the interior, surface and atmosphere: the whole being more than the sum of the parts. References Basilevsky and McGill (2007), Surface evolution of Venus, in Exploring Venus as a Terrestrial Planet, Geophysical Monograph Series 176, edited by L. W. Esposito, E. R. Stofan and T. E. Cravens, pp. 23-43, American Geophysical Union, Washington, DC

Last year’s rumblings beneath New Zealand’s Taupō supervolcano, the site of Earth’s most recent supereruption, lend new urgency to research and outreach efforts in the region.

Understanding the origins of the mantle melts that drive voluminous silicic volcanism is challenging because primitive magmas are generally trapped at depth. The central Taupō Volcanic Zone (TVZ; New Zealand) hosts an extraordinarily productive region of rhyolitic caldera volcanism. Accompanying and interspersed with the rhyolitic products, there are traces of basalt to andesite preserved as enclaves or pyroclasts in caldera eruption products and occurring as small monogenetic eruptive centers between calderas. These mafic materials contain MgO-rich olivines (Fo79–86) that host melt inclusions capturing the most primitive basaltic melts fueling the central TVZ. Olivine-hosted melt inclusion compositions associated with the caldera volcanoes (intracaldera samples) contrast with those from the nearby, mafic intercaldera monogenetic centers. Intracaldera melt inclusions from the modern caldera volcanoes of Taupō and Okataina have lower abundances of incompatible elements, reflecting distinct mantle melts. There is a direct link showing that caldera-related silicic volcanism is fueled by basaltic magmas that have resulted from higher degrees of partial melting of a more depleted mantle source, along with distinct subduction signatures. The locations and vigor of Taupō and Okataina are fundamentally related to the degree of melting and flux of basalt from the mantle, and intercaldera mafic eruptive products are thus not representative of the feeder magmas for the caldera volcanoes. Inherited olivines and their melt inclusions provide a unique “window” into the mantle dynamics that drive the active TVZ silicic magmatic systems and may present a useful approach at other volcanoes that show evidence for mafic recharge.

Hydrothermal alteration zoning and processes provide insights into the evolution of heat source(s) and fluid compositions associated with geothermal systems. Traditional petrological techniques, combined with hydrothermal alteration studies, stable isotope analyses and geochronology can resolve the nature of the fluids involved in hydrothermal processes and their changes through time. We report here new findings along with previous unpublished works on alteration patterns, fluid inclusion measurements and stable isotope data to provide insights into the thermal and chemical evolution of the Kawerau geothermal system, New Zealand. These data indicate the presence of two hydrothermal events that can be coupled with chronological data. The earlier period of hydrothermal activity was initiated at ~400 ka, with the heat driving the hydrothermal system inferred to be from the magmatic system that gave rise to rhyolite lavas and sills of the Caxton Formation. Isotopic data fingerprint fluids attributed to this event as meteoric, indicating that the magma primarily served as a heat source driving fluid circulation, and was not releasing magmatic fluids in sufficient quantity to affect the rock mineralogy and thus inferred fluid compositions. The modern Kawerau system was initiated at ~16 ka with hydrothermal eruptions linked to shallow intrusion of magma at the onset of activity that gave rise to the Putauaki andesite cone. Likely associated with this later event was a pulse of magmatic CO2, resulting in large-scale deposition of hydrothermal calcite enriched in 18O. Meteoric water-dominated fluids subsequently overwhelmed the magmatic fluids associated with this 18O-rich signature, and both the fluid inclusion microthermometry and stable isotope data reflect a change to the present-day fluid chemistry of low salinity, meteoric-dominated waters.

The links between large-scale silicic volcanism and plutonism offer insights into the dynamics of crustal magmatic systems and growth of continental crust. In Hong Kong, voluminous silicic ignimbrites and linked plutons record a ~26 Myr period of magmatism from ~164 to 138 Ma. We present data from these linked volcanic-plutonic assemblages at the Lantau and High Island caldera complexes, with an emphasis on the ~143–138 Ma activity from the latter. To track the evolution of these magmatic systems, U-Pb dating and trace element analyses using secondary-ion mass spectrometry (SIMS) were carried out on zircons from 21 samples from both volcanic and plutonic samples. The SIMS age data sets divide into two groups across volcanic and plutonic origins: (1) seven samples with unimodal age spectra [five of which have the same mean value as the published Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) age from the same sample]; and (2) 14 samples yielding multiple age components. Age patterns from both groups suggest that the previously separated ~143 Ma Repulse Bay (RBVG) and ~141–140 Ma Kau Sai Chau volcanic groups (KSCVG) instead represent activities over a single ~5 Myr period. Direct linkages previously proposed between some volcanic and plutonic units for this period (e.g., High Island Tuff, Kowloon Granite) are no longer supported, and magmatism represented by exposed plutons continued until 137.8 ± 0.8 Ma (Mount Butler Granite). Under CL imagery, a wide range of zircon textures identified in both volcanic and plutonic samples is indicative of complex processes, some of which are identified through trace element data coupled with textural characteristics. Overall, intra-grain (cores vs. rims; sector-zonation) and intra-sample variations in trace element abundances and ratios are larger than those between samples. Zircon chemistries in both volcanic and plutonic samples fall into two groups during the ~5 Myr history of the High Island caldera magmatic system. One group (RBVG and “cold” granites) includes inherited grains back to 164 Ma and wider ranges in Hf, Y, total trivalent elements, Th and U concentrations and Th/U, Yb/Gd, and U/Yb ratios than the other (KSCVG and “hot” granites). Two possible evolutionary models of the High Island caldera magmatic system are: (1) the system randomly tapped a single crustal domain that fluctuated in temperature as a result of varying interactions of hotter melts, or (2) the volcanic and plutonic records reflect the interplay of two crustal domains with contrasting “low-” and “high-temperature” characteristics. In Hong Kong, some plutonic bodies were comagmatic with large-scale volcanism, while others were emplaced at shallow crustal levels independently of volcanism, matching the current two end-member views of the volcanic-plutonic relationship.

The Auckland Volcanic Field (AVF), which last erupted ca. 550 years ago, is a late Quaternary monogenetic basaltic volcanic field (ca. 500 km 2 ) in the northern North Island of New Zealand. Prior to this study only 12 out of the 53 identified eruptive centres of the AVF had been reliably dated. Careful sample preparation and 40 Ar/ 39 Ar analysis has increased the number of well-dated centres in the AVF to 35. The high precision of the results is attributed to selection of fresh, non-vesicular, non-glassy samples from lava flow interiors. Sample selection was coupled with separation techniques that targeted only the groundmass of samples with 10 μm wide, coupled with ten-increment furnace step-heating of large quantities (up to 200 mg) of material. The overall AVF age data indicate an onset at 193.2 ± 2.8 ka, an apparent six-eruption flare-up from 30 to 34 ka, and a ≤ 10 kyr hiatus between the latest and second-to-latest eruptions. Such non-uniformity shows that averaging the number of eruptions over the life-span of the AVF to yield a mean eruption rate is overly simplistic. Together with large variations in eruption volumes, and the large sizes and unusual chemistry within the latest eruptions (Rangitoto 1 and Rangitoto 2), our results illuminate a complex episodic eruption history. In particular, the rate of volcanism in AVF has increased since 60 ka, suggesting that the field is still in its infancy. Multiple centres with unusual paleomagnetic inclination and declination orientations are confirmed to fit into a number of geomagnetic excursions, with five identified in the Mono Lake, two within the Laschamp, one within the post-Blake or Blake, and two possibly within the Hilina Pali.

The geological record of volcanic eruptions suggests that scientists are some way from being able to forecast eruptions at many of the world9s volcanoes. There are three reasons for this. First, continuing geological discoveries show that our knowledge is incomplete. Second, knowledge is limited about why, how, and when volcanic unrest turns into eruptions, and over what timescales. Third, there are imbalances between the studies of past eruptions, and the geophysical techniques and observations on modern events, versus the information needed or demanded by society. Scientists do not yet know whether there are other, presently unknown, factors that are important in controlling eruptions, or if there is an inherent unknowability about some volcanic systems.

Volatile releases through volcanic eruptions are one of the major processes contributing to the global nitrogen cycle. Past studies have often estimated the magnitude of this flux using volcanic emission measurements, which are limited to currently active systems and sensitive to atmospheric contamination. Another possible approach is the measurement of nitrogen in melt inclusions, which are parcels of magmatic melt trapped prior to eruption. This methodology requires appropriate analytical parameters for nitrogen analysis in silicate glasses by secondary ion mass spectrometry (SIMS), which have not yet been established. To this end, calibrations for nitrogen were obtained using ion implanted basaltic and rhyolitic glasses. We demonstrate that water content significantly affects the ion yields of 14N + and 14N16O −, as well as the background intensity of 14N + and 12C +. Application of implant-derived calibrations to natural samples provide the first reported concentrations of nitrogen in melt inclusions. These measurements were made on samples from the Bishop Tuff in California, the Huckleberry Ridge Tuff of the Yellowstone Volcanic Center, and material from the Okaia and Oruanui eruptions in the Taupo Volcanic Center. In studied material, we often find maximum nitrogen contents of less than 45 ppm and that nitrogen concentration varies positively with CO2 concentration, which reflects a partial degassing trend. Using the maximum measured nitrogen contents for each eruption, we find that the Bishop released >3.4 × 1013, the Huckleberry Ridge released >1.4 × 1014, the Okaia released >1.0 × 1011, and the Oruanui released >4.5 × 1013 g of nitrogen. Simple calculations suggest that with concentrations such as these, rhyolitic eruptions may ephemerally increase the nitrogen flux to the atmosphere, but are insignificant compared to the 4 × 1021 g of nitrogen stored in the atmosphere.

The Auckland Volcanic Field (AVF) represents the youngest and northernmost of three subjacent Quaternary intraplate basaltic volcanic fields in the North Island, New Zealand. Previous studies on AVF eruptive products suggested that their major- and trace-element, and Sr-, Nd- and Pb-isotopic signatures primarily reflect their derivation from the underlying asthenospheric and lithospheric mantle. All AVF lavas however ascend through a ca. 20–30 km thick continental crust, and some do carry crustal xenoliths, posing the question whether or not crustal contamination plays a role in their formation. Here we present new Os and Pb isotopic data, and Os and Re concentrations for 15 rock samples from 7 AVF volcanic centres to investigate mantle and crustal petrogenetic processes. The samples include the most primitive lavas from the field (Mg# 59–69) and span a range of eruption sizes, ages, locations, and geochemical signatures. The data show a large range in Os concentrations (6–579 ppt) and 187Os/188Os isotope ratios from mantle-like (0.123) to highly radiogenic (0.547). Highly radiogenic Os signatures together with relatively low Os contents in most samples suggest that ascending melts experienced contamination primarily from metasedimentary crustal rocks with high 187Os/188Os ratios (e.g., greywacke). We further demonstrate that < 1% metasedimentary crustal input into the ascending melt can produce the radiogenic Os isotope signatures observed in the AVF data. This low level of crustal contamination has no measurable effect on the corresponding trace element ratios and Sr-Nd-Pb isotopic compositions. In addition, high Os contents (195–578 ppt) at slightly elevated but mantle-like Os isotopic compositions (187Os/188Os = 0.1374–0.1377) in some samples suggest accumulation of xenocrystic olivine-hosted mantle sulphides from the Permian-Triassic ultramafic Dun Mountain Ophiolite Belt, which traverses the crust beneath the Auckland Volcanic Field. We therefore infer that the AVF Os isotopic compositions and Os contents reflect contamination from varying proportions of heterogeneous crustal components, composed of Waipapa and Murihiku terrane metasediments, and ultramafic rocks of the Dun Mountain Ophiolite Belt. This demonstrates, contrary to previous models that primitive lavas from the Auckland Volcanic Field do show evidence for variable interaction with the crust.

How exceedingly large, volcanic supereruptions begin provides crucial information on the storage, ascent and release of silica-rich magma in catastrophic events. Initial fall deposits of the 2.08 Ma, 2500 km 3 Huckleberry Ridge eruption are multiply bedded and in several places contain reworked intervals, indicating time breaks in the opening phases of the eruption. A 2.5 m section of these fall deposits was sampled at nine levels below the earliest ignimbrite (member A) at Mount Everts (Mammoth, Wyoming). We analyzed major and trace elements and volatiles in quartz-hosted melt inclusions (MIs), reentrants (REs; unsealed melt inclusions) and associated obsidian pyroclasts (thick-walled shards) to establish quartz crystallization and storage depths and melt compositional groupings. Systematic relationships between Rb and other incompatible elements (U, Cl, B) indicate ∼55% fractional crystallization between the least and most evolved glass compositions. In contrast, H 2 O concentrations in MIs show scattered relationships with trace elements and are interpreted to reflect variable loss of H 2 O by diffusion through the quartz host during magma ascent. The wide H 2 O variations (1.0–4.7 wt.%) in MIs from individual fall horizons imply as much as ∼14 days of diffusive loss, reflecting highly variable and surprisingly slow decompression conditions. Water and CO 2 gradients in reentrants, however, are consistent with final ascent times of ∼ 0.3 – 1.5 m / s ), similar to those represented by MIs that we infer to have experienced little to no diffusive H 2 O loss. The wide range of ascent rates for co-erupted crystals mirrors that of intermittent explosive activity at Mount St. Helens in summer 1980, and implies that the Huckleberry Ridge magma body was not strongly overpressured at eruption onset. Restored entrapment pressures and geochemical data for MIs provide evidence for six distinct populations of quartz that originally crystallized in geochemically distinct magma domains. The compositions of REs and obsidian pyroclasts, by comparison, show that by the onset of eruption, the quartz had been brought together into three discrete magma bodies, which we interpret to have been cupolas on the roof of the main magma body. These cupolas were erupted sequentially and episodically from separate vents to generate the fall deposits before escalating activity led to generation of voluminous pyroclastic flows, and this pattern of activity suggests that tectonic triggering may have destabilized multiple magma bodies. Supereruptions as large as the Huckleberry Ridge event may start hesitatingly if the parental magma bodies are not strongly overpressured, with small-scale episodic activity that is modulated by external controls that may leave no other geological evidence for their presence.

Silicic volcanic systems provide timed snapshots at the Earth's surface of the magmatic processes that also build complementary plutons in the crust. Links between these two realms are considered here using three Quaternary (

A major challenge in volcanology is determining the factors that control the frequency and magnitude of eruptions at hazardous caldera volcanoes. Understanding the critical sequence of events that may lead to future eruptions is vital for volcanic monitoring and risk assessment. Here we use magma chemistry and mineral diffusion modeling to interpret the magmatic processes and time scales involved in the youngest three eruptions (2.15–1.7 ka) from Taupo volcano (New Zealand), which peaked with the voluminous A.D. 232 eruption. Of the rhyolites erupted since ca. 12 ka, the 2.75 ka) rhyolites that were tapped from the same magma reservoir. Orthopyroxene Fe-Mg diffusion time scales indicate that the onset of rapid heating and priming of the host silicic mush occurred

The Taupo Volcanic Zone (TVZ) in the North Island of New Zealand is one of the world's most spectacular and productive areas of Quaternary silicic volcanism and geothermal manifestations. The TVZ is only the latest manifestation of NNE–SSW-orientated arcs that have migrated in step-wise fashion to the SSE over the past ca. 16 Ma. The TVZ began erupting around 2 Ma, with early andesitic volcanism being joined and rapidly swamped by voluminous rhyolitic volcanism. The zone shows a pronounced segmentation into northern and southern extremities with andesite composite cones, no calderas and only limited vent-hosted geothermal systems, and a 125 km long rhyolite-dominated central segment. About four times as much magma is trapped at depth below the central TVZ than is erupted, feeding heat, volatiles and chemicals into 23 geothermal systems with a total of ca. 4.2 GW thermal energy release. The modern (post-61 ka) TVZ is an actively rifting arc, widening at 7 mm/year at the south end to 15 mm/year at the Bay of Plenty coastline, with an associated zone of young to active faulting (Taupo Fault Belt: TFB, or the Taupo Rift), but the axes of the modern TFB and TVZ are offset by 15–20 km through much of the central TVZ. Although there is a dominant NNE–SSW tectonic grain within the central TVZ, there are also influences of deeper basement structures that sometimes extend outside the limits of the zone, such as NW–SE, arc-perpendicular accommodation zones linking local domains of extension as well as N–S orientated structures related to the Hauraki Rift that may control fluid flow into the roots of the geothermal systems. Models for the geothermal systems favour either a source in a relatively shallow localised magmatic intrusion (e.g., Kawerau, Ngatamariki) or treat the systems as reflecting large-scale fluid dynamical instabilities from an evenly heated ‘hot plate’ at ∼7 km depth. Where controls from dating of host lithologies are available, systems at Kawerau and Ngatamariki are seen to represent renewed activity superimposed on a fossil system fed by past intrusions, and it is unclear what is meant by the lifetime of any single geothermal system. TVZ geothermal systems appear in turn to react too sluggishly to respond to disruptive episodes of volcanism, and recover within geologically short periods of time, as seen at Waimangu and Taupo. In the central TVZ, there are complex inter-relationships between volcanism, magmatism, and tectonism. Magmatism and volcanism are obviously linked, but it is uncertain why intense magmatism at Taupo and Okataina should yield voluminous rhyolite volcanism, whereas more intense magmatism in the Taupo-Reporoa Basin has not yielded significant silicic volcanism but instead feeds multiple large geothermal systems. The central TVZ is unique for an arc segment in the intensity of its magmatic-volcanic-geothermal flux (matching the Yellowstone system), and the cause(s) of this uniqueness are not yet established. Any explanation needs to address the segmented nature of the zone, and why the thermal flux should be so geographically and temporally constrained.

Detailed mapping and geochronological investigations of edifice-forming materials reconstruct the growth history of Tongariro volcano, New Zealand, and subdivide the edifice into thirty six distinct units which are organised into twelve formations and constituent members. Twenty nine new 40Ar/39Ar age determinations, along with published K/Ar ages combined with volume estimates, petrographic observations and rock chemistry provide an integrated history of the volcano's growth through edifice-forming lavas and pyroclastic deposits. The oldest lava (512 ± 59 ka, 2 s.d.) is a small inlier of basaltic-andesite on Tongariro's NW sector that may reflect a nearly buried independent volcano. The next oldest material that can be confidently attributed to a Tongariro source is 304 ± 11 ka andesite, incorporated as boulders in late Pleistocene ejecta from the Tama Lakes area. In-situ lavas at Tongariro date from 230 ka to present, including numerous flows erupted during glacial periods and building the edifice unevenly due to emplacement against valley-filling ice bodies. Tongariro has a total edifice volume of ~90 km3, 19 km3 of which is represented by exposed map units, with glacial deposits amounting to

The central part of the Taupo Volcanic Zone (TVZ), New Zealand, is notable for the number (23) and size of its high-temperature geothermal systems. Of these, the Wairakei-Tauhara geothermal system collectively represents the largest commercially developed example. Natural thermal output prior to utilisation, i.e. before 1950 was ∼530 MW. Since 1950, drilling of over 300 closely spaced wells for power generation has provided geological data for a comprehensive stratigraphic framework. Within this framework, the strata are dominantly rhyolitic pyroclastic rocks and lava bodies, with interbedded fluvial and lacustrine deposits. Until now, no subsurface units had been directly dated and their ages were constrained only by their positions relative to the surficial 25.4 ka Oruanui ignimbrite, or to buried ignimbrite that is correlated with the 349 ka Whakamaru Group. This paper presents 33 suites of U-Pb age data and a limited number of reconnaissance U-Th disequilibrium age data from magmatic zircon crystals (zircons) to date the rocks of the Wairakei-Tauhara system. The oldest units dated are intermediate lavas from ca. 1.7 Ma, while the youngest locally vented eruptives of Tauhara dacite are tentatively dated to ca. 60 ka. Placing the age data within the established drillhole record allows for the recognition of major volcano-tectonic events and spatially clustered local volcanism that represents magmatic heat sources for geothermal activity within the Wairakei-Tauhara area. In contrast to the Rotokawa and Ngatamariki geothermal systems 10–20 km to the northeast, the Wairakei-Tauhara area was affected by two episodes of caldera collapse associated with deposits dated at 349 ka and 310 ka, leading to major faulting and abrupt variations in thicknesses of units. The faulting and deposit thickness variations established in these two events remain major controls on present-day permeability of the geothermal system. The new age data and stratigraphy demonstrate development of distinct magma systems through time which may have stimulated previous periods of hydrothermal activity. However, from zircon age data from the rhyolite-andesite mixed Tauhara dacite, we propose that the current geothermal system was initiated at ca. 60 ka and is thermally maintained by continuing contributions from the same magmatic system that has sporadically vented distinctive rhyolite since.

A U-Pb zircon geochronology for the Rotokawa geothermal system, central Taupō Volcanic Zone (TVZ), New Zealand, provides new constraints on the chronostratigraphy and volcanic and structural evolution of this area and the broader TVZ. The 3-km-thick volcanic sequence at Rotokawa is mainly composed of rhyolitic ignimbrites linked to large caldera-forming events from sources outside the field area, but locally sourced andesite and rhyolite lavas and intrusions are also present. Crystallisation age spectra (and consequent estimates of eruption age) have been obtained on zircons from hydrothermally altered magmatic rocks by Secondary Ion Mass Spectrometry techniques using a SHRIMP-RG instrument. The oldest rock dated is a Tahorakuri Formation ignimbrite (eruption age estimate of 1.87 ± 0.03 Ma) which, along with comparable-age units at other nearby TVZ geothermal systems (Ngatamariki, Ohaaki), is among the oldest silicic volcanic deposits known in the TVZ. These ignimbrites collectively onlap a basal andesite lava pile, up to 1.2 km thick at Rotokawa, that in turn rests on the Mesozoic basement greywacke. The base of the lava pile is more faulted than its top surface, implying that rifting and graben formation had started along the line of the modern TVZ arc prior to 1.84 Ma and is not a younger feature. Between ~1.8 Ma and 700 ka, there are no rocks represented at Rotokawa, with the next oldest lithology being a 720 ± 90 ka rhyolite lava. At 350 ka, the Rotokawa area was buried by regionally extensive ignimbrites of the Whakamaru Group, which have since subsided by ~700 m but not been greatly faulted. Ignimbrites and sediments of the Waiora Formation were then emplaced, coevally with widespread and volumetrically greater volcanism in the Maroa and Ngatamariki areas 13 km northwest and 8 km north of Rotokawa, respectively. Local rhyolites of the Oruahineawe Formation, dated at ~100 ka, were emplaced both as extrusive domes and shallow intrusions below the area. Sedimentary rocks of the Huka Falls Formation and deposits of the 14C-dated 25.4 ± 0.2 ka Oruanui eruption capped and sealed the system, which has since been disrupted by hydrothermal eruption events. The largest of these occurred at ~6.8 ka (14C date) broadly coincident with a resumption of eruptive activity at Taupō volcano, 20 km to the south-southwest. Notable aspects of the evolution of the Rotokawa area are the early onset of rifting and subsidence along the line of the modern arc, the lack of volcanic activity for >1 Myr from 1.84 Ma to 720 ka, the lack of faulting and only modest subsidence since 350 ka, and the contrasts in volcanic and subsidence histories with other, nearby geothermal systems.

During the climactic Plinian phase of the 1886 basaltic eruption of Tarawera, New Zealand, vents along the 17 km fissure erupted explosively with a wide range of dispersal. The 8 km long segment of the fissure which cuts across Mt Tarawera contains approximately 50 vents and includes the sources of both the weakest and most intense activity of the 5 h eruption. We seek to explain (1) what allowed the intensity to reach Plinian values that are rarely achieved by basaltic magma, and (2) what caused adjacent vents to erupt with very different dispersals and intensities despite identical magma composition. AU juvenile clasts stuthed from this eruption have relatively high vesicle number densities (c. 106cm-3) and exceptionally high microlite crystallinities (60-90% of the groundmass), unlike the typical products of weaker Hawaiian and Strombolian basaltic explosions. Textural analysis of juvenile pyroclasts suggests that all the erupted magma experienced the same decompression history through to fragmentation. The Tarawera magma experienced a sudden, large, decompression producing nucleation of bubbles and microlites. The high microlite content was the primary means by which the magma's viscosity increased, which kept the bubble population well coupled to the magma and allowed it to fragment explosively in a manner analogous to that postulated for silicic Plinian eruptions. The main differences at different sites along the Mt Tarawera fissure segment are in the amount and grain size of the wall rock lithic component of the deposits. We suggest that conduit/vent erosion and incorporation of significant volumes of cold wall rock into the eruptive jet prevented some vents from achieving Plinian intensity. Bubble size analysis suggests that coalescence led to open-system degassing, ending the Plinian phase. © IAVCEI2009.