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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.

Although it is widely accepted that large silicic calderas are associated with voluminous synvolcanic intrusive complexes at depth, geological evidence for caldera-forming eruptions preserved in plutonic rocks has largely been elusive. Here, we document a rare example of such evidence in the Cadillac Mountain intrusive complex, Maine (USA), where erosion has revealed a remarkable marginal “shatter zone” that records evidence for a major caldera-forming eruption. This shatter zone, up to >1 km wide, is bounded by a steep ring fault at its outer margin, which grades inward into Cadillac Mountain granite. Its outer margins are characterized by intensely brecciated and deformed country rock injected by felsite veins, reflecting explosive fragmentation associated with eruptive decompression. This marginal facies grades inward to a chaotic mélange of variably rounded and remelted country rock blocks in granitic matrix, reflecting debris eroded from ring fault conduit walls and milled in an eruptive jet before collapsing onto crystal mush. Further inward, blocks up to 80 m in size were stoped from the collapsing chamber roof and settled onto strong mush. Textural and chemical variations in the shatter zone matrix reveal syneruptive ascent of distinct silicic and more mafic magma from depth, which was likely drawn through the highly permeable shatter zone toward areas of low pressure beneath active vents. The Cadillac Mountain shatter zone provides clear evidence for a major eruption preserved in the plutonic record and supports the origin of some granites as the cumulate roots of large silicic volcanic systems.

Deposits of the ca. 110 ka Halarauður eruption of Krafla caldera (reconstructed volume = 7 ± 6 km3 dense rock equivalent) include the only spatter-rich ignimbrite known in Iceland, and an exceptionally rare lava-like basaltic ignimbrite. We present a revised stratigraphy and new whole-rock major-element data set for products of this unusual event, one of only three Quaternary ignimbrite eruptions identified in Iceland. Compositions of Halarauður products span a broad range (50.0–74.6 wt% SiO2), reflecting mixing of rhyolite with underplating basalt. Small-volume, valley-ponded, basal pumice- and spatter-bearing lithic breccias and ignimbrite (rhyolite to andesite) reflect rapid column collapse during early opening of ring-fault vents. A transition to voluminous, regionally dispersed spatter agglomerates (dacite to basaltic andesite) marks an abrupt eruptive intensification, as gas-poor magma was squeezed into a developing ring-fault system by the subsiding chamber roof. Spatial heterogeneities in ascent rates and outgassing through this variably dilated fault system caused coeval formation of collapsing plumes and spatter fountains at separate vents. Spatter was entrained into flows from the more explosive vents, which deposited proximal spatter agglomerates and more distal spatter-bearing ignimbrite. Overlying lava-like ignimbrite deposits (basaltic andesite to basalt) reflect a final opening of vents, as mafic magma from deep levels of the chamber was squeezed through a dilated ring-fault system by the subsiding roof block and erupted at uncharacteristically high mass flux. Development of a mature ring-fault conduit system during early tapping of silicic magma appears to be a prerequisite for the emplacement of welded basaltic ignimbrites, and it should be considered as a possible eruption scenario in basalt-dominated systems where silicic magma has been known to also accumulate. Poor preservation of the Halarauður deposits exemplifies the challenges of studying ignimbrite eruptions in frequently glaciated regions like Iceland, where they may be more common than the geological record suggests.

We present a detailed petrologic study of rhyolites from seven eruptions spanning the full (∼190 ky) history of rhyolitic volcanism at Krafla volcano, northeast Iceland. The eruptions vary widely in size and style, but all rhyolites are crystal-poor (

Unintentional encounters with silicic magma at ~2–2.5 km depth have recently occurred during drilling at three volcanoes: Kilauea (Hawaii), Menengai (Kenya), and Krafla (Iceland). Geophysical surveys had failed to warn about shallow magma before each encounter, and subsequent surveys at Krafla have been unable to resolve the size or architecture of its silicic magma body. This presents a conundrum for volcano monitoring: Do such shallow “covert” magma bodies pose an eruption risk? Here, we show that Krafla’s most recent explosive eruption, a mixed hydrothermal-magmatic event in 1724 C.E. that formed the Víti maar, involved rhyolite essentially indistinguishable in composition from magma encountered during drilling in 2009. Streaks of quenched basalt in some Víti pumices provide direct evidence for interaction between co-erupted rhyolitic and basaltic magmas, but crystals in these pumices show no evidence for late-stage heating or re-equilibration with more mafic melt, implying mixing time scales of at most several hours. Covert silicic magma thus presents an eruption risk at Krafla and may be mobilized with little warning. Difficulties in resolving magma bodies smaller than ~1 km3 with geophysical surveys mean that covert silicic magma may exist at many other volcanoes and should be considered in hazard and risk assessments.

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.

Understanding the petrogenesis of silicic magmas is critical for understanding the volcanic hazards they pose, their geothermal energy potential, and the creation of continental crust. In this study we explore the origin of rhyolitic magmas in basaltic crust at the Krafla Central Volcano in Iceland. We present laser fluorination oxygen isotope analyses of plagioclase, pyroxene, and groundmass from eight rhyolites and six selected basalts, as well as in situ oxygen isotope analyses and U-Th geochronology of zircons from three rhyolitic domes erupted around the caldera margins. Zircon U-Th geochronology for the rhyolite domes yields ages of 88.7 ± 9.9 ka for Jorundur, 83.3 ± 9.2 ka for Hliðarfjall, and 85.5 ± 9.4 ka for Gaesafjallarani, some 20–30 ka after the eruption of the zoned rhyolite to basalt Halarauður ignimbrite during a major collapse of the Krafla caldera. We suggest that the domes represent a renewed episode of silicic magma production in the pre-heated crust. Oxygen isotope analyses of single and bulk plagioclase and pyroxene identify some instances of isotopic disequilibrium with groundmass (~3.5‰) reflecting assimilation of diverse low δ18O crustal material. However, zircon is largely in equilibrium with groundmass analyses, suggesting it crystallized directly from low δ18O magma. Zircon trace elements (Hf, Yb, Th, U) for all three domes show trends indicative of fractional crystallization. Pairing these observations with two-dimensional thermal modeling using the Heat2D model, and chemical modeling using the Magma Chamber Simulator, we suggest that petrogenesis of rhyolitic magma at Krafla requires at least two-steps: the δ18O of basaltic parental magmas are first lowered through assimilation of hydrothermally altered material (generated in the high temperature region in the crust surrounding the magma chamber) to produce low δ18O mafic to intermediate magmas, which then ascend from magma generation zones into colder crust where they undergo further fractional crystallization at shallower depths. Our models suggest that prior hydrothermal alteration of the mafic crust greatly increases the volume of partial melt that can be produced and assimilated, and we thus suggest that long-lived hydrothermal systems may play an important role in further encouraging the production of larger volumes of rhyolitic magmas in basalt-dominated environments.

The mutual affinity between bubbles and oxide crystals (especially magnetite) is well established and their tendency to remain in contact once they become connected (either by nucleation of one upon the other, or by attachment) has led to models of oxide transport via bubbles in natural melts. However, despite the widespread acceptance of bubble–oxide association, there is little direct textural evidence for these processes. We present results from a series of decompression experiments on andesitic melts, during which aggregates of bubbles and oxides formed because of hydrogen loss through the capsule walls causing oxidation of the melt. Experimental charges were imaged using 3D X-ray computed tomography that revealed complex bubble + oxide aggregates, with small oxide crystals coating part of the outer bubble surfaces in a shell-like morphology. These shells have smooth inner and rugose outer surfaces. Sometimes, additional concentric shells or partial shells can be found around bubbles, in the glass between the bubble wall and another shell. We quantified the volumes of bubbles and oxides and the oxides’ compositions. We measured the surface area where the bubbles and oxides are in contact, thus quantifying their interface in 3D, and used these measurements to investigate the process of oxide shell formation. The complexity of the oxide textures when studied in 3D reveals a range of bubble–oxide interactions, from continuous generation, detachment and disintegration. These processes carry important implications on why such textures seem to have a low preservation potential in natural environments. Nevertheless, we have found natural samples that resemble our experimental results in a range of rock compositions from different geological environments that could have formed either due to rapid oxidation via the fluid phase or by bubbles harvesting different crystals.

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 km3 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 km3) 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.