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Magmatic-hydrothermal fluids play a key role in a variety of geological processes, including volcanic eruptions and the formation of ore deposits whose metal content is derived from magmas and transported to the site of ore deposition by means of hydrothermal fluids. Here, we explain the causes and consequences of fluid saturation in magmas, the corresponding fluid-phase equilibria, and the behavior of metals and ligands during the transition from magma to an exsolved hydrothermal fluid. Much of what we know about magmatic-hydrothermal systems stems from the study of fluid inclusions, which are minute droplets of fluids trapped within minerals during mineral growth.

The publishing process relies on the work of volunteer reviewers, and evaluating the interdisciplinary papers published in G-Cubed can be challenging. As Editors and Associated Editors, we would like to give our appreciation to all reviewers and would like to acknowledge them in this editorial. G-Cubed published 322 manuscripts out of 577 submissions in 2021, thanks to the efforts of 912 dedicated reviewers. Their names are listed below, and in italics are those who provided three or more reviews. A big thank you from the G-Cubed team!

Chalcophile element concentrations in melt inclusions and matrix glasses may be used to investigate low pressure degassing processes, as well as sulfide saturation during crustal fractionation, and mantle melting. Erupted products from Kīlauea Volcano, Hawaiʻi, record three stages of sulfide saturation (in the mantle, crust, and within lava lakes), separated by episodes of sulfide resorption (i.e., sulfide undersaturation) during ascent through the thick Hawaiian lithosphere, and during syn-eruptive degassing. The presence of residual sulfides in the mantle source throughout the melting interval accounts for the high S concentrations of Kīlauean primary melts (1387–1600 ppm). Residual sulfides retain chalcophile elements during melting, decoupling the variability of these elements in high MgO melts from that of lithophile elements. Decompression associated with magma ascent through the thick Hawaiian lithosphere drives an increase in the sulfide concentration at sulfide saturation (SCSS2-), resulting in shallow storage reservoirs (∼1–5 km depth) being supplied with sulfide-undersaturated melts. A drop in temperature, coupled with major element changes during the fractionation of olivine, causes the SCSS2- to decrease. Combined with an increase in melt S contents during fractionation, this initiates a second stage of sulfide saturation at relatively high MgO contents (∼12 wt% MgO). Syn-eruptive degassing of S drives the resorption of sulfides in contact with the carrier liquid. The covariance structure of Cu, MgO and Ni contents in melt inclusions and matrix glasses indicates that the dissolution of sulfides effectively liberates sulfide-hosted Cu and Ni back into the melt, rather than the vapour phase. The contrasting behaviour of Cu, Ni, Se and S during sulfide resorption indicates that the chalcophile element signature of the Kīlauean plume is largely controlled by silicate melt-vapour partitioning, rather than sulfide-vapour partitioning. The participation of dense sulfide liquids in shallow degassing processes may result from their direct attachment to buoyant vapour bubbles, or olivine crystals which were remobilized prior to eruption. Sulfide resorption obscures the textural and chemical record of sulfide saturation in matrix glasses, but not in melt inclusions, which are isolated from this late-stage release of chalcophile elements. The partitioning of S between the dissolving sulfide, melt and the vapour phase accounts for approximately 20% of the total S release into the atmosphere.

Recent studies investigating magmatic volatile contents indicate widespread enrichment of carbon, sulfur, and halogens in ocean island basalts (OIBs). At El Hierro in the Western Canary Islands, magmas with exceptionally high CO2 and S contents have been erupting throughout the Holocene. High S content of up to 5200 ppm requires an oxidised mantle source, but estimates of initial magmatic oxygen fugacity (fO2) are sparse. Here, we present estimates of fO2 and magmatic temperature for El Hierro together with a global mantle potential temperature dataset to evaluate redox and temperature conditions in the early stages of melt evolution for volatile-rich OIBs. Oxygen fugacities calculated using vanadium partitioning between melt inclusions (MIs) and their olivine hosts are >FMQ + 2.9 (2.9 log10 units above the fayalite-magnetite-quartz buffer), indicating that El Hierro magmas are highly oxidised. MI and matrix glass sulfur speciation data record fO2 between FMQ-1 to FMQ + 2; these values strongly depend on the position of the S2− to S6+ transition relative to the FMQ buffer. Nonetheless, glass sulfur speciation data record lower oxygen fugacity than V partitioning data, indicating MIs were able to maintain Fe3+/ΣFe and S6+/ΣS equilibrium with the surrounding melt during their evolution. The high fO2 of El Hierro magmas is coupled with an average mantle potential temperature estimate of 1443 ± 66°C (1σ, n = 17) for the broader Canary Islands, which is slightly higher than the average potential temperature estimated for adjacent mid-ocean ridge segments (1427 ± 33°C, 1σ, n = 474), albeit the two values are well within error. We find that ~98% of Canary Island rock compositions are not suitable for calculation of mantle potential temperatures using currently available methods. This is caused by the presence of substantial pyroxenite and volatile-enriched peridotite mantle domains under the Canary Islands. A wider compositional calibration of various petrological models is necessary to precisely determine mantle potential temperatures for volatile-rich alkali basalts. Our high oxygen fugacity estimates for El Hierro magmas reflect the fertile, fusible, and volatile-enriched nature of the mantle source beneath the Western Canary Islands.

Magmas with matrix glass compositions ranging from basalt to dacite erupted from a series of 24 fissures in the first two weeks of the 2018 Lower East Rift Zone (LERZ) eruption of Kīlauea Volcano. Eruption styles ranged from low spattering and fountaining to strombolian activity. Major element trajectories in matrix glasses and melt inclusions hosted by olivine, pyroxene and plagioclase are consistent with variable amounts of fractional crystallization, with incompatible elements (e.g., Cl, F, H2O) becoming enriched by 4-5 times as melt MgO contents evolve from 6 to 0.5 wt%. The high viscosity and high H2O contents (∼2 wt%) of the dacitic melts erupting at Fissure 17 account for the explosive Strombolian behavior exhibited by this fissure, in contrast to the low fountaining and spattering observed at fissures erupting basaltic to basaltic-andesite melts. Saturation pressures calculated from melt inclusions CO2-H2O contents indicate that the magma reservoir(s) supplying these fissures was located at ∼2-3 km depth, which is in agreement with the depth of a dacitic magma body intercepted during drilling in 2005 (∼2.5 km) and a seismically-imaged low Vp/Vs anomaly (∼2 km depth). Nb/Y ratios in erupted products are similar to lavas erupted between 1955-1960, indicating that melts were stored and underwent variable amounts of crystallization in the LERZ for >60 years before being remobilized by a dike intrusion in 2018. We demonstrate that extensive fractional crystallization generates viscous and volatile-rich magma with potential for hazardous explosive eruptions, which may be lurking undetected at many ocean island volcanoes.

Funder: Leverhulme Trust; Id: http://dx.doi.org/10.13039/501100000275, Funder: Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics, Funder: University of Cambridge Jesus College, University of Cambridge (Jesus College); Id: http://dx.doi.org/10.13039/501100000644, Two of the most widely observed co‐eruptive volcanic phenomena—Ground deformation and volcanic outgassing—Are fundamentally linked via the mechanism of magma degassing and the development of compressibility, which controls how the volume of magma changes in response to a change in pressure. Here we use thermodynamic models—Constrained by petrological data—To reconstruct volatile exsolution and the consequent changes in magma properties. We use the fraction of SO2 exsolved during decompression to predict co‐eruptive SO2 flux and magma compressibility to predict co‐eruptive surface deformation (both normalized by erupted volume). We conduct sensitivity tests using properties of typical basalts to assess how varying magma volatile content, crustal properties, and chamber geometry affect co‐eruptive deformation and degassing. We find that magmatic H2O content has the most impact on both SO2 flux and volume change. Our findings have general implications for typical basaltic systems in arc and ocean island settings. The higher water content of arc magmas makes them more compressible than ocean island magmas and leads to muted or non‐existent deformation being observed during arc eruptions. Our models are consistent with observation: Deformation has been detected during 48% of basaltic eruptions in ocean island settings (16/33) during the satellite era (2005–2020), but only 11% of basaltic eruptions in arc settings (7/61).

Sulfur isotope ratios are among the most commonly studied isotope systems in geochemistry. While sulfur isotope ratio analyses of materials such as bulk rock samples, gases, and sulfide grains are routinely carried out, in-situ analyses of silicate glasses such as those formed in magmatic systems are relatively scarce in the literature. Despite a number of attempts in recent years to analyse sulfur isotope ratios in volcanic and experimental glasses by secondary ion mass spectrometry (SIMS), the effects of instrumental mass fractionation (IMF) during analysis remain poorly understood. In this study we use more than 600 sulfur isotope analyses of nine different glasses to characterise the matrix effects that arise during sulfur isotope analysis of glasses by SIMS. Samples were characterised for major element composition, sulfur content, and sulfur isotope ratios by independent methods. Our glasses contain between 500 and 3400 ppm sulfur and cover a wide compositional range, including low-silica basanite, rhyolite, and phonolite, allowing us to investigate composition-dependent IMF. We use SIMS in multi-collection mode with a Faraday cup/electron multiplier detector configuration to achieve uncertainty of 0.3‰ to 2‰ (2σ) on measured δ34S. At high sulfur content, the analytical error of our SIMS analyses is similar to that of bulk analytical methods, such as gas-source isotope ratio mass spectrometry. We find IMF causes an offset of −12‰ to +1‰ between bulk sulfur isotope ratios and those measured by SIMS. Instrumental mass fractionation correlates non-linearly with glass sulfur contents and with a multivariate regression model combining glass Al, Na, and K contents. Both ln(S) and Al-Na-K models are capable of predicting IMF with good accuracy: 84% (ln(S)) and 87% (Al-Na-K) of our analyses can be reproduced within 2σ combined analytical uncertainty after a correction for composition-dependent IMF is applied. The process driving IMF is challenging to identify. The non-linear correlation between glass S content and IMF in our dataset resembles previously documented correlation between glass H2O abundance and IMF during D/H ratio analyses by SIMS, and could be attributed to changes in 32S− and 34S− ion yields with changing S content and glass composition. However, a clear correlation between S ion yields and S content cannot be identified in our dataset. We speculate that accumulation of alkalis at the SIMS crater floor may be the principal driving force of composition-dependent IMF. Nonetheless, other currently unknown factors could also influence IMF observed during S isotope ratio analyses of glasses by SIMS. Our results demonstrate that the use of multiple, well-characterised standards with a wide compositional range is required to calibrate SIMS instruments prior to sulfur isotope analyses of unknown silicate glasses. Matrix effects related to glass Al-Na-K contents are of particular importance for felsic systems, where alkali and aluminium contents can vary considerably more than in mafic magmas.

We review our understanding of the exsolved volatile phase co-existing with magmas during pre-eruptive storage at the pressures and temperatures corresponding to crustal magma reservoirs. We explore the consequences and implications of such a volatile phase for magma and ore body petrogenesis and the fluid dynamics of magma reservoirs. We outline the geochemical constraints on the size and composition of the exsolved volatile phase that may co-exist with magmas in the crust. We distinguish between decompression-driven and crystallization-driven exsolution, and describe the implications of the volatiles for the dynamics of the magma reservoir, using key natural examples and case studies. We discuss eruptions triggered by second boiling, and the various regimes of magma mixing and magma overturn that may be induced by second boiling in a layered reservoir. We also explore the control of the volatile content of the magma on the mass erupted during an eruption episode, and compare our models to eruption datasets. We then turn to the mechanisms for magma-volatile separation, noting that in crystal-poor melts convective separation of exsolved volatiles may dominate while in crystal-rich melts, volatiles may generate channels or permeable-flow pathways through the crystal mush, thereby separating from the parent magma. We discuss the implications of the accumulation of the exsolved volatile phase at the roof zones of crystal-rich reservoirs for the large gas emissions observed during explosive eruptions, and for the development of metal-rich porphyry deposits.

\ud Crystallization temperatures of primitive olivine crystals have been widely used as both a proxy for, or an intermediate step in calculating, mantle temperatures. The olivine-spinel aluminum-exchange thermometer has been applied to samples from mid-ocean ridges and large igneous provinces, yielding considerable variability in olivine crystallization temperatures. We supplement the existing data with new crystallization temperature estimates for Hawaii, between 1282 ± 21 and 1375 ± 19°C. Magmatic temperatures may be linked to mantle temperatures if the thermal changes during melting can be quantified. The magnitude of this temperature change depends on melt fraction, itself controlled by mantle temperature, mantle composition and lithosphere thickness. Both mantle composition and lithosphere thickness vary spatially and temporally, with systematic differences between mid-ocean ridges, ocean islands and large igneous provinces. For crystallization temperatures to provide robust evidence of mantle temperature variability, the controls of lithosphere thickness and mantle lithology on crystallization temperature must be isolated. We develop a multi-lithology melting model for predicting crystallization temperatures of magmas in both intra-plate volcanic provinces and mid-ocean ridges. We find that the high crystallization temperatures seen at mantle plume localities do require high mantle temperatures. In the absence of further constraints on mantle lithology or melt productivity, we cannot robustly infer variable plume temperatures between ocean-islands and large igneous provinces from crystallization temperatures alone; for example, the extremely high crystallization temperatures obtained for the Tortugal Phanerozoic komatiite could derive from mantle of comparable temperature to modern-day Hawaii. This work demonstrates the limit of petrological thermometers when other geodynamic parameters are poorly known.\ud \ud Plain Language Summary\ud The temperature inside the Earth varies a lot. There are many ways of measuring the mantle's temperature in the present-day, but to understand how our planet has changed through time, we need to know how hot its interior was in the past. One of the ways we can estimate mantle temperatures from ancient and modern rocks is from their crystal chemistry. By measuring the aluminum content in crystals of olivine, we can estimate their crystallization temperature. We do this for crystals from Hawaii. To turn the crystallization temperatures into the mantle temperature we need to know the proportions of minerals the mantle is made of. However, we often don't know all of this information. Using a new model of mantle melting we can calculate how uncertain the mantle temperature is when we only have a crystallization temperature. We find that the mantle under Hawaii is 1582 ± 65°C, much hotter than normal mantle, which has a temperature 1364 ± 23°C. We also apply our method to crystallization temperatures from other locations, including ancient volcanic rocks. We find that crystallization temperatures from large igneous provinces, formed by unusually hot mantle, are consistent with their mantle having a similar temperature to Hawaii

Copper, sourced from porphyry deposits formed in arc settings, is a critical resource, and is primarily sourced from magmas. However, the processes that shape the copper contents of arc magmas are up for debate. Existing models place emphasis on different petrological agents that explain large-scale trends in copper systematics. Previous studies have noted the 'Cu paradox,' where the magmas with high Sr/Y ratios, indicative of ore-forming potential, have the lowest copper concentrations. Here we compile a multidimensional database of volcanic whole rock compositions and couple it with simple petrological models to elucidate the controls on volcanic whole rock compositions with respect to Cu. We show that calc-alkaline, high Sr/Y magmas undergo major element modification caused by extensive amphibole and/or garnet fractionation, which promotes sulphide precipitation and copper depletion. We demonstrate the importance of amphibole fractionation as a globally important process that promotes both calc-alkaline differentiation and sulphide fractionation in arc magmas, as well as its role in signalling the right set of chemical conditions in magmas that ultimately feed copper porphyry deposits. This work also raises the possibility of amphibole as a geochemical and petrological indicator of potential porphyry-forming conditions in a magma, which we show should be readily detectable by a combination of different geochemical metrics. Despite their paucity in copper, high Sr/Y magmas are associated with porphyry deposits, implying that the propensity of magmas to form such deposits depends on factors other than a magma’s bulk copper content.

The 2018 lower East Rift Zone (LERZ) eruption and the accompanying collapse of the summit caldera marked the most destructive episode of activity at Kı̄lauea Volcano in the last 200 years. The eruption was extremely well‐monitored, with extensive real‐time lava sampling as well as continuous geodetic data capturing the caldera collapse. This multi‐parameter dataset provides an exceptional opportunity to determine the reservoir geometry and magma transport paths supplying Kı̄lauea’s LERZ. The forsterite contents of olivine crystals, together with the degree of major element disequilibrium with carrier melts, indicates that two distinct crystal populations were erupted from Fissure 8 (termed High‐ and Low‐Fo). Melt inclusion entrapment pressures reveal that Low‐Fo olivines (close to equilibrium with their carrier melts) crystallized within the Halema’uma’u reservoir ( ∼2 km depth), while many High‐Fo olivines ( > Fo81.5; far from equilibrium with their carrier melts) crystallized within the South Caldera reservoir ( ∼3–5 km depth). Melt inclusions in High‐Fo olivines experienced extensive post‐entrapment crystallization following their incorporation into cooler, more evolved melts. This favoured the growth of a CO2 ‐rich vapor bubble, containing up to 99% of the total melt inclusion CO2 budget (median=93%). If this CO2 ‐rich bubble is not accounted for, entrapment depths are significantly underestimated. Conversely, reconstructions using equation of state methods rather than direct measurements of vapor bubbles overestimate entrapment depths. Overall, we show that direct measurements of melts and vapor bubbles by SIMS and Raman Spectroscopy, combined with a suitable H2 O −CO2 solubility model, is a powerful tool to identify the magma storage reservoirs supplying volcanic eruptions.

Understanding magma storage and differentiation in the East African Rift underpins our understanding of volcanism in continental rift settings. Here we present the geochemistry of melt inclusions erupted in Main Ethiopian Rift transitional basalts, trachytes and peralkaline rhyolites, produced by fractional crystallisation. Basalts stored on‐ and off‐axis are saturated in an exsolved volatile phase at up to 18 km, in the upper crust. Much of the CO2 outgassed from the magmas is likely lost through diffuse degassing. Observed CO2 fluxes require the intrusion of up to 0.14 km3 of basalt beneath the rift each year. On‐axis peralkaline rhyolites are stored shallowly, at ~4‐8 km depth. In the Daly Gap, magmas saturate in sulfide and an exsolved volatile phase, which promotes magma rise to shallower levels in the crust. Here, magmas undergo further protracted fractional crystallisation and degassing, leading to the formation of a substantial exsolved volatile phase, which may accumulate in a gas‐rich cap. The exsolved volatile phase is rich in sulfur and halogens: their projected loadings into the atmosphere during explosive peralkaline eruptions in the MER are predicted to be substantially higher than their metaluminous counterparts in other settings. The high fraction of exsolved volatiles in the stored magmas enhances their compressibility and must be considered when interpreting ground displacements thought to be caused by magma intrusion at depth, otherwise intruding volumes will be underestimated. Pockets of exsolved volatiles may be present at the roof zones of magma reservoirs, which may be resolvable using geophysical techniques.

Much of Earth's carbon resides in the “deep” realms of our planet: sediments, crust, mantle, and core. The interaction of these deep reservoirs of carbon with the surface reservoir (atmosphere and oceans) leads to a habitable surface environment, with an equitable atmospheric composition and comfortable range in temperature that together have allowed life to proliferate. The Earth in Five Reactions project (part of the Deep Carbon Observatory program) identified the most important carbon-bearing reactions of our planet, defined as those which perhaps make our planet unique among those in our Solar System, to highlight and review how the deep and surface carbon cycles connect. Here we review the important reactions that control the concentration of carbon dioxide in our atmosphere: outgassing from magmas during volcanic eruptions and during magmatic activity; and uptake of CO2 by alkaline surface waters. We describe the state of our knowledge about these reactions and their controls, the extent to which we understand the mass budgets of carbon that are mediated by these reactions, and finally, the implications of these reactions for understanding present-day climate change that is driven by anthropogenic emission of CO2.

Carbon is one of the most important elements on Earth. It is the basis of life, it is stored and mobilized throughout the Earth from core to crust and it is the basis of the energy sources that are vital to human civilization. This issue will focus on the origins of carbon on Earth, the roles played by large-scale catastrophic carbon perturbations in mass extinctions, the movement and distribution of carbon in large igneous provinces, and the role carbon plays in icehouse–greenhouse climate transitions in deep time. Present-day carbon fluxes on Earth are changing rapidly, and it is of utmost importance that scientists understand Earth's carbon cycle to secure a sustainable future.

International audience; The publishing process relies on the work of volunteer reviewers, and evaluating the interdisciplinary papers published in G-Cubed can be challenging. As Editors and Associate Editors, we would like to give our appreciation to all reviewers and would like to acknowledge them in this editorial. G-Cubed published 326 manuscripts out of 650 submissions in 2019, thanks on the efforts of 860 dedicated reviewers. Their names are listed below, and in italics are those who provided three or more reviews. A big thank you from the G-Cubed team!

Mt. Etna is among the largest global volcanic outgassers with respect to carbon and sulfur, yet questions remain regarding the source of these volatiles and their systematics in the crust and mantle. The importance of heterogeneous mantle sources, mixing, crustal assimilation and disequilibrium degassing are investigated using melt inclusions erupted during the A.D. 1669 eruption of Mt. Etna, Italy. We find that the melt inclusion compositions define a mixing array between two geochemically distinct melts. One end‐member melt is depleted in light rare Earth elements (LREE) and enriched in strontium (Sr), carbon and sulfur; the other is enriched in LREE and depleted in Sr, carbon and sulfur. We infer, through modeling, that the melts may either have been generated by melting a mantle source that includes a recycled oceanic crustal component; or they may have assimilated carbonate material in the crust. The resulting LREE‐depleted, Sr‐enriched melts were also alkali‐rich, which enhanced the solubility of carbon and sulfur. The LREE‐depleted, Sr‐ and volatile‐rich melt ascended through the crust and likely became supersaturated with respect to CO2 and sulfur. The melt intruded into a LREE‐enriched, relatively degassed magma body in the shallow crust, cooled rapidly and vesiculated, likely triggering eruption. The melt inclusion array trapped by growing olivines during this intrusion process records a snapshot of incomplete mixing between the two melts. Mt. Etna is renowned for the large increases in CO2 gas fluxes shortly before and during eruption. The intrusion of supersaturated, CO2‐enhanced magmas into shallow reservoirs may be a common process at Mt. Etna.

Basaltic volcanism contributes significant fluxes of volatiles (CO2, H2O, S, F, Cl) to the Earth’s surface environment. Quantifying volatile fluxes requires initial melt volatile concentrations to be determined, which can be accessed through crystal-hosted melt inclusions. However, melt inclusions in volatile-rich mafic alkaline basalts, such as those erupted at ocean islands, often trap partially degassed melts, meaning that magmatic volatile fluxes from these tectonic settings are often significantly underestimated. We have measured major, trace element and volatile concentrations in melt inclusions from a series of young ( 20 ka) basanites from El Hierro, Canary Islands. Our melt inclusions show some of the highest CO2 (up to 3600 ppm) and S (up to 4290 ppm) concentrations measured in ocean island basalts to date, in agreement with data from the recent 2011–2012 eruption. Volatile enrichment is observed in melt inclusions with crystallisation-controlled major element compositions and highly variable trace element ratios such as La/Yb. We use volatile-trace element ratios to calculate original magmatic CO2 contents up to 4.2 wt%, which indicates at least 65% of the original CO2 was degassed prior to melt inclusion trapping. The trace element contents and ratios of El Hierro magmas are best reproduced by 1–8% partial melting of a garnet lherzolite mantle source. Our projected CO2 (200–680 ppm) and S (265–450 ppm) concentrations for the source are consistent with upper estimates for primitive mantle. However, El Hierro magmas have elevated F/Nd and F/Cl in comparison with melts from a primitive mantle, indicating that the mantle must also contain a component enriched in F and other volatiles, most probably recycled oceanic lithosphere. Our modelled original magmatic CO2 contents indicates that, per mass unit, volatile fluxes from El Hierro magmas are up to two orders of magnitude greater than from typical mid-ocean ridge basalts and 1.5–7 times greater than from recent Icelandic eruptions, indicating large variability in the primary volatile content of magmas formed in different geodynamic settings, or even within different ocean islands. Our results highlight the importance of characterising mantle heterogeneity in order to accurately constrain both short- and long-term magmatic volatile emissions and fluxes from ocean island volcanoes.

This project is funded by the Natural Environment Research Council grant NE/L013932/1 (RiftVolc). Abstract For many magmatic systems, crystal compositions preserve a complex and protracted history which may be largely decoupled from their carrier melts. The crystal cargo may hold clues to the physical distribution of melt and crystals in a magma reservoir and how magmas are assembled prior to eruptions. Here we present a geochemical study of a suite of samples from three peralkaline volcanoes in the Main Ethiopian Rift. Whilst whole-rock data shows strong fractional crystallisation signatures, the trace element systematics of feldspars, and their relationship to their host glasses, reveals complexity. Alkali feldspars, particularly those erupted during caldera-forming episodes, have variable Ba concentrations, extending to high values that are not in equilibrium with the carrier liquids. Some of the feldspars are antecrysts, which we suggest are scavenged from a crystal-rich mush. The antecrysts crystallised from a Ba-enriched (more primitive) melt, before later entrainment into a Ba-depleted residual liquid. Crystal-melt segregation can occur on fast timescales in these magma reservoirs, owing to the low viscosity nature of peralkaline liquids. The separation of enough residual melt to feed a crystal-poor post-caldera rhyolitic eruption may take as little as months to tens of years (much shorter than typical repose periods of 300-400 years). Our observations are consistent with these magmatic systems spending significant portions of their life cycle dominated by crystalline mushes containing ephemeral, small (

Mafic enclaves hosted by andesite erupted at the Soufrière Hills Volcano between 1995 and 2010 yield insights into syn-eruptive mafic underplating of an andesite magma reservoir, magma mixing and its role in sustaining eruptions that may be widely applicable in volcanic arc settings. The mafic enclaves range in composition from basalt to andesite and are generated from a hybrid thermal boundary layer at the interface between the two magmas, where the basalt quenches against the cooler andesite, and the two magmas mix. We show, using an analytical model, that the enclaves are generated when the hybrid layer, just a few tens of centimetres thick, becomes buoyant and forms plumes which rise up into the andesite. Mafic enclave geochemistry suggests that vapour-saturated basalt was underplated quasi-continuously throughout the first three eruptive phases of the eruption (the end member basalt became more Mg and V-rich over time). The andesite erupted during the final phases of the eruption contained more abundant and larger enclaves, and the enclaves were more extensively hybridised with the andesite, suggesting that at some time during the final few years of the eruption, the intrusion of mafic magma at depth ceased, allowing the hybrid layer to reach a greater thickness, generating larger mafic enclaves. The temporal trends in mafic enclave composition and abundance suggests that basalt recharge and underplating sustained the eruption by the transfer of heat and volatiles across the interface and when the recharge ceased, the eruption waned. Our study has important implications for the petrological monitoring of long-lived arc eruptions.

The style and intensity of hydromagmatic activity is governed by a complex interplay between the relative volumes of magma and water that interact, their relative viscosities, the depth of subsurface explosions, the substrate properties, and the vent geometry. Fundamental questions remain, however, regarding the role of magmatic vesiculation in determining the dynamics of magma-water interaction (MWI). Petrological reconstructions of magmatic degassing histories are commonly employed to interpret the pre- and syn-eruptive conditions during ‘dry’ magmatic eruptions, but the application of similar techniques to hydromagmatic activity has not yet been fully explored. In this study, we integrate glass volatile measurements (S, Cl, H2O and CO2) with field observations and microtextural measurements to examine the relationship between degassing and eruptive style during the Hverfjall Fires fissure eruption, Iceland. Here, coeval fissure vents produced both ‘dry’ magmatic (Jarðbaðsholar scoria cone complex) and variably wet hydromagmatic (Hverfjall tuff ring) activity, generating physically distinct pyroclastic deposits with contrasting volatile signatures. Matrix glass volatile concentrations in hydromagmatic ash (883 ± 172 [1σ] ppm S; 0.45 ± 0.03 [1σ] wt% H2O; ≤20 ppm CO2) are consistently elevated relative to magmatic ash and scoria lapilli (418 ± 93 [1σ] ppm S; 0.12 ± 0.48 [1σ] wt% H2O; CO2 below detection) and overlap with the range for co-erupted phenocryst-hosted melt inclusions (1522 ± 127 [1σ] ppm S; 165 ± 27 [1σ] ppm Cl). Measurements of hydromagmatic glasses indicate that the magma has degassed between 17 and 70% of its initial sulfur prior to premature quenching at variably elevated confining pressures. By comparing volatile saturation pressures for both magmatic and hydromagmatic glasses, and how these vary through the eruptive stratigraphy, we place constraints on the conditions of MWI. Crucially, our data demonstrate that the magma was already vesiculating when it encountered groundwater at depths of 100–200 m, and that the external water supply was sufficient to maintain MWI throughout the eruption with no significant vertical or lateral migration of the fragmentation surface. We propose that development of an in-vent water-sediment slurry provides a mechanism through which the elevated confining pressures of ~1.6–2.6 MPa (or up to 6 MPa accounting for uncertainty in CO2 below analytical detection) could be maintained and buffered throughout the eruption, whilst enabling vertical mixing and ejection of fragmented juvenile and lithic material from a range of depths. Importantly, these results demonstrate that the volatile contents of hydromagmatic deposits provide valuable records of (1) the environment of MWI (e.g., groundwater versus surface water, vertical migration of the fragmentation level) and (2) the state of the magma at the time of fragmentation and quenching. We further suggest that the volatile content of tephra glasses provides a reliable alternative (or additional) indicator of a hydromagmatic origin, particularly for reduced Ocean Island Basalts where late-stage volatile saturation and degassing (S, H2O) occurs over a pressure range relevant to typical MWI environments.

Activity since 1995 at Soufriere Hills Volcano (SHV), Montserrat has alternated between andesite lava extrusion and quiescence, which are well correlated with seismicity and ground deformation cycles. Large variations in SO2 flux do not correlate with these alternations, but high and low HCl/SO2 characterize lava dome extrusion and quiescent periods respectively. Since lava extrusion ceased (February 2010) steady SO2 emissions have continued at an average rate of 374 tonnes/day (± 140 t/d), and incandescent fumaroles (temperatures up to 610oC) on the dome have not changed position or cooled. Occasional short bursts (over several hours) of higher (∼ 10x) SO2 flux have been accompanied by swarms of volcano-tectonic earthquakes. Strain data from these bursts indicate activation of the magma system to depths up to 10 km. SO2 emissions since 1995 greatly exceed the amounts that could be derived from 1.1 km3 of erupted andesite, and indicating extensive partitioning of sulfur into a vapour phase, as well as efficient decoupling and outgassing of sulfur-rich gases from the magma. These observations are consistent with a vertically extensive, crustal magmatic mush beneath SHV. Three states of the magmatic system are postulated to control degassing. During dormant periods (103 to 104 years) magmatic vapour and melts separate as layers from the mush and decouple from each other. In periods of unrest (years) without eruption, melt and fluid layers become unstable, ascend and can amalgamate. Major destabilization of the mush system leads to eruption, characterized by magma mixing and release of volatiles with different ages, compositions and sources.

Sulfides are a major potential repository for magmatic metals and sulfur. In relatively reduced magmas, there may be a dynamic interplay between sulfide liquids and magma degassing as magmas ascend/erupt. Sulfide-bubble aggregates may segregate to shallow levels. Exsolved fluids may oxidize sulfides to produce SO 2 gas and metals, which can vent to the atmosphere with chalcophile metal ratios reflecting those in their parent sulfide liquids. Sulfide breakdown and/or sequestration timing and balance define the role of sulfides in both ore formation and the environmental impacts of volcanic eruptions, including during the evolution of large igneous provinces, which are key periods of heightened volcanism during Earth history.

The role of volatiles in magma dynamics and eruption style is fundamental. Magmatic volatiles partition between melt, crystal, and vapor phases and, in so doing, change magma properties. This has consequences for magma buoyancy and phase equilibria. An exsolved vapor phase, which may be distributed unevenly through reservoirs, contains sulfur and metals that are either transported into the atmosphere or into ore deposits. This article reviews the controls on volatile solubility and the methods to reconstruct the volatile budget of magmas, focusing particularly on the exsolved vapor phase to explore the role of volatiles on magma dynamics and on eruption style.

The 1959 Kīlauea Iki eruption provides a unique opportunity to investigate the process of shallow magma mixing, its impact on the magmatic volatile budget and its role in triggering and driving episodes of Hawaiian fountaining. Melt inclusions hosted by olivine record a continuous decrease in H2O concentration through the 17 episodes of the eruption, while CO2 concentrations correlate with the degree of post-entrapment crystallization of olivine on the inclusion walls. Geochemical data, when combined with the magma budget and with contemporaneous eruption observations, show complex mixing between episodes involving hot, geochemically heterogeneous melts from depth, likely carrying exsolved vapor, and melts which had erupted at the surface, degassed and drained-back into the vent. The drained-back melts acted as a coolant, inducing rapid cooling of the more primitive melts and their olivines at shallow depths and inducing crystallization and vesiculation and triggering renewed fountaining. A consequence of the mixing is that the melts became vapor-undersaturated, so equilibration pressures cannot be inferred from them using saturation models. After the melt inclusions were trapped, continued growth of vapor bubbles, caused by enhanced post-entrapment crystallization, sequestered a large fraction of CO2 from the melt within the inclusions. This study, while cautioning against accepting melt inclusion CO2 concentrations “as measured” in mixed magmas, also illustrates that careful analysis and interpretation of post-entrapment modifications can turn this apparent challenge into a way to yield novel useful insights into the geochemical controls on eruption intensity.

Volcanic plumes are sites of dynamic chemistry involving halogen gases. Here we present new data on the relative abundances of SO2, BrO and OClO gases emitted from Soufrière Hills Volcano [SHV). They were collected during an eruptive hiatus but during sustained degassing at this halogen-rich volcano. By comparison with data from a previous study during an eruptive phase and application of the data and modeling of Villemant et al. (2008), we suggest that, after consideration of errors, either the rate of HBr conversion to BrO is variable, ranging from ∼30% to ∼15%, and/or the relative partitioning of Cl and Br into the gas phase from the melt changes according to eruptive activity. We examine the potential implications of this for fluid-melt partitioning, and compare our results with data from the experimental literature. Our work contributes toward understanding the controls on the BrO/SO2 ratio for volcano monitoring purposes; the changes in plume chemistry with regard to bromine at the onset of lava extrusion may be large and rapid. OClO was detected in the plume at SHV for the first time. This species has only previously been detected in emissions from Mount Etna (using ground-based methods) and from Puyehue Cordon Caulle (using satellite-based methods). No HCHO or NOy species were detected in the spectra.

Andesitic arc volcanoes degas large quantities of volatiles; evidence for vapour transport in the erupted lavas is rarely preserved and poorly understood, but is crucial for understanding eruption style. We present geochemical evidence for the transport of metal-bearing vapour in shear zones preserved in lavas erupted from Soufriere Hills Volcano, Montserrat. Textural evidence suggests that shear-induced brittle failure occurred in a narrow zone (at the conduit wall or in the lava dome). Elevated metal concentrations (Cu, Au, Ag, Pb, Zn) within the zones indicate that the fractures acted as a transient pathway for metal-bearing magmatic gases. During slip, frictional heating to temperatures of >1000 °C caused partial melting at the slip surface. Resorption of volatiles and metals into the partial melt preserved the geochemical signature of magmatic vapour in the shear zone. Cordierite, which is highly unusual in volcanic rocks, crystallised from the peraluminous partial melt, with metal-bearing sulphides and oxides. The shear zones provide the first geochemical evidence for vapour segregation and transport through viscous andesite magmas and provide an insight into controls on eruption style.

We present a study aimed at quantifying the potential for generating sulfur-rich gas emissions from the devolatilization of sediments accompanying sill emplacement during flood basalt eruptions. The potential contribution of sulfur-rich gases from sediments might augment substantially the magma-derived sulfur gases and hence impact regional and global climate. We demonstrate, from a detailed outcrop-scale study, that sulfur and total organic carbon have been devolatilized from shales immediately surrounding a 3-m thick dolerite sill on the Isle of Skye, Scotland. Localized partial melting occurred within a few centimetres of the contact in the shale, generating melt-filled cracks. Pyrite decomposed on heating within 80 cm of the contact, generating sulfur-rich gases (a mixture of H2S and SO2) and pyrrhotite. The pyrrhotite shows 32S enrichment, due to loss of 34S-enriched SO2. Further decomposition and oxidation of pyrrhotite resulted in hematite and/or magnetite within a few cm of the contact. Iron sulfates were produced during retrogressive cooling and oxidation within 20 cm of the contact. Decarbonation of the sediments due to heating is also observed, particularly along the upper contact of the sill, where increasing I´13C is consistent with loss of methane gas. The geochemical and mineralogical features observed in the shales are consistent with a short-lived intrusion, emplaced in

Between January 2011 and April 2012, Santorini volcano (Greece) experienced a period of unrest characterised by the onset of detectable seismicity and caldera-wide uplift. This episode of inflation represented the first sizeable intrusion of magma beneath Santorini in the past 50 years. We employ a new approach using Rn-δ C systematics to identify and quantify the source of diffuse degassing at Santorini during the period of renewed activity. Soil CO flux measurements were made across a network of sites on Nea Kameni between September 2010 and January 2012. Gas samples were collected in April and September 2011 for isotopic analysis of CO (δ C), and radon detectors were deployed during September 2011 to measure (Rn). Our results reveal a change in the pattern of degassing from the summit of the volcano (Nea Kameni) and suggest an increase in diffuse CO emissions between September 2010 and January 2012. High-CO-flux soil gas samples have δ C ∼ 0 ‰. Using this value and other evidence from the literature we conclude that these CO emissions from Santorini were a mixture between CO sourced from magma, and CO released by the thermal or metamorphic breakdown of crustal limestone. We suggest that this mixing of magmatic and crustal carbonate sources may account more broadly for the typical range of δ C values of CO (from ∼ - 4 ‰ to ∼ + 1 ‰) in diffuse volcanic and fumarole gas emissions around the Mediterranean, without the need to invoke unusual mantle source compositions. At Santorini a mixing model involving magmatic CO (with δ C of - 3 ± 2 ‰ and elevated (Rn)/CO ratios ∼ 10 - 10 Bq kg) and CO released from decarbonation of crustal limestone (with (Rn)/CO ∼ 30-300 Bq kg, and δ C of + 5 ‰) can account for the δ C and (Rn)/CO characteristics of the 'high flux' gas source. This model suggests ∼ 60 % of the carbon in the high flux deep CO end member is of magmatic origin. This combination of δ C and (Rn) measurements has potential to quantify magmatic and crustal contributions to the diffuse outgassing of CO in volcanic areas, especially those where breakdown of crustal limestone is likely to contribute significantly to the CO flux. © 2013 Elsevier B.V. All rights reserved.

Magma movement, fault structures and hydrothermal systems influence volatile emissions at rift volcanoes. Longonot is a Quaternary caldera volcano located in the southern Kenyan Rift, where regional extension controls recent shallow magma ascent. Here we report the results of a soil carbon dioxide (CO2) survey in the vicinity of Longonot volcano, as well as fumarolic gas compositions and carbon isotope data. The total non-biogenic CO2 degassing is estimated at − 1, and is largely controlled by crater faults and fractures close to the summit. Thus, recent volcanic structures, rather than regional tectonics, control fluid pathways and degassing. Fumarolic gases are characterised by a narrow range in carbon isotope ratios (δ13C), from − 4.7‰ to − 6.4‰ (vs. PDB) suggesting a magmatic origin with minor contributions from biogenic CO2. Comparison with other degassing measurements in the East African Rift shows that records of historical eruptions or unrest do not correspond directly to the magnitude of CO2 flux from volcanic centres, which may instead reflect the current size and characteristics of the subsurface magma reservoir. Interestingly, the integrated CO2 flux from faulted rift basins is reported to be an order of magnitude higher than that from any of the volcanic centres for which CO2 surveys have so far been reported.

The Soufrière Hills Volcano in Montserrat erupts a Cl-rich, porphyritic andesite. HCl degassing accompanies eruption and is dependent on the growth rate of the lava dome. The magma contains hornblende phenocrysts that show repetitive zoning in most elements, including Cl. On the basis of the zoning data, (Cl/OH) ratios in the melt, calculated from partitioning data, increase rimward through each zone, indicating that the phenocrysts formed under conditions of varying (Cl/OH)m. An empirical relationship between A-site occupancy in the hornblende and temperature implies that crystallisation of each zone is also accompanied by increasing temperature. Each zone ends at a resorption horizon, and crystallisation recommences at lower temperature and (Cl/OH)m. Melt inclusion H2O and Cl contents for the 8th January 2007 explosive eruption can be explained by closed-system degassing with DClfl-m between 5 and 30, or by open-system degassing accompanied by a small amount of crystallisation. However, neither simple closed-system degassing nor convective circulation of magma can explain the positive correlation of (Cl/OH)m with temperature. We suggest that the zoning can be caused by accumulation of CO2-rich vapour in the andesite, probably as a result of mafic magma injection into the chamber. Decreasing H2O fugacity and/or increasing Clm result in increasing (Cl/OH)m while heat transferred with the volatiles causes the rise in temperature. Intermittently, the accumulated fluid is lost to the surface, possibly as a result of renewed eruptive activity. This model requires the CO2-rich fluid to be decoupled from the magma, consistent with previous observations of continuous CO2 emissions at the surface. © 2009 Elsevier Ltd.

Petrological constraints on the timescales of pre-eruptive crystal storage and magma degassing provide an important framework for the interpretation of seismic, geodetic and gas monitoring data in volcanically active regions. We have used Fe–Mg diffusion chronometry in 86 olivine macrocrysts from the AD 1783–1784 Laki eruption on Iceland's Eastern Volcanic Zone to characterise timescales of crystal storage and transport in the lead-up to this eruption. The majority of these olivines have core compositions of Fo < 76, and rim compositions in the range Fo69–Fo74 that are close to equilibrium with the Laki melt. Diffusion modelling using the greyscale intensity of backscattered electron images as a proxy for olivine composition reveals that the most probable Fe–Mg diffusion timescale for Laki olivines is 7.8 days, which reflects the characteristic olivine residence time in the carrier melt prior to eruption. A small population of Fo > 81 olivines record Fe–Mg diffusion timescales of ∼124 days; these crystals are likely to have formed in mid-crustal magma chambers, been transferred to storage at shallower levels and then entrained into the Laki melt prior to eruption. Typical Fe–Mg diffusion timescales of 6–10 days are shorter than the average time interval between discrete episodes of the Laki eruption, indicating variable or pulsed disaggregation of stored crystals into the carrier liquid prior to the onset of each episode. The diffusion timescales coincide with historical accounts of strong and frequent earthquakes in southeast Iceland, which we interpret as being associated with mush disaggregation related to melt withdrawal and the initiation of dyke propagation from a crustal magma reservoir at ∼6 ± 3 km depth to the surface. We calculate pre-eruptive CO2 fluxes of 2–6 Mt d−1, assuming a pre-eruptive CO2 outgassing budget of 189.6 Mt for the Laki eruption and a constant rate of CO2 release in the 6–10 days preceding each eruptive episode. Our dataset indicates that petrological constraints on the timescales of magmatic processes occurring in the days leading up to historic eruptions may enhance our ability to forecast the onset of future large eruptions, both in Iceland and further afield.

South Soufriere Hills and Soufriere Hills volcanoes are two km apart at the southern end of the island of Montserrat, West Indies. Their magmas are distinct geochemically, despite these volcanoes having been active contemporaneously at 131-129 ka. We use the water content of pyroxenes and melt inclusion data to reconstruct the bulk water contents of magmas and their depth of storage prior to eruption. Pyroxenes contain up to 281 ppm H2O, with significant variability between crystals and from core to rim in individual crystals. The Al content of the enstatites from Soufriere Hills Volcano (SHV) is used to constrain melt-pyroxene partitioning for H2O. The SHV enstatite cores record melt water contents of 6-9 wt%. Pyroxene and melt inclusion water concentration pairs from South Soufriere Hills basalts independently constrain pyroxene-melt partitioning of water and produces a comparable range in melt water concentrations. Melt inclusions recorded in plagioclase and in pyroxene contain up to 6.3 wt% H2O. When combined with realistic melt CO2 contents, the depth of magma storage for both volcanoes ranges from 5 to 16 km. The data are consistent with a vertically protracted crystal mush in the upper crust beneath the southern part of Montserrat which contains heterogeneous bodies of eruptible magma. The high water contents of the magmas suggest that they contain a high proportion of exsolved fluids, which has implications for the rheology of the mush and timescales for mush reorganisation prior to eruption. A depletion in water in the outer 50-100 microns of a subset of pyroxenes from pumices from a Vulcanian explosion at Soufriere Hills in 2003 is consistent with diffusive loss of hydrogen during magma ascent over 5-13 hours. These timescales are similar to the mean time periods between explosions in 1997 and in 2003, raising the possibility that the driving force for this repetitive explosive behaviour lies not in the shallow system, but in the deeper parts of a vertically protracted crustal magma storage system.

[Figure][1] Any quantitative interpretation of the formation conditions of igneous rocks requires methods for determining crystallization temperature. Accurate application of such thermobarometers relies on the attainment of equilibrium in the system to be studied. This may be particularly difficult in silicic magmas, where diffusivities are low and crystallization kinetics sluggish. Moreover, progressive degassing of volatile-rich magmas during ascent can result in continuous changes in effective undercooling, causing particular problems in achieving equilibrium between melt and crystals that grow in response to decompression. We consider these problems in the context of plagioclase-melt equilibria for magmas undergoing decompression and degassing-driven crystallization, using two published thermometers. The two thermometers show similar trends with key parameters but absolute temperatures can vary significantly. Analysis of decompression experiments conducted at constant temperature shows systematic variations in calculated temperature and equilibrium constant with varying decompression rate and quench pressure. This indicates that an unrecognized lack of equilibration could result in significant temperature overestimates and potentially spurious results. This highlights the need to assess for equilibrium, and we discuss problems associated with some commonly used indicators of equilibration. Finally, retrospective analysis of published plagioclase-hosted melt inclusion suites from five subduction zone volcanoes shows systematic increases in calculated temperature and decreases in equilibrium constant with decreasing H2O concentration. While this could represent the signature of latent heat of crystallization, we suggest that such patterns should be treated with caution unless there is clear evidence of sustained equilibrium between plagioclase and melt during decompression. [1]: pending:yes

Volcanic emissions link the oxidation state of the Earth's mantle to the composition of the atmosphere. Whether the oxidation state of an ascending magma follows a redox buffer – hence preserving mantle conditions – or deviates as a consequence of degassing remains under debate. Thus, further progress is required before erupted basalts can be used to infer the redox state of the upper mantle or the composition of their co-emitted gases to the atmosphere. Here we present the results of X-ray absorption near-edge structure (XANES) spectroscopy at the iron K-edge carried out for a series of melt inclusions and matrix glasses from ejecta associated with three eruptions of Kīlauea volcano (Hawai‘i). We show that the oxidation state of these melts is strongly correlated with their volatile content, particularly in respect of water and sulfur contents. We argue that sulfur degassing has played a major role in the observed reduction of iron in the melt, while the degassing of H$_{2}$O and CO$_{2}$ appears to have had a negligible effect on the melt oxidation state under the conditions investigated. Using gas–melt equilibrium degassing models, we relate the oxidation state of the melt to the composition of the gases emitted at Kīlauea. Our measurements and modelling yield a lower constraint on the oxygen fugacity of the mantle source beneath Kīlauea volcano, which we infer to be near the nickel nickel-oxide (NNO) buffer. Our findings should be widely applicable to other basaltic systems and we predict that the oxidation state of the mantle underneath most hotspot volcanoes is more oxidised than that of the associated lavas. We also suggest that whether the oxidation states of a basalt (in particular MORB) reflects that of its source, is primarily determined by the extent of sulfur degassing.

Volcanic plume samples taken in 2008 and 2009 from the Halemàumàu eruption at Kīlauea provide new insights into Kīlauea's degassing behaviour. The Cl, F and S gas systematics are consistent with syn-eruptive East Rift Zone measurements suggesting that the new Halemàumàu activity is fed by a convecting magma reservoir shallower than the main summit storage area. Comparison with degassing models suggests that plume halogen and S composition is controlled by very shallow (77%) as gaseous elemental mercury at the point of emission. Sulphate is an important aerosol component (modal particle diameter ∼0.44μm). Aerosol halide ion concentrations are low compared to other systems, consistent with the lower proportion of gaseous hydrogen halides. Plume concentrations of many metallic elements (Rb, Cs, Be, B, Cr, Ni, Cu, Mo, Cd, W, Re, Ge, As, In, Sn, Sb, Te, Tl, Pb, Mg, Sr, Sc, Ti, V, Mn, Fe, Co, Y, Zr, Hf, Ta, Al, P, Ga, Th, U, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Tm) are elevated above background air. There is considerable variability in metal to SO 2 ratios but our ratios (generally at the lower end of the range previously measured at Kīlauea) support assertions that Kīlauea's emissions are metal-poor compared to other volcanic settings. Our aerosol Re and Cd measurements are complementary to degassing trends observed in Hawaiian rock suites although measured aerosol metal/S ratios are about an order of magnitude lower than those calculated from degassing trends determined from glass chemistry. Plume enrichment factors with respect to Hawaiian lavas are in broad agreement with those from previous studies allowing similar element classification schemes to be followed (i.e., lithophile elements having lower volatility and chalcophile elements having higher volatility). The proportion of metal associated with the largest particle size mode collected (>2.5μm) and that bound to silicate is significantly higher for lithophiles than chalcophiles. Many metals show higher solubility in pH 7 buffer solution than deionised water suggesting that acidity is not the sole driver in terms of solubility. Nonetheless, many metals are largely water soluble when compared with the other sequential leachates suggesting that they are delivered to the environment in a bioavailable form. Preliminary analyses of environmental samples show that concentrations of metals are elevated in rainwater affected by the volcanic plume and even more so in fog. However, metal levels in grass samples showed no clear enrichment downwind of the active vents. © 2011 Elsevier Ltd.

We present the results of the first systematic study of melt compositions at Pantelleria, based on both melt inclusions and matrix glasses in pantellerites from 10 eruptions during the last eruptive cycle (

Subduction zones play a fundamental role in the geochemical cycle of carbon, and related arc volcanism is believed to exert primary control on atmospheric CO2 concentrations over geological time. Arc volcanism may have been particularly important in the most recent Greenhouse of the late Cretaceous, where it has been hypothesized that the subduction of the carbonate-rich Tethys contributed to overall higher volcanic CO2 outgassing rates and thus a warmer climate. To test this hypothesis, the decarbonation efficiencies of modern subduction zones were calculated through a geochemical database that compared subaerial arc CO2 fluxes with the subducting crust and sediment geochemistry. The modern data are used to postulate a CO2 recycling and degassing scenario for arc volcanism related to the closure of the Tethys. Our analysis indicates that the thermal structure of subduction zones controls the extent and depth of slab decarbonation, while the sediment geochemistry (e.g. the amount of carbonate sediment) may be of secondary importance. The calculated decarbonation efficiency of modern arcs ranges from 18 to 70%. Our calculations support recent models predicting carbon recycling through infiltration-driven decarbonation, and limited by water availability at sub-arc depths. This analysis allows us to make inferences about the potential volcanic CO2 flux from subduction of the Tethys during the Cretaceous, suggesting between an 8 and 222% increase over modern CO2 outgassing. We suggest that the primary reason for the increase in CO2 outgassing in the Cretaceous is contamination of arc magmas by platform carbonates in the overlying crust and increased decarbonation efficiency.

The major magmatic volatile components—H2O, CO2, S, Cl, and F— play an important role in the formation, evolution, and eruption of magma. Knowledge of magmatic concentrations and fluxes of these volatiles is thus important for understanding explosive eruptive behavior of volcanoes, recycling of volatiles in subduction zones, formation of magmatic-hydrothermal ore deposits, fluxes of volcanic gases to Earth’s atmosphere, and potential climatic impacts of large volcanic eruptions. Over the past 30 years, new analytical techniques for measuring volatiles in melt inclusions and glasses from volcanic rocks and new developments in remote sensing technology used for quantifying volcanic emissions have led to major advances in our understanding of volatiles in magmatic systems and their fluxes from Earth’s mantle to the crust and hydrosphere. Sulfur plays a particularly important role in many of the processes noted above because it affects partitioning of metals into sulfide phases or vapor in magmas during crustal storage, and when released to the atmosphere, it forms sulfuric acid aerosol droplets that catalyze ozone destruction, influences other aspects of atmospheric chemistry, and blocks incoming solar radiation. In addition, S may play a role in causing oxidation of the mantle wedge above subduction zones (Kelley and Cottrell 2009). In silicate melts, the solubility behavior, activity-composition relations, and vapor-melt partitioning of S are complex due to multiple valence states and species (S2−, S6+ in melt; H2S, S2, SO2, SO3 in vapor) and the occurrence of non-volatile S-rich phases (immiscible Fe-S-O liquid, pyrrhotite, monosulfide and intermediate solid solutions, anhydrite). Sulfur dioxide (SO2) is the easiest of the main magmatic volatiles to measure in volcanic plumes using ground- and satellite-based remote sensing techniques because of its relatively high concentration in volcanic plumes relative to background values. More …

A study of volcanic gas composition and matrix glass volatile concentrations has allowed a model for halogen degassing to be formulated for Kilauea Volcano, Hawai[modifier letter turned comma]i. Volcanic gases emitted during 2004-2005 were characterised by a molar SO2/HCl of 10-64, with a mean of 33; and a molar HF/HCl of 0-5, with a mean of 1.0 (from approximately 2500 measurements). The HF/HCl ratio was more variable than the SO2/HCl ratio, and the two correlate weakly. Variations in ratio took place over rapid timescales (seconds). Matrix glasses of Pele's tears erupted in 2006 have a mean S, Cl and F content of 67, 85 and 173 ppm respectively, but are associated with a large range in S/F. A model is developed that describes the open system degassing of halogens from parental magmas, using the glass data from this study, previously published results and parameterisation of sulphur degassing from previous work. The results illustrate that halogen degassing takes place at pressures of < 1 MPa, equivalent to < ~ 35 m in the conduit. Fluid-melt partition coefficients for Cl and F are low (< 1.5); F only degasses appreciably at < 0.1 MPa above atmospheric pressure, virtually at the top of the magma column. This model reproduces the volcanic gas data and other observations of volcanic activity well and is consistent with other studies of halogen degassing from basaltic magmas. The model suggests that variation in volcanic gas halogen ratios is caused by exsolution and gas-melt separation at low pressures in the conduit. There is no evidence that either diffusive fractionation or near-vent chemical reactions involving halogens is important in the system, although these processes cannot be ruled out. The fluxes of HCl and HF from Kilauea during 2004-5 were ~ 25 and 12 t/d respectively.

To improve interpretation of volcanic SO2 flux data, it is necessary to quantify and understand reactions involving SO2 in volcanic plumes. SO2 is lost in volcanic plumes through a number of mechanisms. Here we report SO2 measurements made with miniature ultraviolet spectrometers at Soufrière Hills volcano, Montserrat; a low altitude volcano (~ 1000 m above sea level) whose plume entrains humid marine air in the planetary boundary layer. Traverses very near (< 400 m) beneath the ash-free plume were made at various distances from the source (from ~ 2 km to ~ 16 km), thereby spanning plume ages of about 6 to 35 min with minimal attenuation. We find average SO2 loss rates of ~ 10− 4 s− 1 (e-folding time of ~ 2.78 h), slightly lower than estimated previously for Soufrière Hills. These are in the fast end of the range of loss rates measured at other volcanoes (10− 3–10− 7 s− 1, e-folding times of 0.28–2778 h), indicating that Montserrat plumes have short SO2 lifetimes. This work is more detailed and precise than previous work and is likely to represent the general case at Montserrat. SO2 flux measurements made > 2 km downwind from Soufrière Hills volcano significantly underestimate at-source SO2 emission rates, on the order of 70–146%, when not accounting for the decay rate. Similar SO2 loss is likely to occur in plumes from other tropical low altitude volcanoes under conditions of high relative humidity (~ 20% of active volcanoes worldwide). These results suggest that the global volcanic SO2 emission rate may be underestimated as the estimates are based on measurements taken downwind of volcanoes, by which time significant loss of SO2 may have taken place. The loss rates calculated here could be used, in conjunction with downwind SO2 fluxes, to estimate at-source SO2 emission rates from volcanoes with similar environmental conditions to those at Soufrière Hills volcano.

Mafic magmas are common in subduction zone settings, yet their high density restricts their ascent to the surface. Once stalled in the crust, these magmas may differentiate, and assimilate crust and other melts and crystal mushes to produce hybridized intermediate magmas. The Soufrière Hills Volcano on Montserrat is a ‘type locality’ for such hybridization processes and yet, just 3?km south of the crater, voluminous basalts have erupted from the South Soufrière Hills volcano within the same time period as the Soufrière Hills Volcano was erupting hybrid andesites (131–128?ka). Basaltic South Soufrière Hills magmas have 48–53?wt % SiO2 and 4–6?wt % MgO. They were hot (970–1160°C), volatile-rich (melt inclusions contain up to 6·2?wt % H2O) and were stored at 8–13?km depth prior to eruption (based on olivine- and pyroxene-hosted melt inclusion volatile geochemistry). Melt inclusions do not preserve basaltic liquids: they are andesitic to rhyolitic in composition, related to one another by a line of descent controlled by simple closed-system fractionation. Whole-rock compositions, however, are best described by a hybridization model involving ‘back-mixing’ of andesitic to rhyolitic melts with mafic crystal phases such as magnetite, olivine, orthopyroxene and clinopyroxene. Phenocryst zoning illustrates repeated mixing events between evolved melts and mafic phenocrysts; this feature, when coupled with the heterogeneity of crystal compositions, strongly suggests that although the bulk compositions are basaltic (containing Fo80 olivine), they were assembled from disparate ingredients, probably derived from mafic crystal mushes and more evolved melt lenses of variable composition. The mixing events occur days to weeks prior to eruption. We propose that the South Soufrière Hills basaltic magmas, with their higher bulk density relative to andesites from neighbouring volcanoes, ultimately may have been eruptible owing to both the transtensional tectonics imposed by offshore grabens (related to oblique subduction in the Lesser Antilles arc) and surface unloading caused by large-scale edifice collapse. Our observations support the idea that compositional changes in arcs might reflect not only changes in source compositions, but also effects caused by variations in crustal strain and tectonics.

The 12–13 July 2003 dome collapse at Soufriere Hills Volcano, Montserrat, was the largest event of its kind during the eruption thus far (1995–2005), involving the removal of 210 million m 3 of the lava dome complex over 18 h. Less than 2% of the total volume of material involved in the dome collapse was deposited on land. A pyroclastic density current deposit alongshore and inland from the Tar River Fan was generated from a single blast originating at the shoreline. The blast was caused by the interaction of pyroclastic flows with seawater. We propose that at the peak of the lava dome collapse, a sharp increase in the volume flux of pyroclastic flows caused substantial displacement of seawater from the shoreline, followed by inrush of seawater when the flux decreased a few minutes later. The tsunami allowed penetration of seawater into the main body of the pyroclastic flow at the shoreline, which led to explosive fragmentation of pyroclastic blocks. Tephra fall deposits accumulated at a high rate on Montserrat, causing extensive damage to vegetation and buildings. Their stratigraphy recorded fallout from high co-pyroclastic flow clouds, from a vulcanian explosion cloud at the peak in collapse rate (caused by the fragmentation of degassed lava dome) and from four vulcanian explosion clouds after the dome collapse (caused by fragmentation of bubbly magma in the conduit). The total tephra fall volume is estimated at 10–20 million m 3 .

The lava dome collapse of 12–13 July 2003 was the largest of the Soufriere Hills Volcano eruption thus far (1995–2005) and the largest recorded in historical times from any volcano; 210 million m3 of dome material collapsed over 18 h and formed large pyroclastic flows, which reached the sea. The evolution of the collapse can be interpreted with reference to the complex structure of the lava dome, which comprised discrete spines and shear lobes and an apron of talus. Progressive slumping of talus for 10 h at the beginning of the collapse generated low-volume pyroclastic flows. It undermined the massive part of the lava dome and eventually prompted catastrophic failure. From 02:00 to 04:40 13 July 2003 large pyroclastic flows were generated; these reached their largest magnitude at 03:35, when the volume flux of material lost from the lava dome probably approached 16 million m3 over two minutes. The high flux of pyroclastic flows into the sea caused a tsunami and a hydrovolcanic explosion with an associated pyroclastic surge, which flowed inland. A vulcanian explosion occurred during or immediately after the largest pyroclastic flows at 03:35 13 July and four further explosions occurred at progressively longer intervals during 13–15 July 2003. The dome collapse lasted approximately 18 h, but 170 of the total 210 million m3 was removed in only 2.6 h during the most intense stage of the collapse.

We report here the first results from an automated, telemetered UV scanning spectrometer system for monitoring SO2 emission rates at Soufriere Hills Volcano, Montserrat. Two spectrometers receive light by way of a motor-driven stepping prism and telescope in order to make vertical scans of the volcanic plume. Spectral data from these spectrometers, situated 2,800 m apart and 4,500 m from the volcano, are relayed back to the observatory every 4–5 s via radio modems. A full scan of the plume is accomplished every 1–6 min by the (time-synchronised) spectrometers and a SO2 emission rate is calculated using the SO2 slant concentrations, scan angles and plume speeds estimated from the wind speed from a telemetered weather station near to the volcano. The plume's position and dimensions are calculated using the angular data from the two spectrometers. The plume height varies significantly diurnally and seasonally and is important in order to minimise the error on SO2 emission rates. The new scanning system (Scanspec) provides SO2 emission rates from 08:00 to 16:00 h local time every day. Preliminary results highlight a number of features of the SO2 time series and plume dynamics and give our first indications of the errors and limits of detection of this system. SO2 emission rates vary widely on all time scales (minutes, days, months). This new system has already provided the first real and consistent indication that SO2 emission rates vary on a minutes to hours basis, which can be correlated with volcanic activity (for example, rockfall and pyroclastic flow activity). It is anticipated that this system at Soufriere Hills will yield information on shallow processes occurring on short time scales (periods of minutes to hours) as well as deep processes relating to magma supply rates, which will be associated with longer wavelength SO2 signals of weeks to months.

For 30 years, the correlation spectrometer (COSPEC) has been the principal tool for remote monitoring of volcanic SO 2 fluxes. During this time, the instrument has played a prominent role in volcanic hazard assessment. COSPEC data also underpin estimates of the global volcanic SO 2 flux to the atmosphere. Though innovative for its time, COSPEC is now outdated in several respects. Here we report the first measurements with a potential replacement, using a low cost, miniature, ultraviolet fibre-optic differential optical absorption spectrometer (mini-DOAS). Field experiments were conducted at Masaya Volcano (Nicaragua) and Soufriere Hills Volcano (Montserrat). The mini-DOAS was operated from a road vehicle and helicopter, and from a fixed position on the ground, indicating fluxes of ∼4 and 1 kg s −1 at Masaya Volcano and Soufriere Hills Volcano, respectively. Side-by-side observations with a COSPEC on Montserrat indicate a comparable sensitivity but the mini-DOAS offers several advantages, including the collection of broadband ultraviolet spectra. It has immense potential for geochemical surveillance at volcanoes worldwide.

HCl:SO2 mass ratios measured by open path Fourier transform spectroscopy (OP-FTIR) in the volcanic plume at Soufrière Hills Volcano, Montserrat, are presented for the second phase of dome building between November 1999 and November 2000. HCl:SO2 mass ratios of greater than 1 and HCl emission rates of greater than 400 t day-1 characterise periods of dome building for this volcano. The data suggest that chlorine partitions into a fluid phase as the magma decompresses and exsolves water during ascent. This is substantiated by a correlation between chlorine and water content in the melt (derived from the geochemical analysis of plagioclase melt inclusion and matrix glasses from phase I and II of dome growth). The matrix glass from the November 1999 and March 2000 domes indicate an open system degassing regime with a fluid-melt partition coefficient for chlorine of the order of 250-300. September 1997 glasses have higher chlorine contents and may indicate a switch to closed system degassing prior to explosive activity in September and October 1997. The OP-FTIR HCl time series suggests that HCl emission rate is strongly related to changes in eruption rate and we infer an emission rate of over 13.5 kt day-1 HCl during a period of high extrusion rate in September 2000. A calculation of the HCl emission rate expected for varying extrusion rates from the open-system degassing model suggests a HCl emission rate of the order of 1-4 kt day-1 is indicative of an extrusion rate of between 2 and 8 m3 s-1. Monitoring of HCl at Soufrière Hills Volcano provide a proxy for extrusion rate, with changes in ratio between HCl and SO2 occurring rapidly in the plume. Order of magnitude changes occur in HCl emission rates over the time-scale of hours to days, making these changes easy to detect during the day-to-day monitoring of the volcano. Mean water emission rates are calculated to range from 9-24 kt day-1 during dome building activity, calculated from the predicted mass ratio of H2O:HCl in the fluid at the surface and FTIR-derived HCl emission rates.