Closed-system Degassing Research Articles

Magmatic water content in HIMU basalts from the Cook-Austral Islands: constraints on degassing processes and source composition from clinopyroxene phenocrysts

The amount of water recycled during subduction is unclear. Water/Ce estimates in HIMU (high-238U/204Pb) basalts are variable, ranging from < 100 to > 250 in glasses and melt inclusions. Because clinopyroxene (cpx) is a common early liquidus phase and the cpx/melt partitioning of water is well constrained, cpx phenocrysts provide an additional constraint on magmatic water contents. We present water and trace element concentrations in HIMU-basalt-hosted cpx phenocrysts from the Austral Islands. Calculated melt [H2O] (up to 4.3 wt.%) and H2O/Ce ratios (22–825) are higher than in olivine-hosted inclusions, as are estimated equilibration pressures. Correlation between estimated melt [H2O] and crystallization or entrapment pressure suggests significant water loss during magma ascent. Most ocean island basalts (OIBs) span a limited range in H2O (~ 1–1.5 wt.%), and low (< 100) H2O/Ce ratios are primarily observed in melts with unusually high [Ce] (up to 350 ppm). Additionally, [H2O] and H2O/Ce in some suites correlate with entrapment pressure despite having quench pressures high enough to prevent significant water loss from open- or closed-system degassing (< 100 MPa). Polybaric “sparging”, whereby low-P melts re-equilibrate with CO2-rich fluids exsolved at higher pressure, may result in water loss at pressures less than CO2 saturation. This may more accurately describe OIB degassing processes than open or closed system degassing. After correcting for degassing, primary Australs melts likely have H2O/Ce of ~ 300–600. If applicable to OIB sources in general, this limits the total water budget of the mantle, including the mantle transition zone, to < 2.4 ocean masses.

The role of pre-eruptive gas segregation on co-eruptive deformation and SO2 emissions

The presence of exsolved gas bubbles influences measurements of both volcanic surface deformation and SO2 emissions. In a closed-system, exsolved volatiles remain within the melt but in an open-system, the decoupled gas phase can either outgas or accumulate, leading to large variations magmatic gas fraction. Here we investigate the role of gas volume fraction and gas segregation processes on magma properties and co-eruptive monitoring data. First we use thermodynamic models of gas exsolution to model gas volume fraction and magma compressibility, and use these to calculate SO2 emissions and co-eruptive volume change. We find that volume change is equally sensitive to magma compressibility and chamber compressibility over realistic parameters ranges, and both must be considered when interpreting surface deformation data. Reservoir depth and magma composition are the dominant controls on gas volume fraction, but the initial content of H2O and S have strong influences on volume change and SO2 emissions, respectively. Pre-eruptive gas accumulation produces increased SO2 emissions and muted co-eruptive deformation, while degassing has the opposite effect. We then compare our models to a compilation of data from 20 recent eruptions where measurements of volume change, SO2 emissions and erupted volume are available. To the first order, shallow reservoirs produce smaller volume changes per volume erupted and silica-poor magmas yield greater co-eruptive volume changes than silica-rich systems, consistent with closed system degassing. Co-eruptive degassing causes high SO2 emissions during effusive eruptions. Comparison between model predictions and observations suggests that all magmatic systems experience a certain degree of outgassing prior to an eruption. Our findings are consistent with current conceptual models of transcrustal magmatic systems consisting of heterogeneous mixtures of gas and melt and have important implications for the interpretation of surface deformation and SO2 emission signals at all stages of the eruption cycle.

Conduit processes in crystal-rich dacitic magma and implications for eruptive cycles at Guagua Pichincha volcano, Ecuador

Stratovolcanoes are commonly characterised by cyclic eruptive activity marked by transitions between dome-forming, Vulcanian, Subplinian and Plinian eruptions. Guagua Pichincha volcano (Ecuador) has been a location of such cyclicity for the past ~ 2000 years, with Plinian eruptions in the first and tenth centuries AD (Anno Domini/after Christ), and CE (Common Era) 1660, which were separated by dome-forming to Subplinian eruptions, such as the recent 1999–2001 eruption. These cycles are therefore a prominent example of effusive-explosive transitions at varying timescales. Here, we investigate the reasons for such shifts in activity by focusing on degassing and outgassing processes within the conduit. We have coupled a petrophysical and textural analysis of dacites from the CE 1660 Plinian eruption and the 1999–2001 dome-forming/Vulcanian eruption, with different percolation models in order to better understand the role of degassing on eruptive style. We demonstrate that the transition from dome-forming to Plinian activity is correlated with differences in phenocryst content and consequently in bulk viscosity. A lower initial phenocryst content and viscosity is inferred for the Plinian case, which promotes faster ascent, closed-system degassing, fragmentation and explosive activity. In contrast, dome-forming phases are promoted by a higher magma viscosity due to higher phenocryst content, with slower ascent enhancing gas escape and microlite crystallization, decreasing explosivity and yielding effusive activity.ResumenLos estratovolcanes se caracterizan comúnmente por presentar actividad eruptiva cíclica, marcada por transiciones entre erupciones formadoras de domos y erupciones de tipo Vulcanianas, Subplinianas y Plinianas. El volcán Guagua Pichincha (Ecuador) ha dado lugar a tal ciclicidad durante los últimos ~ 2000 años, con erupciones Plinianas tanto en los siglos Primero y Décimo, como en el año 1660, las cuales estuvieron intercaladas por erupciones formadoras de domos y de tipo Subplinianas, tal como ocurrió durante la erupción reciente de 1999–2001. Estos ciclos son, por lo tanto, ejemplos destacados de transiciones eruptivas de tipo efusiva-explosiva a escalas de tiempo variadas. En este trabajo, investigamos las razones de tales cambios de actividad enfocándonos en procesos de exsolución y pérdida de gases del magma en el conducto (desgasificación en sistemas cerrado y abierto). Hemos acoplado análisis petrofísicos y texturales tanto de dacitas de la erupción Pliniana de 1660, como de la erupción formadora de domos/Vulcaniana de 1999–2001, junto con diferentes modelos de percolación, para así comprender mejor el rol de la exsolución de volátiles en el estilo eruptivo. Demostramos que la transición desde una actividad efusiva formadora de domos a una Pliniana está correlacionada con diferencias en el contenido de fenocristales y, subsecuentemente, con la viscosidad total del magma. Un contenido inicial menor de fenocristales y una menor viscosidad se infiere para el caso Pliniano, lo que promueve un ascenso más rápido, desgasificación en sistema cerrado, fragmentación y finalmente actividad explosiva. Por el contrario, las fases formadoras de domos son promovidas por una viscosidad mayor debido a un contenido mayor de fenocristales, con ascenso más lento promoviendo a su vez el escape de gases y la cristalización de microlitos, disminuyendo la explosividad y produciendo actividad efusiva.

Decompression experiments for sulfur-bearing hydrous rhyolite magma: Redox evolution during magma decompression

Abstract We performed decompression experiments on sulfur-bearing hydrous rhyolite magma at a temperature of 800 °C to investigate redox evolution during magma decompression. The magma was continuously decompressed from 100 MPa to 10–50 MPa at rates of 10 and 100 MPa h–1. The evolution of the ferric to total iron ratio (Fe3+/Fetotal) during decompression was investigated using XANES, and redox evolution was determined based on a thermodynamic calculation and measured Fe3+/Fetotal. Before decompression, the sample was buffered from NNO to NNO+1, and the pre-exsolved fluid phase and sulfide crystal coexisted. Sulfide crystals were found in all decompressed samples, and Fe3+/Fetotal showed a slight decrease with decompression. It was confirmed that the sample in a gold capsule was not influenced by the change in redox conditions outside of the capsule for the timescale of the decompression experiments; thus, the decompressed sample reflected the redox evolution in magma during decompression. Our experiments indicated that magma decompression causes a slight reduction when it includes water and sulfur. This evolution is qualitatively explained by sulfur degassing and fluid-melt redox equilibria. During the fluid-melt redox equilibria, magma is reduced if the existence of a pre-exsolved fluid phase is assumed, while the model calculation shows that magma is oxidized when it contains only water or no pre-exsolved fluid phases. This is because sulfur buffers the oxidation of magma through a reaction with oxygen in the fluid phase. Therefore, we inferred that the redox condition of magma is not oxidized during explosive volcanism with a pre-exsolved fluid phase and closed-system degassing. In contrast, if magma experiences open-system degassing, it may be oxidized, resulting in the breakdown of sulfide crystals as observed in some pyroclasts and lavas.

Magma Pressure-Temperature-Time Paths During Mafic Explosive Eruptions

We have constrained syneruptive pressure-temperature-time (P-T-t) paths of mafic magmas using a combination of short-timescale cooling and decompression chronometers. Recent work has shown that the thermal histories of crystals in the last few seconds to hours of eruption can be constrained using concentration gradients of MgO inside olivine-hosted melt inclusions, produced in response to syneruptive cooling and crystallization of olivine on the inclusion walls. We have applied this technique to the study of melt inclusions erupted by arc and ocean island volcanoes, including the 1974 subplinian eruption of Fuego volcano; the 1977 fire-fountain eruption of Seguam volcano; and three eruptions of Kilauea volcano (episode 1 of the 1959 Kilauea Iki fire-fountain eruption, the 1500 CE vigorous fire-fountain eruption, and the 1650 CE subplinian eruption). Of the eruptions studied so far, melt inclusions from the 1959 Kilauea Iki eruption record the highest syneruptive cooling rates (3–11°C/s) and the shortest cooling durations (4–19 s), while inclusions from the 1974 Fuego eruption record the slowest cooling rates (0.1–1.7°C/s) and longest cooling durations (21–368 s). The high cooling rates inferred for the Kilauea Iki and Seguam fire fountain eruptions are consistent with air quenching over tens of seconds during and after fragmentation and eruption. Melt inclusions sampled from the interiors of small (∼6 cm diameter) volcanic bombs at Fuego are found to have cooled more slowly on average than inclusions sampled from ash (with particle diameters < 2 mm) during the same eruption, as expected based on conductive cooling models. We find evidence for a systematic relationship between cooling rates and decompression rates of magmas, in which rapidly ascending gas-bearing magmas experience slower cooling during ascent and eruption than slowly ascending magmas. Our magma P-T-t constraints for the Kilauea Iki eruption are in broad agreement with isentropic models that show that the dominant driver of cooling in the conduit is adiabatic expansion of a vapor phase; however, at Fuego and Seguam, our results suggest a significant role for latent heat production and/or open-system degassing (both of which violate assumptions required for isentropic ascent). We thereby caution against the application of isentropic conduit models to magmas containing relatively high initial water concentrations (e.g., arc magmas containing ∼4 wt% water). We note that several processes that have been inferred to occur in volcanic conduits such as magma stalling, magma mingling, open- and closed-system degassing, vapor fluxing, and vapor accumulation (in foam layers or as slugs of gas) are associated with different implied vapor volume fractions during syneruptive ascent. Given the sensitivity of magma P-T-t paths to vapor volume fraction, the syneruptive thermometer presented here may be a means of identifying these processes during the seconds to hours preceding the eruption of mafic magmas.

Triggering of the powerful 14 July 2013 Vulcanian explosion at Tungurahua Volcano, Ecuador

The 14 July 2013 Vulcanian explosion at Tungurahua occurred after two months of quiescence and was extremely powerful, generating some of the highest infrasound energies recorded worldwide. Here we report on how a combination of geophysical data, textural measurements, and physical and mechanical tests on eruptive products allowed us to determine the processes that led to the pressurization of the conduit and triggering of this large Vulcanian event. Two weeks prior to the 14 July event, daily seismic counts and radial tilt began to steadily increase, indicating the probable intrusion of a new batch of magma into the edifice. The 14 July explosion produced three different juvenile products that were each sampled for this study: airfall; juvenile, vesicular, pyroclastic density current material; and dense plug rocks in the form of ballistic ejecta. Feldspar microlite textures and vesicle size distributions were measured, and used jointly with a two-step recompression model to characterize the spatial distribution of gas in the conduit and the decompression histories of the erupted samples. Model results reveal a vertically stratified conduit with regards to porosity, crystallinity and volatile content prior to the explosion. Overall, our data suggests a complex sequence of events that led eventually to this powerful explosion. 1) Remnants of magma from the previous eruptive phase in May stalled in the shallow conduit, triggering crystallization which, coupled with efficient outgassing, formed a dense (< 2% porosity), highly crystalline plug. This plug had a very low matrix permeability (10−17 to 10−18 m2) and high tensile strength (9 to 13 MPa), forming an efficient rigid seal and preventing significant outgassing from the conduit. 2) Just below the plug, a high porosity zone (up to 50%) formed, acting as a gas storage zone, with pressurization occurring partly under closed-system degassing. 3) The shallow conduit became pressurized due to the combination of both gas accumulation and the resistance of the plug to magma column extrusion in response to a new influx of magma. 4) On the 14th of July a critical gas over-pressure was reached, overcoming the strength of the dense plug of magma, trigging extreme decompression and evacuation of the top two kilometres of the conduit. 5) The newly intruded magma was not directly involved in the explosion and continued to ascend during the week following the explosion.

Geochemical and textural constraints on degassing processes in sub-Plinian eruptions: case-study of the Greenish Pumice eruption of Mount Somma-Vesuvius

Plinian eruptions are characterized by high intensity and an overall steady character, and result in a stable convective column. The main processes controlling the dynamics of such steady and stable plume systems have been extensively investigated. Conversely, sub-Plinian eruptions are unsteady, as recorded by the large variability of the products and deposits. Our knowledge of the processes creating this unsteadiness on various timescales remains limited, and still requires more observations as well as theoretical and experimental investigation. Here, we focus on the sub-Plinian eruption of the Greenish Pumice (GP, 19,265 ± 105 BP), Mt. Somma-Vesuvius (Italy). On the basis of coupled geochemical and textural analyses of samples from the well-established stratigraphy of the GP deposits, we investigate volatiles (H2O, CO2, F, Cl) to better constrain the unsteady sub-Plinian eruptive style. This allows us to carry out a detailed study of the degassing processes in relation to the eruption dynamics. We find that degassing by open-system processes generally dominates throughout the entire eruption, but alternates with episodes of closed-system degassing. The fluctuating degassing regimes, responsible for the variable magma ascent rate within the conduit, are also responsible for the eruptive column instability. Volatile behavior is well correlated with textural heterogeneities of the eruptive products. Both reflect higher conduit heterogeneity than for Plinian eruptions, where we find a higher horizontal gradient in magma ascent velocity due to a smaller conduit diameter.

Experimental and petrological constraints on long-term magma dynamics and post-climactic eruptions at the Cerro Galán caldera system, NW Argentina

Cerro Galán in NW Argentina records >3.5Myr of magmatic evolution of a major resurgent caldera complex. Beginning at 5.72Ma, nine rhyodacitic ignimbrites (68–71wt% SiO2) with a combined minimum volume of >1200km3 (Dense Rock Equivalent; DRE) have been erupted. The youngest of those ignimbrites is the eponymous, geochemically homogenous, caldera-forming 2.08±0.02Ma Cerro Galán Ignimbrite (CGI; >630km3 DRE). Following this climactic supereruption, structural and magmatic resurgence led to the formation of a resurgent dome and post-climactic lava domes and their associated pyroclastic deposits. A clear transition from amphibole to sanidine-bearing magmas occurred during the evolution of Cerro Galán and is inferred to represent a shallowing of the magma system. We test this hypothesis here using experimental phase equilibria.We conducted a series of phase equilibria experiments on the post-climactic dome lithologies under H2O-saturated conditions using cold seal Waspaloy pressure vessels with an intrinsic log fO2 of NNO+1±0.5 across a temperature-pressure range of 750–900°C and 50–200MPa (PH2O=Ptotal), respectively. Petrologic and geochemical analysis of the post-climactic lithologies shows that the natural phase assemblage (plagioclase+quartz+biotite+sanidine+Fe-Ti oxides±apatite±zircon) is stable at <50MPa (PH2O) and 805–815°C. Applying experimental results to the CGI pumice, which has the same phenocryst phase assemblage and modal abundance, whole rock and phenocryst chemistry, and overlapping temperature and fO2 as the post-climactic deposits, suggests that these pre-eruptive conditions (PH2O<50MPa) are relevant for the magmas that sourced the climactic CGI supereruption as well.Amphibole in the early Cerro Galán ignimbrites (Toconquis Group; 5.72–4.51Ma, and the Cueva Negra Ignimbrite, 3.77±0.08Ma) records crystallization across a range of pressures (500 to 200±60MPa). In the interval between the eruption of the Cueva Negra ignimbrite and the CGI (2.08±0.02Ma) the complex magma system shallowed and stalled at low pressures (<100MPa), resulting in a more simple magma reservoir configuration represented by a large-volume, geochemically homogenous magma body. The shallowing of the Cerro Galán magma system during this time explains the marked transition from amphibole to sanidine-bearing magmas and seems to characterize many large silicic caldera-forming magma systems that erupt over million year timescales to generate long-lived volcanic complexes.The post-climactic history of Cerro Galán is informed through a detailed investigation of the textural differences among the post-climactic dome lithologies, and a comparison of those textures with previously published decompression experiments. These suggest that the highly vesiculated, pumiceous clasts with rare microlites represent magma stored within the core of the lava dome that decompressed relatively rapidly (0.003–0.0003MPas−1) and evolved via closed system degassing. Resulting over-pressure of the dome may have triggered superficial explosion. In contrast, dense clasts with abundant crystalline silica precipitates represent more typical dome-forming magmas that decompressed more slowly (<0.00005MPas−1), evolved via open system degassing, and form the outer carapace of a lava dome. Integrating decompression histories with results from new phase equilibria experiments suggests that during post-climactic volcanic activity at Cerro Galán, remnant CGI dome-forming magmas ascended from the shallow magma reservoir (<4km) to motivate resurgent uplift and erupt as lava domes either explosively as vesiculated clasts or effusively as dense clasts that make up the outer structure of lava domes.

Pre- and syn-eruptive degassing and crystallisation processes of the 2010 and 2006 eruptions of Merapi volcano, Indonesia

The 2010 eruption of Merapi (VEI 4) was the volcano’s largest since 1872. In contrast to the prolonged and effusive dome-forming eruptions typical of Merapi’s recent activity, the 2010 eruption began explosively, before a new dome was rapidly emplaced. This new dome was subsequently destroyed by explosions, generating pyroclastic density currents (PDCs), predominantly consisting of dark coloured, dense blocks of basaltic andesite dome lava. A shift towards open-vent conditions in the later stages of the eruption culminated in multiple explosions and the generation of PDCs with conspicuous grey scoria and white pumice clasts resulting from sub-plinian convective column collapse. This paper presents geochemical data for melt inclusions and their clinopyroxene hosts extracted from dense dome lava, grey scoria and white pumice generated during the peak of the 2010 eruption. These are compared with clinopyroxene-hosted melt inclusions from scoriaceous dome fragments from the prolonged dome-forming 2006 eruption, to elucidate any relationship between pre-eruptive degassing and crystallisation processes and eruptive style. Secondary ion mass spectrometry analysis of volatiles (H2O, CO2) and light lithophile elements (Li, B, Be) is augmented by electron microprobe analysis of major elements and volatiles (Cl, S, F) in melt inclusions and groundmass glass. Geobarometric analysis shows that the clinopyroxene phenocrysts crystallised at depths of up to 20 km, with the greatest calculated depths associated with phenocrysts from the white pumice. Based on their volatile contents, melt inclusions have re-equilibrated during shallower storage and/or ascent, at depths of ~0.6–9.7 km, where the Merapi magma system is interpreted to be highly interconnected and not formed of discrete magma reservoirs. Melt inclusions enriched in Li show uniform “buffered” Cl concentrations, indicating the presence of an exsolved brine phase. Boron-enriched inclusions also support the presence of a brine phase, which helped to stabilise B in the melt. Calculations based on S concentrations in melt inclusions and groundmass glass require a degassing melt volume of 0.36 km3 in order to produce the mass of SO2 emitted during the 2010 eruption. This volume is approximately an order of magnitude higher than the erupted magma (DRE) volume. The transition between the contrasting eruptive styles in 2010 and 2006 is linked to changes in magmatic flux and changes in degassing style, with the explosive activity in 2010 driven by an influx of deep magma, which overwhelmed the shallower magma system and ascended rapidly, accompanied by closed-system degassing.

Explosive origin of silicic lava: Textural and [formula omitted]–H2O evidence for pyroclastic degassing during rhyolite effusion

A long-standing challenge in volcanology is to explain why explosive eruptions of silicic magma give way to lava. A widely cited idea is that the explosive-to-effusive transition manifests a two-stage degassing history whereby lava is the product of non-explosive, open-system gas release following initial explosive, closed-system degassing. Direct observations of rhyolite eruptions indicate that effusive rhyolites are in fact highly explosive, as they erupt simultaneously with violent volcanic blasts and pyroclastic fountains for months from a common vent. This explosive and effusive overlap suggests that pyroclastic processes play a key role in rendering silicic magma sufficiently degassed to generate lava. Here we use precise H-isotope and magmatic H2O measurements and textural evidence to demonstrate that effusion results from explosion(s)—lavas are the direct product of brittle deformation that fosters batched degassing into transient pyroclastic channels (tuffisites) that repetitively and explosively vent from effusing lava. Our measurements show, specifically that D/H ratios and H2O contents of a broad suite of explosive and effusive samples from Chaitén volcano (hydrous bombs, Plinian pyroclasts, tuffisite veins, and lava) define a single and continuous degassing trend that links wet explosive pyroclasts (∼1.6 wt.% H2O, δD=−76.4‰) to dry obsidian lavas (∼0.13 wt.% H2O, δD=−145.7‰). This geochemical pattern is best fit with batched degassing model that comprises small repeated closed-system degassing steps followed by pulses of vapour extraction. This degassing mechanism is made possible by the action of tuffisite veins, which, by tapping already vesicular or brecciated magma, allow batches of exsolved gas to rapidly and explosively escape from relatively isolated closed-system domains and large tracts of conduit magma by giving them long-range connectivity. Even though tuffisite veins render magma degassed and capable of effusing, they are nonetheless the avenues of violent gas and particle transport and thus have the potential to drive explosions when they become blocked or welded shut. Thus the effusion of silicic lava, traditionally thought to be relatively benign process, presents a particularly hazardous form of explosive volcanism.

Explosive subglacial rhyolitic eruptions in Iceland are fuelled by high magmatic H2O and closed-system degassing

Research Article| February 01, 2013 Explosive subglacial rhyolitic eruptions in Iceland are fuelled by high magmatic H2O and closed-system degassing Jacqueline Owen; Jacqueline Owen 1Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK Search for other works by this author on: GSW Google Scholar Hugh Tuffen; Hugh Tuffen 1Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK Search for other works by this author on: GSW Google Scholar David W. McGarvie David W. McGarvie 2Department of Environment, Earth and Ecosystems, The Open University, Milton Keynes MK7 6AA, UK Search for other works by this author on: GSW Google Scholar Author and Article Information Jacqueline Owen 1Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK Hugh Tuffen 1Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK David W. McGarvie 2Department of Environment, Earth and Ecosystems, The Open University, Milton Keynes MK7 6AA, UK Publisher: Geological Society of America Received: 18 May 2012 Revision Received: 13 Aug 2012 Accepted: 12 Sep 2012 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2013 Geological Society of America Geology (2013) 41 (2): 251–254. https://doi.org/10.1130/G33647.1 Article history Received: 18 May 2012 Revision Received: 13 Aug 2012 Accepted: 12 Sep 2012 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jacqueline Owen, Hugh Tuffen, David W. McGarvie; Explosive subglacial rhyolitic eruptions in Iceland are fuelled by high magmatic H2O and closed-system degassing. Geology 2013;; 41 (2): 251–254. doi: https://doi.org/10.1130/G33647.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Rhyolitic eruptions beneath Icelandic glaciers can be highly explosive, as demonstrated by Quaternary tephra layers dispersed throughout northern Europe. However, they can also be small and effusive. A subglacial rhyolitic eruption has never been observed, so behavioral controls remain poorly understood and the influence of pre-eruptive volatile contents is unknown. We have therefore used secondary ion mass spectrometry to characterize pre-eruptive volatile contents and degassing paths for five subglacial rhyolitic edifices within the Torfajökull central volcano, formed in contrasting styles of eruption under ice ∼400 m thick. This includes the products of the largest known eruption of Icelandic subglacial rhyolite of ∼16 km3 at ca. 70 ka. We find pre-eruptive water contents in melt inclusions (H2OMI) of up to 4.8 wt%, which indicates that Icelandic rhyolite can be significantly more volatile-rich than previously thought. Our results indicate that explosive subglacial rhyolite eruptions correspond with high H2OMI, closed-system degassing, and rapid magma ascent, whereas their effusive equivalents have lower H2OMI and show open-system degassing and more sluggish ascent rates. Volatile controls on eruption style thus appear similar to those for subaerial eruptions, suggesting that ice plays a subsidiary role in influencing the behavior of subglacial rhyolitic eruptions. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Magma degassing and eruption dynamics of the Avellino pumice Plinian eruption of Somma–Vesuvius (Italy). Comparison with the Pompeii eruption

The eruptive history of Mt. Somma–Vesuvius is characterised by large explosive events: Pomici di Base eruption (22,030±175yrcal BP), Mercato (8890±90yrcal BP), Avellino (3945±10yrcal BP) and Pompeii (79AD). Pre-eruptive conditions and sin-eruptive degassing processes of the Avellino eruption, the highest-magnitude Plinian event, have been investigated, using volatile contents (F, Cl, H2O) in melt inclusions and residual glass, and textural characteristics of pumice clasts of the 9 fallout layers sampled in detail in a representative sequence. The sequence displays an up-section sharp colour change from white to grey, corresponding to variations in both magma composition and textural characteristics. The pre-eruptive conditions have been constrained by systematic measurements of Cl content in both melt inclusions and matrix glass of pumice clasts. The pumice glass composition varies from Na-rich phonolite (white pumice) to K-rich phonolite (grey pumice). The measured Cl values constantly cluster at 5200±400ppm (buffer value), whatever the composition of the melt, suggesting that the entire magma body was saturated with sub-critical fluids. This Cl saturation constrains the pre-eruptive pressures and maximum H2O contents at 200±10MPa and 6.3±0.2wt.% H2O for the white pumice melt and 195±15MPa and 5.2±0.2wt.% H2O for the grey pumice melt. The fluid phase, mainly composed of a H2O-rich vapour phase and brine, probably accumulated at the top of the reservoir and generated an overpressure able to trigger the onset of the eruption. Magma degassing was rather homogeneous for the white and grey eruptive units, mostly occurring through closed-system processes, leading to a typical Plinian eruptive style. A steady-state withdrawal of an H2O-saturated magma may explain the establishment of a sustained Plinian column. Variation from white to grey pumice is accompanied by decrease of mean vesicularity and increase of mean microcrystallinity and permeability related to significant vesicle coalescence. Despite this, the ascending magma column still evolves under closed-system degassing, without significant gas loss through conduit walls. The Avellino eruption shows numerous similarities with the 79AD Pompeii eruption in pre-eruptive conditions, degassing processes and eruptive style which are discussed here.

The melt inclusion record from the rhyolitic Kos Plateau Tuff (Aegean Arc)

The >60 km3 rhyolitic Kos Plateau Tuff provides an exceptional probe into the behavior of volatile components in highly evolved arc magmas: it is crystal-rich (30–40 vol% crystals), was rapidly quenched by the explosive eruptive process, and contains abundant homogeneous melt inclusions in large quartz crystals. Several methods for measuring major, trace and volatile element concentrations (SIMS, FTIR, Raman spectroscopy, electron microprobe, LA–ICPMS) were applied to these melt inclusions. We found a ~2 wt% range of H2O contents (4.5–6.5 wt% H2O, measured independently by SIMS, FTIR, and Raman spectroscopy) and relatively low CO2 concentrations (15–140 ppm measured by FTIR, with most analyses <100 ppm). No obvious correlations between H2O, CO2, major and trace elements are observed. These observations require a complex, protracted magma evolution in the upper crust that included: (1) vapor-saturated crystallization in a chamber located between 1.5 and 2.5 kb pressure, (2) closed-system degassing (with up to 10 vol% exsolved gas) as melts percolated upwards through a vertically extensive mush zone (2–4 km thick), and (3) periodic gas fluxing from subjacent, more mafic and more CO2-rich magma, which is preserved as andesite bands in pumices. These processes can account for the range of observed H2O and CO2 values and the lack of correlation between volatiles and trace elements in the melt inclusions.

Sulfur dioxide emissions and degassing behavior of Erebus volcano, Antarctica

Emission rates of sulfur dioxide (SO 2) were measured at Erebus volcano, Antarctica in December between 1992 and 2005. Since 1992 SO 2 emissions rates are normally distributed with a mean of 61 ± 27 Mg d − 1 (0.7 ± 0.3 kg s − 1 ) ( n = 8064). The emission rates vary over minutes, hours, days and years. Hourly and daily variations often show systematic and cyclic trends. Long-wavelength, large amplitude trends appear related to lava lake area and both are likely controlled by processes occurring at depth. Time series analysis of continuous sequences of measurements obtained over periods of several hours reveals periodicity in SO 2 output ranging from 10 to 360 min, with a 10 min cycle being the most dominant. Closed and open-system degassing models are considered to explain observed variable degassing rates. Closed-system degassing is possible as rheological stiffening and stick/slip may occur within the system. However, the timescales represented in these models do not fit observations made on Erebus. Open-system degassing and convection fits the observations collected as the presented models were developed for a system similar to Erebus in terms of degassing, eruptive activity and process repose time. We show that with the observed emission rate (0.71 kg s − 1 ) and a crystal content of 30%, magma will cool 65 °C to match observed heat fluxes; this cooling is sufficient enough to drive convection.

Andesitic magma degassing investigated through H 2O vapour–melt partitioning of halogens at Soufrière Hills Volcano, Montserrat (Lesser Antilles)

Magma degassing at Soufrière Hills Volcano (SHV) is characterised by an almost permanent SO 2 flux and a HCl production rate which mainly depends on dome growth rate. Degassing processes have been studied through textural, H 2O and halogen analyses of clasts collected between 1995 and 2006 on the dome and in pyroclastic flows and vulcanian eruption deposits. Cl, Br and I are strongly depleted in melts during H 2O degassing with no significant Cl–Br–I fractionation, whereas F is almost unaffected. All magmas erupted at SHV have followed a multi-step degassing path from the magma chamber up to a shallow depth (∼ 1 km, P ∼ 20 MPa). From that depth, however, effusive and explosive paths are distinct; vulcanian eruptions are the result of closed system degassing (CSD), while effusive dome growth is the result of CSD up to a very shallow depth (≤ 200 m, P ∼ 5–2 MPa) followed by open system degassing (OSD). CSD is modelled using the H 2O solubility law, the perfect gas law and partition coefficients of halogens between a rhyolitic melt and H 2O vapour ( d v − l i ). Gas loss characteristic of OSD is modelled using a Rayleigh law. Degassing induced crystallisation is introduced through the ratio of crystallisation and degassing rates, which ranges from 150–500. d v − l Cl for OSD ranges between 50–300, increasing with melt Cl content. For CSD, the lower effective d v − l Cl (∼ 20) is attributed to kinetic effects. Dome forming activity has a greater impact on atmospheric chemistry than vulcanian eruptions because OSD is much more efficient at extracting halogens. The model shows that HCl flux is a good proxy for the dome forming eruption rate. Comparison between model and measured gas compositions suggests a high HBr–BrO conversion rate (BrO/Total Br ∼ 1/3) in the SHV gas plume. The degassing behaviour of Cl, Br and I implies similar Cl/Br (∼ 160) and Br/I (∼ 90) in initial melts, volcanic clasts and high temperature gases. The low Cl/Br at SHV compared to other island arcs (∼ 250–300) is attributed to a shallow, pre-eruptive Br enrichment. The almost permanent dome extrusion at SHV since 1995 has likely had a significant regional atmospheric impact because of the very efficient effusive degassing and the high conversion rate of halogens into reactive species within the gas plume.

Breadcrust bombs as indicators of Vulcanian eruption dynamics at Guagua Pichincha volcano, Ecuador

Vulcanian eruptions are common at many volcanoes around the world. Vulcanian activity occurs as either isolated sequences of eruptions or as precursors to sustained explosive events and is interpreted as clearing of shallow plugs from volcanic conduits. Breadcrust bombs characteristic of Vulcanian eruptions represent samples of different parts of these plugs and preserve information that can be used to infer parameters of pre-eruption magma ascent. The morphology and preserved volatile contents of breadcrust bombs erupted in 1999 from Guagua Pichincha volcano, Ecuador, thus allow us to constrain the physical processes responsible for Vulcanian eruption sequences of this volcano. Morphologically, breadcrust bombs differ in the thickness of glassy surface rinds and in the orientation and density of crack networks. Thick rinds fracture to create deep, widely spaced cracks that form large rectangular domains of surface crust. In contrast, thin rinds form polygonal networks of closely spaced shallow cracks. Rind thickness, in turn, is inversely correlated with matrix glass water content in the rind. Assuming that all rinds cooled at the same rate, this correlation suggests increasing bubble nucleation delay times with decreasing pre-fragmentation water content of the melt. A critical bubble nucleation threshold of 0.4-0.9 wt% water exists, below which bubble nucleation does not occur and resultant bombs are dense. At pre-fragmentation melt H2O contents of >~0.9 wt%, only glassy rinds are dense and bomb interiors vesiculate after fragmentation. For matrix glass H2O contents of ≥1.4 wt%, rinds are thin and vesicular instead of thick and non-vesicular. A maximum measured H2O content of 3.1 wt% establishes the maxi- mum pressure (63 MPa) and depth (2.5 km) of magma that may have been tapped during a single eruptive event. More common H2O contents of ≤1.5 wt% suggest that most eruptions involved evacuation of ≤1.5 km of the conduit. As we expect that substantial overpressures existed in the conduit prior to eruption, these depth estimates based on magmastatic pressure are maxima. Moreover, the presence of measurable CO2 (≤17 ppm) in quenched glass of highly degassed magma is inconsistent with simple models of either open- or closed-system degassing, and leads us instead to suggest re-equilibration of the melt with gas derived from a deeper magmatic source. Together, these observations suggest a model for the repeated Vulcanian eruptions that includes (1) evacuation of the shallow conduit during an individual eruption, (2) depressurization of magma remaining in the conduit accompanied by open- system degassing through permeable bubble networks, (3) rapid conduit re-filling, and (4) dome formation prior to the subsequent explosion. An important part of this process is densification of upper conduit magma to allow repressur- ization between explosions. At a critical overpressure, trapped pressurized gas fragments the nascent impermeable cap to repeat the process.

Degassing at the Soufriere Hills Volcano, Montserrat, Recorded in Matrix Glass Compositions

Compositions of matrix glasses from the current eruption of Soufrière Hills Volcano indicate that decompression-driven crystallization results in 20–70 wt % groundmass crystallization during eruption and variable degassing. Variations in crystallinity and volatile contents (water and chlorine) of matrix glasses are attributed to variations in extrusion rates and residence times in the lava dome. Residual water contents in pumice clasts (0·2–0·6 wt %) indicate minimum pressures of 1·1–3·7 MPa in 1997 Vulcanian explosions. Residual water contents of 1·6 wt % in a ballistic block ejected in sub-Plinian explosive activity on 17 September 1996 imply larger pressure drops (∼20 MPa). Variable residual water contents in dome samples are consistent with pressure variations of up to 9 MPa in the lava dome interior. Large variation in chlorine contents between lava blocks compared with explosion pumice clasts indicates that shallow-level processes dominate degassing. Low melt chlorine contents of dome samples cannot be explained by open- or closed-system degassing, especially when crystallization is taken into account. Instead, heterogeneous chlorine leaching by circulation of groundwater vapour in the dome is proposed. Variable and elevated matrix glass δD values can also be attributed to interaction with isotopically heavy surface waters. HCl emission during the current eruption can be accounted for by Cl loss from the melt, consistent with the melt being undersaturated in chlorine.

Volatiles in Alkalic Basalts form the North Arch Volcanic Field, Hawaii: Extensive Degassing of Deep Submarine-erupted Alkalic Series Lavas

The North Arch volcanic field is a submarine suite of alkali basaltic to nephelinitic lavas of the seafloor north of Oahu at water depths of 3900–4380 m. Glasses from these lavas were analyzed for H2O, CO2, Cl, S, Fe3+/∑Fe, and noble gases to investigate the role of volatiles in the generation, evolution, and degassing of these alkalic series lavas. In contrast to the systemic negative correlation between concentrations of SiO2 and nonvolatile incompatible elements (e.g. P2O5), the behavior of the volatile components is much more irregular. Concentrations of H2O in glasses vary by a factor of two ( 0.69–1.42 wt %) and show a poor correlation with melt composition, whereas concentrations of dissolved CO2 in glasses (260–800 p.p.m) increase with increasing alkalinity of the glasses. The H2O2 concentrations in the glasses are in equilibrium with an H2O–CO2 vapor at the depth of eruption (400 bar pressure). Samples collected directly from vent structures are highly vesicular, suggesting that these samples were gas rich upon eruption. Estimated bulk volatile contents of the two most vesicular vent samples are high (1.9±0.1 wt % H2O and 5.4±0.4 CO2) and are interpreted to have formed by closed system degassing. Estimated bulk volatile contents in four other vesicular vent samples are lower 1.3±0.2 wt % H2O and 2.0±0.4 wt % CO2), and these samples are interpreted to have lost some gas during eruption. Glass samples from inflated, flat lava flows are nonvesicular and interpreted to have lost essentially all exsolved gas during eruption and flow. Forward degassing models can predict the observed range in dissolved H2O and CO2 contents, calculated vapor compositions, and vesicularity as a function of SiO2. The models involve open to closed system degassing of an H2O–CO2 vapor phase from melts initially having H2O/P2O5=3 and CO2H2O=1–4 mass. Cl concentrations (400–1360 p.p.m) in glasses correlate with concentrations of nonvolatile, incompatible elements. Concentrations of noble gases measured on bulk glass samples are low compared with mid-oceanic ridge basalt (MORB). The low concentrations result mainly from extensive vapor exsolution from the magma. The helium isotopic ratios for gases released from vesicles are similar to MORB values [6.8–8.5 times the air ratio (RA)], whereas those released from glasses are lower than MORB values as a result of in situ decay of U and Th. The S contents (0.11–0.22 wt %) of most of the alkali olivine basaltic and basanitic glasses are sufficient to saturate the silicate melt with immiscible Fe–S–O liquid at the T and P of eruption and quenching. However, two vesicular samples appear to hae lost some dissolved S owing to eruptive degassing. Magmatic oxygen fugacitites estimated from Fe3+/∑Fe range from ΔFMQ=-0.8 to +0.7, with the nephelinitic glasses being more oxidizing than the less alkalic glasses. We infer that the mantle source region for the North Arch magmas was homogeneous with respect to Fe3+/∑Fe and that melting occurred in the absence of graphite or CH4- rich fluid. The effect of variable partial melting on magmatic oxygen fugacity may be a common feature of Hawaiian volcanism. These complex data pint to a simple result, namely that parental magma compositions can be derived by variable extents of melting of a homogeneous source followed by olivine crystallization and degassing at 400 bar. If the parental liquids are produced by 1.6–9.0% partial melting (±20% relative), then mantle volatile contents are estimated to be 525±75 p.p.m. H2O, 1300±800 p.p.m. CO2 and 30± p.p.m. Cl.

Volatile characteristics of peralkaline rhyolites from Kenya: an ion microprobe, infrared spectroscopic and hydrogen isotope study

Peralkaline rhyolites of the Greater Olkaria Volcanic Complex, Kenya Rift Valley, were derived from separated, though closely related, magma chambers. Ion microprobe analyses of glass inclusions in quartz phenocrysts show pre-eruptive water contents of up to 3.4 wt% contrasting with previous estimates that the magmas were anhydrous. The values approach predicted solubility levels corresponding to water saturation at low crustal pressures (1 kbar). The glass matrices of the rhyolites have low water contents, ranging from 0.07 to 0.46 wt%, suggesting significant degassing during, or prior to, eruption. Infrared measurements of the matrix glasses show variation in the relative proportions of the two hydrous species dissolved in the glasses. The amount of molecular water, determined semi-quantitatively, apparently increases with increased fluorine content and peralkalinity. This suggests a competition between hydroxyl groups and fluoride ions for similar sites within the melt structure. The mechanism of degassing has been investigated using hydrogen isotopes. The range of δD values in most rocks can be produced by varied degrees of open-system degassing of rhyolite melt initially in equilibrium with water of a fixed, or limited, δD value. There is evidence to suggest that closed-system degassing may also have been a significant component in some rhyolites. The exact mechanisms of degassing remain uncertain. Particular problems include the relative contribution of open-and closed-system degassing during eruption and the initial vapour compositions and solubility relationships.