Abstract
The global carbon dioxide (CO
2
) flux from subaerial volcanoes remains poorly quantified, limiting our understanding of the deep carbon cycle during geologic time and in modern Earth. Past ...attempts to extrapolate the global volcanic CO
2
flux have been biased by observations being available for a relatively small number of accessible volcanoes. Here, we propose that the strong, but yet unmeasured, CO
2
emissions from several remote degassing volcanoes worldwide can be predicted using regional/global relationships between the CO
2
/S
T
ratio of volcanic gases and whole-rock trace element compositions (e.g., Ba/La). From these globally linked gas/rock compositions, we predict the CO
2
/S
T
gas ratio of 34 top-degassing remote volcanoes with no available gas measurements. By scaling to volcanic SO
2
fluxes from a global catalogue, we estimate a cumulative “unmeasured” CO
2
output of 11.4 ± 1.1 Mt/yr (or 0.26 ± 0.02·10
12
mol/yr). In combination with the measured CO
2
output of 27.4 ± 3.6 Mt/yr (or 0.62 ± 0.08·10
12
mol/yr), our results constrain the time-averaged (2005–2015) cumulative CO
2
flux from the Earth’s 91 most actively degassing subaerial volcanoes at 38.7 ± 2.9 Mt/yr (or 0.88 ± 0.06·10
12
mol/yr).
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Quantifying the global volcanic CO2 output from subaerial volcanism is key for a better understanding of rates and mechanisms of carbon cycling in and out of our planet and their consequences for the ...long‐term evolution of Earth's climate over geological timescales. Although having been the focus of intense research since the early 1990s, and in spite of recent progress, the global volcanic CO2 output remains inaccurately known. Here we review past developments and recent progress and examine limits and caveats of our current understanding and challenges for future research. We show that CO2 flux measurements are today only available for ~100 volcanoes (cumulative measured flux, 44 Tg CO2/year), implying that extrapolation is required to account for the emissions of the several hundred degassing volcanoes worldwide. Recent extrapolation attempts converge to indicate that persistent degassing through active crater fumaroles and plumes releases ~53–88 Tg CO2/year, about half of which is released from the 125 most actively degassing subaerial volcanoes (36.4 ± 2.4 Tg CO2/year from strong volcanic gas emitters, Svge). The global CO2 output sustained by diffuse degassing via soils, volcanic lakes, and volcanic aquifers is even less well characterized but could be as high as 83 to 93 Tg CO2/year, rivaling that from the far more manifest crater emissions. Extrapolating these current fluxes to the past geological history of the planet is challenging and will require a new generation of models linking subduction parameters to magma and volatile (CO2) fluxes.
Key Points
Progress in determining subaerial volcanic CO2 flux has been significant
Challenges remain with regard to extrapolations through time and global coverage of measurements and with regard to diffuse tectonic degassing and dynamic nature of volcanic degassing
Volcanic and tectonic contributions are <2% of current anthropogenic contributions
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The currently available data set of S–Cl–F abundances in volcanic gas plumes and high-temperature fumarolic gas samples from basaltic volcanism is reviewed here in the attempt to derive constraints ...on the modes of halogen degassing from mafic silicate melts. Apart from large volcano-to-volcano variations, reflecting remarkable differences in volatile abundances in the source magmas, each of the explored volcanoes displays large changes of SO
2/HCl and SO
2/HF ratios with the style of volcanic activity, with HCl/HF staying fairly constant. Halogen abundances are low and SO
2/HCl and SO
2/HF are high when fresh (volatile-rich) magmas sustain degassing, as during explosive eruptions, at the onset of eruptive cycles, or shortly before paroxysmal events. Low SO
2/HCl and SO
2/HF ratios are instead characteristic of late stages of volcanic degassing, typically being observed in the concluding stages of basaltic eruptions, or during periods of reduced magma supply at persistently degassing volcanoes. These observations are taken as evidence of halogens being less keen to enter the gas phase (relative to S) during degassing of basaltic magmas; and quantitatively interpreted in light of a Rayleigh-type open-system degassing model. The model, though simple, quantitatively reproduces the range of volcanic gas compositions observed at basaltic volcanoes worldwide, and allows prediction of vapour/melt partitioning contrasts of factors ~
9 and ~
36 for the volatile couples S–Cl and S–F, respectively. These predictions require validation from appropriately designed experiments of halogen partitioning between magmatic vapours and silicate melts over a range of
P–
T–
X conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
During the reawaking of a volcano, magmas migrating through the shallow crust have to pass through hydrothermal fluids and rocks. The resulting magma-hydrothermal interactions are still poorly ...understood, which impairs the ability to interpret volcano monitoring signals and perform hazard assessments. Here we use the results of physical and volatile saturation models to demonstrate that magmatic volatiles released by decompressing magmas at a critical degassing pressure (CDP) can drive volcanic unrest towards a critical state. We show that, at the CDP, the abrupt and voluminous release of H
O-rich magmatic gases can heat hydrothermal fluids and rocks, triggering an accelerating deformation that can ultimately culminate in rock failure and eruption. We propose that magma could be approaching the CDP at Campi Flegrei, a volcano in the metropolitan area of Naples, one of the most densely inhabited areas in the world, and where accelerating deformation and heating are currently being observed.
We report on new volcanic gas composition results acquired in October 2017 at Minas de Azufre, a persistent fumarolic field topping the resurgent Sierra Negra caldera, in the Galápagos archipelago. ...Our results indicate that the Minas de Azufre fumaroles are moderately hydrous (52–64 mol.% H2O) and rich in CO2 (35–46 mol.%), with total sulfur (ST) being 21–35 times less abundant than CO2. SO2, the most abundant S species, is released at an average rate of 19 ± 9 tons/day. Using a volatile saturation model that provides the composition of magmatic gases at equilibrium with western Galápagos basaltic melt (48 wt. % SiO2) in the 400–0.1 MPa pressure range, we infer that Minas de Azufre fumarolic emissions consist of a mixture of (a) magma‐derived gases coexisting with a melt at ∼50–60 MPa and (b) shallow meteoric water. We thus propose that the fumaroles are supplied by outgassing of magma stored in a ∼2 km deep sill‐like reservoir underneath the caldera floor, and that the trapdoor fault system at the western margin of the resurgent caldera block acts as a preferential pathway for magmatic gas ascent and surface discharge. Our results thus suggest that, in contrast to the majority of the volcano‐hosted hydrothermal systems worldwide, Minas de Azufre releases a relatively pristine magmatic gas.
Plain Language Summary
Magmatic gas released by intraplate, hot‐spot related volcanism can offer insight into the abundance and distribution of volatiles in the Earth's upper mantle. Unfortunately however, the available data set for hot spot magmatic gases is sparse and incomplete, due to relatively infrequent eruptions and the remote location of many hot‐spot volcanoes. Here, we present novel information for the gas chemistry and emission rate at the Minas de Azufre fumarolic field, the most active persistent gas manifestation on Sierra Negra volcano, in the western part of the Galápagos hotspot. We interpret our gas observations in tandem with results of a volatile saturation model that calculates the equilibrium composition of magmatic gases coexisting with basaltic melt under P‐T conditions relevant to Galápagos volcanism. From this comparison, we conclude that the Minas de Azufre fumarolic emissions are fed by degassing of basaltic magma stored in a ∼2 km deep sill underneath the Sierra Negra caldera floor, and that the trapdoor fault system at the western margin of the resurgent caldera block acts as a preferential pathway for magmatic gas leakage and surface discharge. Our results are relevant to a better understanding of Sierra Negra volcano and contribute to extending the volcanic gas catalog for hot‐spot volcanism.
Key Points
The composition and mass flux of volcanic gases from Minas de Azufre, in the Sierra Negra Caldera (Galápagos), are determined
Fluids are interpreted as prevalently derived from a shallow (∼2 km deep) magmatic sill underneath the caldera floor
Trapdoor faults at the resurgent caldera block's margin favor magmatic gas leakage and surface discharge
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The Canary Islands, in the eastern Atlantic, are among the most enigmatic Oceanic Island provinces on Earth, as the mantle source feeding its volcanism exhibits wide spatial heterogeneity and a ...multiplicity of sources. Multi-isotope whole-rock studies have long revealed the presence of a recycled oceanic crust/lithosphere component in the mantle source. However, noble gas systematics have been more challenging to interpret, and the available carbon isotope data is limited and cannot support/dismiss this interpretation. Here, we present the very first isotopic characterisation of CO2 and noble gases (He-Ne-Ar) in fluid inclusions (FI) in minerals hosted in mantle xenoliths from El Hierro, the youngest and westernmost island of the Canary volcanic archipelago. Six fresh xenoliths from El Julan cliff valley were analysed (3 spinel lherzolites and 3 spinel harzburgites). We find carbon isotopic compositions of CO2 in FI (δ13C) ranging from −2.38 to −1.23‰ in pyroxenes and from −0.19 to +0.96‰ in olivines. These unusually positive δ13C values, well above the typical mantle range (−8‰ < δ13C < −4‰), prove, for the first time, the presence of a recycled crustal carbon component in the local source mantle. We interpret this 13C-rich component as inherited from a mantle metasomatism event driven by fluids carrying carbon from C. In contrast, our El Hierro xenoliths identify a depleted mantle-like He signature, with an average Rc/Ra ratio (3He/4He normalised to air ratio and corrected for atmospheric contamination) of 7.45 ± 0.26 Ra. The involvement of depleted mantle-like fluids, variably admixed with air-derived components (possibly recycled via paleo-subduction event(s)), is corroborated by Ne-Ar isotopic compositions. The depleted mantle-like He signature suggests instead the involvement of a primordial He source in the local lithospheric mantle and indicates a marginal role played by past subduction events in modifying the local mantle He budget. When put in the context of previous 3He/4He measurements in FI and surface gases along the Canary archipelago, our results confirm an overall west-to-east decrease of Rc/Ra ratios, which may be interpreted as due to increasing contributions from the African sub-continental mantle, the addition of radiogenic 4He during magma migration in the oceanic crust (whose thickness increases eastward) and/or magma ageing.
•δ13C in fluid inclusions tracks recycled crustal carbon in El Hierro mantle source.•Crustal carbon may derive from subducted altered oceanic crust or lithosphere.•Noble gases indicate the MORB-like signature of the mantle beneath El Hierro.•Crustal contribution concurs to the west-to-east 3He/4He decrease in the Canary.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
We present here the first volcanic gas compositional time‐series taken prior to a paroxysmal eruption of Villarrica volcano (Chile). Our gas plume observations were obtained using a fully autonomous ...Multi‐component Gas Analyser System (Multi‐GAS) in the 3 month‐long phase of escalating volcanic activity that culminated into the 3 March 2015 paroxysm, the largest since 1985. Our results demonstrate a temporal evolution of volcanic plume composition, from low CO2/SO2 ratios (0.65‐2.7) during November 2014‐January 2015 to CO2/SO2 ratios up to ≈ 9 then after. The H2O/CO2 ratio simultaneously declined to <38 in the same temporal interval. We use results of volatile saturation models to demonstrate that this evolution toward CO2‐enriched gas was likely caused by unusual supply of deeply sourced gas bubbles. We propose that separate ascent of over‐pressured gas bubbles, originating from at least 20‐35 MPa pressures, was the driver for activity escalation toward the 3 March climax.
Key Points
We present the first volcanic gas compositional time‐series taken prior to a paroxysmal eruption of Villarrica volcano (Chile)
We find evidence for a gas CO2/SO2 ratio precursor to eruption of a carbon‐poor arc magma
We interpret preeruptive evolution toward CO2‐enriched gas as caused by supply of deeply sourced gas bubbles
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Large Igneous Provinces (LIPs) facilitate massive transfers of CO2 and other volatiles from the mantle to atmosphere, contributing to past global warming and environmental disruption. However, the ...scale and evolution of magmatic CO2 fluxes during these events remain uncertain due to the tendency of CO2 to degas deep in magmatic systems. Here we estimate LIP CO2 using an approach based on an observed correlation between gas CO2/S ratios and trace elements in volcanic rocks. We apply this method to a compilation of published geochemical data for tholeiitic LIP lavas and to a new major and trace element dataset for alkaline rocks from the Siberian Traps. Our results indicate that CO2/S and therefore CO2 in tholeiitic and alkaline magma suites from LIPs span 1–2 orders of magnitude, emphasizing that changing CO2 concentrations can combine with magma flux to drive strong variations in CO2 release through the evolution of LIP magmatism.
Measuring the carbon stable isotope ratio (13C/12C, expressed as δ13CCO2) in geogenic CO2 fluids is a crucial geochemical tool for studying Earth's degassing. Carbon stable isotope analysis is ...traditionally performed by bulk mass spectrometry. Although Raman spectroscopy distinguishes 12CO2 and 13CO2 isotopologue bands in spectra, using this technique to determine CO2 isotopic signature has been challenging. Here, we report on in-situ non-destructive analyses of the C stable isotopic composition of CO2, applying a novel high-resolution Raman configuration on 42 high-density CO2 fluid inclusions in mantle rocks from the Lake Tana region (Ethiopia) and El Hierro (Canary Islands). We collected two sets of three spectra with different acquisition times at high spectral resolution in each fluid inclusion. Among the 84 sets of spectra, 58 were characterised by integrated 13CO2/12CO2 band area ratios with reproducibility better than 4‰. Our results demonstrate the determination of δ13CCO2 by Raman spectroscopy in individual fluid inclusions with an error better than 2.5 ‰, which satisfactorily matches bulk mass spectrometry analyses in the same rock samples, supporting the accuracy of the measurements. We thus show that Raman Spectroscopy can provide a fundamental methodology for non-destructive, site-specific, and spatially resolved carbon isotope labelling at the microscale.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
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