Hydrogen and hydrogen sulphide in volcanic gases: abundance, processes, and atmospheric fluxes
Hydrogen (H$_{2}$) and hydrogen sulphide (H$_{2}$S) are typically present at only minor to trace levels in volcanic gas emissions, and yet they occupy a key role in volcanic degassing research in view of the control they exert on volcanic gas reducing capacity (e.g., their ability to remove atmosphe...
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Académie des sciences
2023-09-01
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| Online Access: | https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.5802/crgeos.235/ |
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| author | Aiuppa, Alessandro Moussallam, Yves |
| author_facet | Aiuppa, Alessandro Moussallam, Yves |
| author_sort | Aiuppa, Alessandro |
| collection | DOAJ |
| description | Hydrogen (H$_{2}$) and hydrogen sulphide (H$_{2}$S) are typically present at only minor to trace levels in volcanic gas emissions, and yet they occupy a key role in volcanic degassing research in view of the control they exert on volcanic gas reducing capacity (e.g., their ability to remove atmospheric O$_{2}$). In combination with other major compounds, H$_{2}$ and H$_{2}$S are also key to extracting information on source magma conditions (temperature and redox) from observed magmatic gas compositions. Here, we use a catalogue, compiled by extracting from the geological literature a selection of representative analyses of magmatic to mixed (magmatic–hydrothermal) gases, to review the processes that control H$_{2}$ and H$_{2}$S abundance in volcanic gases. We show that H$_{2}$ concentrations and H$_{2}$/H$_{2}$O ratios in volcanic gases both exhibit strong positive temperature dependences, while H$_{2}$S concentrations and H$_{2}$S/SO$_{2}$ ratios are temperature insensitive overall. The high H$_{2}$ concentrations (and low H$_{2}$S/SO$_{2}$ compositions, of ${\sim }$0.1 on average) in high-temperature (${>}$1000 °C) magmatic gases are overall consistent with those predicted thermodynamically assuming external redox buffering operated by the coexisting silicate melt, at oxygen fugacities ranging from ${\Delta }$FMQ $-$1 to 0 (non-arc volcanoes) to ${\Delta }$FMQ 0 to ${+}$2 (arc volcanoes) (where ${\Delta }$FMQ is oxygen fugacity expresses as a log unit difference relative to the Fayalite–Magnetite–Quartz oxygen fugacity buffer). Lower temperature (${<}$1000 °C) volcanic gases exhibit more oxidizing redox conditions (typically above the Nickel–Nickel Oxide buffer) that are caused by a combination of (i) gas re-equilibration during closed-system (gas-phase only) adiabatic cooling in a gas-buffered system, and (ii) heterogenous (gas–mineral) reactions. We show, in particular, that gas-phase equilibrium in the H$_{2}$–H$_{2}$S–H$_{2}$O–SO$_{2}$ system is overall maintained upon cooling down to ${\sim }$600 °C, while quenching of higher temperature equilibria (at which Apparent Equilibrium Temperatures, AETs, largely exceed measured discharge temperatures) is more frequently observed for higher extents of cooling (e.g., at $T < 600$ °C). In such lower temperature volcanic environments, gas–mineral reactions also become increasingly important, scavenging magmatic SO$_{2}$ and converting it into H$_{2}$S and hydrothermal minerals (sulphates and sulphides). These heterogeneous reactions, when occurring, can also control the temperature dependence of the volcanic gas H$_{2}$/H$_{2}$O ratios. Finally, by using our volcanic gas dataset in tandem with recently published global volcanic SO$_{2}$ and CO$_{2}$ budgets, we provide refined estimates for total H$_{2}$S (median, 1.4 Tg/yr; range, 0.9–8.8 Tg/yr) and H$_{2}$ (median, 0.23 Tg/yr; range, 0.06–1 Tg/yr) fluxes from global subaerial volcanism. |
| format | Article |
| id | doaj-art-1e53895e5922429697205e96e418dc84 |
| institution | Kabale University |
| issn | 1778-7025 |
| language | English |
| publishDate | 2023-09-01 |
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| spelling | doaj-art-1e53895e5922429697205e96e418dc842025-02-07T10:41:28ZengAcadémie des sciencesComptes Rendus. Géoscience1778-70252023-09-01356S18510810.5802/crgeos.23510.5802/crgeos.235Hydrogen and hydrogen sulphide in volcanic gases: abundance, processes, and atmospheric fluxesAiuppa, Alessandro0https://orcid.org/0000-0002-0254-6539Moussallam, Yves1https://orcid.org/0000-0002-4707-8943Dipartimento di Scienze della Terra e del Mare, Università di Palermo, Palermo, ItalyLamont-Doherty Earth Observatory, Columbia University, New York, USAHydrogen (H$_{2}$) and hydrogen sulphide (H$_{2}$S) are typically present at only minor to trace levels in volcanic gas emissions, and yet they occupy a key role in volcanic degassing research in view of the control they exert on volcanic gas reducing capacity (e.g., their ability to remove atmospheric O$_{2}$). In combination with other major compounds, H$_{2}$ and H$_{2}$S are also key to extracting information on source magma conditions (temperature and redox) from observed magmatic gas compositions. Here, we use a catalogue, compiled by extracting from the geological literature a selection of representative analyses of magmatic to mixed (magmatic–hydrothermal) gases, to review the processes that control H$_{2}$ and H$_{2}$S abundance in volcanic gases. We show that H$_{2}$ concentrations and H$_{2}$/H$_{2}$O ratios in volcanic gases both exhibit strong positive temperature dependences, while H$_{2}$S concentrations and H$_{2}$S/SO$_{2}$ ratios are temperature insensitive overall. The high H$_{2}$ concentrations (and low H$_{2}$S/SO$_{2}$ compositions, of ${\sim }$0.1 on average) in high-temperature (${>}$1000 °C) magmatic gases are overall consistent with those predicted thermodynamically assuming external redox buffering operated by the coexisting silicate melt, at oxygen fugacities ranging from ${\Delta }$FMQ $-$1 to 0 (non-arc volcanoes) to ${\Delta }$FMQ 0 to ${+}$2 (arc volcanoes) (where ${\Delta }$FMQ is oxygen fugacity expresses as a log unit difference relative to the Fayalite–Magnetite–Quartz oxygen fugacity buffer). Lower temperature (${<}$1000 °C) volcanic gases exhibit more oxidizing redox conditions (typically above the Nickel–Nickel Oxide buffer) that are caused by a combination of (i) gas re-equilibration during closed-system (gas-phase only) adiabatic cooling in a gas-buffered system, and (ii) heterogenous (gas–mineral) reactions. We show, in particular, that gas-phase equilibrium in the H$_{2}$–H$_{2}$S–H$_{2}$O–SO$_{2}$ system is overall maintained upon cooling down to ${\sim }$600 °C, while quenching of higher temperature equilibria (at which Apparent Equilibrium Temperatures, AETs, largely exceed measured discharge temperatures) is more frequently observed for higher extents of cooling (e.g., at $T < 600$ °C). In such lower temperature volcanic environments, gas–mineral reactions also become increasingly important, scavenging magmatic SO$_{2}$ and converting it into H$_{2}$S and hydrothermal minerals (sulphates and sulphides). These heterogeneous reactions, when occurring, can also control the temperature dependence of the volcanic gas H$_{2}$/H$_{2}$O ratios. Finally, by using our volcanic gas dataset in tandem with recently published global volcanic SO$_{2}$ and CO$_{2}$ budgets, we provide refined estimates for total H$_{2}$S (median, 1.4 Tg/yr; range, 0.9–8.8 Tg/yr) and H$_{2}$ (median, 0.23 Tg/yr; range, 0.06–1 Tg/yr) fluxes from global subaerial volcanism.https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.5802/crgeos.235/HydrogenHydrogen sulphideVolcanic gasesVolcanic gas redoxAtmospheric fluxes |
| spellingShingle | Aiuppa, Alessandro Moussallam, Yves Hydrogen and hydrogen sulphide in volcanic gases: abundance, processes, and atmospheric fluxes Comptes Rendus. Géoscience Hydrogen Hydrogen sulphide Volcanic gases Volcanic gas redox Atmospheric fluxes |
| title | Hydrogen and hydrogen sulphide in volcanic gases: abundance, processes, and atmospheric fluxes |
| title_full | Hydrogen and hydrogen sulphide in volcanic gases: abundance, processes, and atmospheric fluxes |
| title_fullStr | Hydrogen and hydrogen sulphide in volcanic gases: abundance, processes, and atmospheric fluxes |
| title_full_unstemmed | Hydrogen and hydrogen sulphide in volcanic gases: abundance, processes, and atmospheric fluxes |
| title_short | Hydrogen and hydrogen sulphide in volcanic gases: abundance, processes, and atmospheric fluxes |
| title_sort | hydrogen and hydrogen sulphide in volcanic gases abundance processes and atmospheric fluxes |
| topic | Hydrogen Hydrogen sulphide Volcanic gases Volcanic gas redox Atmospheric fluxes |
| url | https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.5802/crgeos.235/ |
| work_keys_str_mv | AT aiuppaalessandro hydrogenandhydrogensulphideinvolcanicgasesabundanceprocessesandatmosphericfluxes AT moussallamyves hydrogenandhydrogensulphideinvolcanicgasesabundanceprocessesandatmosphericfluxes |