Hydrogen and hydrogen sulphide in volcanic gases: abundance, processes, and atmospheric fluxes

Abstract

Hydrogen (H2) and hydrogen sulphide (H2S) 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 O2). In combination with other major compounds, H2 and H2S 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 H2 and H2S abundance in volcanic gases. We show that H2 concentrations and H2/H2O ratios in volcanic gases both exhibit strong positive temperature dependences, while H2S concentrations and H2S/SO2 ratios are temperature insensitive overall. The high H2 concentrations (and low H2S/SO2 compositions, of ∼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 ∆FMQ −1 to 0 (non-arc volcanoes) to ∆FMQ 0 to +2 (arc volcanoes) (where ∆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 H2–H2S–H2O–SO2 system is overall maintained upon cooling down to ∼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 SO2 and converting it into H2S and hydrothermal minerals (sulphates and sulphides). These heterogeneous reactions, when occurring, can also control the temperature dependence of the volcanic gas H2/H2O ratios. Finally, by using our volcanic gas dataset in tandem with recently published global volcanic SO2 and CO2 budgets, we provide refined estimates for total H2S (median, 1.4 Tg/yr; range, 0.9–8.8 Tg/yr) and H2 (median, 0.23 Tg/yr; range, 0.06–1 Tg/yr) fluxes from global subaerial volcanism.