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 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.