1. Hydrogen and hydrogen sulphide in volcanic gases: abundance, processes, and atmospheric fluxes
- Author
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Aiuppa, Alessandro and Moussallam, Yves
- Subjects
Hydrogen ,Hydrogen sulphide ,Volcanic gases ,Volcanic gas redox ,Atmospheric fluxes ,Geophysics. Cosmic physics ,QC801-809 ,Chemistry ,QD1-999 ,Geology ,QE1-996.5 - Abstract
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 (${
- Published
- 2023
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