The role of S−3 ion in thermochemical sulphate reduction: Geological and geochemical implications

- We reveal the formation of S−3 in hydrothermal fluids at T as low as 100 °C.
- The S−3 ion is the key intermediate-valence specie involved in TSR processes.
- The presence of S−3 is required for TSR to proceed efficiently.
- S−3 is stable over a wide range of PVTX conditions.
- It explains the numerous occurrences of TSR in nature.

Thermochemical sulphate reduction (TSR) plays a crucial role in the global sulphur cycle in the Earth's crust, and may affect current and past sulphur isotopic records. However, the extrapolation of experimental reaction rates measured at high temperature (above 200 °C) towards lower temperatures, as well as the interpretation of the sulphur isotopic fractionation recorded in natural samples, require an accurate description of the elementary steps controlling these reactions. We addressed this question through dedicated experiments. Based on in situ Raman spectroscopy measurements, we show that the trisulphur ion S−3 is the dominant intermediate sulphur valence species involved in abiogenic sulphate reduction processes initiated by H2S, over a wide range of temperature (100-350 °C) and solution compositions, whatever the electron donor considered. The in situ spectroscopic data reported here unambiguously demonstrate the presence of S−3 at temperatures as low as 100 °C. The presence of S−3 is critical to achieve rapid sulphate reduction, especially at low temperature. We propose that any dissolved constituent which decreases the dielectric constant of water, or which yields favourable S−3 coordination, will stabilise the trisulphur ion (thus promoting TSR) at T and pH conditions that are less extreme than previously thought. The importance of S−3 in these processes should also be taken into account when discussing the mass-independent sulphur isotopic compositions recorded in natural and/or experimental TSR-related samples.