Submarine magmatic-hydrothermal system represents a unique hydrothermal system caused by the direct mixing of magmatic volatiles with seawater, which generates huge material and heat fluxes, and shows broad range of chemical and physical variability. However, the fluid components, especially gases, vary as a result of drastic temperature and pressure changes during the measurement process, affecting the accurate assessments of the hydrothermal fluxes.
Recently, we developed a high temperature resistant (up to 450°C) Raman insertion probe for hydrothermal vent (RiP-Hv) that can be applied to hydrothermal fluid measurement. Here, a high-precision in situ Raman quantitative technique was used for the hydrothermal fluids at the Onsen site and a newly found site named Faxian in a magmatic-hydrothermal system at the DESMOS caldera, Manus back-arc basin. The maximum in situ H₂ concentration (8.56 mmol/kg) and measured H₂ concentration in the lab (26.51 mmol/kg) at the Onsen site are the highest values ever reported in the similar hydrothermal systems. In contrast, the diffuse fluid at the Faxian site was characterized by rich H₂S (7.78 mmol/kg,) but without H₂. The differences at mixing contents of seawater induced different geochemical reactions and fluxes of hydrogen and hydrogen sulfide. Furthermore, the existence of hydrogen- and sulfide-oxidizing genes suggests that the oxidation of hydrogen and hydrogen sulfide are the energy sources of the microbial communities at the two sites even within the same magma source.
Approximately 75% of submarine volcanic activity are distributed at the volcanic arc and back-arc and accompanied with diverse microbial communities, which suggests widespread submarine magmatic-hydrothermal systems contribute to studying the hydrothermal fluxes, and relationships between early life and volcanism based on in situ detection.