Petit-spot hydrothermal systems simulated: Fluid evolution, chemical fluxes, and chemoautotrophic habitability

Hydrothermal systems at petit-spot volcanoes are unique in that they induce hydrothermal activity around outer rises—i.e., oceanic plates prior to subduction. However, their biogeochemical significance remains poorly understood due to a lack of direct observation. In this study, hydrothermal experiments were conducted at 250°C and 350°C under 500 bar to simulate petit-spot hydrothermal systems. Petit-spot volcanism introduces volatile-rich magma into thick sediment layers. Accordingly, natural petit-spot lava and ambient pelagic sediment were used as initial solid materials, while a CO2-rich NaCl solution served as the initial liquid to supplement CO2 degassed from the lava. The Nd-Sr-Pb isotope ratios of clastics in hydrothermal ferromanganese oxides collected from a petit-spot volcano indicated sediment involvement in water–rock interactions. This sediment involvement enhanced CH4 concentrations in the reacted fluid. Additionally, Ca, Fe, Mn, and Ba concentrations increased, likely due to a pH decrease associated with the production of organic acids. The decrease in Mo concentration due to sediment involvement, and the reduction in the Mn/Fe ratio resulting from lower reaction temperatures, observed in the experiments, do not contradict the geochemical characteristics of hydrothermal ferromanganese oxides collected from a petit-spot volcano. These findings suggest that the experiments successfully simulated petit-spot hydrothermal systems. Although the chemical fluxes from global petit-spot hydrothermal systems to the ocean may be negligible due to their limited fluid discharge, the potential metabolic energy available in the mixture of seawater and hydrothermal fluid near the seafloor is comparable to that of other hydrothermal systems where vent ecosystems have been confirmed. Therefore, petit-spot hydrothermal systems are likely habitable, at least for chemosynthetic microbes. Under certain conditions—such as high fluid flux during the early stages of hydrothermal activity, and the involvement of ultramafic xenoliths and SO2 gas in water–rock interactions—petit-spot hydrothermal systems may even serve as oases for higher organisms on old, cold oceanic plates.