Hydrothermal activity due to the deepest submarine volcanism

Ferromanganese oxides formed by hydrothermal activity have been collected from a petit-spot volcano that erupted on an old and cold oceanic plate prior to subduction. This is the first evidence of hydrothermal activity due to the deepest submarine volcanism.
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We describe the formation process of hydrothermal ferromanganese (Fe–Mn) oxides, obtained from a petit-spot volcano at deeper water depth than any other hydrothermal site known to date, in our paper published in Communications Earth & Environment.

In 2015, when I was an undergraduate student, I became interested in petit-spot volcanoes, which have been reported as a new kind of volcano in Science (after many twists and turns) in 2006. Because petit-spot volcanoes erupt the magma just below tectonic plate in response to plate flexure and fissure, they are the only volcanism that occurs on old and cold oceanic plate without conventional hydrothermal activity. Although submarine hydrothermal activity plays an important role in biogeochemical cycle, active petit-spot hydrothermal site or the vestige of hydrothermal activity were not recognized. Therefore, I started the study of petit-spot hydrothermal activity with my supervisor, Assoc. Prof. Hirano. However, our project had not easily found any evidence of the hydrothermal activity in the rock samples from petit-spot volcanoes.

 

In 2018, when the study was halfway abandoned, the samples were collected (Figure 1) during the cruise KS-18-9 of the research vessel named ‘Shinsei-maru’, at one of the youngest petit-spot volcano on the outer-rise (i.e., convexly flexed zone of oceanic plate) of the Pacific Plate off the northeast Japan. The petit-spot lavas and Fe–Mn oxides (Figure 2) were dredged from a petit-spot volcano at 5.7 km water depth. I soon realized that the Fe–Mn oxides were of hydrothermal origin, because their cross-sections showed a dull metallic luster.

Figure 1: Dredge operation during the cruise KS-18-9. The three persons in the foreground of the photograph are the authors (K. Azami, N. Hirano and S. Machida from the left).
Figure 2: Photographs of the petit-spot lavas and Fe–Mn oxides sampled by the dredge during the cruise KS-18-9. High vesicularity is typical feature of petit-spot lavas. The Fe–Mn oxides show a dull metallic luster.

After the research cruise, we immediately analyzed the Fe–Mn oxide samples. Their chemical and mineral compositions clearly demonstrate that they were precipitated from a low-temperature hydrothermal fluid. This is the evidence that the deepest hydrothermal activity occurred around petit-spot volcano, because a petit-spot volcano is the only volcano in the dredge site at water depth deeper than any hydrothermal site known to date (<4,960 m). In addition, the concentrations of Eu and Mo, which are easily leached from rocks by high-temperature fluid–rock interaction, in them are lower than those in hydrothermal Fe–Mn oxides from conventional volcanoes, suggesting the weaker hydrothermal alteration of oceanic crust by petit-spot volcanoes than conventional volcanisms.

To reveal the formation process of the samples (i.e., the process of hydrothermal activity), we performed independent component analysis (ICA) on the elemental mapping data of the samples. ICA is the mathematical statistical method for extracting original signals from data, which is the mixture of signals from independent signal sources. When applied to geochemical data, components and processes controlling the chemical composition of samples are extracted as independent components. The formation process of the samples (Figure 3) revealed by the results of ICA and geochemical analysis is as follows. (i) A low-temperature hydrothermal fluid, produced by fluid–rock interactions at probably <200 °C, diffused through sediments and precipitated Mn oxides at the sediment–seawater interface. (ii) Downward growth of Mn oxides impedes the escape of ascending hydrothermal fluids. (iii) Fe oxides are precipitated beneath the Mn oxide cap layer. During the hydrothermally active period (19.0–76.3 kyr), silicate debris is altered by the hydrothermal fluid. (iv) A hydrogenetic rim grows on seawater-exposed surface after the cessation of the hydrothermal activity.

Figure 3: Schematic of the formation process of the samples (Azami et al. 2023, Comm. Earth Envi.).

Petit-spot volcanic activities are common on world subduction zone. Therefore, we estimated heat and water volume fluxes from petit-spot hydrothermal activity using a simple geophysical method. Assuming that all heat from petit-spot magma produced hydrothermal fluids with temperatures of 5–50 °C, we estimated the global heat and water volume fluxes from petit-spot hydrothermal systems to be 1.8 × 109 W and 2.9–48 × 1011 kg/yr, respectively, ~0.1% and several percent of those along the global mid-ocean ridge (MOR) axis, respectively. This estimated water volume flux suggests that petit-spot hydrothermal systems exert a nonnegligible influence on global biogeochemical cycle, depending on the chemistry of the hydrothermal fluid.

Hydrothermal fluids upwelling through sediments tend to be up to 100 times richer in CH4 and NH3 than those emanating from sediment-free crusts. Magmatic volatiles are also an important factor controlling the chemistry of hydrothermal fluids. Petit-spot magmas are extremely volatile-rich, with pre-degassing CO2 contents of 10–100 times higher than that of typical MOR basalts. Assuming that the CO2 and CH4 contents in petit-spot hydrothermal fluids are enriched to levels ten times those of MOR hydrothermal fluids, we roughly estimate that global emissions from the petit-spot systems would be 1.7–80 × 1011 mol/yr of CO2 and 0.7–48 × 109 mol/yr of CH4, which are over several tens of percent compared to those from the entire MOR axis. Thus, petit-spot hydrothermal systems, which emit certainly Fe and Mn and possibly CH4 and NH3, may serve as habitable oases for hydrothermal organisms (at least for chemosynthetic bacteria) on the cold oceanic plate, which has been considered as a barren desert for hydrothermal organisms.

Therefore, we are now planning to investigate the dredge site using the Manned Submersible ‘Shinkai 6500’ and also hydrothermal experiments to estimate the fluid chemistry. These future studies will deepen the understanding of the role of petit-spot hydrothermal activity in the context of the global biogeochemical cycle.

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