Mantle-derived helium released through the Japan trench bend-faults

Oceanic subduction zones feature complex fault systems, including bend-faults at the outer trench slope. These structures are suspected to allow seawater infiltration into the crust and mantle. While numerical models suggest this process, direct evidence remains scarce. Seawater penetration into the mantle is difficult to verify due to limited fluid detection methods. Some studies have proposed that bend-faults act as conduits for fluid flow, but the extent and activity of such systems remain uncertain. Prior research has shown that bend-faults are common in subduction zones, yet their role in deep hydration is not fully established. The lack of clear isotopic signatures from mantle-derived fluids has hindered progress in this area. This gap motivated the need for new data on fluid composition and flow patterns. By analyzing helium isotope ratios, researchers can trace fluid sources and pathways in these tectonic settings.

This study aimed to investigate whether bend-faults at the Japan trench facilitate seawater infiltration into the mantle. The researchers sought to determine if such fluid flow is active and detectable through geochemical signatures. They focused on helium isotope ratios in sediment pore water as a tracer for mantle-derived fluids. The study also aimed to assess the spatial extent of fluid infiltration along these faults. By combining geochemical data with seismic reflection imaging, the team aimed to visualize fluid pathways. The goal was to test the hypothesis that bend-faults act as conduits for hydrothermal circulation. This approach could help clarify the role of these faults in crustal and mantle hydration. The findings could provide insights into deep-sea fluid dynamics and subduction zone processes.

The researchers collected sediment pore water samples from the outer trench slope of the Japan trench. They measured helium isotope ratios (³He/⁴He) and neon isotope ratios (⁴He/²⁰Ne) in these samples. Seismic reflection data was used to map fault structures and fluid flow patterns. The study compared isotopic signatures to those of bottom seawater and mantle-derived helium. The team focused on sites near bend-faults to assess localized fluid infiltration. They analyzed the vertical distribution of isotopic ratios to infer fluid sources. The data was integrated with existing geophysical models of the region. This multi-method approach allowed the researchers to evaluate fluid pathways and their activity.

The study found anomalously high ³He/⁴He ratios in sediment pore water at sites near bend-faults. These ratios were close to those of bottom seawater and remained constant with depth. The ⁴He/²⁰Ne ratios also showed consistency with seawater-derived fluids. The data suggests that seawater infiltrates the crust and mantle along these faults. Seismic reflection imaging revealed fluid pathways through the Moho boundary. The findings indicate active hydrothermal circulation through the bend-faults. This system appears to allow mantle-derived fluids to outflow into the crust. The study provides the first evidence of large-scale fluid infiltration via these structures.

The authors propose that bend-faults at the Japan trench facilitate seawater infiltration into the upper mantle. Their data supports the idea that these faults act as active conduits for hydrothermal circulation. The isotopic signatures suggest that seawater-derived fluids reach the mantle and return via the same faults. This process may contribute to crustal hydration in subduction zones. The findings suggest that bend-faults are not passive structures but active fluid pathways. The study highlights the importance of helium isotopes in tracing mantle fluid dynamics. The results align with prior models of fluid flow in subduction zones. These conclusions provide new insights into the role of bend-faults in deep-sea tectonic processes.