Flash vaporization and migration of iodine in the oceanic plate subduction zone

Crustal fluids in subduction zones, such as subsurface aquifers, submarine seeps, and gas hydrate waters, are often rich in iodine (I₂) and methane (CH₄). Large-scale aquifers in the Kanto subduction zone, where the Pacific Plate (PAC) and the Philippine Sea Plate (PHS) are subducting, also exhibit high concentrations of I₂ and CH₄. However, the origin and behavior of I₂ in the subduction zone are unclear, and its coexistence with CH₄ remains unresolved. To investigate this, we compiled the I₂ phase diagram under high-pressure and high-temperature (P–T) conditions to predict its physicochemical properties in the subduction zone. We then applied the P–T paths of subducted PAC and PHS sediments to the I₂ phase diagram. Our findings reveal that I₂ can exist as a liquid in the young and hot PHS subduction zone. Transient decompressions during earthquake ruptures can cause liquid iodine to flash-vaporize and be expelled from subducted sediments. Along with I₂, thermogenic CH₄ and hydrogen (H₂) generated in the subducted sediments are also released and transported upward, likely by slab-dehydrated fluids. Additionally, H₂ may enhance microbial CH₄ production through hydrogenotrophic methanogenesis. In subduction zones of young and hot oceanic plates such as the PHS, crustal fluids are enriched in I₂ and coexist with CH₄ owing to the simultaneous expulsion of I₂, CH₄, and H₂ from the same subducted sediments and their migration via deep fluids. Large subsurface aquifers can act as traps and reservoirs for migrating I₂ and CH₄, forming large-scale I₂ and CH₄ deposits.