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 (I 2 ) and methane (CH 4 ). 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 I 2 and CH 4 . However, the origin and behavior of I 2 in the subduction zone are unclear, and its coexistence with CH 4 remains unresolved. To investigate this, we compiled the I 2 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 I 2 phase diagram. Our findings reveal that I 2 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 I 2 , thermogenic CH 4 and hydrogen (H 2 ) generated in the subducted sediments are also released and transported upward, likely by slab-dehydrated fluids. Additionally, H 2 may enhance microbial CH 4 production through hydrogenotrophic methanogenesis. In subduction zones of young and hot oceanic plates such as the PHS, crustal fluids are enriched in I 2 and coexist with CH 4 owing to the simultaneous expulsion of I 2 , CH 4 , and H 2 from the same subducted sediments and their migration via deep fluids. Large subsurface aquifers can act as traps and reservoirs for migrating I 2 and CH 4 , forming large-scale I 2 and CH 4 deposits. • I 2 in oceanic sediments subducting with the hot oceanic plate may behave as a liquid. • I 2 vaporizes and is expelled from subducted sediments during earthquake ruptures. • I 2 , along with thermogenic CH 4 and H 2 , is co-expelled from subducted sediments. • Crustal fluids in the hot oceanic plate subduction zone are enriched in I 2 and CH 4 .