EPISODIC RELEASE OF MANTLE-DERIVED FLUIDS ALONG THE PLATE INTERFACE: HELIUM ISOTOPE EVIDENCE AFTER THE TOHOKU-OKI EARTHQUAKE

Fluids are essential in subduction zones, including a trigger for partial melting to produce arc magmatism and also for seismic activity; in addition, ore deposits in subduction zones form during hydrothermal activity where fluids transport metals. In a subduction system, fluids together with fluid-mobile metals are incorporated from slabs subducted at the trench, expelled during progressive decent, and discharged through arc volcanoes. Geophysical evidence supports the idea that the interface between subducting and overlying plates are filled with fluids as sediments and slabs release large quantities of water and other volatile elements during their subduction.

The M9.0 Tohoku-oki earthquake in 2011 provided evidence that the movement of fluid during a large earthquake is significantly different from that during quiet periods. The compositions of bottom seawater near the trench have been measured at varying depths since 2007. The record shows steady-state compositions of water with 3He/4He, about ~ 1.15 Ra (Ra= atm. value) before the earthquake, but the value increased sharply to 1.17 -1.19 Ra one month after the earthquake at depths greater than 3500 m. This was accompanied by the release of high¹³C methane. The fluid compositions returned to pre-earthquake values ~ 5 months after the event. The locations and timing of high 3He, primordial gas, indicate the release of mantle-derived fluids along the plate interface during the earthquake. The flux of 3He, ~ 1 atom/cm2/sec, is similar to that for the metalliferous hydrothermal activity in the Okinawa back-arc trough. The Tohoku-oki earthquake ruptured the interface between the subducting and overlying plates and produced a co-seismic slip of more than 50 m. The rupture likely increased permeability for the quick release of overpressured fluids from the mantle to the surface. This displacement produced a large tsunami. More than 22 tsunamis have been reported since the 9^th century in the area. The deposit related to the AD869 event indicates that the displacement associated with the earthquake was comparable or even larger than that of the 2011 Tohoku-oki earthquake. This suggests that large earthquakes, which are common in subduction zones, likely induce periodic release of fluids and other volatiles from the mantle to the shallow crustal levels