Scientists at the University of Liverpool have demonstrated that deep sea fault zones could transport much larger amounts of water from the Earth’s oceans to the upper mantle than previously thought.
Over the age of the Earth, the equivalent of as many as 3.5 present-day oceans of water could be subducted along the Japan subduction zone.
Using seismic modelling techniques the researchers analyzed earthquakes which occurred more than 100 km below the Earth’s surface in the Wadati-Benioff zone.
Analysis of the seismic waves from these earthquakes shows that they occurred with low seismic velocities. Seismic waves travel slower in these fault zones than in the rest of the subducting plate because the sea water that percolates through the faults reacts with the oceanic rocks to form serpentine – a water containing mineral.
Researchers have shown that serpentinized normal faults are present at 50 – 150 km depth and are directly linked to lower plane seismicity of the Wadati-Benioff fault. Normal, or Dip-slip, faults are inclined fractures where the blocks have mostly shifted vertically. They quantified the volume of mineral bound H2O in the mantle lithosphere due to hydration from outer rise faulting, and showed that the mantle lithosphere is the most significant contributor to subducted water, transporting more than 10 times as much water as the oceanic crust and sediments put together.
Further modelling suggests that much of the water carried in the lithospheric mantle may be transported to the transition zone. It also suggests that the slab mantle in Japan is able to carry much larger volumes of water than previously thought, providing a mechanism to explain the hypothesized highly hydrated mantle transition zone.