On the Dynamics of Water Transportation and Magmatism in the Mid-Mantle

The distribution of water within the Earth's mantle has significant implications for the Earth's dynamics and evolution. Recent mineral physics experiments indicate that dense hydrous magnesium silicates can contain large amounts of water stable up to 60 GPa or even beyond along slab geotherms. Here we perform petrological-thermomechanical numerical simulations of water transportation by deep slab subduction and related magmatism in the mid-mantle. Key parameters including those defining the slab thermal parameter and the water storage capacity in the oceanic lithosphere and surrounding mantle are explored. The results show two major dehydration events of ultramafic rocks at around 150 and 750 km by dehydration of serpentine at 600 degrees C and superhydrous phase B in the entrained wet upper mantle, respectively. Large amounts of water, similar to 1.5 wt% at least locally, are carried down to the mantle transition zone and lower mantle. We estimate an upper limit of slab water flux into the mid-mantle of 0.1-0.28 x 10(12) kg/yr, which is similar to 13%-37% of the input water from the serpentinized mantle. Moreover, a substantial fraction of the water released by the slab is absorbed by the entrained mantle and overlying mid-mantle portions, such that similar to 30%-70% of the water injected at the trench could be delivered to the lower mantle. The deepest magmatism is observed at similar to 1,500 km in case of phase H breakdown (MgO-SiO2-H2O system), coinciding with the depth of strong seismic attenuation. Overall, these simulations suggest that up to 0.2 ocean mass per billion years could be transported down to the mid-mantle and beyond. Plain Language Summary Water plays a crucial role in mantle dynamics as it decreases the effective viscosity and melting temperature. Deep diving slabs can carry water to sub-arc, mantle transition zone, and lower mantle depths. However, the amount of water that can be entrained by the slab depends on its thermal structure and the water storage capacity of the oceanic crust and surrounding mantle. With the rapid progress of laboratory experiments, many high-pressure hydrous phases have been reported in recent years. Here we combine different high-pressure high-temperature experimental data sets together with geodynamic models to study the water transportation and related magmatism down to the mid-mantle. With the incorporation of the dense hydrous magnesium silicates, our results show significant slab dehydration due to the breakdown of superhydrous phase B at similar to 800 km and of phase H at similar to 1,500 km (MgO-SiO2-H2O system), which is coincident with the seismic low-velocity and strong attenuation zones at similar to 800-1,500 km globally. Our model further shows that up to 0.2 surface mass ocean can be transported beyond 1,500 km per billion years.