Serpentine and the subduction zone water cycle

This study explores a chemo-thermo-dynamic subduction zone model that solves for slab dehydration during subduction. We investigate how changes in the incoming plate's hydration and thermal structure may effect the efficiency of sub-arc water release from sediments, crust, and serpentinized mantle. We find that serpentinized lithospheric mantle may not only be an important fluid source to trigger arc melting but is also an efficient ‘transport-lithology’ to recycle chemically bound water into the deeper mantle. In fact, an old slab may remain sufficiently cold during subduction to retain up to 40% of its initial ‘mantle’ water at 8 GPa (∼240-km depth) after serpentine transforms to higher pressure hydrous phase A. Furthermore, deep water recycling at subduction zones is parameterized in terms of slab age and speed. Coupling this parameterization to a parameterized mantle convection evolution model allows us to calculate the mantle-surface geologic water cycle throughout the Earth's history. We find that the present-day Earth mantle may be highly outgassed containing only a small fraction of the Earth's water, which would mostly be recycled water from the exosphere