Aqueous Fluid Connectivity in Subducting Oceanic Crust at the Mantle Transition Zone Conditions

Abstract

Experiments were performed at 17–19 GPa and 1000–1200 °C to determine the aqueous fluid-majoritic garnet-majoritic garnet dihedral angle θgrt-grt in a basalt-H2O system. The results show that the θgrt-grt is between 44 ± 2° and 55 ± 3°, decreasing with increasing pressure and temperature. These new data combined with previous data obtained in the aqueous fluid-olivine and aqueous fluid-garnet systems suggest that connected network of aqueous fluids can form in the peridotite part of the subducting slab but may not form in a cold subducting oceanic crust at pressures below 14 GPa. Therefore, aqueous fluids formed by dehydration of a cold slab could be trapped as interstitial fluids in the oceanic crust and transported into the deep mantle. However, at the mantle transition zone (MTZ) conditions, aqueous fluids trapped previously and/or formed lately by the breakdown of hydrous minerals can percolate through the oceanic crust and hydrate the MTZ, providing an important mechanism for the MTZ hydration. Furthermore, aqueous fluids formed by mineral dehydration in a hot slab are readily lost into the mantle wedge at shallow depth, due to low dihedral angles (θ < 60°) of the subducting oceanic crust, resulting in less water available to hydrate the MTZ. The distinct contribution of the cold slab and the hot slab to the MTZ hydration may cause water heterogeneity in the MTZ.