Limited water contents of wadsleyite and ringwoodite coexisting with hydrous minerals in cold subducting slabs

• Hydrogen partitioning between olivine polymorphs and hydrous phases up to 21 GPa. • Nearly dry wadsleyite and ringwoodite coexisting with hydrous phases in a slab core. • Hydrous phases are a main water carrier in a wet slab core. • Dehydration of hydrous phases can cause deep-focus earthquakes below 660 km depth. • Stability of hydrous phases changes depending on water content in a system. How water is distributed in a subducting slab is essential to understand water transport into the deep mantle and mechanisms of deep-focus earthquakes and slab deformation around the 660-km discontinuity. A recent experimental study demonstrated that water contents of olivine and wadsleyite coexisting with hydrous phase A is limited at upper mantle pressures, suggesting strong water partitioning to the hydrous phase. However, water distribution between nominally anhydrous and hydrous minerals at the deeper mantle is not investigated in detail. We determined water contents in wadsleyite and ringwoodite coexisting with hydrous phases down to transition-zone depths along cold slab temperatures. Wadsleyite coexisting with hydrous phase A has ∼200 ppm water at 14–16 GPa and 800 °C. At 21 GPa, ringwoodite coexisting with superhydrous phase B has 8–13 ppm water at 800 °C and 46 ppm at 900 °C. Thus, olivine and its high-pressure polymorphs are kinetically dry along cold slab core conditions even in a wet subducting slab. Slab deformation and stagnation around 660 km depth can be caused by grain-size reduction due to phase transitions of dry olivine and the presence of rheologically weak hydrous phases. The deepest earthquakes below 660 km depth can be caused by dehydration of hydrous phases.