Low Thermal Conductivity of Hydrous Phase D Leads to a Self‐Preservation Effect Within a Subducting Slab

Abstract

Abstract Earth's deep water cycle impacts the physical and chemical properties and geodynamics in its deep interior. However, how dense hydrous magnesium silicates (DHMSs) influence the thermal evolution and dynamics of sinking slabs remains poorly understood. We have precisely measured thermal conductivity of phase D, an important DHMS that could carry large amounts of water from the mantle transition zone to the lower mantle, at high pressure‐temperature conditions. The thermal conductivity of (Al,Fe)‐bearing phase D is lower than those of the pyrolitic mantle and basaltic crust along slab subduction, except for the depth range of ∼800–1,100 km where a spin transition of iron occurs. Numerical simulations indicate that although the spin transition in phase D has minor effects on slab's temperature due to its small volume fraction, the poorly thermally‐conductive hydrous minerals contribute to maintain a cold hydrous layer within a sinking slab, stabilizing slab hydrous minerals and promoting water transportation to the deeper mantle.