Reduced Thermal Conductivity of Hydrous Aluminous Silica and Calcium Ferrite-Type Phase Promote Water Transportation to Earth's Deep Mantle

Subduction of oceanic slabs introduces chemical heterogeneities in the Earth's interior, which could further induce thermal, seismic, and geodynamical anomalies. Thermal conductivity of slab minerals crucially controls the thermal evolution and dynamics of the subducted slab and ambient mantle, while such an important transport property remains poorly constrained. Here we have precisely measured high pressure-temperature thermal conductivity of hydrous aluminous post-stishovite (Λ_Hy-Al-pSt) and aluminum-rich calcium ferrite-type phase (Λ_CF), two important minerals in the subducted basaltic crust in the lower mantle. Compared to the dry aluminous stishovite and pure stishovite, hydration substantially reduces the Λ_Hy-Al-pSt, resulting in ∼9.7–13.3 W m⁻¹ K⁻¹ throughout the lower mantle. Surprisingly, the Λ_CF remains at ∼3–3.8 W m⁻¹ K⁻¹ in the lower mantle, few-folds lower than previously assumed. Our data modeling offers better constraints on the thermal conductivity of the subducted oceanic crust from mantle transition zone to the lowermost mantle region, which is less thermally conductive than previously modeled. Our findings suggest that if the post-stishovite carries large amounts of water to the lower mantle, the poorer heat conduction through the basaltic crust reduces the slab's temperature, which not only allows the slab bringing more hydrous minerals to greater depth, but also increases slab's density and viscosity, potentially impacting the stability of heterogeneous structures at the lowermost mantle.