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Seismic anomalies observed in Earth's deep mantle are conventionally considered to be associated with thermal and compositional anomalies, and possibly partial melt of major lower-mantle phases. However, through deep water cycle, impacts of hydrous minerals on geophysical observables and on the deep mantle thermal state and geodynamics remain poorly understood. Here we precisely measured thermal conductivity of delta-(Al,Fe)OOH, an important water-carrying mineral in Earth's deep interior, to lowermost mantle pressures at room temperature. The thermal conductivity varies drastically by twofold to threefold across the spin transition of iron, resulting in an exceptionally low thermal conductivity at the lowermost mantle conditions. As delta-(Al,Fe)OOH is transported to the lowermost mantle, its exceptionally low thermal conductivity may serve as a local thermal insulator, promoting high-temperature anomalies and the formation of partial melt and thermal plumes at the base of the mantle, strongly influencing thermo-chemical profiles in the region and fate of Earth's deep water cycle.

Plain Language Summary Hydrous minerals subducted to Earth's deep interior may critically affect the thermo-chemical and seismic signatures observed at the bottom of the mantle. We measured thermal conductivity of delta-(Al,Fe)OOH, an important water carrier in deep Earth, to the lowermost mantle pressures. Its thermal conductivity varies drastically across the spin transition of iron and approaches an exceptionally low value of similar to 5 W.m(-1).K-1 at the lowermost mantle conditions, much smaller than the pyrolitic mantle. Such anomalous evolution of thermal conductivity would induce anomalies in heat flux and temperature profile in the lower mantle. It could create a local thermal insulating effect that heats up slab's crust at the lowermost mantle, facilitating dehydration melting of surrounding mantle and affecting thermo-chemical features in the region.