Deep-earth water transport beyond the transition zone via subducted feldspars

Subducted potassium feldspar transforms into a hollandite-type structure, rendering it a plausible carrier for large lithophile elements and volatiles, possibly including molecular water, into the deep mantle. Nevertheless, the capacity of this phase to incorporate and retain such incompatible species under deeper mantle conditions has remained largely unconstrained. We report here the formation of the hydrated hollandite-type structures from subducting K-bearing feldspars in the presence of H₂O near 400 km depth conditions. Combined in situ high-pressure and -temperature synchrotron X-ray diffraction and ex situ synchrotron Fourier-transform infrared spectroscopy reveal molecular H₂O incorporated within the expanded 1D tunnels together with hydroxyls through the aluminosilicate octahedral framework of the hollandite-type structure, yielding up to net ~1.97 wt.% water content. Subsequently, dehydration occurs via framework distortion upon exceeding the lower boundary of the mantle transition zone near 720 to 780 km. We estimate that, over a 200 Ma of subduction cycle, hydrated hollandite-type phases may have contributed the water flux to the topmost lower mantle by the amount to suppress the formation of the representative lower mantle phase, bridgmanite. This process may thus be linked to the apparent depression of 660-km discontinuity in penetrating slabs and their surrounding regions to address key limitations of existing akimotoite- and basalt-derived models.