Potassium isotopic evidence for recycling of surface water into the mantle transition zone

Global water cycling involves interactions between Earth's interior and its surface environment. Geophysical and mineral physics studies have suggested that the mantle transition zone is hydrous, at least locally. However, there are poor constraints on whether water in the transition zone is sourced internally from primordial materials or from Earth's surface via subduction-related processes. Cenozoic volcanism in Northeast Asia is triggered by hot and wet upwelling flows above the stagnated Pacific slab and has produced mantle transition zone-derived volcanic rocks. Potassium behaves geochemically similarly to water during magmatic processes, and hence can potentially be used to constrain the nature of water in the mantle transition zone. Here we report potassium isotopes in a set of well-characterized Cenozoic volcanic rocks in Northeast Asia. Their K isotope ratios (-0.83 parts per thousand to -0.36 parts per thousand) are lower than primitive mantle (-0.42 parts per thousand +/- 0.08 parts per thousand), suggesting crustal potassium inputs and modifications of the mantle transition zone by subducted slab materials. Decoupling of potassium isotopes from radiogenic Sr-Nd-Pb isotopes, combined with a geophysically identified low-resistivity anomaly above the transition zone, requires the input of surficial water from the stagnated subducted slab into the transition zone. This water can then be cycled back to the surface in Northeast Asian volcanics. Water in the mantle transition zone beneath Northeast Asia is sourced from the Earth's surface and introduced by the subducted Pacific slab, according to a study of potassium isotopes from Cenozoic volcanics.