Petrological experiments on oceanic crust samples characterize the recycling of potassium from mid-ocean ridge basalts and sediments. Metasomatism could develop directly and continuously from subducted potassium-bearing crust from shallow levels to a maximum depth of 300 kilometers. Phengite (a potassium-rich mica) is the principal potassium host at subsolidus conditions. It transports potassium and water to depths of up to 300 kilometers and could yield over the entire depth range potassium-rich fluids or melts (depending on the specific geotherm), which are likely to constitute one of the primary metasomatic agents for generation of calc-alkaline magmas.