Deep potassium (K) and sodium (Na) cycles can affect the physiochemical properties and geodynamic processes of the Earth's interior, to some extent. Recently reported natural K- and Na-rich davemaoite inclusion in superdeep diamond (Tschauner et al., 2021), which is still controversial (Walter et al. 2022a; Tschauner et al., 2022), motivated us to reinvestigate the recycling and partitioning behaviours of K and Na in the subducting oceanic crust under deep-mantle P-T conditions. In this study, we experimentally determined the partitioning of Na and K, as well as other major elements, in a natural phengite-rich eclogite system under 5–30 GPa and 850–1550 °C, relevant to P-T conditions from the deep upper mantle to the uppermost lower mantle. We found that the produced davemaoite with the perovskite structure contained minimal amounts of other elements, including K and Na. Therefore, we conclude that K- and Na-rich davemaoite inclusion in the superdeep diamond cannot be inherited from deeply subducted oceanic crusts. In addition, our experimental results further support previous arguments overestimating the K and Na, and other elemental contents in natural davemaoite, which may further indicate that the reported diamond “inclusion” with abnormally high heteroatoms (Tschauner et al., 2021) is not a davemaoite from the lower mantle (Walter et al. 2022a). Finally, we combined the high-pressure phase relationship in the upper continental crust, oceanic basalt, and pyrolite systems from previous studies, and constructed a geodynamical model for the deep K and Na cycles along with the subduction of the oceanic slabs into the uppermost lower mantle.